Macbeth TR924 Densitometer missing knob markings

[Tobywan]

I'm hoping someone here can help me. I recently purchased a used Macbeth TR924 Densitometer and the disc on top of the reflection head is missing. Everyone I've seen on the net is the same. Does it matter which setting I use or if someone has one with the disc could you please post a picture of it. Thanks!

[Bill Burk]

How many detents are there? Four? If so they correspond to a green filter, red filter, blue filter and brown visual filter (Wratten 90).

You could infer the filters by reading something that is red, green then blue with the different filters, the lowest reading (least "density") for each color chip would tell you the color of the filter the light went through. The visual channel is the one you would want to use... so it would be the odd duck out.

[Tobywan]

I'll give that a try. I did check it at each position with a white and black target and they read the same so it may not make a difference.

[Bill Burk]

Well, for reading black and white, silver gelatin prints, no it won't make much difference...

The "right" one to use is the black one.

As for figuring out which one is the "black" one... you should know these were designed to read four color process printed sheets. Prepress operators would put color swatches on the plates which would print in the margins of the press sheets in varying percents of the colors, so the press operators would have four series of patches to read and they would write down some readings to give quality control proof the press is under control...

So the four channels would be marked Yellow, Magenta, Cyan and Black.

Yellow is the mark for the blue-filter position, Magenta is the mark for the green-filter position and Cyan is the mark for the red-filter position.

You would find each position by finding the highest reading for yellow (is the yellow position)... the highest reading for magenta (is the magenta position) and the highest reading for cyan (is the cyan position)....

I told you to try the actual filter color before and look for the lowest reading... now I'm telling you to use the press colors and look for the highest readings...

Black is the mark for the brown (Wratten 90) reading. It's going to be the one that is not obviously highest on yellow, magenta or cyan.

[Tobywan]

Thanks!