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If 7-segment displays on a portable device to be easily legible from say, 10 feet, laid on a surface facing up at a light source (indoor or daylight), what are the minimum luminous intensities I would need for the display to compete with the surrounding (ambient) lighting?

  • Is there a known reference?

Furthermore, if I'm switching (multiplexing or PWM or other(?)), is there a known reference which will establish the relationship between the average luminance, peak luminance, duty cycle and frequency with respect to the light retention of the eye such that I can consider and factor in the benefits of switching?

kando
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    I already wrote something recently on the topic: [here](https://electronics.stackexchange.com/a/662012/330261). I tell you exactly what book to find and buy, as well. (The only really good single book reference I know of on the topic.) (And yes, I'm aware that I'm using the word 'reference' differently than you likely are.) – periblepsis May 02 '23 at 20:45
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    @periblepsis : That one's actually in the mail! It's going to answer that too? Bonus! – kando May 02 '23 at 20:58
  • @periblepsis : Daylight intensity hasn't changed since the early 1980s, has it? (; j) – kando May 02 '23 at 21:04
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    If the book doesn't cover pretty much all of your questions on the topic, then I'd say your remaining questions are either (1) related to newer products sufficiently different that the text doesn't cover it (certain new LED colors, for example) or (2) some highly unusual situation they didn't take time to cover. The book covers so many topics, including multiplexing/PWM, etc. However, it's a learning process. There are no bright line answers to "What's the minimum luminous intensity required for outdoor use?" As the authors state, there are a lot of human factors and environmental issues. – periblepsis May 02 '23 at 21:04
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    And there are a lot of ways you can go in dealing with visibility issues. Lots of things matter. Including the angles at which visibility must exist. Etc. And no. The sun has stayed mostly the same. But the Earth is a big place. And humans have created any number of application requirements. I am very glad to hear the book is on the way to you. I sincerely believe you will want to keep it and read it. If it in any way disappoints you, I will consider the idea of making you whole on it and buy it. But I don't think you'll want to sell it once you get it. – periblepsis May 02 '23 at 21:05

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Since this is too long for a comment, I'll post it as an answer.

easily legible ... known reference ...

Visually, you (better yet, your team - at least one of which is colorblind) should make the determination. That is because of many factors - LED efficiency, batch / vendor differences, LED degradation over time, human perception / acuity differences, temperature effects, ambient changes (direct sunlight versus fluorescent versus dark), the list goes on and on. I've tried meeting standards and results were lackluster at best; always ended up deviating. Also battled with repairing/adjusting devices which didn't care about standards at all (blindingly bright at night and barely bright enough to read in room lighting.) It is a good topic to carefully consider.

You can try to adhere to luminosity standards, but ultimately they are moot in the face of all of these variables. All the hard work using them goes out the window when the user places your device in an environment you didn't envision (like in an airplane with moving sunlight and shadows), or when the piece of red plastic over the display must be sourced from another vendor and the new window suddenly makes the display seem way brighter (or dimmer) than before.

I'm not sure an adjustment pot is the ideal answer here, but it's not a bad idea for the unforeseeable. And an ambient light / photo-detector to servo the brightness is a really good idea if expecting to operate in wide ambient range. The range from dark to sunlight is logarithmic and vast. Could even use an LED and detector internally to compensate for aging if desired. (Have an LED alarm clock from the 80's that still runs (and servos its own brightness), but is quite dim these days - only way it could have been made better would be to compensate for aging.)

PWM ... [Persistence of Vision]

Researching Persistence of Vision will get you going on this topic. The books in the comments are a good idea as well. They will say that about 20 updates per second will "persist" enough to seem continuous. Perhaps they were talking about full-frame motion pictures (maybe even interlaced) but this certainly is not the lower limit for character displays. An update rate of 100/s is still detectible, especially when the eyes or display moves. Many people dislike flickering of any sort. Like allergies, some are much more sensitive to it than others. The faster you can update it, the better. Might make sense to dedicate one micro just to displaying the data quickly with accurate timing, then pipe it the info to display over I2C or SPI.

Also consider matching integer camera shutter/video speeds if you care about the photograph-ability of the display. Interesting experiment - use a cellphone to record video of the (character display) instruments in a private airplane cockpit. Since they were not designed for this, characters will be totally unreadable on video.

rdtsc
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