Mark,

A quick explanation of colour densitometer 'channels':

For comparative purposes (comparing the readings from one densitometer with another) 'Status' colour filters are used. The common ones for colour photographic materials are 'Status M' and 'Status A' and there will be three filters - red, green and blue.

Status M is used for materials that are intended as an intermediate part of the process - eg camera negative film that will be printed. Therefore the M filters are intended to represent the response of colour print materials. They don't do that particularly well, but that's a longer story. I'll explain later if you wish.

Status A is used for the final image, whether it is a transparency or a print.

A transmission densitometer can, therefore, have both M and A, while a reflection densitometer may only have A. The Status filters will be in addition to a 'visual' channel as well which covers all visible wavelengths. This is typically done with a Wratten 106 filter.

Typically, dichroic (spectral interference) filters are used for the Status filters. They are very stable through the years. You can swap out the Status M filters for dye based filters to improve the match with the response of your paper, for example, but those filters won't remain as stable as dichroics.

Other filter types include:
Status T - wide band RGB filters for graphic arts prepress etc use, typically gelatin filters.
Status I - narrow band, graphic arts, typically dichroic.
UV - typically Wratten 18A, for alternative processes.
Ortho - for ortho processes (had you guessed?)

Best,
Helen