Diffusing Rebel LED for Lightbox

[mohakem]

Hello, im looking to make a lightbox to mount posters in. I want to use LED's and have got hold of some Luxeon Rebels.

Ideally I want the lightbox to be as slim as possible which means I will need some sort of material to diffuse the light for uniform output. So, assuming i've got the lightbox as slim as possible, taking into consideration heat, what material would you suggest is good for diffusing the light?

Thanks.

 

[Gunner12]

Maybe a textured plastic tubing cut in half lengthwise would work.

:welcome:

 

[Oznog]

Well you're basically looking at a backlight situation. A real common problem. You'd use emitters on the edge pointing inward and a frosted piece of acrylic. It is ever difficult to achieve an even illumination however.

If you're thinking of putting the LEDs in an array across the back surface ad diffuse the forward light, it's pretty much impossible to diffuse this way without a lot of thickness, 1"-2". It'd work with many dimmer devices close together but not with power emitters.

Backlights are used behind LED displays of course. If you can find a busted laptop display of the appropriate size you could rip out the backlight. It will be CFL but CFL really is a better idea anyways.

 

[SemiMan]

Are you backlighting or front lighting?

Semiman

 

[Steve K]

Well you're basically looking at a backlight situation. A real common problem. You'd use emitters on the edge pointing inward and a frosted piece of acrylic. It is ever difficult to achieve an even illumination however.

If you're thinking of putting the LEDs in an array across the back surface ad diffuse the forward light, it's pretty much impossible to diffuse this way without a lot of thickness, 1"-2". It'd work with many dimmer devices close together but not with power emitters.

Interesting subject! I've got a stained glass window that I wanted to backlight and hang on a wall, and this seems to be a very similar problem.

I like the idea of using a piece of 1/4" or 1/2" acrylic, and graduating the "frosting" or surface texture, to try to create a fairly uniform backlight. In my vision, I'd start with non-frosted acrylic (i.e. smooth), and use a fine abrasive overall, with coarser abrasive at the middle. The trick would be to get the graduation worked out, as well as the overall level of texture. Maybe just a light pass with a fine sandpaper or steel wool would do for the areas near the edges where the LEDs are located, with a coarser sandpaper in the middle?

I can imagine that it might get tricky, since any texture added to one area will let light out, reducing the light at other areas. Overall, it's an interesting concept, especially since I had a semester class in optics in integrated circuits. We spent so much time working on equations that describe light hitting the boundary between two materials with different optical densities, etc... it'd be nice to apply that knowledge even if it is a trivial case.

The other plan was to use an array of 5mm white LEDs, and put layers of frosted acrylic between the LEDs and the stained glass. Would have been pretty bulky.

Steve K.

 

[SemiMan]

They are not the most efficient things, but you can not beat Lux-III side emitters for backlighting. Just put something dark on top so no light goes forward and you just get the side light.

Semiman

 

[Oznog]

I can imagine that it might get tricky, since any texture added to one area will let light out, reducing the light at other areas. Overall, it's an interesting concept, especially since I had a semester class in optics in integrated circuits. We spent so much time working on equations that describe light hitting the boundary between two materials with different optical densities, etc... it'd be nice to apply that knowledge even if it is a trivial case.

The other plan was to use an array of 5mm white LEDs, and put layers of frosted acrylic between the LEDs and the stained glass. Would have been pretty bulky.

Steve K.

Well the key is getting the light to enter the material in the first place. Lambertian emitters stuck in a drilled side hole project a lot of light which will not meet the total internal reflection angle, thus a very bright area where the light is lost. A focused device stuck in the side will have less uncaptured light.

Also optical interfacing gels should help a lot.

The 5mm array in back will require either tight spacing OR significant distance between the devices and the frosted panel. The degree of frosting won't change that. The LED will project a glowing circle onto the frosted surface whose diameter is based on its distance and the LED's projection angle, not the degree of frosting.

 

[Steve K]

Well the key is getting the light to enter the material in the first place. Lambertian emitters stuck in a drilled side hole project a lot of light which will not meet the total internal reflection angle, thus a very bright area where the light is lost. A focused device stuck in the side will have less uncaptured light.

Also optical interfacing gels should help a lot.

any recommended optics for this type of application?

interfacing gels? <scratch head>
Is there anything that's not too difficult to obtain, or that you'd recommend?

thanks,
Steve K.

 

[mohakem]

Thanks for your replies..

I was thinking to use backlighting but it seems like i wont be able to get the uniform light output i want (suitable for say, a movie poster size lightbox). Is that right?

I'm not sure how side emitters would be mounted?I guess you would attach them to the side of the frame, but, in front of or behind the poster?

Thank you very much.

 

[SemiMan]

With side emitters, you would create a light box approximately 2" thick. You could likely get by with less, but 2" should be enough. The LEDS are mounted to the back of the light box. There light goes sideways, hence no direct light ever hits the front. If you mounted them on a 6" matrix spacing, it would be quite even. You may even be able to get by with more than 6". The inside of your box should be painted with a diffuse white paint that reflects as much light as possible. In a pinch, a good flat white paint will do. For the front of your light box, you can experiment with anything from clear to a milky white diffuse plastic front. The white plastic front will give the most even look as it will help internal reflection in the box, and diffuses the last little hot spots out.

Semiman

 

[mohakem]

Thanks for your detailed reply Semiman.

I guess I will have to do a bit of searching and experimenting to find the best diffuser. Just one more thing, have you had any experience with these 'frosted glass' type sprays? If so do they do a good job of diffusing the light?

Thanks a lot for your help.

 

[LukeA]

Thanks for your detailed reply Semiman.

I guess I will have to do a bit of searching and experimenting to find the best diffuser. Just one more thing, have you had any experience with these 'frosted glass' type sprays? If so do they do a good job of diffusing the light?

Thanks a lot for your help.

I haven't yet used that stuff on a flat panel, but it diffuses 5mm LEDs very nicely when applied to their domes.

 

[SemiMan]

No experience with a frosted glass spray, sorry.

Semiman