Thread: LED Backlight for LCD

Originally Posted by turbocad6

im not saying the touchscreen, but the lcd itself....

Doesn't matter...I'm referring to the total light transmission of whatever complete assembly is between light source and the viewer...LCD + touchscreen + glare filter + whatever. The point is that the entire assembly, whatever its component parts, MUST transmit more than 10% of the available light if the light source doesn't put out more than 10 watts.

Originally Posted by parksgm

Some how I doubt that the color temp degrades that quickly...otherwise they really wouldn't be that useful for any applications. Do you have any data to backup that assertion?

Talk to studio photographers (my wife is who I got the info from). Photographers can deal with this as they can use light meters to find the current temp and adjust their cameras accordingly.

It's not a significant drop off (the average person isn't going to see it unless you have two side by side), but it is there and critical for non-tolerant applications.

I would guess that you could probably (anecdotel guess) get much longer use for LCD purposes, but sooner or later (and i'd guess some where in the area of 20 hours tops) the color temp is going to degrade to the point that the colors on your screen don't look right.

I would also suggest you go hold your hand 6" in front of one of these bulbs to see how hot it gets. There is a reason that studio lamps are made out of metal where they will be getting the full brunt of the heat.

-dave

many have tried what your thinking, but it will not be easy..... if it was easy there would be plenty of these sucess stories with them.... I don't know of any other than the one I did... your formulas are not the whole picture. but they may help to determine how much you need...

the manufactures have an advantage, as they have the resources to r&d an efficient light guide....it is designed to distribute the light 100%.... you can't duplicate that really... the light guide only has so much entrance surface area, so you can't add plenty to it, overdriving the ccfl can help, but shorten it's life...

the only option for screen mods past that is rear illumination..... you will need extreme light.... adding one ccfl that duplicates the existing original in the lcd will not double the brightness..... by your formulas maybe, but not in its aplication.... you would need to add like 4 more like the existing one to double the output..... the reason is you will have a large loss to the diffusion you will need or else you'll see a screen with 4 bright stripes across the back.....

you are right about the bulbs loosing light as they back away, but what your not factoring is the diffusion, which will improve as the light backs up, meaning less additional diffusion is necissary.... you need to kinda focus the light... how it's projected plays a big part...

so now if you knock your available light down by a maybe 50%-70% with your diffusion layer, then loose a little over 90% of whats left of that.... how much light do you need?

you need a lot of light....

the only easy way would I know of would be EEFL's... I knew this a year ago when i did mine, but choose not to wait, I did try hard to get them... they would have been so much better than what I winded up having to do...

now, a year later they may be easier to get.... I would really look hard for them before exploring any other option if I was to build another screen without using the luxeons.... the eefl's weren't expensive really... I mean a few hundred or less should get you that 1000 nit holly grail from these with no heat..... if you can find someone to sell them to you let me know, I'll take a bunch

I built a home made projector set up and with a 600 watt (halogen i believe) projector bulb....it still was dim in direct sun light.........let alone the projection itself....

What i did was just added a second CCFL and inverter...i know it sounds simple and straightforward...but it worked like a charm...the screen is almost perfect in direct sunlight....and it wasnt all that much thicker or hotter....

Originally Posted by parksgm

No way there's 85 to 90% loss. Here's why:

A typical screen consumes about 10 watts (or 1amp at 12volts)...that's to run the electronics and the backlight. If you take the electronics out of the equation AND assume that the backlight is 100% efficient, then there can only be 10 watts of illumination produced by the backlight. Period, 'cause Energy in = energy out - conversion to some other form.

If the touchscreen blocked 90-95% of that light, then you would be left with a measly 1 watt or so of illumination, which would be dim even in a dark room. So, the touchsceen simply cannot block that much light.

Maybe those figures are a bit more fuzzy then you think ;-)

Of course, the upside of the above is that since <10 watts of illumination provides ALMOST enought brightness, then a relatively modest 50 watts at the light source should be VERY bright indeed.

Another point to consider is that one either needs an EXTREMELY BRIGHT (your emphasis) point light source that is well diffused, or a somewhat less than extremely bright light source that is more evenly diffused from the beginning...like compact flourescent bulbs.

Finally, the heat produced by the assembly will be function of two things:

1) The efficiency of the light source

2) The light absorbance of the screen...the more light the screen blocks, the more energy it will absorb (in both the visible and non-visible spectrums), and the hotter it will get.

I think issue #2 is the most concerning, as it has the most potential to damage the LCD in the sort of setup I envision, with significant space between the light source and the LCD. In your setup, the LEDs sound like they touch (or almost touch) the screen, necessitating that the entire assembly be cooled. I don't have to care if my light source gets hot as long as it doesn't cause the light source itself to fail.

Finally, I found this thread: Backlight using compact fluros and a ligtbox?

did you check my thread? it's in my sig.... I'm not trying to beat you up or prove you wrong, I'm just trying to help...

the whole lightbox will need to be sealed or if vented it'll have to be extremely well filtered with a duct system & a fan maybe.... the back of the lcd is open to this box & dust would be bad...

Originally Posted by Sidewalksalvage

I built a home made projector set up and with a 600 watt (halogen i believe) projector bulb....it still was dim in direct sun light.........let alone the projection itself....

What i did was just added a second CCFL and inverter...i know it sounds simple and straightforward...but it worked like a charm...the screen is almost perfect in direct sunlight....and it wasnt all that much thicker or hotter....

how did you add it? to the edge like the original one? the oposite edge?

if you get it on the edge of the lightguide I think one bulb & inverter can double the light, or somewhere around there

what screen was it?

well I did ti fairly simply....

I removed the white paper/plastic backing stuff off the original backlight of the LCD then I had an backlight diffusion layers and all from the LCD i used in my projector so i removed everything from that except the main plastic part (i hope this is making sense its kinda late and im kinda tired) and placed it behind the first oriented so that there was part of the CCFL on each side (one L shaped CCFL covered two sides the second L shaped CCFL covered the final two)

I guess you took advantage of the light guide, & this let you get 100% of the second bulbs output too... awsome....

lightguides adapted from other lcds may be something to look into, I thought about that too, but went more for the all out extremely bright & rear lit...

yours sounds cool, got any pictures?