**Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles**

J. Alejandro Betancur *EAFIT University Colombia* 

## **1. Introduction**

242 Augmented Reality – Some Emerging Application Areas

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The command dashboard of an automobile is the instrument where most of the information related to the current state of the vehicle is displayed, and visually it is the way the driver can have access to that information. Nowadays, it is a great risk not only for the driver, but also for the passengers that the driver has to focus his attention off the road to focus on the dashboard information; on situations like this, are when Head-Up Display (HUD) systems represent a significant breakthrough in terms of automotive safety.

The kind of HUDs here analyzed are the Optical See-through Augmented Reality systems, where the objects of an outer environment are combined with previously structured additional information, and the result is visualized by mean of a translucent display (combiner), which is generally the windshield of an automobile. On this type of system, what we want to accomplish is an interaction with the user through a series of visual stimuli and artificial scenes.

This chapter focuses on the approach of the functional design requirements that must include an HUD system applied to current automobiles, in which the fast acquisition of the vehicle's available functional information in its panel would imply a positive impact in terms of the vehicle's simplicity to be driven, security, handling information, communication, among others. The analysis of the functional aspects of this technology will be covered by the theoretical analysis and the instrumental implementation, so that we can understand the behavior of optic phenomena that are involved in these systems, in order to be able to determine which parameters are the most critical in the possible construction of an HUD. The appreciations and considerations expressed in this chapter are defined after several years of applied and exploratory research that gathers documental fields and laboratory aspects, using the characteristics from the scientific method as aid, determining what is transcendent and what is possible from the facts.

The general objective of this chapter is to evaluate the scientific and engineering phenomena from an HUD, in order to set main characteristics and crucial parameters in terms of physics, thus, determining what considerations are fundamental for the proper functioning of these types of augmented reality systems, and identifying the functional and physical characteristics from the basic optical elements of HUD's visualization systems adapted to automobiles.

This chapter is addressed in four phases: the first phase approaches the analysis of the conceptual information about what is currently structured like HUDs applied to

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 245

a) b)

c) d)

Although, there is still a lot more analysis to be done in these augmented reality systems with the intention of establishing regulation and utilization rules, it must be settled that the quantity of HUDs and HDDs applications found so far in automobiles are really efficient in

Next, Figure 2 represents the flow of information between the main sections of many augmented reality system; this, as a starting point to begin a discussion about the main considerations that must be taken into account in the configuration of the HUD system here proposed.

The HUD system that is suggested here uses catadioptric elements as the main resource for the presentation of the information to the observer; according to Figure 2, it must be considered that independently from the principle of functioning of the HUD, three (3) essential sections

Currently, a large quantity of options for emissive displays can be found, some of which will be mentioned on Table 1, where some essential relations and comparisons are included.

are present in this kind of augmented reality system, which are detailed here below.

Fig. 1. (a) Right HUD; (b) Middle HUD; (c) Left HUD; (d) HDDs.

Fig. 2. Sections of the augmented reality system proposed.

**3. Classification of physical components** 

terms of the driver's safety.

**3.1 Opto-electronical system** 

automobiles; the second phase focuses on describing design alternatives for the conception of an HUD applied to an automobile; the third stage is where all the ideas presented in the previous stages (according to their real development potential) converge in a structured design proposal; finally, on the fourth phase we reach conclusions related to the most relevant data depicted in the chapter, reporting interpretations, relationships, reaches, and the structuring of a possible application.

Something very notorious in current HUD's applications is the degree of shyness in these systems in terms of images projected, since a level of sophistication has not been reached yet that allow to discard completely the tachometer and the different instruments clusters, by projected images on the windshield or on another type of element that works as a combiner; this situation proves that although significant goals have been achieved within the last few years, it is an understatement that more analysis and numerous resources are still needed with the intention of commercialize these type of devices to a higher level.

Finally, the HUDs technologies applied to automobile industries, should not only be considered as their exclusive field of application, since the use of augmented reality systems such as these are being incorporated into daily activities, i.e. answering the phone, studying, etc.

The study and the appropriation of concepts referred to this technology is appealing because it contributes to the design and redesign of several visual mobile technologies, and according to previous researches (Amezquita, 2010; Dawson et al., 2001; Betancur, 2010; Pinkus & Task, 1989), a tendency in most visualization devices to adopt the elements and the configurations that are present in HUD systems is noted.

## **2. Definition and justification of HUDs**

Nowadays, we find a notable tendency in automobiles to offer a lot more information to the driver, which has its own direct repercussions in terms of driving, due to the lack of concentration that implies when having so many options, reason why, it is necessary to think in proposing a quick way to access the information that an automobile offers.

Starting from this point, we must think about a much more efficient communication interface between the user and the automobile that will not interfere with the user's driving; thus, augmented reality devices are born as facilitators and boosters that will give us application ideas to significantly reduce the time needed to be able to interact with the vehicle's diverse options (Brown et al., 1988).

Up until now, the best way to supply the need that we previously mentioned is with the augmented reality devices HUDs and HDDs (Head-Down Displays), which can be set up in a way that will provide to the driver the quick acquisition of information about vehicle's state, thus, obtaining a direct repercussion in the elimination of several risk situations.

In this chapter, we are looking to explain in a sequential way some coherent alternatives to link the driver to the systems and options offered by an automobile through the use of HUDs (being these systems our main subject of study) and HDDs. The operation context of these technologies can be seen in the Figure 1, where it is clear how the driver is influenced by the surroundings of an augmented reality through an image generated from an optoelectronical system.

