why does nonone use fluoresent lamps for backlights?

WhiteRabbit · 05-25-2008, 05:29 PM

tons to thought and energy have gone into DIY construction of back lights. And it seems to me under certain conditions standard fluorescent lamps may be an ideal solution. some brainstormed ideas:

4" fluorescent lamps can be inexpensive when compared to high intensity LED banks combined with diffusers.

using an array prevents the need for a fancy diffuser at all.

Fluorescent bulbs are available in any temperature from 4000-10,000 easily and anywhere in between. probably out of this range too if desired.

power consumption must not be a factor in the design.

why arent these used more often compared to LED banks?

Rob Withey · 05-25-2008, 05:50 PM

What do you think a CCFL is?
They are very commonly used.

WhiteRabbit · 05-26-2008, 01:13 PM

not cold cathode. standard fluorescent. the standard is a way to decrease cost while increasing area of light production (by using many tubes 1/2" to 3/4" rather than 2-4 tubes 1-2mm) while simultaneously allowing the installer to eliminate (theory) the diffuser by creating 100% coverage of the LCD panel (due to the large tube diameter)

key word used was DIY.

The typical forum DIY method are to use very high output LED strings as the light source and as much thought and technology as possible put into the light diffuser to compensate for the point sources of light.

I just pulled four CCFL tubes from a 19" computer monitor. Agreed; seems common enough.

I've never seen a production monitor (obviously never will, either) using inexpensive standard fluorescents in an array but more importantly never seen anyone here DIY their own.

Does that make more sense? I may not have been clear enough with my initial line of reasoning.

infamous_t · 05-31-2008, 12:22 AM

Standard fluorescent tubes aren't used because their lifespan is significantly shorter than CCFL.
Also CCFL are instant on, standard fluorescent are not.

LED is better suited than both in a car due to the fact that the tubes are sensitive to shock, as anyone thats had the backlight stop working can tell you.

WhiteRabbit · 05-31-2008, 02:17 PM

sound like very good reasons. inarguable, in my opinion, for industry applications.

It's still unclear why not for the home based application. With all the homebrew development into shock resistance planning for hard drives, the same effort here could easily be put into tube protection (off the top of my head I can think of at least one way to use hot glue to enhance shock resistance of tubes)

The home brew user wont care about startup time and will compensate for brittleness.

One also has to wonder why BOTH cant be used. A standard backlight we all know and understand, edge lit glass as comes standard with all LCD for night use. With the monitor apart, we can remove the opaque glass backing and replace that with fluorescent tubes to be used WITH the CCFL backlight tubes in an attempt to bring daylight readability to the party.

seems feasible, and can be controlled as needed with a switch hidden anywhere in the dash like an in car ricer neon. Or more sophisticated control is easily possible.

And the conbination system should be (should be) more easily integrated than the LED strip. Yes?

wolrah · 05-31-2008, 09:04 PM

CCFLs have heat and durability advantages over regular fluorescent bulbs, so that's why those are chosen. LEDs are even more durable, lower power, brighter, and over all better, thus you see that modern high end laptops are now switching to LEDs (I'm typing this from a Macbook Pro with LED backlighting which is much better than my old CCFL Macbook).

WhiteRabbit · 06-01-2008, 01:20 AM

LED's aren't brighter off the bat. It takes MANY LED to equal the power of one Fluorescent bulb.

We've already covered lower power and durability in previous posts. Modern laptops DEFINITELY do not count because of the size constraint we don't have in the car.

Again, this is DIY.
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Tired of irrelevant conjecture, I decided to experiment myself to try to see why its not instantly obvious they are not used as stated in the post #1 brainstorm.

I agree heat is a problem. easy way around this:

Purchased solutions have their own hoods. this protects the top, bottom, sides, and read of the bulb. this just leaves the front of the bulb (LCD) that requires protection. This can be done via distance with some functional testing.

The bigger problem with the way I described the potential solution is that the LCD turns transparent when pixels are opened:

Bulbs are CLEARLY visible, and unless 100% coverage is achieved to exactitude, it is easy to see dark spots.

