effective diffusers for overhead lights

Hello folks,

I have a bunch of COB LED emitters that I want to attach to my ceiling and have light shine down. Most likely it will be done in a ghetto manner. I just started reading about diffusers and wanted to make sure that I get the "right" kind that doesn't waste light. For example, I read that milky colored diffusers are wasteful because they absorb too much light, while "light shaping diffusers" are the right kind to get.

What I have right now is two 4ft t8 fluorescent tubes on the ceiling, spaced about 4 ft apart. Above each tube, I put a 6 inch wide strip of clear plastic film, but spray-painted white, so it mostly reflects, but still transmits a little bit so the ceiling above the lights isn't dark. It works ok in that it gives relatively soft shadows. So I would like something as good or better. I plan to take the LEDs and space them from one to a few feet apart, depending on their brightness and how much of the room I want to light up. What would be a good and not too expensive film or other shape to use?

For example, Tap plastics has some matte / frosty / crackled panels, but I don't know how good they are at spreading light.
http://www.tapplastics.com/product_info/plastics_information/light_panels

There's the Inventables films, which at least have specs:
https://www.inventables.com/categories/materials/light-diffusers

Then I guess there are the official "light shaping diffusers", although I didn't find an easily available source for them.

Even Ikea has cloth lamp shades for cheap, but I don't know how much light they lose.

One idea I had was to simply mount a few LEDs on an aluminum channel, and then take some diffusing film and roll it around.

At minimum, I want to avoid obvious multiple shadows, and it had better not burn my eyes to look directly at the fixture.

Any suggestions?

Thanks

PS: Also, it shouldn't mess up the CRI of my lighting

This is an interesting subject... one that I know just enough about to be dangerous.

My only experience in diffusing light was with an LED backlight that I built for a modest sized piece of stained glass. The goal was to create a backlight with uniform intensity (if that's the right term) with a minimal depth. I should have prototyped it before building the whole thing... oops.
The design used a 1" (25mm) grid of small surface-mount LEDs, with a diffuser mounted about 1 inch in front of the LEDs.

The result was not great, but not awful. 1 inch between the LEDs and the diffuser is just too close to avoid the appearance of a grid of LEDs.

My guess is that the idea solution is to do it the same way that LED computer monitors and TVs do it.... have a textured piece of plastic sheet, squirt the light into the edges of the sheet (i.e. an optical waveguide), and use surface texture on the emitting face of the plastic to let out just the right amount of light. The downside of this approach is that you actually have to know what you are doing and be able to create the required amount of surface texture. That's why I didn't do it.

For your application, I think that your variables to adjust are as follows:
1. spacing between LED emitters
2. spacing between the LED emitters and the diffuser material
3. type of diffuser
4. # of stages of diffusion.

Obviously, using a 1" grid of small LEDs would really make it easy to diffuse the light, but trust me, it's a pain to build.

The spacing between the LEDs and diffuser is probably limited by the fixture or available space. I'm guessing that it is a few inches at best? You could use reflection as a way to increase the spacing, where the LEDs are mounted at the bottom of the fixture and are aimed upwards into a reflector material that directs the light down into a diffuser. The reflector itself could be textured or painted surface that would provide additional diffusion.

That's all I've got. Let's see what the experts say!

I just bought a sheet of that prisms plastic used in floro lights from HD. Works well imho.

So I tried the prismatic diffuser sheets at Home Depot. They work, but only so-so. The problem is that they only work well if the sheet is far from the LEDs. The LEDs are almost points sources, but instead of spraying uniformly, they shoot the majority of light forwards. So for example, they start off as a 1/2" disk of blinding light. If the diffuser is only 4 inches away, it now becomes a 3" disk of blinding light. The most recent version of a fixture I made spaces out the diffuser about 1 foot away, and now it becomes something like 9" disks of not-quite blinding light.

I think what I am looking for is some kind of optics that can take the light and spread it out more uniformly, or even more light from the sides and less from the front. That way, the light that hits the diffuser is more even.

Would something like a concave lens work? (In conjunction with the diffuser panel) On one hand, it is supposed to spread light rays outwards. On the other hand, the light would still be coming from a small source (the size of the lens), so I am not sure how much it would help.

I have 3 leds spaced evenly along a 4" channel of aluminum as the "backbone".

beanbag said:

So I tried the prismatic diffuser sheets at Home Depot. They work, but only so-so. The problem is that they only work well if the sheet is far from the LEDs. The LEDs are almost points sources, but instead of spraying uniformly, they shoot the majority of light forwards. So for example, they start off as a 1/2" disk of blinding light. If the diffuser is only 4 inches away, it now becomes a 3" disk of blinding light. The most recent version of a fixture I made spaces out the diffuser about 1 foot away, and now it becomes something like 9" disks of not-quite blinding light.

I think what I am looking for is some kind of optics that can take the light and spread it out more uniformly, or even more light from the sides and less from the front. That way, the light that hits the diffuser is more even.

Would something like a concave lens work? (In conjunction with the diffuser panel) On one hand, it is supposed to spread light rays outwards. On the other hand, the light would still be coming from a small source (the size of the lens), so I am not sure how much it would help.

I have 3 leds spaced evenly along a 4" channel of aluminum as the "backbone".

Click to expand...

What you want or expect to happen is not going to happen with a simple diffuser. The light looks like a 3" disk of blinding light because that is how big the cone of light from the LED is when it hits the diffuser. Milky or light shaping is not going to change that. The only way to change that is distance, at least with a COB since that is your only variable.

Conceptually, you can "try" to enhance the distance till the light hits the diffuser. Your lens essentially does this by spreading the cone out ... but losing light in the process. Others would be to shine the light up or sideways to again increase the effective distance.

COBs really were not meant to be used this way.

Well theres these

http://m.rs-online.com/h5/mobile/uk/catalog?url=/web/p/led-lenses/7851111/

Or these

http://m.rs-online.com/h5/mobile/uk/catalog?url=/web/p/led-lenses/7107313/

Or something more oblong

http://m.rs-online.com/h5/mobile/uk/catalog?url=/web/p/led-lenses/7107319/

And point it at a mirror and use the mirror to reflect it back onto the diffuser to even the light. I dont really know what your trying to do, but old flouresant tube holders have very good mirrors in the back of them.


Edit

Or theres these I found on ali expreess

50 for 8 dollars street lighting 120x60 degrees. Bit of a laugh!

http://m.aliexpress.com/item-desc/1981843456.html