My wife is a Black and White film photographer. I was wondering if there is a color code that I could use as a basis for a helper app?

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    You're talking about using a mobile device display as a light source, right? The problem is that they don't display "frequencies", just combinations of red, green, or blue... an OLED display showing only red is probably the best you could do. Whether it would be "safe" or not is something somebody who knows more about film processing will have to tell you... – junkyardsparkle Jun 10 '16 at 6:24
  • hmm. thanks. You are correct in that I'm thinking about using her mobile device. It does not (yet) have an oled screen. Perhaps filter paper over the device? – ICL1901 Jun 10 '16 at 6:43
  • @DavidDelMonte Filter paper? – Stan Jun 11 '16 at 21:30

Typical phone screens have too broad a spectrum to use as safelights (there's too much orange in the red channel). The red safelight spectrum here indicates that there's no light below 600nm. You'll get too much leakage below that from any LCD. With OLED you might be OK I've seen spectra that indicate that you would, and that you wouldn't (e.g. here, though there's no data on how they took the spectra). So using a phone as a safelight seems to be out. The main effect would be a reduction in contrast.

If you want something like a timer app, you might have more luck. OLED might be good here as the black background would be really black rather than leaking short wave light.

For a red-on-black timer app, unless you point it directly at your paper, you're likely to get away with it even with LCD as the photon count is so low, especially if you keep the backlight turned down. The red would be #FF0000.

Of course, if you're using your phone in the darkroom and a bright white notification pops up for a text, your print is probably ruined wherever the phone is pointing.

  • @junkyardsparkle is right, I've added some more detail from a spectroscopy point of view. Last time I did any wet developing mobiles had black LCD screens with no backlight at all. Despite my negative answer I suggest an experiment but also not going on to sell any app as you don't know what screen your customer would have – Chris H Jun 10 '16 at 6:57
  • Great info. thanks.. especially the testing and the possibility of interrupting notifications / calls / messages.. If I can solve that one, what about the idea of a (plastic) color filter? Any chance that could work? My wife is using one to cover a regular bulb and uses that as a safelight. This works ok.. I should add, that this darkroom is only used to make contact sheets and work prints. – ICL1901 Jun 10 '16 at 7:23
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    @DavidDelMonte I'd go for a filter designed for fluorescent tubes if you can get one. The filters designed only for low-wattage incandescent bulbs don't have to block much in the blue/UV region of the spectrum. – Chris H Jun 10 '16 at 7:49
  • Perfect. Thanks for all the advice..I'll look for an appropriate filter – ICL1901 Jun 10 '16 at 8:07

If you had sheets of Rubylith masking film by ULANO™ you could lay a sheet over any device for a darkroom safe-light filter. If you peel off the strip-able layer, and it will cling right to the phone or pad screen as a Mylar layer does without any glue or moisture. It is perfectly transparent and you can see the tiniest details through the red layer. It's also waterproof like plastic wrap so it's darkroom friendly. I have a lifetime supply here in Montreal. :)


Edit: If you use Rubylith over a fluorescent tube, it must be replaced every few weeks as the UV bleaches the red dye used in the film rendering it useless.

  • @DavidDelMonte I have actually tested and used this successfully for years under actual production conditions. When you find other solutions are fogging your paper, I think you'll come bace to this actual tested solution. The stuff IS a darkroom filter for orthochromatic materials. – Stan Jun 11 '16 at 21:26
  • I would probably do this even on a device where the spectrum of the red channel was deemed "safe", just to prevent nasty surprises when a "low battery" alert or something pops onto the screen... – junkyardsparkle Jun 11 '16 at 22:29
  • @junkyardsparkle You could even have a "safe" flashlight to locate things or read labels in the low light conditions of the average darkroom. (That sounded odd when I re-read what I wrote.) – Stan Jun 11 '16 at 22:35

Try this with iPhones running iOS 10.1 and higher: Go to Settings > General > Accessibility > Display Accommodations > Color Filters > slide to 'on' (right) position Then, in the drop-down menu that appears, select 'Color Tint' and slide INTENSITY and HUE to the extreme right side.

This will shift the display to a photo-safe red, and should be fine for viewing any app in situations requiring red safelight work. That said, I would still recommend reducing the overall screen brightness level to less than 50%. If you slide the upper screen display's palette viewer from the default pencil array to the middle ROYGBP grid, you will notice that the 'actinic' green to purple wavelengths are now displaying as variations of mid red/grey to black.

To switch back to the normal display, go to Settings > General > Accessibility > Display Accommodations > Color Filters > slide to 'off' (left) position.

From this point on, every time you want to switch to the red channel display, navigating to Color Filters and sliding to the right will turn the display to red.

I teach photography and have worked in wet lab situations for over 30 years, and this little-known gift from Apple has saved a lot of photo materials from unwanted exposure!


There's a product idea here, but it's not just an app. You need to produce screen protectors that are also safelight filters, so that people can safely use any app in the darkroom. A companion app with timers, calculators, chemistry data, etc., would complete the package.

  • Thanks Caleb. The basic app is easy, and almost complete. You're right that with the correct help, it could be a product, perhaps having a wider dimension. I'm just trying to help my wife though, and have no energy for a new business endevour. David – ICL1901 Jun 10 '16 at 9:35
  • You could experiment with lighting gels to see if any brands are compatible with touch screens. There's probably a gel in the right color. – Caleb Jun 10 '16 at 10:02
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    My wife has some gels so we will test later on... @chris, you can avoid all interruptions by putting the device in airplane mode ;) – ICL1901 Jun 10 '16 at 10:43

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