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I am currently designing a LED light indicator for use in an aircraft cockpit replica.

I’m asking for expert advice on the type of lenses to use in order to get a result similar to shown pictures.

When illuminated, the indicators display a really crisp text legend. When off, the legend is not visible at all, even when in direct light.

As you can see on the picture, the indicator appears flat/matte black when off :

enter image description here

It looks like there is some kind of black matte plastic filter with kind of a microscopicly rough surface (as if it was sandblasted really fine).

When illuminated, the text looks crisp, with zero light leak :

enter image description here

The indicator consists of a wide angle LED inside a closed chamber, with a label filter on top (probably photo etched with really high resolution for the text to be crisp). My question is more precisely on the surface filter : what filter to use to get a similar result ? i.e. not let any light in, any reflection, flat matte black appearance, but still lets the indicator shine when on ?

I’ve looked for optical filters that match those characteristics but no luck.

Tt looks like such optical feature is called Secret-until-lit or dead font or black panel effect.

Thanks for your help.

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  • $\begingroup$ "As you can see on the picture, the indicator appears flat/matte black when off." It looks very light grey in your top photo. Can you not examine what's underneath? $\endgroup$
    – Transistor
    Commented Aug 5, 2021 at 22:19
  • $\begingroup$ It would be hard to inspect non-destructively. I think it’s casted in resin. I thought it would be a frosted acrylic at first but I think I would distinguish a front layer. The light grey color you observe are sun reflection that are diffused - as opposed to the small frames that reflect it. $\endgroup$
    – GunMan
    Commented Aug 6, 2021 at 6:42
  • $\begingroup$ Looks like I can't edit my question but I wanted to add a small detail - it looks like such optical feature is called "Secret-until-lit" or "dead font" or "black panel effect". $\endgroup$
    – GunMan
    Commented Aug 6, 2021 at 8:33
  • $\begingroup$ I do not know about the suitability for aviation related use nor the effectiveness on black glass, but adhesive tapes are available which can be pasted onto clear glass to make it "frosted". Web search for frosted glass film. $\endgroup$
    – AJN
    Commented Aug 6, 2021 at 11:52
  • $\begingroup$ you could ask at aviation.stackexchange.com $\endgroup$
    – jsotola
    Commented May 4, 2022 at 4:03

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I don't think you can achieve this with backlighting. At the least you'd need to have a "light-pipe" layer with sidelighting; the light feeds into the lettering on that layer and thus forms the letters.

If the real device is using backlighting, then my guess is that the illumination level is quite high and the display (cover glass as well as the layer with lettering) has high-quality antireflective coatings. It appears "black" due to the contrast with the bright illuminated region. Compare with your TV set, which looks grey-ish when off, but appears to have deep-black portions when generating images which are brighter than you think they are.

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    $\begingroup$ The real device indeed uses backlighting. I discovered this technique is quite common in the automotive industry and several companies specialize in offering this. Indeed, it looks like a very low transmission lens is put in front of the legend overlay, and it is illuminated with a highly powered light source, of which 95% is lost. Info can be found there : gmnameplate.com/resource-center/blogs/dead-front-printing - looks like it will be hard to replicate in a DIY/cheap fashion. Thanks for your inputs ! $\endgroup$
    – GunMan
    Commented Aug 7, 2021 at 9:24

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