DIY LCD Backlight Brightness

Cocide · 04-06-2008, 12:44 AM

So I'm in the middle of planning an install, and so far I have yet to find a display that is good quality, sunlight readable, and cheap.

I was thinking about getting this panel:
http://www.logicsupply.com/products/800tpc_l_of
because of the native resolution and contrast ratio.

The big problem is that I have a convertible and need something that is sunlight readable. With the prices of transreflective displays I was thinking about either adding a film of plastic with a two way mirror coating on it between the backlight and the LCD or to take out the ccfl tubes and replace each with a row of SMD LEDs.

It comes down to me being worried about the two way mirror effecting image quality and lowering the backlight even more for viewing not in sunlight. Plus if I make the backlight LED then I can use less power and dim the system with a photovoltaic based system, maybe even using a fusion brain.

Essentially what I am looking for is numbers. The research I have done (or at least what I understood) is that the backlight without the panel in front of it is roughly ten times brighter the the end result brightness. I have also heard that 600mcd is just at the daylight readable point with 800-1000mcd being good.

So basic logic (assuming the above is true) says that I want to produce 10,000mcds of diffuse light.

The part that confuses me is while I keep reading that I need to use very bright LEDs, none of them say a numerical value. I have shopped around for surface mount LEDs and can find a single 2,500 mcd LED very easily, I have even found a few at 10,000 mcds each.

All the places I have looked say that I need to get very bright LEDs, but logic says that if I use the most amount of LEDs possible to make the most diffuse light that reaches my target then the backlight will look better.

So, did I do the estimating correct? Do I need to make 10,000 mcds or did I forget something?

Should I get 370mm (width of the display * 2 for top and bottom to replace the ccfl tubes) worth of the lowest power LEDs that add up to the 10,000mcd goal? Or should I go brighter then that keeping in mind that if I go too bright the color will change when I dim the setup low enough not to blind me at night.

nobb · 04-06-2008, 11:02 AM

I'm interested to do an led upgrade to my screen too.

In regards to the two way mirror idea, I dont think that would work. Simply reflecting back the incoming sunlight is not good enough...you need to diffuse that light, or else you might just be seeing reflections on your screen. Maybe a sheet of 3M's ESR behind the diffuser panel in your LCD would work.

As for the LEDs, you can get small SMD leds from china on ebay very cheap. Like a bag of 100 of these for a very reasonable price. Quality wise I think they would be ok, but not sure if their brightness specs are realistic or not. I would just take out the ccfl tube and try to fit in as many leds as you can. Although a single led is bright, the light is very focused and not very diffused, so you might need to overdo it a bit. For a typical 7 inch screen, I think you can fit about 30 of these on the width of the screen.

I'd love to see pics if you decide to go with this.

Cocide · 04-06-2008, 11:29 AM

Quote:

In regards to the two way mirror idea, I dont think that would work. Simply reflecting back the incoming sunlight is not good enough...you need to diffuse that light, or else you might just be seeing reflections on your screen. Maybe a sheet of 3M's ESR behind the diffuser panel in your LCD would work.

Thats exactly what I was afraid of, but I had not thought of that solution. As for now I am going to try just the LED upgrade.

Quote:

As for the LEDs, you can get small SMD leds from china on ebay very cheap. Like a bag of 100 of these for a very reasonable price. Quality wise I think they would be ok, but not sure if their brightness specs are realistic or not. I would just take out the ccfl tube and try to fit in as many leds as you can. Although a single led is bright, the light is very focused and not very diffused, so you might need to overdo it a bit. For a typical 7 inch screen, I think you can fit about 30 of these on the width of the screen.

Actually that is almost exactly what I was going to do, but the ones I had found were 1.2mm wide, the screen is about 185mm wide total, which means around 300 LEDs ($60 or so worth).

The LEDs I'm looking are rated at 2,500 mcds each, this is based off of the product spec sheet from the manufacture and I would like to hope this is correct.

Now heres the part where I bet that I am wrong. 300 LEDs times the 2500 mcd rating each is 750,000 mcds total. Assuming the bit about 1/10 the light of the panel being the end result is correct that means the end result is a 75,000 mcd bright display?!? That is MUCH brighter then my 800 to 1000 mcd target.