244 Augmented Reality – Some Emerging Application Areas

automobiles; the second phase focuses on describing design alternatives for the conception of an HUD applied to an automobile; the third stage is where all the ideas presented in the previous stages (according to their real development potential) converge in a structured design proposal; finally, on the fourth phase we reach conclusions related to the most relevant data depicted in the chapter, reporting interpretations, relationships, reaches, and

Something very notorious in current HUD's applications is the degree of shyness in these systems in terms of images projected, since a level of sophistication has not been reached yet that allow to discard completely the tachometer and the different instruments clusters, by projected images on the windshield or on another type of element that works as a combiner; this situation proves that although significant goals have been achieved within the last few years, it is an understatement that more analysis and numerous resources are still needed

Finally, the HUDs technologies applied to automobile industries, should not only be considered as their exclusive field of application, since the use of augmented reality systems such as these are being incorporated into daily activities, i.e. answering the phone, studying,

The study and the appropriation of concepts referred to this technology is appealing because it contributes to the design and redesign of several visual mobile technologies, and according to previous researches (Amezquita, 2010; Dawson et al., 2001; Betancur, 2010; Pinkus & Task, 1989), a tendency in most visualization devices to adopt the elements and

Nowadays, we find a notable tendency in automobiles to offer a lot more information to the driver, which has its own direct repercussions in terms of driving, due to the lack of concentration that implies when having so many options, reason why, it is necessary to

Starting from this point, we must think about a much more efficient communication interface between the user and the automobile that will not interfere with the user's driving; thus, augmented reality devices are born as facilitators and boosters that will give us application ideas to significantly reduce the time needed to be able to interact with the

Up until now, the best way to supply the need that we previously mentioned is with the augmented reality devices HUDs and HDDs (Head-Down Displays), which can be set up in a way that will provide to the driver the quick acquisition of information about vehicle's state, thus, obtaining a direct repercussion in the elimination of several risk situations.

In this chapter, we are looking to explain in a sequential way some coherent alternatives to link the driver to the systems and options offered by an automobile through the use of HUDs (being these systems our main subject of study) and HDDs. The operation context of these technologies can be seen in the Figure 1, where it is clear how the driver is influenced by the surroundings of an augmented reality through an image generated from an opto-

think in proposing a quick way to access the information that an automobile offers.

with the intention of commercialize these type of devices to a higher level.

the configurations that are present in HUD systems is noted.

**2. Definition and justification of HUDs** 

vehicle's diverse options (Brown et al., 1988).

electronical system.

the structuring of a possible application.

etc.

Fig. 1. (a) Right HUD; (b) Middle HUD; (c) Left HUD; (d) HDDs.

Although, there is still a lot more analysis to be done in these augmented reality systems with the intention of establishing regulation and utilization rules, it must be settled that the quantity of HUDs and HDDs applications found so far in automobiles are really efficient in terms of the driver's safety.

Next, Figure 2 represents the flow of information between the main sections of many augmented reality system; this, as a starting point to begin a discussion about the main considerations that must be taken into account in the configuration of the HUD system here proposed.

Fig. 2. Sections of the augmented reality system proposed.

## **3. Classification of physical components**

The HUD system that is suggested here uses catadioptric elements as the main resource for the presentation of the information to the observer; according to Figure 2, it must be considered that independently from the principle of functioning of the HUD, three (3) essential sections are present in this kind of augmented reality system, which are detailed here below.

#### **3.1 Opto-electronical system**

Currently, a large quantity of options for emissive displays can be found, some of which will be mentioned on Table 1, where some essential relations and comparisons are included.

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 247

Next, on Table 2 a description of the potential general stages of an HUD optical system is made, with the purpose of examining and inquiring about the importance of some optical

Description

It is necessary to determine the location of the HUD inside the vehicle, since 95% of the subsequent configurations depend on this

The position of the emissive display in relation to the suggested optical system is critical in terms of the augmentation of the

The light generated by the emissive display that can be assumed as omni-directional in 180º, must become collimated light in order to






The aberrations introduced in the image through an optical system are compensated through the windshield, reason why a legible

The image generated by the HUD system must have some specified visual requirements that guarantee their own integrity and acceptance from the user, considering that independently from the quality of the image showed to the observer, we are looking to

obtain a virtual, augmented, straight and collimated image.

generated image, if it is real, virtual, straight or inverted.

from mirrors, chromatic aberrations are not generated.

be manipulated easier in further stages.

aberration introduced by the windshield.

of digital imaging pre-processing.

has to be approximately spherical.

image would be appreciated.

of anti-refractive coatings.

considerations to take into account in this type of augmented reality systems.

initial parameter.

**3.2 Optical system** 

General stages of the optical system

Determination of the position and the occupied space by the optical system.

Positioning of the emissive display

Collimation of the image coming from the emissive display

Introduction of aberrations to the projected images

Compensation of the projected image

Generation of the final image

Table 2. General optical stages in HUD systems.


Table 1. Types of opto-electronical devices.

It is important to mention that an inefficient system in terms of illumination could have a direct repercussion in a large quantity of consumed energy by the emissive display, and consequently, in the use of refrigeration systems due to the generation of heat, making the HUD systems more complex and expensive (Pinkus & Task, 1989).

## **3.2 Optical system**

246 Augmented Reality – Some Emerging Application Areas

It is a device that is based on the modulation of the intensity and/or phase of a light beam; this modulation depends on the position of incidence of the previously mentioned beam onto the recording mean of the SLM

It is a device that projects a beam of electrons over a phosphorus covered screen (this material lights up when impacted by electrons beams). The mini colored CRTs are made out of three (3) phosphorus layers: red, green

It is a semi-conductive device that emits a reduced light spectrum when polarized in a direct way and electric

It is a screen made up of a number of color or mono chrome pixels placed in front of a light source. It is based on a layer of aligned molecules between two (2) transparent electrodes and two

It is the result of a liquid crystal screen (LCD) with a better image quality than the later due to the use of thin-film

 It is a diode that uses an organic component film that reacts to a specific electric stimulation, generating and emitting light by themselves (Dawson & Kane, 2001).

It is a group of OLED pixels that are made up of an assembly of Thin-Film Transistors (TFT) in order to form a pixel matrix (Dawson & Kane, 2001).

HUD systems more complex and expensive (Pinkus & Task, 1989).

It is important to mention that an inefficient system in terms of illumination could have a direct repercussion in a large quantity of consumed energy by the emissive display, and consequently, in the use of refrigeration systems due to the generation of heat, making the

current flows through it.

(2) polarization filters.

transistors (TFT).

Table 1. Types of opto-electronical devices.

(Amezquita, 2010).

and blue.

Functioning principle Relations

user's eyes.

than LCDs.

consumption.

contrast range.

(Dawson, 1998).

