DIY densitometer

[jisner]

Tomorrow when the sun is out I will try a UV meter and Stouffer scale. There’s a chance it will block UV

We use Stouffer step wedges with UV light all the time. We use it to determine exposure, to get density data for characteristic curves, and (now, hopefully), to determine digital negative densities.

 

[jisner]

I solved the DIY densitometer problem. Here's how. I am fortunate to have two UV dose meters: one that controls my UV exposure unit (just set the dose and walk away), and a small battery-powered UV meter designed for industrial use. The small meter is round and flat, like an oversized old-fashioned pocket watch, with the LCD on one side and the sensor on the other (you can get one on eBay -- search for "UV-Integrator"). I placed small meter inside my exposure unit with the sensor facing up toward the bulbs. I placed my negative on top of the small meter's sensor, emulsion-side down. I exposed for a constant dose of 800 units (about 15 minutes). With shorter exposures I could not read densities over 1.5. Anyway, I exposed each of the steps of a 21-step Stouffer 4x5. For each exposure, I recorded the reading on the small meter and the (known) Stouffer step density, then reset the small meter to 0 for the next exposure. This gave me 21 data points. I fit an exponential curve to the 21 points. It was a beautiful fit with R=0.999. This is my calibration curve. Now I can take a sample negative, expose it in exactly the same way, and use the reading from the small meter to find the corresponding density on the calibration curve (I actually use the equation of the best-fit curve). The only drawback of this solution is the length of the exposure -- fifteen minutes for one density measurement! This is not a good general solution, but it works for me. The samples that I'm measuring are small pieces (say 2cm x 2cm) of solid black digitally printed on transparency film. No "maneuvering" is required to position the sample over the sensor.

 

[dkonigs]

So here we go again.. As @koraks suggested here https://www.photrio.com/forum/threads/diy-light-meter-for-darkroom.169398/page-2#post-2204322 that I should make a densitometer.. Film photography is interesting hobby; sometime you wonder where to use a densitometer and then in next picture one is actually designing one and measuring with it. Oh well..

I keep thinking about this as a side-project idea as well. However, its still pretty easy to get a B&W densitometer (even brand new), even if it does cost a bit of money.
The real shortfall is in color (RGB) densitometers. I honestly know of absolutely no new currently-in-production product capable of replacing the functionality of the X-Rite 810 (or similar) that I bought off eBay last year. I keep wondering if its worth figuring out how to make something like your rig (assuming I can figure out the optical parts) using an RGB sensor like the TCS34725 (or something else in its product category). However, then I struggle to understand how to manipulate its RGB outputs (based on its spectral sensitivity) into equivalents within the "Status A" and "Status M" wavelengths. Its probably possible if you know what math to do on the data, but I really don't have a clue.

 

[ProfessorC1983]

I keep thinking about this as a side-project idea as well. However, its still pretty easy to get a B&W densitometer (even brand new), even if it does cost a bit of money.

But where's the FUN in just buying something that you could pour hundreds of hours of frustration into designing, building, calibrating, and troubleshooting yourself??

@OP, thanks so much for posting this as I'm going down a similar path myself. Ideally I want to build a darkroom exposure meter that I can use as a poor man's densitometer under an enlarger, to allow me to compute density range and contrast index for negatives under both UV and white/blue light. I've pretty much settled on the VEML6075 you linked to as the UV sensor I want to try, but can I ask why you picked the TSL2591 over the VEML7700? The latter seems to have greater range from what I can tell, and the resolutions look similar, but I haven't compared the datasheets in detail.

 

[radiant]

However, its still pretty easy to get a B&W densitometer (even brand new), even if it does cost a bit of money.

I would have bought the densitometer if there was reasonable price devices available. But all that I could found was over 1000 euros which is just too much. Can you give links to cheaper devices?

I've pretty much settled on the VEML6075 you linked to as the UV sensor I want to try, but can I ask why you picked the TSL2591 over the VEML7700? The latter seems to have greater range from what I can tell, and the resolutions look similar, but I haven't compared the datasheets in detail.

Uhm, it was only low lux sensor I could find at that time. But now looking at datasheet, I would go with VEML7700 for sure! Adafruit even has breakout for it https://www.adafruit.com/product/4162

 

[ProfessorC1983]

Adafruit even has breakout for it

Yes exactly, at $5 it is definitely worth playing around with! And much faster shipping than ordering from China. For UV it does seem they have discontinued the VEML6075 but they have the VEML6070 on a breakout board which only lacks the UVB diode... not a problem for my purposes, since my light source is UVA only. So I went ahead and ordered both. Will post here if I get some meaningful results. Cheers!