So either my 'math' is wrong, I forgot to factor something into the formula, or I need to get much dimmer LEDs.

nobb · 04-06-2008, 11:46 AM

Where did you find a 1.2mm wide led? Are you sure a 1.2mm wide led can produce 2500mcd? Thats a very small led.

By 300 leds, I am assuming you are doing the top and the bottom. It is going to be a real pain to solder up all that, you might want to just try going with one edge first just to test it out. So 150 led's an edge. But keep in mind these leds are very tiny and its going to be surgical work to be able to solder 150 of these leds back to back. So realistically, maybe 100 leds/side is more feasible.

I'm not exactly sure how they do these mcd ratings, but you definitely will not be getting 75 000 mcd total. I think each of the 2500 mcd coming from each led will have to be diffused and spread out across the whole area of the panel. Dont know how to do the math on this one. You probably should start off with the brightest leds you can find, rather than going with "dimmer" leds. If you find the panel is too bright, you can always desolder some leds. Its easily to make a bright panel dim, but you cant make a dim panel bright, so maybe its better to be safe than sorry.

Also, what were your plans to arrange and support these leds? For my case, I was thinking of using two parallel strands of wire then solder the leds in parallel sort of like a ladder. Then use clear epoxy on the back of the led as structural support. With this method, you could arrange more leds to a side since you are using the width of the led but the downside is you will need 2.5v at a fairly high current. Or maybe I was thinking to have the leds in series then use diodes to rectify the high voltage AC output from the inverter to a high voltage DC for the leds.

Cocide · 04-06-2008, 12:07 PM

Quote:

Where did you find a 1.2mm wide led? Are you sure a 1.2mm wide led can produce 2500mcd? Thats a very small led.

I did a double check on the size and I was wrong, they are 1.6mm not 1.2mm, but still very small
http://cgi.ebay.com/ebaymotors/50p-1...spagenameZWDVW
That same LED, but in larger quantity of course.

Quote:

By 300 leds, I am assuming you are doing the top and the bottom. It is going to be a real pain to solder up all that, you might want to just try going with one edge first just to test it out. So 150 led's an edge. But keep in mind these leds are very tiny and its going to be surgical work to be able to solder 150 of these leds back to back. So realistically, maybe 100 leds/side is more feasible.

Yes I am planning on lighting the top and bottom. I do know the size and challenges of surface mount components, an easy way to work with them is not to solder them on but to use solder paste and a heat gun.

Taking the LEDs off is a good point, but I was also thinking that if I used a PWM control system (pulses the LEDs very quickly to change the intensity, longer % on makes them appear brighter) then the discoloration would not be a problem, they would be very controllable.

Quote:

Also, what were your plans to arrange and support these leds? For my case, I was thinking of using two parallel strands of wire then solder the leds in parallel sort of like a ladder. Then use clear epoxy on the back of the led as structural support.

for this I was hoping to simply etch 2 thin boards to have a positive and a negative rail down the length (the hard part would be getting the board the right dimensions). That would allow me to put 2 lines of solder paste, put all the LEDs on, then go down the line with the heat gun. My second choice would be to do the whole ladder thing, to help with the attaching some double stick tape could hold the loose ends of the LEDs and allow for a few to be put in place at a time, but it could still turn into a huge mess.

Well so a mcd is a cd/m2, I'm thinking thats a candela over a square meter. Would I then need to find the surface area of the LEDs used and compare that to the total brightness?

nobb · 04-06-2008, 12:23 PM

The board idea would work, but I have the feeling it would take up alot of room. For most screens, the actual panel itself is housed in some sort of metal cage and there is not alot of room to work with. Maybe you will be luckier. I was thinking to just slide out the existing ccfl tube and make an led backlight that would slide right into the same space.

Since a mcd is a cd/m^2, that makes more sense now of how they are able to quote such high brightness values for those tiny leds. I am thinking maybe you can do a simple ratio to figure out how much theoretical brightness you will get.

