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I am trying to build a night clock projector, inspired by this project: https://microengineer.eu/2018/05/01/diy-night-clock-projector/

As I am more a computer scientist than an electrical engineer, I prefer ready-to-use/plug-and-play solutions (I am not very good at soldering, and not at all at SMD soldering...).

The linked project recommend to use a small LCD screen with a power LED. But I had a hard time finding such LCD screen, compared to the cheap OLED alternative: https://www.buydisplay.com/serial-spi-i2c-white-0-42-inch-arduino-raspberry-pi-oled-display-72x40

However, this screen is advertised as 120 cd/m^2, and it doesn't seem enough to be projected to a ceiling 2 meters away (I did not manage to get anything projected, other than the activity LEDs of my RPi ^^')

In comparison, the LCD alternative (with pre-mounted backlight) is bigger and is advertised as 200 cd/m^2 (which is a rough average value for LCD). Finally I am not even sure that it is bright enough for clear projection.

Finally, I found this LCD screen (with no backlight) which is FAR more expensive, require a backlight (like this LED) and some soldering to connect all these things together. Here, it is hard to get some brightness comparison with other solutions.

What I come to is:

  • 120 cd/m^2 times the size of the screen gives 0.008 cd for the pre-mounted OLED screen
  • 200 cd/m^2 times the size of the screen gives 0.136 cd for the pre-mounted LCD screen
  • 87 lm over 85 deg gives 30 cd for the LCD + LED

Is this comparison correct?
Is there a way to compute the required brightess/luminance/luminous flux/luminous intensity/illuminance for this kind of application?
Is there some "in-between" solution, with more than 0.136 cd but less than 30?

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What you are missing in terms of understanding is that the project writeup is not using the stock LCD backlight. Rather it's stripping that out and instead running what is basically an "LED flashlight" through the bare LCD glass. No off the shelf integral display intended for direct view is going to be bright enough for this, no matter if it's LCD+ordinary backlight or OLED.

Because LCDs are obstructive displays which darken to impede some other light source, if you can mechanically get access to both sides then you can use a bright light source up to the point where you overheat the display.

OLEDs in contrast generate their own light; as such there's no way you can increase their output beyond the point at which the tiny manufactured in organic LEDs (or else their drivers) would be damaged. You can't use an OLED for this project.

If you want something actually workable off the shelf, it would have to be the element out of or intended for an existing projection product. Something like that might be possible, but beware that display interconnects can be quite fragile - repurposing simple displays out of dirt cheap products where you can just start over if you break on is one thing, doing it for moderate cost products a bit more risky.

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  • \$\begingroup\$ Thanks. You basically answered the implicit question: "No off the shelf integral display [...] is going to be bright enough". So, from you point of view is it that hard to dismount some integral display to keep only the LCD glass ? Which kind of LED should I use (is a 5mm red LED enough, or should I use a flashlight LED) ? \$\endgroup\$ – AlexisBRENON Nov 24 '20 at 8:39
  • \$\begingroup\$ The answers to these questions are in the document you started with. \$\endgroup\$ – Chris Stratton Nov 24 '20 at 15:09

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