[ATM] How dark is dark

[Michael Wheatley]

If I understand a densitometer correctly it uses a low angle of 
incident light and a 90 degree angle for measurement.  These are the 
conditions for viewing a photograph.  This combination would would 
cause gloss black to seem darker that matte black.  In a simple 
telescope tube you would have a low angle of incident light and a 
high angle (170 deg ?) of measurement so that gloss black could be 
much brighter than matte black.  A photographic densitometer would 
not give you information that is useful inside a telescope.

For a percent reflectance to have any value in our application you 
also need to specify the angle of incidence and the angle of measurement.

For gloss black to work in a telescope tube you would need careful 
and specific design.  I expect matte black is a much safer choice.

Mike Wheatley