Brightness of LED x Area(led) = Brightness of screen x Area screen

So for a n number of leds, the brightness would be:

Brightness of Screen = n x (brightness of single led) x (area of single led) / (area of screen)

Units of brightness is mcd = cd/m^2.
Area is in m^2

So using the leds in your link, for 100 leds and a 7 inch screen, brightness is only 63 mcd. Seems a bit low...maybe my calculations are wrong?

Cocide · 04-06-2008, 12:49 PM

I think you miss understood the layout of the board, let me try to explain in a text drawing

if you were looking down the end of the setup, with the screen to the left, and ccfl tubes were on the top and bottom:
the display is represented with ||
a LED is an *
and = shall represent the circuit board

=
*
||
||
||
||
||
*
=

from the top, looking down on the lcd pannel:
the # in this one are the panel
once again * for a LED
and = for the board
========
*********
########
########
########
*********
========

so basically the boards would be the width of the panel by the height of the LED, seeing as that would go where the ccfl tube went it should fit.

Quote:

Since a mcd is a cd/m^2, that makes more sense now of how they are able to quote such high brightness values. I am thinking maybe you can do a simple ratio to figure out how much theoretical brightness you will get.

Brightness of LED x Area(led) = Brightness of screen x Area screen

So for a n number of leds, the brightness would be:

Brightness of Screen = n x (brightness of single led) x (area of single led) / (area of screen)

So using the leds in your link, for 100 leds and a 7 inch screen, brightness is only 63 mcd. Seems a bit low...maybe my calculations are wrong?

Well I checked the calculations based on the 'math' and for a 8.4 4:3 with 200 I got 77mcd, basically the same. The one thing that dose stand out is that I have seen it as cd/m2 not cd/m^2, I was guessing that they meant square meter, but I would think with a LED that bright those numbers have to be somewhere in the middle of the 750,000 and 77 mcds.

I do have some 1000mcd LEDs, they are not SMD so I could not use them, They are extremely bright, its almost like staring into the sun if they hit you the right way. So by comparison I would think that those are plenty bright, not to mention that most of the places I have looked will draw the LED / super bright LED line at 1000 mcd.

coucouillu · 04-06-2008, 03:23 PM

have you seen that?

Cocide · 04-06-2008, 10:03 PM

Quote:

have you seen that?

No I had not, and it seems that I should be worried about heat and not bright enough.

Coucouillu, in the pictures of your backlight upgrade, are those 2 10,000mcd LEDs with a screen in front of them? Did you end up using an LED system? What have you tried and what were the results?

The Osram LWW5SG LEDs are about 3.87 times larger, 4 times brighter, and about 20x more expensive each (thats if I have to pay $4 each, digikey has em for over 6 but they are expensive, maybe I could find them cheaper but I highly doubt I could find them for $0.80 which would be the point where the build cost would be identical to get the same light intensity and about the same length). I do love the idea of such a ridiculously bright LED and actually when did my original research the only reason that I did not emphasize the possibility of using those or similar was the cost.

If I use 170mm as a guess estimate for the panel width, 1.6mm as the width of the 2,500mcd LEDs, and 6.2mm (thats me hoping the unlabeled measurements on the data sheet are mm) for the 10,000mcd LEDs then:

I can fit 27 10,000mcd LEDs along the top or bottom of the LCD for a total of 54. That brings the total brightness (incorrect math of just adding the values) to 540,000mcds. If the LEDs cost $4 a pop then thats $216 + shipping

I can also fit 106 2,500mcd LEDs along the top or bottom of the LCD for a total of 212. That brings the total brightness (incorrect math of just adding the values) to 530,000mcds. From ebay thats $43 + shipping.

If I guessed the data sheets unlabeled units of measure wrong then the LEDs might be smaller which could make things more expensive but brighter, if they are larger then its cheaper but dimmer. Either way I am not wanting to spend $200 when I could spend $50 and get the same result.

Also, the more I think about the heat problem the more it seems for my application the ladder type assembly thing could be better, I might be able to find a way to attach the assembly to the enclosure of the panel to allow better cooling just by direct heat transfer, plus I plan on posting a step by step when I do this in a few weeks and that makes it more realistic to follow because I imagine that not everyone here wants to play with etch solution. I am still amazed that heat is really a problem, one of the selling points of LEDs is that they are cool, my laptop backlight (which is LED not CCFL) dose not get warm at all, but then again I do not have it on full that often.

