DIY ND filter

[336v]

While repairing old Luna Pro light meter I've noticed that the 6EV filter that gets inserted in the light path for high scale is flaking off - its surface looks like tiny droplets of
some substance etched away filtering material leaving dirty spots on filter's glass. IPA does not clean it. Anyway, since finding good pare filter of this size is not possible,
I had to come up with alternative.

I managed to get about right attenuation by printing wide gray scale wedge on a piece of transparency film, and cutting out small square
section that happen to be required density. It actual took stack of two pieces overlapping each other, but inserting/removing them blocks light 6 stops worth, which is what
the stock filter did. But quality of the surface, while not critical, is not great, even printed at 600 dpi.

Then I had an idea I wanted to run by collective wisdom here. I have variety of color gelatin theatrical projector filters (like these:
https://www.bhphotovideo.com/c/product/1016197-REG/colourlite_mafp_213_vivid_colors_filter_pack.html ). Since we know white light roughly
consists of equal amount of red, green and blue light, by the same thinking, if I overlap red, green and blue filters of about equal density, I get ABOUT neutral "gray" filter.

Is this accurate assumption? I could adjust overall density by having two or more layers of each color. What do you think?

Granted, I can probably just find and buy gray filter online, but the best I found are way too thick, minimum set of two large ones (like 6x6 inches while I need just
8mm x 8mm square, and I'm reluctant to waste at least ~$15 to throw away practically all of it buy the tiny piece. For a light meter printed wedge works just fine, but
I'm thinking laser jet's toner won't last long on transparency. Gelatin is far more durable.

If you have any other ideas, please share. The filter in Luna Pro cannot be more than about 0.3mm thick.

 

[bernard_L]

But quality of the surface, while not critical, is not great, even printed at 600 dpi.

Does not matter IMO. This is not an imaging configuration. As long as the global attenuation is right, and the area is approximately uniform, you should be OK.

if I overlap red, green and blue filters of about equal density, I get ABOUT neutral "gray" filter.

Don't overthink the neutrality. When intercomparing some of my light meters (Selenium; CdS, Si) I noted offsets between the meters (of course, nothing is perfect) that would change according to the color of the object being metered; even warm gray versus cold gray. So...

If you have any other ideas, please share.

Why not make if from B/W film? That is how Stouffer wedges are made, and nobody complains about their durability. First iteration will give you a rough idea of the exposure needed, next time take a pic (MF) of a slight gradient at about the right exposure, then select on the negative the spot within the gradient with the exact amount of density; don't worry about the gradient within the small area that you use.

 

[koraks]

Is this accurate assumption?

No.

But if you want to make neutral density, you can get fairly close with a neutral-base B&W film (something like Fomapan 100 in sheet film format for instance) and develop it to get neutral-toned silver layer. If you follow this link you'll find a post that links to a developer that will do this. Getting the desired density will be a process of experimentation.