problems with macbeth transmission densitometer

[Terence Falk]

I own an old Macbeth transmission densitometer, model TD502. I cannot get the unit to zero out, even trying the adjustment screw on the back. I took the snout apart and there is a reddish/ orange colored filter in it, very damaged. There was also corrosion in the area, which I cleaned. Now, without the crud and without the filter, the unit now zeros.
The question is: have I screwed this up now that the filter has been removed? What is the filter for in the first place? I tried to replace the snout unit, but I cannot find a replacement (calling numerous places) and even if I found the part, that one could have the same problem. I did find another unit and the inside of the snout was just as bad.

Any suggestions would be welcome.

[nolindan]

There is no need for an orange filter in a B&W densitometer and I have no idea why the filter would be there in the first place. And there is no reason for the presence of any crud or corrosion in the measuring area - unless your meter was used for something very odd, which may explain the filter.

If you have removed the filter and cleaned the thing up and it zeros and indicates the right density for some known density sample then I wouldn't worry. Absolute accuracy isn't as important as relative accuracy and linearity.

[Terence Falk]

thanks for the input; its been very frustrating in trying to restore this unit. I will shoot a film test and get a stouffer calibrated wedge to see how it all works out.

[Bill Burk]

The orange filter was likely Wratten #106 for Visual Density measurements.

I don't know how badly your "modification" would affect black and white film readings. It would ruin the accuracy of Visual Density readings off color film, because the filter minimizes the influence of color.

[nolindan]

The orange filter was likely Wratten #106 for Visual Density measurements.

Bill's right - needed for an S-4 cathode photomultiplier tube so that it has about the same intensity response as the eye. PM tubes aren't a first thought when it comes to ordinary optical measurement these days - it is an old densitometer, though.

Black and white's density is made up of metalic silver. The silver denisty can be read any any reasonable wavelength(s).

The densitometer, without the filter, will read the blue/purple base density differently than your eye and as Bill stated the density of color material will be seen differently without the filter. OTOH, photographic paper sees things differently than your eye, so correcting the PM tube for 'visual' response doesn't make that much sense to begin with. The S4 style tube peaks at 400nM and so it can be said that it is a closer match to enlarging paper without the filter than with.

It should be expected that this densitometer has lost sensitivity over the years: the PM tube may be gassy and/or the HV power supply's voltage multiplier capacitors may be electrically leaky.

[Bill Burk]

OTOH, photographic paper sees things differently than your eye, so correcting the PM tube for 'visual' response doesn't make that much sense to begin with. The S4 style tube peaks at 400nM and so it can be said that it is a closer match to enlarging paper without the filter than with.

It should be expected that this densitometer has lost sensitivity over the years: the PM tube may be gassy and/or the HV power supply's voltage multiplier capacitors may be electrically leaky.

Good points. My interpretation...

1. Probably don't need the filter.
2. Old electronics shouldn't be left with power on unattended.

[Terence Falk]

I did some research and it turns out it was a wratten 106 filter, so I am getting a new one to replace the old one, which was shot anyway. There is also a bluish glass
disk in the snoot that was filthy, so I cleaned that.
I always unplug this device when I am not using it; I don't trust the safety of the electronics either.