Application and Optimisation of the Spatial Phase Shifting ...

162 Improvements on SPS
To obtain performance data for our approach, we use the σ d values from a simple out-of-plane tilt. The
test object is a white painted metal plate that scatters with strong depolarisation (ρ = 0.78 ± 0.01). The
light scattered off the object is imaged with a lens L2 onto the target of the CCD camera, with 1024×768
pixels. A polariser PF in front of the camera target selects either the vertical or horizontal SOP of the
scattered light. The measured correlation coefficient for the corresponding speckle fields S vi (x,y) and
S hi (x,y) is c = 0.02 ± 0.005 which is in acceptable agreement with the value of c = 0.03 ± 0.003 expected
for the measured depolarisation coefficient. Since S v /S h 0.78, the plane of polarisation of the
reference wave is set to 48° instead of 45° by the POC to obtain P vb (x,y)P hb (x,y), this is, we intend to
replace some 50% of ϕ O,vi (x,y) (ϕ O,vf (x,y)) by entries from ϕ O,hi (x,y) (ϕ O,hf (x,y)), whereby the best
utilisation of both speckle patterns is assured.
The reference wave's fibre end is placed in the aperture plane A of the imaging system and positioned to
yield α x = 120°/column on the CCD sensor.
The phase stabilisation works as follows: part of the object light is reflected by the small mirror M1
mounted on the object. It passes through the lens L2 and is then reflected by another small mirror M2,
close beside the CCD chip, towards the plane S. On the opposite side of the CCD sensor, a small
beamsplitter BS reflects a part of the reference wave towards S. By proper adjustment of M1, M2 and BS,
both waves can interfere in S, forming an interference pattern of concentric fringes as shown in Fig. 6.21.
This is far easier to achieve than broad fringes of stable shape.
D2
D1
Fig. 6.21: Interference pattern in the plane S of the PID unit; the white squares indicate the locations and areas of
the photodiodes. The circular boundary of the pattern is due to the imaging aperture.
A photodiode D1 is placed in the centre of this pattern where a broad fringe occurs. Another one, D2, is
placed outside the centre, integrating the intensity distribution over some 12 fringes. Thus, the output of
D2 is insensitive to phase variations and tracks the intensity fluctuations of the laser instead. Whenever
phase changes occur between the object and the reference wave, the intensity in the centre of the fringe
system changes. This variation is detected by D1, while intensity fluctuations of the laser are detected by