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COMPARISON OF OPTICAL CONTOURING METHODS 119 
fidelity of the contours obtained. The signal- 
to-noise ratio of the moiré contour patterns 
depends on the contrast ofthe grating and the 
projected shadow and hence it is difficult to 
achieve a good signal-to-noise ratio at very 
high resolution. In order to improve the con- 
trast ofthe contours itis sometimes necessary 
to paint the surface with a suitable structure. 
The variation in the thickness layer of the 
paint often introduces error in measurement. 
The metric accuracy and reliability are de- 
termined by the geometrical imaging charac- 
teristics of the recording optical system used. 
The contours produced localize on or near 
the object surface. In order to reduce the data, 
the contours must be recorded on a photo- 
graphic film. As explained later in this sec- 
tion, this requirement poses a serious problem 
in achieving the desired metric accuracy. The 
overall systems used in moire contouring are 
very simple compared to photogrammetric 
systems. 
In all contouring methods based on holo- 
graphic interferometry, the range and resolu- 
tion are limited by the availability of suitable 
laser sources. In the multiple index method 
the problem of handling the object limits the 
size. Contour intervals of 1 to 2 micrometers 
have been obtained experimentally using 
holographic contouring methods. The 
signal-to-noise ratios associated with 
double-exposure holography govern the 
signal-to-noise performance of these contour 
systems. The speckle associated with diffuse 
holographic images usually causes serious 
noise problems whenever a high density of 
contours is encountered. Surface preparation 
such as painting improves the contrast of the 
fringes obtained. The metric accuracy of the 
holographic images along with the super- 
posed contours can be very high if suitable 
precautions are taken during the holographic 
recording procedure. However, since the 
contours exist in three dimensional space, 
considerable effort is needed to maintain the 
same accuracy during the data reduction pro- 
cedure. This point is discussed later in this 
section. Holographic systems are far more 
complex and are not often applicable to large 
objects. Because of the extreme stability re- 
quirements for holographic recording, these 
systems are practical only under research en- 
vironments. 
Optical processing of stereo-pairs permits 
the automation of the stereo-compilation proc- 
ess and in every other respect its perfor- 
mance characteristics are similar to conven- 
tional stereo-photogrammetric systems. The 
resolution in.parallax measurement, which in 
turn determines the ultimate resolution, not 
only depends upon photogrammetric 
parameters such as base-to-height ratio but 
also depends on the image structure. The 
width of the contours obtained is inversely 
proportional to the spatial frequency content 
in the image. The contrast and the brightness 
of the contours depend on the perfect match- 
ing of the conjugate image points. The 
signal-to-noise performance of these systems 
degrade markedly under conditions of large 
base-to-height ratios and imagery involving 
large slopes. From the arguments presented 
above, it is clear that the stereo- 
transparencies have to be either vertical or 
rectified to be vertical. Since the contours 
relate to the photo coordinates rather than to 
the model space, the contours are perspective 
in nature. However, since the controls for the 
stereo-pair are known, they can be reduced 
easily to orthoscopic form either optically in 
real time or digitally. Since the performance 
of these systems depends greatly on the 
image structure, desired high frequency pat- 
terns such as speckles can be superimposed 
on the object to improve contrast and the res- 
olution of the output. Moiré contouring can 
be considered as an example ofthis approach. 
Contouring from holographic images effec- 
tively combines the advantages offered by 
holography with the measurement charac- 
teristics of photogrammetry. The resolution 
of these systems is determined either by the 
pointing accuracies or by the depth-of-focus 
of the auto focus system. Coherent speckle 
associated with the diffuse surfaces is the 
main source of noise in the system. Since the 
measurement is made in the model space, the 
data reduction involved becomes minimal. 
Problems arising from the environmental re- 
quirements for holographic recording are the 
main sources of inconvenience. 
Most of the investigations relating to the 
generation of elevation contours have not ad- 
dressed the problem of data reduction once 
the contours are generated. Once the record 
of the contours is made on a two dimensional 
photographic film, the contours become 
perspective. Each contour must be individu- 
ally adjusted for magnification to provide the 
correct representation of the object. Also 
there is no way of identifying a contour repre- 
senting a hill from a corresponding one rep- 
resenting a valley. The three-dimensional 
appearance of the object aids in discrimina- 
tion during visual display, but this informa- 
tion is lost in the record. Alternative methods 
of data extraction are necessary before holo- 
graphic interferometric and moiré techniques 
can provide very reliable and accurate metric 
information.