Only 1% of the generated light from LEDs (under daytime operating conditions) will get to the user. Due to the energy loss by transmission on the windshield, less than 10% of the luminescence produced by a LCD will reach the

Although the LCDs usually have better poly-chromatic contrast than Mini CRTs, this last one usually has a higher mono-chromatic contrast

LCD panels tend to have a limited vision angle in relation to Mini CRT

The organic layers of OLEDs are much more luminescent, malleable and thin than crystalline layers of a TFT-LCD, which results in saving space as well as energy

When emitting their own light, an OLED screen is much more visible under standard daytime lighting conditions than a LCD, in addition to offering a higher chromatic and

"*AMOLED screens are very thin and light with an enormous flexibility, with the possibility of "enrolling", even when being active, which is an interesting point of view when creating a display with self-distorted images*"

screens and plasma screens.

Emissive display

SLM (Spatial Light Modulator)

Mini-CRT (Cathode Ray Tube)

LED (Light Emitted Diode) array

> LCD (Liquid Cristal Display)

TFT-LCD (Thin-Film Transistors)

> OLED (Organic Light Emitting Diode)

AMOLED (Active Matrix OLED)

Next, on Table 2 a description of the potential general stages of an HUD optical system is made, with the purpose of examining and inquiring about the importance of some optical considerations to take into account in this type of augmented reality systems.


Table 2. General optical stages in HUD systems.

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 249

Configuration 3

Observations

The surface of mirror (3) is covered with an aluminum semi-reflective coating (25%). The surface (4) is a hyperboloid, and it is covered with an anti-reflective magnesium fluoride coating (5), its objective is to reflect a percentage of light that was reflected by the surface of mirror (3), and to additionally establish that a percentage from light of the outside is not transmitted towards the observer, thus, the outer light and the projected

The thickness of the combiner is defined as *e*, but sometimes that parameter cannot be assumed as a constant, since surfaces (4) and (6) that define the combiner are considered as

The angle ø is determined to correct the parallax errors on the image presented to the

The surfaces used to collimate the light from the emissive display are (3), (4) and (6), these surfaces are proposed to satisfy the requirement that an image is formed from combiner (2)

Configuration 4

Fig. 5. Configuration 3 (Frithiof, 1972).

Fig. 6. Configuration 4 (Suzuki et al., 1988).

Elements

image by the combiner compete at an equal level.

with the same sagittal and southern magnification.

Elements

5. Distance between the virtual image (showed

7. Distance between the combiner and the

8. Distance between the combiner and the

4. Distance of the virtual image.

by the combiner) and the observer.

none parallel, making the windshield behaves as a wedge.

1. Emissive display. 2. Combiner. 3. Mirror.

4. Back surface of the combiner. 5. Reflective coating (aspheric surface). 6. Frontal surface of the combiner.

7. Body of the combiner.

8. Observer.

observer.

1. Combiner. 2. Optical system.

6. Emissive display.

optical system.

observer.

3. User.

#### **3.2.1 Image forming optical system**

The type of HUD system here described is an Optical see-through augmented reality, where the objects of an external environment are combined with additional information previously configured (avatars: overlapped objects on the scene), the result is visualized through a translucent screen or combiner; next, on Table 3, different types of configurations imposed to these HUD systems are analyzed in order to highlight the main elements that shapes these types of systems.

248 Augmented Reality – Some Emerging Application Areas

The type of HUD system here described is an Optical see-through augmented reality, where the objects of an external environment are combined with additional information previously configured (avatars: overlapped objects on the scene), the result is visualized through a translucent screen or combiner; next, on Table 3, different types of configurations imposed to these HUD systems are analyzed in order to highlight the main elements that shapes

Configuration 1

Configuration 2

Fig. 3. Configuration 1 (Furukawa, 1991).

Fig. 4. Configuration 2 (Griffiths &

Friedemann, 1975).

**3.2.1 Image forming optical system** 

Elements

Observations In this type of configuration, astigmatism aberrations are not present, nor distortion, but a geometrical deformity is present, consequently, the projected image is visualized with a trapezoidal profile according to the incidence angle of the projected image

Elements

Observations It could be considered that certain region in the windshield might possess approximately spherical geometry (its central region), where the α angle formed between the chief ray of the projected image and the optical axis of the spherical region is considered, then, this angle defines the degree of astigmatism in the image

Under this previous spherical consideration, we could think of cylindrical lenses to eliminate the astigmatic effect presented in the

1. Flat combiner external to the windshield.

these types of systems.

2. Emissive display.

over the combiner.

1. Optical system. 2. Windshield. 3. Incandescent lamp. 4. Diffusing screen. 5. Projected image. 6. Exit platform. 7. Observer.

presented to the observer.

image projected to the user.

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 251

Observations The position of emissive display (1) in relation to optical system (3) (a convergent lens for this type of configuration) defines the augmentation of the virtual image (augmentation = focal distance of the optical system / d1) that is visualized by the observer and the distance

The length of the lenses largely determines the exit-pupil of the optical system, for this application a squared formatted lens is used, to be able to generate a squared formatted exit-pupil that will adapt better to the head movements of the driver in the automobile. Next, on Figure 9, the IFOV effect can be appreciated, since due to from its extension the eye-box also increases (defined as the volumetric space where the user can place his/her head and see the whole extension of the virtual image); it is noticeable on Figure 9 that with the IFOV's angle, the user may move and will continue seeing the whole virtual image generated, it also must be noted that the user will generally move his/her head in X and Y dimensions, reason why the eye-box must be rectangular, thus, rectangular lenses must be used. As a requirement, it is set out that the IFOV must be at least 1.5 times bigger than the

Fig. 9. Visual representation of the eye-box applied to configuration 5 (Brown et al., 1988). The optical trajectory of this kind of HUD system might be fold through the use of mirrors as shown on Figure 10, which implies a reduction in the space needed to contain the

Fig. 10. Folding of the optical trajectory on configuration 5 (Brown et al., 1988).

In these devices, binocular vision is involved; this vision handles a horizontal FOV of 120°

The first requirement for good binocular vision is that the eyes converge to a single spot of the visual range, by using binocular vision inappropriately to see the outside scene and the generated image by the HUD device, may cause stress and vision difficulties such as

On the other hand, with binocular vision if the luminance level is the same for both eyes, the scene that is being observed will be perceived brighter than what it really is, otherwise, if

that it is going to be visualized from the optical system (d2).

