Friday, February 11, 2011

Filter glass requirements for camera modification

Modification of a camera dedicated to IR or UV needs the replacement of the internal cut filter in front of the camera sensor by a suitable glass filter of similar quality but different spectral transmission.

The challenge with a filter in front of a sensor is, that it has to meet high requirements:

- optically exactly parallel polished
- bubble and inclusion free
- streak and striae free

This very hard to meet since off the shelf filter glass does not provide that usually, as it does not matter in front of a lens - but in front of a sensor (i.e. about in focus), all that gets visible in your image.

A good converter checks his filter glass under a microscope, locates the needed size which fulfills the a.m. requirements and then cuts that small out from a larger piece.

Personally I would rather buy such a filter precut from a professional camera converter, as he would be liable for the quality and not try risky experiments since "cheap is way to expensive" in the end.

An example borrowed, showing a bubble in SCHOTT IR filter glass:


Example for striae in glass:

(c) glassphotonics

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site