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10. Baffles and Bellows
In the last paragraph ("bundle shape"), we learned that we'd better not disturb the useful imaging ray bundle by any kind of hardware.
Considering paragraph 8 ("flaws") also, you'll understand that even a lamp   that shines onto the front lens inadvertently   will more or less veil the image in stray light.
Since the early days of photography, everybody knows that baffles and bellows are used to protect image quality against stray light.

Here I offer some hints regarding the design of baffles and bellows:

A. Color
The interior must be absorptive, that is, black. Make sure that your black paint is not glossy and that it really absorbs all the wavelengths it should. For example, many black anodising colors do not absorb infrared.

B. Stops
In most applications, optics and mechanics have to be as slim and lean as possible. So bellows will rather tightly be wrapped around the imaging bundle (para. 9). And this will cause many rays to hit it at grazing angles.
Now light at grazing angles is reflected very efficiently, no matter how mat and dull your paint is. That's why good bellows contain a number of stops. These stops must be arranged in such a way that it takes multiple reflections at rather steep angles, before stray light can reach the lens. (See fig. 10)
So at every reflection the light can be attenuated to a factor of, say,   0.1
... achieving a fantastic total attenuation of
          0.1 ^ n
where           n   =   (total number of necessary reflections).

fig.10: Bellows against stray light (15kByte)

C. Position
As an example, imagine an orbiting telescope trying to image a faint star while the sun directly shines onto the main mirror. It takes a very near-to-ideal mirror to handle this task. Paramount expenses can be spent on mirror quality and on keeping this expensive element absolutely clean.

By far less expensive   --   and more effective   --   is a baffle.

Baffles must be arranged in such a way that they keep these strong sources from radiating onto imaging optics. In short:

Baffle is to be placed between irritating light source and imaging optics.
At best, direct lamp light does not even reach the inner surface of bellows.

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Last modified Nov.26th, 2002 18:22