TFOV.

**3.3 User** 

elements of the optical system.

Table 3. Types of configurations of an HUD.

and vertical of 130° (being 60° upwards and 70° downwards).

diplophia (when the user sees two separate images at the same time).

Observations

It assumes that it wants to generate a virtual image Y´ over a flat combiner, which characteristics include being augmented, straight, collimated and without double refraction (or an acceptable parallax), so an optical system is configured (represented due to reasons of ease through a single lens).

There is also an attempt to reduce the parallax of the visualized image making sure the light beams of the projected image are as parallel as possible among them. We are looking to find, based on certain considerations of the geometrical optics, a way to get closer to the designing requirements: distance of the virtual image=infinite, L1/L2=0, and the minimum angle β, as it is observed on figures 6 and 7.

The binocular parallax is the horizontal and vertical dephase that the reflected image by the first interface of the windshield (air-glass) has in relation to the reflection of its second interface (glass-air); it can be corrected with a prism which works as a wedge, redirecting the light. As found on literature (Betancur, 2010), horizontal parallax is not corrected since it produces stereoscopic images that are tolerable by the user.

Fig. 7. Detail Figure 6 (Suzuki et al., 1988).


Observations

The position of emissive display (1) in relation to optical system (3) (a convergent lens for this type of configuration) defines the augmentation of the virtual image (augmentation = focal distance of the optical system / d1) that is visualized by the observer and the distance that it is going to be visualized from the optical system (d2).

The length of the lenses largely determines the exit-pupil of the optical system, for this application a squared formatted lens is used, to be able to generate a squared formatted exit-pupil that will adapt better to the head movements of the driver in the automobile.

Next, on Figure 9, the IFOV effect can be appreciated, since due to from its extension the eye-box also increases (defined as the volumetric space where the user can place his/her head and see the whole extension of the virtual image); it is noticeable on Figure 9 that with the IFOV's angle, the user may move and will continue seeing the whole virtual image generated, it also must be noted that the user will generally move his/her head in X and Y dimensions, reason why the eye-box must be rectangular, thus, rectangular lenses must be used. As a requirement, it is set out that the IFOV must be at least 1.5 times bigger than the TFOV.

Fig. 9. Visual representation of the eye-box applied to configuration 5 (Brown et al., 1988). The optical trajectory of this kind of HUD system might be fold through the use of mirrors as shown on Figure 10, which implies a reduction in the space needed to contain the elements of the optical system.

Fig. 10. Folding of the optical trajectory on configuration 5 (Brown et al., 1988).

Table 3. Types of configurations of an HUD.

## **3.3 User**

250 Augmented Reality – Some Emerging Application Areas

Observations

characteristics include being augmented, straight, collimated and without double refraction (or an acceptable parallax), so an optical system is configured (represented due to reasons

The binocular parallax is the horizontal and vertical dephase that the reflected image by the first interface of the windshield (air-glass) has in relation to the reflection of its second interface (glass-air); it can be corrected with a prism which works as a wedge, redirecting the light. As found on literature (Betancur, 2010), horizontal parallax is not corrected since

Configuration 5

Fig. 8. Configuration 5 (Brown et al., 1988).

There is also an attempt to reduce the parallax of the visualized image making sure the light beams of the projected image are as parallel as possible among them. We are looking to find, based on certain considerations of the geometrical optics, a way to get closer to the designing requirements: distance of the virtual image=infinite, L1/L2=0, and the minimum

It assumes that it wants to generate a virtual image Y´ over a flat combiner, which

of ease through a single lens).

angle β, as it is observed on figures 6 and 7.

Fig. 7. Detail Figure 6 (Suzuki et al., 1988).

1. Emissive display.

θ. Total Field Of View (TFOV). ϴ. Instant Field Of View (IFOV).

2. Observer. 3. Optical system. 4. Virtual image.

Elements

it produces stereoscopic images that are tolerable by the user.

In these devices, binocular vision is involved; this vision handles a horizontal FOV of 120° and vertical of 130° (being 60° upwards and 70° downwards).

The first requirement for good binocular vision is that the eyes converge to a single spot of the visual range, by using binocular vision inappropriately to see the outside scene and the generated image by the HUD device, may cause stress and vision difficulties such as diplophia (when the user sees two separate images at the same time).

On the other hand, with binocular vision if the luminance level is the same for both eyes, the scene that is being observed will be perceived brighter than what it really is, otherwise, if

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 253

configured (Frithiof, 1972).

inner and outer surface.

Deformity ≈ 0 Related to the trapezoidal deformity effects on the image seen

viewed by the user may be proposed.

Also, here below, an analysis on perceptual factors must be done to those HUDs applied to

Most of the time that the driver spends on driving, his attention is directed to the road, however, there are short periods of time where the driver looks at the command board of the vehicle, which although very small, might mean the loss of a critical event; the use of an HUD while driving allows the reduction of observation time for the variables in the vehicle's panel from 0.25 to 1 second, that time can be crucial on a dangerous situation.

The virtual image generated that is visualized by the user is typically located at a higher distance from the observer than the command dashboard and within the visual range of the user (looking towards the road), then, less accommodation is required when the driver

switches to focus the virtual image of the HUD to the outside environment.

≈ 0 Depending on the location of the display source in relation to the user, the horizontal binocular parallax produces stereoscopic images that can be acceptable, but the vertical binocular parallax produces fatigue on the user's sight, due to it is intended to be minimized by the use of prisms specially

≈ 0 It must be minimized, looking to diminish the angle of incidence between the emissive display (with collimated light) and the combiner, and additionally using an anti-reflective coating over the inner surface of the combiner; on the other hand, this effect may be eliminated by including a wedge effect into the combiner, eliminating the parallelism between it

Depends on the visual capacity of the user (Hecht & Zajac,

In relation to the literature (Velger, 1997; Brown et al., 1988), if we consider a system with a combiner reflectance of 30%, an optical system transmittance of 85%, and a contrast appropriate (1.3:1) for the view on a sunny day (10.000 ft-L), the image luminescence value (viewed by the user) may be calculated.

