2.2 Measurement of glass block tilt using fringe projection Fringe projection techniques are commonly used for measuring the surface profile of an object. The detailed information of the surface profile is contained in the distorted fringe pattern. Measurement of the absolute height of a specimen block using fringe projection includes calculating the distance δx between the break of a ray on the block surface and the flat background as shown in Fig. 2(a). From the fringe shift δx, the height of the block h can be calculated using the simple triangulation equation:(h=一个数学公式,跳过)
where θ is the angle of projection.
When collimated light is used the angle of projection θ is constant across the area of incidence. This is not the case for non-collimated projection where the projection angle θ varies along the length of block L (Fig. 2(b)). For accurate height calculation using Eq. (1) the projection angle must be known at each point on the block surface.
The basic concept in fringe projection is the shift of fringe δx on the block surface relative the background caused by the height h. The distance δx can be determined by measuring the shift of the non-collimated light ray on the block relative to the background. In order to determine θ for various locations along the length of block L a relationship between θ and L must be established. This
relationship will enable the calculation of θ for all fringe breakpoints on the block surface.