Mixing LED colours...

Hi, this really is an optocs question, hopefully someone can help.

I'm trying to make an LED backlight for and LCD (about 40x30mm). I really want it to be multicolour......so i was thinking..RGB LED's. But, these are still pretty expensive, and I cannot find a suitable light guide for side-lighting (requires one, maybe two LED's) so I'm going for direct lighting (LED array, diffuser, behind the LCD). This needs a minimum of 4 LED's (about ￡15/S25 where I am)

So my question is, if I was to use separate LED chips, red, green, and blue, could a typical diffuser plate 'mix' these colours well enough? Or would I get spots of green, and red/blue? I assume I would have to group them together, as close as possible. And most SMD LED's have a 100-120 degrees angle.

Also, if anyone knows how to make light-guides, light pipes, diffusers or just LED backlights, I would be grateful for any info. I'm doing this on the cheap, I can't find a pre-built backlight that ins't terribly expensive :( And if I could find one, RGB LED's are significantly larger than standard 0805 size, which most light guides are designed for.

Again, any info would be welcome.


BuriedCode.

I don't think you will be able to satisfactorily mix colour with 3 separate LEDs ..
Try KingBright type of RGB diodes: http://home.wanadoo.nl/electro1/avr/rgbled.htm
They shouldn't cost more than S5 .. check out with RS Components ..
Regards,
IanP

Hi IanP,

Thanks for your reply.

I did an experiment with 0805 SMD LED's (not the smallest, but good enough) and although the colour mixed for 90% of the diffuser, there are indeed component colours around the edge. :( Guess its RGB's after all. Although I used scrap plastic as a diffuser, which is only 'slightly' diffuse (cheap polypropelene).

So, I've decided to use the RGB's in an array (only 40x40mm) and to make it as thin as possible, what diffuser would I need? I opened up my LCD module, and looked at the backlight. Its a 16x2 character with 5x2 LED's. The diffuser film is very thin, and bendy white plastic, it looks completely opaque but lets light through and is an excellent diffuser.

I've got 'some' plastics here and I've tested them, but their either not diffuse enough, or are too 'coarse' and do not make a smooth light. Tried:

CD case.
Safety goggles plastic.
Plastic milk bottle plastic
clear plastic with paper on one side/both sides.
White Baking paper - great diffuser, but doesn't let much light trough, and its rough.


Again, I cannot find any of these types of backlights for sale in the UK. Unless I buy another backlighted LCD, but that costs ￡￡￡.

I'm pretty sure I could knock up a white plastic light box, with the LEDs glued in, but the diffuser is a problem. Some 'milky-white' plastic would be great.
Any *cheap* solutions would be very welcome. I'm completely stumped on this one.

BuriedCode.