Considering the night luminescence of 4000 ft-L, together with the optical and contrast parameters indicated in daylight luminescence, a value of night luminescence in the image

Requirements Value Description

Vertical binocular parallax

Double refraction

Daylight luminescence

Night luminescence

automobiles:

Accommodation ≤ ±0.25

Dpt

≤ 9000 ft-L

≥ 3639 ft-L

Table 4. Primordial visual requirements.

**3.3.1 Increase of visual time on the road** 

**3.3.2 Reduction of re-accommodation time** 

2005).

by the user.

one of the eyes is subjected to more illumination than the other one, the visualized image might look less bright than what it is, thus, the importance of keeping constant illumination on the image that the HUD generates.

Additionally, the spectral sensibility of the eye must be considered in order to generate the maximum luminescence possible in HUD devices, that is, when the luminescence levels are higher than 3.18\*100.5 Cd/m2 we talk about Photopic vision; on the other hand, for luminance levels under 3.18\*10-3.5 Cd/m2 we talk about Scotopic vision; when the eye is being used in one of these two regions its spectral sensibility changes, being higher for the Photopic region in 505nm and for the Scotopic region in 555nm, between the region that covers the spectrums of green and yellow respectively, that is why these colors tend to be used in HUD devices. If the wavelength emitted by the emissive display cannot be changed, it might be used a wavelength that is common on both levels of luminescence (525nm approximately).

Otherwise, retinal illumination is mentioned as the quantity of light that goes through the eye in relation to the scene's luminescence, this concept describes the Stiles and Crawford effect that talks about the visual sensation of the user, which is different for equal quantities of light in different places of the retina, being the rays that go through the center of the pupil (and which impact on the retina) more effective by a factor of 8 (Betancur, 2010).

In relation to visual sharpness (negatively affected by Scotopic vision and the little graphic details that the projected information by the emissive display has), we notice for the projected image that a user with low visual sharpness would not be able to differentiate the fine details that it possesses, therefore, this parameter should be considered when defining the type of image that the HUD device will generate.

Next, on Table 4 is detailed some of the most important visual requirements to be taken into account when an image is being presented in these HUD systems (Betancur, 2011; Díaz, 2005).



Table 4. Primordial visual requirements.

252 Augmented Reality – Some Emerging Application Areas

one of the eyes is subjected to more illumination than the other one, the visualized image might look less bright than what it is, thus, the importance of keeping constant illumination

Additionally, the spectral sensibility of the eye must be considered in order to generate the maximum luminescence possible in HUD devices, that is, when the luminescence levels are higher than 3.18\*100.5 Cd/m2 we talk about Photopic vision; on the other hand, for luminance levels under 3.18\*10-3.5 Cd/m2 we talk about Scotopic vision; when the eye is being used in one of these two regions its spectral sensibility changes, being higher for the Photopic region in 505nm and for the Scotopic region in 555nm, between the region that covers the spectrums of green and yellow respectively, that is why these colors tend to be used in HUD devices. If the wavelength emitted by the emissive display cannot be changed, it might be used a wavelength that is common on both levels of luminescence (525nm

Otherwise, retinal illumination is mentioned as the quantity of light that goes through the eye in relation to the scene's luminescence, this concept describes the Stiles and Crawford effect that talks about the visual sensation of the user, which is different for equal quantities of light in different places of the retina, being the rays that go through the center of the pupil

In relation to visual sharpness (negatively affected by Scotopic vision and the little graphic details that the projected information by the emissive display has), we notice for the projected image that a user with low visual sharpness would not be able to differentiate the fine details that it possesses, therefore, this parameter should be considered when defining

Next, on Table 4 is detailed some of the most important visual requirements to be taken into account when an image is being presented in these HUD systems (Betancur, 2011; Díaz,

> ≈ 0 Considering that the projected image is polychromatic, the use of mirrors is necessary to avoid the chromatic aberration, or apochromatic doublets for the correction of such aberration

≤ 1.5 % This is especially noticeable when observing the image on the

not as good as the center of itself.

It is present by the toroidal geometry of the windshield and it is compensated commonly with cylindrical lenses. This aberration tends to null itself when the angle α (which defines the degree of astigmatism in the presented image to the observer) formed between the chief ray of the projected image and the optical axis of the projection region of the windshield (considered as spherical) tends to zero (Freeman, 1998).

eye-box limit, where the visualized image is generated by the optical system's periphery, here the formation of the image is

(and which impact on the retina) more effective by a factor of 8 (Betancur, 2010).

the type of image that the HUD device will generate.

≤ 0.25 Dpt

Requirements Value Description

(Velger, 1997).

on the image that the HUD generates.

approximately).

2005).

Chromatic aberration

Astigmatism aberration

Distortion aberration Also, here below, an analysis on perceptual factors must be done to those HUDs applied to automobiles:

#### **3.3.1 Increase of visual time on the road**

Most of the time that the driver spends on driving, his attention is directed to the road, however, there are short periods of time where the driver looks at the command board of the vehicle, which although very small, might mean the loss of a critical event; the use of an HUD while driving allows the reduction of observation time for the variables in the vehicle's panel from 0.25 to 1 second, that time can be crucial on a dangerous situation.

#### **3.3.2 Reduction of re-accommodation time**

The virtual image generated that is visualized by the user is typically located at a higher distance from the observer than the command dashboard and within the visual range of the user (looking towards the road), then, less accommodation is required when the driver switches to focus the virtual image of the HUD to the outside environment.

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 255

As it can be appreciated, Figure 11 is a detailed view of Figure 2 previously described, being this last one the starting point for the design of any HUD. From the opto-electronical stage on Figure 11, the digital pre-processing stage of the projected image stands out, which is not very common in this type of augmented reality system, and its effect and reach is better

Fig. 12. Corrective pre-processing of images in the HUD system proposed (Betancur, 2011).

Now, based on the previously established considerations and in order to test other

From the instrumental set up described on Figure 13, these components are specially

2. Windshield: corresponding to the RENAULT LOGAN 2009 model reference

3. Projection system: as a result of the evaluation of the previously indicated alternatives on Table 3, an optical system proposal was established; whose designing requirements are detailed next on Table 5 and it is also represented schematically in figures 14 and 15.

additional ones, the instrumental set up described on Figure 13 was developed.

