**2.2.1 Surface digitisation technology**

Surface digitisation is a procedure by which digital data is acquired from a surface of interest in order to obtain more knowledge of the shape and dimensions of the object. This is achieved through a series of interactions between the measurement equipment and the surface being assessed. Digitisation usually provides a string of 3-D co-ordinates in ASCII format.

Fig. 2. A classification of data acquisition methods.

There are many different methods for acquiring co-ordinate data, as shown in Figure 2. Essentially, each method uses some mechanism or phenomenon for interacting with the surface of the object of interest.

been advanced in the fields of medicine and dentistry (Chadwick, 1989; Seymour *et al*. 1996; Kabban, *et al.* 2001; Zou, *et al.* 2001, 2009; Cherukara GP *et al.* 2002; Theocharopoulos *et al.*,

The interface between the CMM and the measurement object or surface is a probe. The probe plays a very important role in obtaining the co-ordinates of a surface, and there are several kinds of probes commercially available at the present time. **Touch trigger probes** are by far the most commonly used with CMMs due to their simplicity and low cost. One of the problems with touch probes is that even under well-controlled conditions and at a generally accepted measuring speed and force, damage to the surface being scanned can occur because of the inertia effects of some probe parts (Win *et al.*, 1998). Win also indicated that initial plastic yielding of a surface can easily occur when probing with a sphere, and at higher probing speeds, multiple bouncing of the probe may occur, which reduces the accuracy for all mechanical probe systems. **The optical probe** has the great advantage of non-contact with the surface being measured. By the late 1980's, non-contact laser probes were widely accepted for surface measurement, especially in free form surface digitisation techniques due to its fast speed in the digitisation. However the accuracy in measurement when using optical probes is affected by the reflectivity of the surface, for example, a rough or dark coloured surface which is often encountered, results in empty or distorted data

Surface digitisation is a procedure by which digital data is acquired from a surface of interest in order to obtain more knowledge of the shape and dimensions of the object. This is achieved through a series of interactions between the measurement equipment and the surface being assessed. Digitisation usually provides a string of 3-D co-ordinates in ASCII

Interferometry

There are many different methods for acquiring co-ordinate data, as shown in Figure 2. Essentially, each method uses some mechanism or phenomenon for interacting with the

DATA ACQUISITION METHODS

Contact methods

Scanning probe

Touch Trigger

probe

2010**;** Morris *et al.* 2011 ).

readings at areas due to the surface low reflectivity.

Non-contact methods

Structured Lighting

Fig. 2. A classification of data acquisition methods.

surface of the object of interest.

**2.2.1 Surface digitisation technology** 

format.

Optical Triangulation Contact methods use mechanical contact through a trigger or scanning probe at the end of an arm or the column of a CMM (Jarvis, 1983) to obtain surface geometrical information. Development of this method was driven by reverse engineering (Milroy *et al.*, 1996; Dalton, 1998; Chow, 1997). In each case, an appropriate sensor determines a precise position on the object's surface.

Non-contact methods use scanning–type CMMs to capture larger numbers of sampling points, and have been used successfully for measuring surface form (Bradley and Vickers, 1992; Lee *et al.*, 2001; Chang and Lin, 1999). A recent study comprising questionnaire respondents from research institutions and companies engaged in surface topography in the European Community, revealed that non-contact optical techniques are more popular and widely used than previously thought (Dong *et al.*, 1994). Three dimensional non-contact measurement methods started to be used in the 1980's (Bosch, 1995; Stout, 1994).

The result of data acquisition is a discrete 'point cloud', through a 3-D analysis software it is interpolated into a 3-D surface and can be visualised as a 3-D object, furthermore to be analysed in its shape and dimensions.

With regard to the previously mentioned taper or convergence angle of prepared teeth; an angle measurement is taken at the middle of the mesial-distal plane where the central fissure runs through. The taper is the angle between the two lines along the two walls in this plane that passed the most outer points A and B which are located at the mesial and distal aspects as shown in Figure 3.

Fig. 3. The angle measurement between mesial and distal walls
