Morphometric Data - An Invariant Approach to Statistical Analysis of Shapes - page 30

Optical metric systems
The Reflex 3-Metric System includes instruments that provide mea-
surements from stationary objects in three dimensions without
directly touching the specimen and requires interface with a personal
computer. MacLarnon (1989) described these instruments in detail.
The Reflex Microscope involves the use of the stereoscopic ability of the
observer to pinpoint the surface of an image to within a few microns
along the Z-axis, from a distance of typically 65mm. This allows the
examination of objects that have a very uneven surface. An illuminat-
ed spot of a selectable size is used to precisely locate the feature to be
measured. The selected size of the illuminated spot allows very precise
delineation of the part of the image under examination. The act of mea-
surement does not degrade the image and no direct contact is made
with the specimen under view. The microscope represents a hybrid
between a small coordinate measuring machine and a surface profiler
and is ideal for measuring three dimensional distances and angles
between small surface features with irregular surfaces. It is also capa-
ble, in principle, of quantifying sub-surface features in transparent or
semi-transparent materials, although to do this, the influence of the
material refractive index has to be accounted for.
When digitizing, the object is viewed through an adapted stereo-
scopic microscope where a small light spot appears in the field of view.
Using the motorized stage the light spot can be moved until the loca-
tion of the relevant surface feature coincides with the spot. The depth
coordinate is set by the observer using stereoscopic vision to locate the
spot “on” the surface of the object. The X-, Y-, and Z-coordinates are
monitored continuously via linear encoders and can be stored on com-
mand in the computer so that single points or streams of points that
describe curves can be collected and written to a file for future analy-
sis. The object stage, driven by motors, can be guided manually using
high resolution joysticks or automatically under software command,
allowing movements to be constrained to predetermined planes or
directions if required or to pre-set patterns.
Measurement error studies of earlier models of the Reflex instru-
ments are available (Scott, 1981; Setchell, 1984; Speculand et al., 1988a,
1988b; see also MacLarnon, 1989). The latest version of the microscope
(January, 1999) provides an absolute accuracy of better than ± 3 µm in
XY and ± 4 µm along the Z-axis. Repeatability is said to depend largely
on the skill of the operator and is expected to be better than ±2 microns
in X- and Y-, and ±4- microns in Z-, using the highest standard magnifi-