1. Windshield holder: this allows the rotation of the windshield in two of its axes.

Fig. 11. Proposed augmented reality system.

appreciated next on Figure 12.

highlighted:

G000463620\_V02\_01.

4. User's position.

## **3.3.3 Distance of the virtual image**

The distance of the virtual image must be between 2.5 and 4 meters from the eyes of the driver, in order to increase the time of recognition and accommodation.

## **3.3.4 Accommodation effect**

When superimposed objects are located within the observer FOV, the object that has a shorter distance from the observer's focus, tends to dominate the accommodation answer, this situation is shown in HUD systems when using an external combiner instead of the windshield.

#### **3.3.5 Intolerance to visual disorder**

The results of case studies of drivers whose vehicles had an HUD system, showed that sometimes these system could be uncomfortable due to the quantity of information that is displayed toward the driver; although, this is currently discussed, because the analysis of this uncomfortable situation is highly related to the type of information being displayed, its organization and relevance to the driver.

#### **3.3.6 Illumination interference**

The overlapping of the symbology generated by the HUD over the driver's external scene, will tend to hide the outside objects via illumination interference, which results if the luminance of the symbology of the HUD is higher than the luminance of the outer objects of the vehicle, this can be a big problem according to the quantity of information shown by the HUD.

#### **3.3.7 Cognitive capture**

Under high working conditions there is a high level of temporary uncertainty for the unexpected appearance of outer objects, consequently, it is likely that a cognitive capture occurs by the HUD operation, that is, an attention deficiency to the surroundings due to the exposed symbology by the HUD, generating delays in the answers to outside events, which can result in the loss of external objectives.

#### **3.3.8 Symbology of the display**

The figures of the conformation symbology move with the elements of the outside world, this kind of symbology increases the acceptance of HUD systems among drivers.

#### **3.3.9 Location**

It is not appealing that the HUD simbology is located in the middle of the driver's visual range; many research documents suggest that the drivers prefer to locate this symbology below the line of the horizon, in a more peripheral position.

#### **4. Implementations**

Based on what was previously mentioned and as a starting point for this chapter proposal, an augmented reality system was set out based on phases, components and flows, as indicated on Figure 11.

Fig. 11. Proposed augmented reality system.

254 Augmented Reality – Some Emerging Application Areas

The distance of the virtual image must be between 2.5 and 4 meters from the eyes of the

When superimposed objects are located within the observer FOV, the object that has a shorter distance from the observer's focus, tends to dominate the accommodation answer, this situation is shown in HUD systems when using an external combiner instead of the

The results of case studies of drivers whose vehicles had an HUD system, showed that sometimes these system could be uncomfortable due to the quantity of information that is displayed toward the driver; although, this is currently discussed, because the analysis of this uncomfortable situation is highly related to the type of information being displayed, its

The overlapping of the symbology generated by the HUD over the driver's external scene, will tend to hide the outside objects via illumination interference, which results if the luminance of the symbology of the HUD is higher than the luminance of the outer objects of the vehicle, this

Under high working conditions there is a high level of temporary uncertainty for the unexpected appearance of outer objects, consequently, it is likely that a cognitive capture occurs by the HUD operation, that is, an attention deficiency to the surroundings due to the exposed symbology by the HUD, generating delays in the answers to outside events, which

The figures of the conformation symbology move with the elements of the outside world,

It is not appealing that the HUD simbology is located in the middle of the driver's visual range; many research documents suggest that the drivers prefer to locate this symbology

Based on what was previously mentioned and as a starting point for this chapter proposal, an augmented reality system was set out based on phases, components and flows, as

this kind of symbology increases the acceptance of HUD systems among drivers.

below the line of the horizon, in a more peripheral position.

can be a big problem according to the quantity of information shown by the HUD.

driver, in order to increase the time of recognition and accommodation.

**3.3.3 Distance of the virtual image** 

**3.3.5 Intolerance to visual disorder** 

organization and relevance to the driver.

can result in the loss of external objectives.

**3.3.8 Symbology of the display** 

**3.3.9 Location** 

**4. Implementations** 

indicated on Figure 11.

**3.3.6 Illumination interference** 

**3.3.7 Cognitive capture** 

**3.3.4 Accommodation effect** 

windshield.

As it can be appreciated, Figure 11 is a detailed view of Figure 2 previously described, being this last one the starting point for the design of any HUD. From the opto-electronical stage on Figure 11, the digital pre-processing stage of the projected image stands out, which is not very common in this type of augmented reality system, and its effect and reach is better appreciated next on Figure 12.

Fig. 12. Corrective pre-processing of images in the HUD system proposed (Betancur, 2011).

Now, based on the previously established considerations and in order to test other additional ones, the instrumental set up described on Figure 13 was developed.

From the instrumental set up described on Figure 13, these components are specially highlighted:


Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 257

this configuration is located in the first lens. Exit pupil (Ex.P.) The exit pupil represents the produced image from the system's

virtual image that the user can observe.

the user of the system's second lens.

the exit pupil's diameter (D1).

0.5 meter must be established.

Table 5. Designing requirements of the optical system proposed.

Mt=M1\*M2 (M1 =-Si1/So1, M2 =-Si2/So2).

User distance (U.D.) It is proposed in relation to previous conducted researches (Wood

It is intended that the distance between the optical system's lenses

The entrance pupil is where light enters to the optical system, in

first lens (opening diaphragm) by the system's second lens, and additionally, according to its size, it limits the cone of rays of the

It is the element that limits the entrance of light from the object to the optical system, in this configuration is represented by the first

et al., 1990; Velger, 1997) that there is a distance of 0.8 meters from

To know the size of the exit pupil, the distance between both lenses of the optical system (D) must be known, which (for the exit's pupil diameter calculation) will be recognized as the object distance of the second lens (So2), and it is also necessary to know the focal distance of the second lens (f2); this way, by applying Gauss' equation for thin lenses we will be able to know the image's distance of the second lens (Si2), then, through the longitudinal augmentation relation (M=–Si2/So2) the augmentation can be known; finally, since the object and image distances are proportional to their heights, and knowing the opening diaphragm's diameter, it is possible to know

To know the size of the virtual image generated by the configuration of this particular optical system, it is necessary to multiply the generated augmentations for each of the lenses

For the configuration of the optical system indicated a distance of

According to previous research (Griffiths & Friedemann, 1975; Suzuki et al., 1988) and to preserve a good response from the user, it is established that this distance must not be lower than 2.5 meters.

