DIY light meter for darkroom

dkonigs · Apr 18, 2021

DMJ said:
I recently developed an incident light meter for Android (actually my second one) and works very well. The Android SDK only provides integer lux values so a phone cannot be used for an enlarging meter.

I did a lot of experimentation with using an Android phone as an under-the-enlarger incident light meter several months back when I was trying to see if I could use it as a quick-and-dirty way of calibrating the PrintSizer app I was working on at the time. I'm pretty sure I got floating point results. The real problem I had, however, was that whatever filtering Android did before giving me the data was effectively a showstopper. Basically, any change below a certain threshold would be ignored and not passed back through the API. Of course the variations in low light levels that are kinda important for enlarger usage fall in this range.
For the use case of a camera light meter you're working with a lot more light, so this isn't really an issue.

dkonigs · Apr 18, 2021

DMJ said:
Thank you guys for the replies, your comments are very helpful. I'm going to look into Ilford's RC paper specs and see if I can get useful times.

Its hard to actually go from Ilford's data sheet to useful times, because the way they measure light (to determine paper speed) may not match your measurement setup at all. I suspect their ISO(P) values are based on measurements w/o a contrast filter and exposures /w a contrast filter, but there are so many extra variables that your own results may still not line up.

However, the ISO(R) contrast range values are likely to be a close match to what you get, at least in my own experience.

Unfortunately, there's really no way to "connect" these two numbers from published information, so you have to consider them completely independent measurements.

Nicholas Lindan · Apr 18, 2021

DMJ said:
I was reluctant [to purchase the Darkroom Automation meter] because they use an order form.

Most purchasers just send an email and pay with PayPal. The order form is for figuring shipping & handling or charges for upgrading a product. Some people do send in the form with a check using, gasp, first class mail.

Meters are now made to order and take 2-3 weeks to ship.

Meters are $94, the Pyro model for metering pyro negatives is $134. In both cases S/H for domestic Priority Mail shipping is $12; international first class air parcel shipping is $32.

albada · Apr 18, 2021

DMJ said:
How was your buying experience/shipping times?

My experience was very good. As Matt King pointed out, Darkroom Automation is operated by Nicholas Lindan, who posts here on Photrio.
The meter was shipped within a few days. I believe these meters are built when ordered, causing a delay of a couple of days.
Anyway, I suggest *not* using the USPS for shipping, as they were still slow as of a few weeks ago.

DMJ · Apr 18, 2021

Nicholas Lindan said:
Most purchasers just send an email and pay with PayPal. The order form is for figuring shipping & handling or charges for upgrading a product. Some people do send in the form with a check using, gasp, first class mail.

Meters are now made to order and take 2-3 weeks to ship.

Meters are $94, the Pyro model for metering pyro negatives is $134. In both cases S/H for domestic Priority Mail shipping is $12; international first class air parcel shipping is $32.

Great! I just sent you an email.

DMJ · Apr 19, 2021

dkonigs said:
I did a lot of experimentation with using an Android phone as an under-the-enlarger incident light meter several months back when I was trying to see if I could use it as a quick-and-dirty way of calibrating the PrintSizer app I was working on at the time. I'm pretty sure I got floating point results. The real problem I had, however, was that whatever filtering Android did before giving me the data was effectively a showstopper. Basically, any change below a certain threshold would be ignored and not passed back through the API. Of course the variations in low light levels that are kinda important for enlarger usage fall in this range.
For the use case of a camera light meter you're working with a lot more light, so this isn't really an issue.

I couldn't get floating point numbers from the SDK but it doesn't matter because, as you said, there is a filtering threshold; the main used of the light (ambient) sensor in phones is to adapt the brightness of the screen.
My device is 2x2.5in. and includes temperature and accelerometer sensors. I'm thinking on adding these additional data to the app, perhaps it can be useful for those buying this new enlarger that sits on a tripod and needs leveling. Temperature is always useful.

https://developer.android.com/reference/android/hardware/Sensor#TYPE_LIGHT

Nicholas Lindan · Apr 19, 2021

albada said:
I recently purchased [the Darkroom Automation enlarging meter] It measures EV, where EV 0 is defined as the exposure that produces D-max on Ilford RC IV paper when exposed for one minute...

