ImagePro>Calibration problem: trying to Measure Angles

["Graham Ellis" <g.ellis[at]microsharp.co.uk>]

We use Image-Pro Plus (3.0.1) to measure defects and deformities in
microscopic samples (for example, of polarising filters and diffraction
gratings).  We capture images from a CCD camera atop a fairly standard
bench microscope.  The common measurement (aside that of length of the
observed Feature) is the angle between features.  We mostly want to
measure that features are at 90degrees to each other.

In its previous set-up it just happens that the equipment -- when
calibrated in both dimensions by a graticule -- gave us an aspect ratio
of 1.04; this was quite adequate for us to judge that Angles we draw
with the Measurement tool were indeed 90degrees.

The equipment has been resited and now the aspect ratio has changed
quite dramatically.  Nothing we do can correct this, but that's what the
aspect ratio setting was for, or so we had always assumed.  Upon
recalibrating by graticule, the aspect ratio comes out at about 0.854.
This means that all 90degree corners appear quite distorted on screen,
but distance measurements are still made quite accurately by Image-Pro.
However, when we select the Angle measurement tool, it measures the
_apparent_ angle as it appear _on-screen_ which is no good to us
whatsoever!

Currently we are working around this by Length measuring all three lines
of a triangle and then doing a spot of trig. to decide the real angle,
but of course as the angles approach 90degrees the slightest
inaccuracies in measurement of these lengths are translated into far
greater inaccuracies in angle the closer we get to exactly 90.

Is Image-Pro capable of measuring 'real' angles using the calibrated
aspect ratio?
Are we just doing something wrong?
Or do we have to continue doing careful trig. until we can reconfigure
the equipment to give us a realistic aspect ratio?

Help!  Our brains are slowly melting...  (Although that might be the
green laser in the corner of the lab ;-)

G.J.Ellis
I.S. Support
Microsharp Corporation Ltd.
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