In relation to the optical system that was proposed, the FOV-θ is defined as the vision angle necessary that the user must have to visualize the vertical extension of the virtual image generated.

In relation to the optical system that is proposed here, the FOV-α is defined as the vision angle that the exit pupil allows the user to have of the vertical extension of the virtual image generated. In relation to Figure 15, it can be noticed that the higher the FOV-α is

to the FOV-θ, then, the bigger will the system's eye-box be.

Requirement Description

lens of such system.

is never higher than 40 cm.

Distance between the system's lenses (D)

> Entrance pupil (En.P.)

Diaphragm of aperture (D.A.)

The exit's pupil diameter (D1)

Image size generated by the optical system. (D2)

User distance from the exit pupil (U.D.1)

User distance from the generated image (U.D.2)

Vertical visual range from the generated image (FOV-θ)

Vertical visual range generated by the exit pupil (FOV-α)

Fig. 13. Instrumental set up proposed.


256 Augmented Reality – Some Emerging Application Areas

Fig. 13. Instrumental set up proposed.

Quantity of lenses in the optical system

Object distance of the system's first lens (So1)

Image distance of the system's first lens (Si1)

Object distance of the system's second lens (So2)

Image distance of the system's second lens (Si2)

Longitudinal augmentation (M)

Requirement Description

two converging lenses.

brighter final image (Si2).

(Si1).

distance of the system's second lens.

combiner, will be minimally of 1.8 meters.

resolution will be drastically decreased.

It is proposed as a starting point, an optical system configured by

The main goal is that the object distance of the system's first lens to be as lower as possible (a maximum of 15 cm), so that this lens will collect as much light as possible and can consequently generate a

It is intended that this image distance be generated near the focal

The definition for this system is: the distance (D) that separates the two lenses of the system minus the image distance of the first lens

It is intended that this parameter, that is the distance from which the driver will see the final image due to its reflection in the

It is intended that the image is augmented by a maximum factor of 9, since, if the image's original dimensions are augmented, its


Table 5. Designing requirements of the optical system proposed.

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 259

Fig. 16. Development of the optical system proposed.

described implementations (Betancur, 2011).

4. Diffusing screen: that creates an image of the emissive display.

5. Emissive display: for this analysis a Pico-projector from MICROSYSTEMS ® was used, where the image of the projection is transferred from a computer that pre-processes the

Next, in order to show what the user would perceive from his/her driving position and under the implementations that were proposed previously; here below on Figure 17 a) and b) the proposed visualization of the HUD and the real visualization of the HUD are shown

a) b)

Fig. 17. (a) Proposed image; (b) Visualization of the proposed image under previously

image according to the specific design parameters previously indicated.

1. Windshield. 2. Lens 1. 3. Lens 2.

respectively.

Fig. 14. Optical system proposal.

Fig. 15. FOV-θ in comparison to FOV-α.

The values used for the set up of the proposed optical system (figures 15 and 16), are indicated here below on Table 6.


Table 6. Values for the optical system proposed.

Next, on Figure 16, it can be seen the proposed optical system described on figures 14 and 15; Fresnel lenses were used due to their low bulkiness, with a squared format in order to accomplish a squared exit pupil that is more appropriate in relation to the user's movements.

Fig. 16. Development of the optical system proposed.


258 Augmented Reality – Some Emerging Application Areas

The values used for the set up of the proposed optical system (figures 15 and 16), are

Next, on Figure 16, it can be seen the proposed optical system described on figures 14 and 15; Fresnel lenses were used due to their low bulkiness, with a squared format in order to accomplish a squared exit pupil that is more appropriate in relation to the user's

Parameter Value So1 249 mm f1 135 mm D 400 mm f2 110 mm Si2 2375.4 mm Mt 8,7 Ex.P. 151 mm D1 41,525 mm D2 261 mm U.D. 551 mm U.D.1 551 mm U.D.2 2926,4 mm FOV-θ 2,35° FOV-α 2,96°

Fig. 14. Optical system proposal.

Fig. 15. FOV-θ in comparison to FOV-α.

Table 6. Values for the optical system proposed.

movements.

indicated here below on Table 6.


Next, in order to show what the user would perceive from his/her driving position and under the implementations that were proposed previously; here below on Figure 17 a) and b) the proposed visualization of the HUD and the real visualization of the HUD are shown respectively.

Fig. 17. (a) Proposed image; (b) Visualization of the proposed image under previously described implementations (Betancur, 2011).

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 261

Fig. 19. (a) Specific configuration for the HUD system proposed; (b) Detail of the operation

Now, as a complement for Figure 19, it is proposed through Figure 20 to determine what other options should be taken into account specifically for this type of augmented reality

mode from the HUD system proposed.

8. Command buttons for the HDD 1.

2. Illumination of the presented image. 3. Proximity of the presented image.

From Figure 19.b) the following items stand out:

system and under the interaction of a companion.

1. HUD 1. 2. HUD 2. 3. HUD 3. 4. HDD 1.

5. Controls for HUD 1. 6. Controls for HUD 2.

1. On/Off HUD 1.

From Figure 19.a) the following items stand out:

7. Controls for HUD 3 (spherical scroll button –Mouse-).

4. Handling of the information from HUD 1 (Enter button).

## **5. Applications**

Next on Figure 18, a possible automotive configuration for this type of augmented reality systems is proposed.

Fig. 18. General set up for the proposed HUD system.

Then, on Figure 19 we specify some of the main options that an augmented reality system should have under the configuration from Figure 18.

Fig. 19. (a) Specific configuration for the HUD system proposed; (b) Detail of the operation mode from the HUD system proposed.

From Figure 19.a) the following items stand out:

1. HUD 1.

260 Augmented Reality – Some Emerging Application Areas

Next on Figure 18, a possible automotive configuration for this type of augmented reality

**5. Applications** 

systems is proposed.

