Re: calibration of color cameras

On Apr 3, 12:55 pm, heini <buerg...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
wrote:
Hello,
we have some color cameras on our microscopes and somehow we never get
identical results with them.
We are analyzing blood smears and good color reproduceability is
absolutely necessary for automated image analysis.
So we started to look for color calibration tools -- and found none so
far!
OK, there are sheets one can photograph (with indirect light), but no
calibration slides for transmitted light we could use in the
microscopes without taking the cameras off.
Or, am i wrong?
We would love slides with more that 3 colors on them and known RGB or
whatever spectrum.
Maybe someone in this group knows where to find calibration slides for
color cameras or has a better idea?

Thanks

Heini

Heini,

Are your cameras all of the same type? It's difficult to say what to
do without more details, but lots of the cameras that are used for
microscopes have an "auto white balance" function, or something
similar, that usually works quite well. This is found somewhere in
the software that's driving the camera.

In the simplest case, you move the slide away from the specimen so
that you are only looking at the transmitted light going through the
blank slide. At this point you bring up the "auto white balance" or
"white balance" function and activate it. The camera takes a picture
of this background, analyzes it and corrects it so the image is white
in color. After that, the correction is applied to the pictures you
take of the specimen.

Some software packages have a "color selection" tool or an eyedropper
that you can point to an area that you want to make white. You click
on this area, and the same thing happens: that area is made white, and
the correction is applied to the whole field of view.

Other cameras, especially consumer digital cameras, do this all by
software. They analyze and correct the color from the entire field of
view. Usually you have to set this up by telling the software what
kind of illumination you're using (incandescent, fluorescent,
daylight, etc.) Some trial and error may be needed to get the correct
color, or something quite close to what you're seeing through the
microscope. With these types of cameras, you can often set up and use
"custom" settings that will have the necessary color correction in
them.

A note of caution: it depends on the camera and the software, but it's
not unusual to have to correct the color or change the settings
whenever you change the magnification or adjust the illumination. A
different objective or a change in the voltage to the bulb can change
the color of the illumination. So you might need a different setting
if you go from a 10x to a 40x objective or if you decrease or increase
the brightness knob.

In the latter case I'd recommend that you use neutral density filters
to cut down or increase the brightness of the illumination rather than
constantly increasing and decreasing the voltage to the lamp in the
microscope.

Good luck, hope this helps!

Cheers,

Bob Chiovetti
swpinet dot com
.

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