There are several types of heat absorbing filters. For enlargers and projectors, we must have the type that passes visible light but absorbs excess heat from the light passing through.

They’re made of glass of a special alloy containing an iron compound. The iron compound absorbs heat from the light passing through it and that increases the temperature of the filter. Since they can get hot, heat absorbing filters are tempered to prevent the filter from breaking due to thermal shock.

Even so, if the filter gets too hot it can still break. The specifications usually state a maximum operating temperature of 300F (149C). Most often the filter simply splits into two pieces.

If a heat absorbing filter is split, so long as the retainer holds the pieces edge-to-edge in the proper configuration, the filter will continue to absorb excess heat.

Since these filters are almost always outside of the optical path from negative to print the split in the filter won’t affect the image.

When you encounter a split heat absorbing filter, the break is almost always the result of thermal shock, not physical trauma.

Dichroic filters are glass filters with thin metallic elements deposited onto the glass in a precisely controlled THICKNESS. These operate by the mechanism of thin film interference. The coating material has little color of its own.

It’s only when deposited onto the glass in a precisely controlled thickness that we have the sharp cutting filtration so that only a specific color is passed: cyan, yellow, or magenta for color enlargers and blue or green for VC light sources.

http://en.wikipedia.org/wiki/Filter_(optics)

http://www.tpub.com/content/construc.../14044_148.htm

http://en.wikipedia.org/wiki/Dichroic_filter