Fig. 18. General set up for the proposed HUD system.

should have under the configuration from Figure 18.

Then, on Figure 19 we specify some of the main options that an augmented reality system


From Figure 19.b) the following items stand out:


Now, as a complement for Figure 19, it is proposed through Figure 20 to determine what other options should be taken into account specifically for this type of augmented reality system and under the interaction of a companion.

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 263

a) b)

c)

The considerations taken into account from figures 18 to 21 are imposed on Figure 22, which is considered as a good approximation to the functional design of the augmented reality

Fig. 21. (a) Configuration of the projections of the HUD 1; (b) Configuration of the

projections of the HUD 2; (c) Kind of projections in the HUD 2.

system proposed in this chapter.

The projection configurations of the HUD 1 and 2 are indicated here below on Figure 21.

Fig. 20. Specific set up for the HUD system proposed.

From Figure 20 the following items stand out:


262 Augmented Reality – Some Emerging Application Areas

Fig. 20. Specific set up for the HUD system proposed.

4. Reticules projected onto the central driving mirror.

3. HUD 3 on the command panel is the HDD 2 (handled by touch screen).

7. USB ports to connect a keyboard or any other device that helps the co-driver to manipulate the HUD 3 (this is not recommendable for the driver due to the big risk that

From Figure 20 the following items stand out:

5. Control for the HUD 4 and HUD 5. 6. On/Off buttons for all the HUDs.

1. HUD 4 is the same HUD 1. 2. HUD 5 is the same HUD 2.

could represent).

The projection configurations of the HUD 1 and 2 are indicated here below on Figure 21.

Fig. 21. (a) Configuration of the projections of the HUD 1; (b) Configuration of the projections of the HUD 2; (c) Kind of projections in the HUD 2.

The considerations taken into account from figures 18 to 21 are imposed on Figure 22, which is considered as a good approximation to the functional design of the augmented reality system proposed in this chapter.

Physical Variable Analysis Involved in Head-Up Display Systems Applied to Automobiles 265

collimated, this group of qualities are the main requirements (demanded by the user) that

3. The approach of a two lens optical system is interesting when satisfying the established requirements between the longitudinal augmentation, the size and the exit pupil's position, the generated virtual image, the object location in relation to the optical system, among others; since, for a lens, to accomplish these requirements the same way the optical system proposed does, may be very complex and expensive; also, a three lens system for this type of application might be unnecessary, since, as it can be appreciated, a two lens system is

4. The use of lenses with a squared format becomes necessary when trying to obtain a more appropriate eye-box in relation to the user's movements; in the same way, this format allows a presentation of images to the user, according to the square projection shapes from most of

5. It is clear that the presence of optical aberrations (astigmatism, double refraction, chromatic, distortion, etc.) it is notorious in these kinds of systems, the way it corrects from analogical elements and through the stages of digital pre-processing are not yet completely generalized, so in consequence, being able to present a tolerable image in terms of such

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these augmented reality systems must satisfy.

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the emissive displays.

**7. References** 

Fig. 22. Configuration of HUDs and HDDs in the augmented reality system proposed.

## **6. Conclusions**

1. On the Figure 17 it is barely noticeable the aberrant component over the final image presented to the observer, in the same way, even if the virtual image presented is not perfectly coherent to the one initially proposed, it is adequately and without any major problem recognizable by the user; then, this image has not overcome the limits established as appropriate for the presentation of the information in HUD systems.

2. The optical qualities used to set up an HUD are diverse, being the needs required by the driver their main parameter of design; this way, the instrumental proposal here depicted, mainly focuses on presenting a virtual image to the user that is straight, augmented and collimated, this group of qualities are the main requirements (demanded by the user) that these augmented reality systems must satisfy.

3. The approach of a two lens optical system is interesting when satisfying the established requirements between the longitudinal augmentation, the size and the exit pupil's position, the generated virtual image, the object location in relation to the optical system, among others; since, for a lens, to accomplish these requirements the same way the optical system proposed does, may be very complex and expensive; also, a three lens system for this type of application might be unnecessary, since, as it can be appreciated, a two lens system is enough to provide the basic requirements demanded.

4. The use of lenses with a squared format becomes necessary when trying to obtain a more appropriate eye-box in relation to the user's movements; in the same way, this format allows a presentation of images to the user, according to the square projection shapes from most of the emissive displays.

5. It is clear that the presence of optical aberrations (astigmatism, double refraction, chromatic, distortion, etc.) it is notorious in these kinds of systems, the way it corrects from analogical elements and through the stages of digital pre-processing are not yet completely generalized, so in consequence, being able to present a tolerable image in terms of such aberrations implies much more analysis and research.

#### **7. References**

264 Augmented Reality – Some Emerging Application Areas

Fig. 22. Configuration of HUDs and HDDs in the augmented reality system proposed.

as appropriate for the presentation of the information in HUD systems.

1. On the Figure 17 it is barely noticeable the aberrant component over the final image presented to the observer, in the same way, even if the virtual image presented is not perfectly coherent to the one initially proposed, it is adequately and without any major problem recognizable by the user; then, this image has not overcome the limits established

2. The optical qualities used to set up an HUD are diverse, being the needs required by the driver their main parameter of design; this way, the instrumental proposal here depicted, mainly focuses on presenting a virtual image to the user that is straight, augmented and

**6. Conclusions** 


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4.961.625

## *Edited by Andrew Yeh Ching Nee*

Augmented Reality (AR) is a natural development from virtual reality (VR), which was developed several decades earlier. AR complements VR in many ways. Due to the advantages of the user being able to see both the real and virtual objects simultaneously, AR is far more intuitive, but it's not completely detached from human factors and other restrictions. AR doesn't consume as much time and effort in the applications because it's not required to construct the entire virtual scene and the environment. In this book, several new and emerging application areas of AR are presented and divided into three sections. The first section contains applications in outdoor and mobile AR, such as construction, restoration, security and surveillance. The second section deals with AR in medical, biological, and human bodies. The third and final section contains a number of new and useful applications in daily living and learning.

Photo by liuzishan / iStock

Augmented Reality - Some Emerging Application Areas

Augmented Reality

Some Emerging Application Areas

*Edited by Andrew Yeh Ching Nee*