**6.2 Spectroradiometer input optics**

The purpose of the spectroradiometer input optics is to couple the desired radiation flux from the source into the monochromator as efficiently and accurately as possible. The type of coupling used should take into account the characteristics of the complete system that influence the accuracy of our measurements. Some issues to consider are:


Fig. 17. Source size and monochromator optics

If the radiation from the source overfills the monochromator optics, the overfill radiation will become stray scattered radiation within the monochromator. The problem with this stray radiation is that radiation of wavelengths other than those we are measuring can enter our detection system and cause an erroneous addition to our output signal. A small source, or a source placed at a large distance from the monochromator, will underfill the monochromator optics. In addition to being inefficient, this underfilling of the optics is susceptible to any non-uniformity in the optics. Any motion of the source will cause the radiation to pass through a different part of the spectroradiometer optics, which may result in a different response from the monochromator. This will become more of a problem if another source is used to calibrate the spectroradiometer. This other source will probably be a different size and therefore pass through different optical paths in the system.

v. An input optic lens or mirror system that collects more radiation from the source and is sized to fit the monochromator input is often used. The schematic of Figure 16 shows this configuration. In this case the filament of the lamp is focused onto the input slits of the monochromator. There are two problems with this arrangement: 1.) Since an image of the filament is produced on the slits, any vibration of the radiation source or spectroradiometer components will cause a dramatic variation in the amount of radiation that passes through the slits and enters the monochromator. 2.) The spatial structure of the radiation at the input slits will be imaged onto the output slits of the monochromator, and then into the detector. If there is any non-uniformity in the detector spatial responsivity, our output signal will not be representative of the amount of input radiation.

There are, however, instances when we do use this type of input optics. These are usually when the source is a uniform source, such as a Lambertian diffuser, or when we

iii. Imperfections in the optical components will cause scattering of some of the radiation from its ideal trajectory. This causes both a loss in signal and stray radiation problems. iv. The efficiency of our optical design and components will affect our signal-to-noise and accuracy. Since the signals we are usually measuring with a spectroradiometer are small, one of the prime considerations is to collect as much radiation as possible, and to use it as efficiently as possible, without undue loss anywhere in the optical path. To this end, we should use as much of the optical system, particularly that of the monochromator, as possible. The basic ideas of filling the input optics of the

If the radiation from the source overfills the monochromator optics, the overfill radiation will become stray scattered radiation within the monochromator. The problem with this stray radiation is that radiation of wavelengths other than those we are measuring can enter our detection system and cause an erroneous addition to our output signal. A small source, or a source placed at a large distance from the monochromator, will underfill the monochromator optics. In addition to being inefficient, this underfilling of the optics is susceptible to any non-uniformity in the optics. Any motion of the source will cause the radiation to pass through a different part of the spectroradiometer optics, which may result in a different response from the monochromator. This will become more of a problem if another source is used to calibrate the spectroradiometer. This other source will probably be a different size and

v. An input optic lens or mirror system that collects more radiation from the source and is sized to fit the monochromator input is often used. The schematic of Figure 16 shows this configuration. In this case the filament of the lamp is focused onto the input slits of the monochromator. There are two problems with this arrangement: 1.) Since an image of the filament is produced on the slits, any vibration of the radiation source or spectroradiometer components will cause a dramatic variation in the amount of radiation that passes through the slits and enters the monochromator. 2.) The spatial structure of the radiation at the input slits will be imaged onto the output slits of the monochromator, and then into the detector. If there is any non-uniformity in the detector spatial responsivity,

our output signal will not be representative of the amount of input radiation.

There are, however, instances when we do use this type of input optics. These are usually when the source is a uniform source, such as a Lambertian diffuser, or when we

monochromator are shown in Figure 17.

Fig. 17. Source size and monochromator optics

therefore pass through different optical paths in the system.

are imaging the radiation emitted from a certain portion of a ribbon filament of an incandescent lamp. In both of these instances, the source has very little spatial structure. vi. In many radiometric measurements, the input system involves a diffuser, either a flat plate (Figure 17) or an integrating sphere. While this may cause a considerable loss of

radiation, it does reduce the errors caused by geometrical structure in the radiation source, component vibration, polarization effects, and any spatial non-uniformity in the monochromator optics and/or detector responsivity. The input optics to the monochromator shown in Figure 15 is an integrating sphere, together with a baffle assembly. More detail for this configuration is shown in Figure 18.

Fig. 18. Integrating sphere input to monochromator

The input port to the sphere is the area that defines what flux is to be measured, so it acts as part of the defining optics for the radiation source measurement. The required distance is that between the source and the defining aperture at the sphere input. The size of the sphere and its ports should be arranged such that the radiation that enters the monochromator has had more than one 'bounce' within the sphere. Figure 18 shows a configuration such that the region of the sphere wall that emits radiation into the monochromator does not overlap with the region of the sphere wall that is directly irradiated by the source. Integrating spheres accept radiation from all directions and emit radiation into all directions at any open port of the sphere. This must be controlled to avoid stray radiation from entering the sphere and to prevent an overfill of the input optics to the monochromator as was shown in Figure 17. To prevent on overfill at the monochromator, the size of the sphere output port and the distance of the sphere output port from the monochromator input slits can be adjusted such that the optics of the input to the monochromator are just filled, as shown in Figure 18.

A baffle assembly may be used at the input to the integrating sphere to reduce unwanted stray radiation from entering the sphere. Apertures and baffles must be considered carefully, since the edges of the aperture or baffle, no matter how thin, will reflect significant radiation, especially from the direct input beam, into the measured portion of the input. The baffle assembly shown in Figure 18 includes only one baffle, the limiting front aperture, that directly views the input radiation from the source. The baffles interior to the assembly are only used to prevent scattered radiation from the interior walls of the baffle assembly from re-entering the input beam to the sphere. The limiting aperture is large enough that it does not define, or vignette, the final size of the input radiation beam that enters the sphere. Its purpose is to limit as much as possible the amount of off-angle stray radiation that enters the integrating sphere.

The multiple reflections at the interior of the sphere cause a change in the relative spectral distribution of the sphere by a factor of ( ) /(1 ( )) , where ( ) is the reflectance of the material of the sphere wall (CIE 084, 1989). Any irregularities or changes in ( ) with wavelength will become amplified in the sphere output radiation. With the typical white diffusers used in integrating spheres, such as PTFE or BaSO4, this is usually observed in the lower wavelengths below approximately 400 nm. If a more uniform behaviour with wavelength is desired, a compromise is to reduce the reflectance ( ) from its high values near 99% to approximately 85% by addition of dark absorbers into the coating.

In summary, although the integrating sphere input configuration has several disadvantages, the advantages that it provides in reducing the spatial non-uniformities of sources and detectors outweigh the disadvantages. The disadvantages may be mitigated by careful consideration of each of the components of the system when assembling a measurement system.
