Thread: LED Backlight for LCD

& again, what seems bright may not be bright enough when behind an lcd... you need an extreme amount of light for this to have decent results....

Originally Posted by turbocad6

& again, what seems bright may not be bright enough when behind an lcd... you need an extreme amount of light for this to have decent results....

You mention valid concerns...dust, heat, and color temperature...but all solvable (and fairly cheaply, I would think) with good forethought and research before execution. Seems easier to address those concerns with a more mainstream, off the shelf solution if I have adequate space than try to rig a solution with underdeveloped/expensive technologies. After all, smaller usually = more expensive, at least initially.

My point is that the limitation that people are trying to work around with a thin (and expensive) solution is mainly space behind the LCD...and if there is adequate space for another, cheaper solution, then why not go for it?

Thanks for the concerns you listed...always good to have multiple troubleshooters!

...so, now I'll stop hijacking this thread and start a new thread if I decide to persue this avenue.

If you have the space behind for a big assembly how about a square funnel with a domestic bulb in it

What about a 11 watt energy efficiant bulb, the flourescent type, and running it from a 12v ballast/inverter

IE http://www.maplin.co.uk/images/Full/46289i0.jpg

I think you can get this shape now in 12v in some countrys.

Anyhow for a mains 110/220 just take the tube off, and connect it to the gubbins from one of these

http://www.newenglandsolar.com/image...(Dscn0102).JPG

So you have a round white light, I think an 11w tube is about 50w of light?

how bright would an lcd be with the power of 50w (headlight?) behind it?

Originally Posted by parksgm

My point is that the limitation that people are trying to work around with a thin (and expensive) solution is mainly space behind the LCD...and if there is adequate space for another, cheaper solution, then why not go for it?

High power incandecent bulbs that have color temp ratings aren't exactly cheap. The other issue with them is that the color temp degrades (relatively quickly, ~4 hours) as they are used.

Tungsten bulbs don't have the degredation issue over their life, but they produce far more heat and are substancially more expensive.

The upside of a LED or **FL backlight is that you don't need ready access to it as they have long lifespans. "Normal" blubs will most likely need to be replaced more often.

-dave

Originally Posted by iamgnat

High power incandecent bulbs that have color temp ratings aren't exactly cheap. The other issue with them is that the color temp degrades (relatively quickly, ~4 hours) as they are used.

Tungsten bulbs don't have the degredation issue over their life, but they produce far more heat and are substancially more expensive.

The upside of a LED or **FL backlight is that you don't need ready access to it as they have long lifespans. "Normal" blubs will most likely need to be replaced more often.

-dave

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?

In any case, determining the amount of light needed is a simple physics equation or two away...light decreases in intesity with the square of the distance from the source (as I remember). As I see it, this has two important implications...

1) The futher behind the screen the light source is, the brighter it must be...

2) The futher away from the screen your eyes are, the brighter the light source must be...I guess that's something to consider when choosing a screen size and mounting location.

What I don't readily know (but will work on) is how to convert light output in watts to a brightness equivalent in "nits". A formula that allowed conversion from one to the other would be useful for determining the amount of illumination necessary for the screen to be sunlight readable. As I remember, sunlight readable screens are generally >1000 nits, while the typical LCD display (like those sold on mp3car.com) are in the 300-500 nit range.

I'm going to head to the ole physics book to do some research on this tomorrow.

EDIT

Found some info:

http://www.marktechopto.com/Engineer...easurement.cfm

http://www.reefkeeping.com/issues/2006-04/sj/index.php

http://www.du.edu/~jcalvert/optics/lumens.htm

LUMINANCE:
1 lm/m2/sr (lumens per sq. meter per steradian)
= 1 candela/m2 (cd/m2)
= 1 nit
= 10-4 stilb (sb) (or 1 candela/cm2)
= 9.290 x 10-2 cd/ft2
= π apostilbs (asb)
= π x 10-4 lamberts (L)
= 2.919 x 10-1 foot-lamberts (fL)

the nit brightness is a mesurement of light through the panel.... every panel will be different & it will even be different in different spots on a given lcd... the brightness rating is an average of different points of the lcd

regular consumable bulbs could be a pain, but again, we need EXTREME BRIGHTNESS... like a good projector bulb(expensive & hot... color?) a xenon bulb/hid (expensive & hot) or whatever else is extremely bright.... you loose like 90% of the available light to the panel alone.... that's extreme..... I did all my research a year ago, so my numbers could be a little off from memory, but I think it was between 85%-95%...... that's a lot of loss..... to make the amount of light you'll need will bring heat..... a lot of light will be a lot of heat

I choose my led's specifically because all the heat they generate is disapated through the aluminum mounting pad, which gives me a way to extract & remove the heat..... a lot of high bright screens in kiosks & stuff use fans....

Originally Posted by turbocad6

the nit brightness is a mesurement of light through the panel.... every panel will be different & it will even be different in different spots on a given lcd... the brightness rating is an average of different points of the lcd

regular consumable bulbs could be a pain, but again, we need EXTREME BRIGHTNESS... like a good projector bulb(expensive & hot... color?) a xenon bulb/hid (expensive & hot) or whatever else is extremely bright.... you loose like 90% of the available light to the panel alone.... that's extreme..... I did all my research a year ago, so my numbers could be a little off from memory, but I think it was between 85%-95%...... that's a lot of loss..... to make the amount of light you'll need will bring heat..... a lot of light will be a lot of heat

I choose my led's specifically because all the heat they generate is disapated through the aluminum mounting pad, which gives me a way to extract & remove the heat..... a lot of high bright screens in kiosks & stuff use fans....

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?

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

only 5-10% of what you start with will remain by the time the light makes it to the front of the lcd.... I did this, I have a workingf high brite screen in my car & I did tons of research, check here for a pdf discribing some aspects of lcd's

trust me, you need an extreme amount of light to get any sort of decent results