Furthermore, these are night shots. I don't have test data to suggest whether or not daylight readability is even possible via this method. will test tomorrow.

The solution is to modify the design slightly. Because the LCD has its own backlight, the diffusers are still there and can be added without using them all. This prevents bulb visibility without using ALL the diffusers which limit light emmitance.

But the practical solution must include cost, and there is no way around that. So I have to ask. What are people paying here to DIY their own LED backlights to achieve daylight readability?

I found a fluorescent desk lamp at Home Depot for $21 after tax. The hood is slightly over sized for a 7" screen, and the bulb slightly undersized. From an overall performance perspective, this is less than ideal because no matter what there will be a slightly dark spot to one side of the screen, left or right. Alternate bulbs exist, but would double the hardware cost.

The hood does not get hot at all but the bulb does, as stated before. The idea is by placing it one inch back dark spots will be eliminated while heat becomes a non-issue.

That is the theory, anyways.

WhiteRabbit · 06-01-2008, 05:17 AM

some fun reading while I was looking up other's DIY projects. (there are not many on this board)

http://www.mp3car.com/vbulletin/lcd-...backlight.html
http://www.mp3car.com/vbulletin/lcd-...take-look.html
http://www.mp3car.com/vbulletin/lcd-...ment-idea.html
http://www.mp3car.com/vbulletin/lcd-...tra-ccfls.html
http://www.mp3car.com/vbulletin/lcd-...ght-lcd-7.html

number of completed projects from these fellows......0.....

rijndael · 06-01-2008, 10:16 AM

http://www.mp3car.com/vbulletin/show...oled-dell.html

development of the screen starts at post 94. its not flourescent...but its a DIY that is finished...

WhiteRabbit · 06-01-2008, 01:04 PM

figured that one went without saying. Any others? And I dont mean projects like the one I posted above, "non finished". Plenty of in progress testing out there.

Noone ever completed a project and put it into service. Except one.

WhiteRabbit · 06-01-2008, 04:18 PM

Daylight readability passes with flying colors.

But change of plan. Combination of factors

Heat testing passed proof-of-concept with flying colors. Shielding the LCD simply requires two panes of glass separated by approximately 1/8th of an inch, spaced maybe 1/2 inch from the bulb. 3/4 better.

Name of the game isn't heat dissipation, it's heat containment. In my application, the screen is NEVER on for more than two hours (and even that is really pushing it). So as long as the case can sustain 2 hours of continuous use without meltdown, then no issues.

We already know the existing hood can take the heat indefinitely. The front face merely becomes an issue of what material can sustain high temperatures. I chose simple glass.

Again, the lamps containing a 500 degree plasma, I'm not so worried about trying to dissipate that. I just dont want that to damage any other component.

The proof of concept was the desklamp hood. The next step was to build a mock-up using a high temperature insulator as the new dash-friendly hood complete with glass cover. The insulator I was going to test was car exhaust wrap inside an MDF/hardboard case. Again, goal being to keep the outside wrap temperature within MDF service temperature for hours.

Noone leaves these monitors on 24 hours a day.
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But I dont have the room in the underdash. Just enough for the LCD and lamps and hood, but then I run out of space for the LCD control boards.

and vice versa. Not unless the control boards sit between the lamps and LCD.

testing was initiated using a 2 lamp shoplamp though, which showed no signs of "failing" bright or darkspots with the diffusor in place. Retesting for daylight readability passes as well.

So the plan is to drop the 4-tube bulb system in preference to a 2 bulb system to take advantage of a reduced size hood within the underdash. Another goal is to also find a bulb that is compatible with a standard socket to prevent carrying the cost of a ballast to power a single bulb.

The hood will change from a cubic shape to a trapezoid where the top and bottom flares to the LCD size, somewhat in the spirit of a CRT shape. This is compatible with my dashboard while theoretically giving me space to install the LCD circuit boards. While simultaneously generating half the heat by using a 13 watt bulb rather than a 27 watt bulb.