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6. Color Correction
You know these beautiful rainbow colors that prisms make out of sunlight.
They show that the angle of refraction is wavelength dependent.

This universal law governs lenses too: Use a simple convex lens element to image a black/white edge. Examine the edge image with a magnifying glass or with a microscope. You're going to find rainbow colors in the edge image. Somewhat exaggerated, I've shown this in fig. 6.

fig.6: lacking color correction (7kByte) The lens designer can minimize these color artifacts in the image by using different glass types in alternating lens elements.

If your image is evaluated in the grey level domain only, then you can rely on the influence that color artifacts have on MTF. Regarding MTF alone will supply enough information so that you can assess the lens safely.

But if the image is going to be evaluated in color, then you'd better carefully consider the effect of colored edge artifacts. They might render the lens unusable.

The wavelength dependent effect is refraction of light in glass.

So tell your lens designer if you intend to use glass windows on the object- and/or on the image side.
He will ask for thickness, refractive index and position of your glass sheets.
This is necessary for good MTF   a n d   for good color correction.

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Last modified Nov.20th, 2002 21:45