Cocide · 04-07-2008, 04:47 AM

So I was browsing for solutions to regulate the brightness in a cost effective manor and came across 500,000mcd and 750,000mcd LEDs. While looking into them I found that they were often referred to by the wattage consumption. I also remembered that while the LWW5SG LEDs are larger, they are also MUCH more power intensive for only a 4x mcd rating increase. This got me thinking that the math that makes perfect sense about brightness over a square meter.

I think the formula was correct or close to.

I ran numbers for both LEDs (the ebay ones and the LWW5SG)

for the display I used 170mm by 120mm to approximate an 8.4 in 4:3 display (these are the only numbers I have, and its a fairly rough guess). That means the surface area is 20400mm^2

I figured 200 2,500 mcd LEDs or 50 10,000 mcd LEDs with a surface area of 3.15mm^2 and 42.16m^2 respectively.

So that means the outputs are 77.2 and 1033.3 mcds respectively, although both numbers are still WAY to low if the 1/10 brightness transmission threw the LCD guideline is correct.

Well that raises the question would these numbers be accurate as to what was used by coucouillu and the results that followed.

Also, the panel I am looking at says its typical draw is 8w, the LED backlight using the LWW5SGs comes to a grand total of 55w, much higher the the stock ccfl system and probably much more heat too. Hopefully I am failing at the math ...again... or there is something that is not taken to account because the goal is for it to be cheap, not set my car on fire hot, and sunlight readable.

If someone wants to pwn me w/ knowledge here, recommend some other LCD panel / backlight / touchscreen combo or individual units, or just general help I would greatly appreciate it.

I'm starting to think that something like this might be better to do, although more costly and just at the readable point. But I am planning to make the screen removable with like a 3' cable so if it was not viewable then I could just pop it out and use it.

EDIT: corrected units of measure

pilot · 04-07-2008, 12:24 PM

For your info i have upgraded the ccfl EBY-701 with smd leds 2500 mcd each.The final result was very impressive, much brighter and vivid than before.
http://cgi.ebay.com/50-PCS-PLCC-2-PO...QQcmdZViewItem
Pics of my first attempt attached.

Cocide · 04-07-2008, 01:12 PM

Very interesting....

I see that you are only using 24 LEDs, will that be or was that the final count?
How are you powering them? Is the brightness controlled?

Quote:

The final result was very impressive, much brighter and vivid than before.

How bright was the factory backlight? Is it sunlight readable or close to with the LCD and touchscreen on?

pilot · 04-07-2008, 03:24 PM

My first attempt included 24 leds , power consumption at 3.5 v was 1.5A. Now( first pic) i made another setup with 50 leds, which should be the maximum number of leds placed together in a row . The problem ( not big) in that case was heat.I managed to control heat ,powering the whole led row at 3.2v max.Total brightness was 40 -50 % ( 24 leds) ,and 60 - 80 % ( 50 leds) better than the original ccfl. Brightness can be controled easily from 2.6v (night) to 3.5v ( day) ,with a cheap voltage regulator ,(12v input to 3.5v output)something like LM317.You can do it also easily with a 12v relay , and toggle with car lights switch ,to those prefix settings.Much better viewable results than standard softwear solution will be achieved controling backlight power .
The original ccfl does not dim at all when changing brightness from front panel switch.
I also use all available 3m films as, ESR, BEF X 2 ,DBEF 45 maximizing total readability.
Even all this significant led upgrade ,if i could find the very good touchscreen ,used in MP3CAR transflective lilliput,with the AR coating, the final readability ,would be unbelievable better.

coucouillu · 04-07-2008, 03:45 PM

Hi Cocide,

I have understood your maths.

I will look into your project with interrest.

Cocide · 04-07-2008, 03:48 PM

Have you tried using a variable output voltage regulator? I think that using 2 pots and a photovoltaic cell it might be passable to have it automated based on the ambient light.

So am I correct to understand that with 50 LEDs it is reasonably readable in direct sunlight?

Out of curiosity, what screen are you using?