It's not really EV - more like the numbers on a light meter dial. The numbers on a light meter, in combination with the film's ASA, give an EV number. The numbers on the enlarging meter, in combination with the time in stops (0 stops of time = 1 second, 1 stop = 2 seconds, 2 stops = 4 seconds...), give the total exposure in stops. http://www.darkroomautomation.com/em-faq.htm

The range on the meter is 10 stops, equivalent to an exposure range of f2.0 to f64 or 1 to 1024 seconds. A reading of 9.9 results in max black on MGIV, filter #2, in ~1 second.

albada · Apr 21, 2021

albada said:
It measures EV, where EV 0 is defined as the exposure that produces D-max on Ilford RC IV paper when exposed for one minute.

I need to correct an error in my posting quoted above. From the manual (http://www.darkroomautomation.com/support/eminstructionsj1.pdf), a reading of 0.00 is the light intensity that will produce full black on Ilford MGIV RC paper in 1024 seconds (not 60 seconds).

And Nicholas is correct: Those numbers are not Ev. Should they be called Lv (light value)?
The equation would be: Ev = Lv + Tv, where time_in_seconds = 2^Tv (^ means "raise to power").
Mark Overton

RalphLambrecht · Apr 21, 2021

vedostuu said:
I've been experimenting building my own light meter for darkroom printing. Idea is to get the time to a "ballpark" and maybe reduce one test strip in the process. I've been using digital light sensor which should have "light ranges from 188 uLux up to 88,000 Lux".

For example on my Fujimoto G70 on f2.8 aperture on 50mm nikkor lens, distance 450mm, I get lux values ranging from 0.066 to 0.8 lux.

However I'm struggling how to process this information. If I convert this information to EV and assume paper "film" ISO is about 8, I get totally wrong exposure times. Let's just throw that the EV is -2 at that 0.6 lux. That gives 30 seconds exposure time on f2.8 at film ISO 100. Multiply that to whatever "paper ISO" you want and you end up to many minutes exposure time, which is totally wrong.

Is there a problem that I'm measuring illuminance and I should calculate luminance some how?

Any insights how to use lux meter for darkroom printing would be really appreciated

Datasheet for the sensor I'm using: https://cdn-shop.adafruit.com/datasheets/TSL25911_Datasheet_EN_v1.pdf

I really see no need for a self-made meter because there are some very good darkroom meters on the market.

distributed · Apr 21, 2021

RalphLambrecht said:
I really see no need for a self-made meter because there are some very good darkroom meters on the market.

I find it fun and instructive to build one. It combines my work skills with my hobby, which is nice.

DMJ · Apr 21, 2021

RalphLambrecht said:
I really see no need for a self-made meter because there are some very good darkroom meters on the market.

I don't know why are you posting this here but I'm replying anyway:
Aside from the fun, building something is a great way to learn a subject. In my case it costs nothing, I already have the materials, the knowledge and skills. It is also a good project for my students too, they will learn computer science, mobile development, engineering and art. In the US we have something called STEAM which is a combination of science and arts and projects like this fit perfectly.

And I think this is one of the reasons why photography appeals to many people from different backgrounds: it is a form of art that can be as scientific or technical. I've been into this for 40 years and still learning!

radiant · Apr 25, 2021

I made some measurements after long pause and I've been thinking of developing my UI a bit. However here are shots from my measurement (highlight & shadow):

You can see that in that frame the shadows are at lux 0.0342 and highlights at 0.1371 lux. Just for reference if anyone is doing similar device.

That measured frame has 2.05 stops range and density 0.62 (if I've calculated right..).

radiant · Apr 25, 2021

Maybe I have to explain a bit.

The "measured" value is in f-stops so luxes converted to f-stop (log2). To get some positive f-stop numbers I multiply lux by 1000.

"fdiff" shows this currently measured value against EEPROM stored value. Of coure it is meaningful to set this to the value that gives first details in highlights, aka "almost" white.

What is still a bit stupid is that I need to know the exposure time which I used. So I need to manually calculate that. Lets say I exposed for 4 stops to get correct exposure when making the "calibration". Now I change the negative and for example end up with "fdiff" of value 1, I know that the highlights on this second negative pass more light to the paper. I now know I need to decrease the exposure by 1 stop and expose for 3 stops.

This is how it works in theory.. Feel free to question this

DIY light meter for darkroom

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DIY light meter for darkroom

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