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fringe pattern, since it is localized on or near 
the surface of the object or the image of the 
object. 
CONTOUR GENERATION WITH OPTICAL 
PROCESSING OF STEREO-PHOTOGRAPHS 
In the last few years several coherent opti- 
cal processing systems for generating instan- 
taneous contours and profiles from stereo- 
pairs have been proposed and demonstrated. 
All these systems depend on optical correla- 
tion for the detection of conjugate images. In 
a pair of vertical stereo-photographs, the 
height information is stored in the form of 
x-parallax differences. The locus of all the 
points having the same x-parallax difference 
hence represents a contour on the photo 
coordinates. These contours are known as 
perspective contours and, based on the 
geometry of the stereo-pairs, the data can be 
reduced to represent orthoscopic contours 
either optically or digitally. These methods 
are different from conventional photogram- 
metric systems since a direct visual display of 
contour or profile information is possible. A 
few ofthe systems that have been reported in 
the literature are the positive-negative sys- 
temé, the scatter plate system”, the image fil- 
ter systems, the matched filter profile genera- 
tion system, and the interferometric contour- 
ing systems!9,1!1, In most of these systems, a 
superposition of a stereo-pair of transparen- 
cies gives rise to a band representing points 
of conjugate image coincidence. For a pair of 
vertical photographs this band represents a 
contour. One of the transparencies is trans- 
lated along the x-direction to generate con- 
tours of different elevations. Detailed de- 
scriptions of each of these methods, even 
briefly, is beyond the scope of this paper. A 
good review ofthese systems as they relate to 
close-range photogrammetry can be found in 
the references!?, 
CONTOURING FROM HOLOGRAPHIC IMAGES 
The reconstructed holographic images at 
unity magnification exhibit extreme degrees 
of metric fidelity!?. The application of photo- 
grammetric pointing methods as well as auto 
focusing methods have resulted in powerful 
techniques of measurement for close-range 
objects. This technique can not be classified 
as a direct optical contouring method but 
however is included in this evaluation be- 
cause of its novel approach to the measure- 
ment problem. 
The geometric fidelity is dependent 
greatly on the similarity of the recording and 
reconstructon geometries of the hologram. 
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1976 
Considerable work has been done with re- 
gard to the geometric fidelity of holographic 
reconstructed images and the results are well 
documented elsewhere!3,14, In the pointing 
approach, a self-illuminated dot attached to 
an XYZ coordinate measuring device and 
placed in the virtual image space is used as a 
measuring mark. During measurement the 
floating dot is brought in coincidence with 
the image point of interest and its coordinates 
are directly read from the coordinate measur- 
ing device. By maintaining the apparent con- 
tact of the dot with the surface, a contour is 
generated during translation in a single 
plane. 
In the auto focus system the real image 
rather than the virtual image is used for meas- 
urement. A sinusoidal pattern is projected 
on the surface while recording the hologram. 
The detection of the contrast of this pattern in 
the real image space permits an unambiguous 
determination of the location of the focused 
image surface. An image dissector attached to 
an XYZ measuring device and used in the 
detection of the contrast permits automatic 
and quick data reduction. 
The overall system can be either simple or 
very complex depending upon the resolu- 
tion, speed, and sophistication of the data re- 
quired. Spatial and depth resolutions of 25 to 
50 micrometers have been experimentally 
demonstrated using these systems. 
EVALUATION OF THE CONTOURING METHODS 
The factors that must be considered in 
evaluating the performance characteristics of 
the various contouring systems are (a) resolu- 
tion and range, (b) the signal-to-noise ratio, 
(c) metric accuracy and reliability, (d) the spa- 
tial resolution, and (e) the complexity of the 
system to meet the overall metric accuracy. 
Inherently the optical contouring systems 
offer speed and simplicity of configuration. 
These two main characteristics account for 
the widespread interest in optical contouring 
systems. 
Moiré contouring techniques are capable 
of high resolution, but only at the expense of 
reduced range. Without resorting to exotic 
Optics in the imaging systems, itis possible to 
obtain contour intervals of about 25 mi- 
crometers easily. The elevation range for this 
resolution in conventional moiré contouring 
is about 2 to 3 mm. It is possible to increase 
this elevation range by at least an order of 
magnitude by adopting the phase transpa- 
rency method. However, the instability ofthe 
photographic emulsion during bleaching 
might cause concern regarding the metric