Choosing a filter for a 7 segment LED display

[e100]

I bought some cheap green LED 7 segment voltmeters from ebay and they came without a front filter so under normal lighting you can see the non-illuminated segments which results in poor contrast.

I went looking for some green transparent plastic thinking that this would be a common item for displays but apparently not. I tried the lens from a pair of sunglasses and that looked OK, so I went looking for neutral grey transparent plastic which lead me to ND2, ND4 and ND8 filters designed for photography.

I've never seen these filters in person and it's difficult to tell from the photos which of these would be roughly equivalent to the lens in a pair of sunglasses. Can anyone suggest which density grade is likely to give best results? 

[ar__systems]

Why don't you just try? Get transparent film for laser printer, print few samples at different densities, and choose the one you like.

[sokoloff]

Another thread with some pics of window tint film (including light transmission percentages):
https://www.eevblog.com/forum/projects/aesthetics-led-bar-graph-in-aluminium-panel/msg1210023/#msg1210023

[e100]

Another thread with some pics of window tint film (including light transmission percentages):
https://www.eevblog.com/forum/projects/aesthetics-led-bar-graph-in-aluminium-panel/msg1210023/#msg1210023

After some more research it seems that regular sunglasses have a transmittance of about 25% (optical density of 0.6), therefore a ND4 filter should do the job.

[David Hess]

Art supply stores carry acetate or Mylar (polyester) colored film in different colors.

[mikeselectricstuff]

A circular polariser is what you ideally want

[knotlogic]

A circular polariser is what you ideally want

I'm curious, why a circular polariser?

[mikeselectricstuff]

I think the idea is that when circularly polarised light gets reflected ( off the surface of the display), teh reflection is polarised in the opposite direction, and so gets attenuated.
I've used circular polarised filters in the past on displays and they work well.
 http://www.apioptics.com/circular-polarizer-display-filters.html

[floobydust]

You can gut an old LCD monitor panel to get a diffuser and polarizer to experiment with.

Gray is OK, the LEDs are dimmer but contrast ratio goes up so you can read the display in bright light.
My latest red LED DMM has a purple window which works really well for red LED's.

I've bought plexiglass plastic for displays from ali and ebay for a few $; keyword: acrylic plastic sheet green

[mikeselectricstuff]

You can gut an old LCD monitor panel to get a diffuser and polarizer to experiment with.

Gray is OK, the LEDs are dimmer but contrast ratio goes up so you can read the display in bright light.
My latest red LED DMM has a purple window which works really well for red LED's.

I've bought plexiglass plastic for displays from ali and ebay for a few $; keyword: acrylic plastic sheet green

LCD polarisers are linear, not circular AFAIK

[james_s]

I don't know where you are but there's a place around here called Tap Plastics which carries all kinds of plastic including colored plexiglass (acrylic) in a rainbow of colors and various thicknesses.

[reboots]

I had the same issue with colored 7-segment displays. Reasoning that the best brightness and contrast would be provided by a filter approximating the wavelength of the emitters, and rejecting other ambient wavelengths, I used a filter matching the color. I've found that approach works very well.

First I used theatrical lighting gels. These are great because they come in a wide variety of hues and densities, and have a very well-characterized spectral response. But they require tape or some other form of adhesion, and may be difficult to source.

Then I found a vendor of colored transparent vinyl tape (paperstreetplastics on eBay, no affiliation). The tape color was ideal for my application, and the adhesive seems to be strong and long-lasting.

https://www.ebay.com/itm/380356230567
https://paper-street-plastics.myshopify.com

So that's what I recommend.

[tweezak]

I know this is old but I found it through a search so maybe someone else will find this comment useful.

If your LED 7-segment display came with a protective plastic film on it, don't peel it off. Get a Sharpie marker the color of the display and fill in the plastic making your own film/gel for free. I tried this with a small red $3 volt meter from Digi-Key and it worked great at muting the white segments when they are not turned on. In a bright room the display is much easier to read.

It's ghetto but it really does work. Also free to try.

If you've already peeled the film try scotch tape.

[reboots]

For a thorough analysis of this subject, check out Avago Technologies' Application Note 1015: Contrast Enhancement Techniques for LED Displays.

Direct PDF link: https://docs.broadcom.com/wcs-public/products/application-notes/application-note/333/52/5964-6129e-an_1015-15jul10.pdf

[shabaz]

For simple hobby stuff, plastic camera filter sheets are cheap. They are almost the same thickness as a PCB (the plastic sheets are 1.7mm thick), which can provide physical protection too.

I used a gray neutral density filter to replace the scratched-up red filter when I was restoring my ancient frequency counter. It looks better now, and is very readable, as can be seen in the photo.