Re: LED Apparent Brightness

On Tue, 10 May 2005 19:08:27 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:

>On Tue, 10 May 2005 16:13:15 -0700, John Larkin
><jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
>>On Tue, 10 May 2005 14:03:02 -0700, Charlie Edmondson
>><edmondson@xxxxxxxx> wrote:
>>
>>>Rich Grise wrote:
>>>
>>>> On Mon, 09 May 2005 15:05:21 -0700, John Larkin wrote:
>>>>
>>>>>On Mon, 09 May 2005 14:13:16 -0700, Charlie Edmondson
>>>>>
>>>>>>There is only one good reason to pulse a light source to get brighter
>>>>>>illumination when multiplexing is not needed... if you are matching to a
>>>>>>shuttered imager. I had an application that I was developing, in which
>>>>>>we were going to use an IR filter in front of a standard CMOS imager,
>>>>>>adn then use a pulsed IR source with a short pulsewidth matched to an
>>>>>>equally short shutter time on the camera. By sync'ing these together,
>>>>>>the IR illuminator could be the predominate light source, overpowering
>>>>>>even daylight, while still maintaining eye safety.
>>>>>
>>>>>Too bad LCDs are so slow. The ideal backlight would be sequentially
>>>>>pulsed R-G-B led's.
>>>>
>>>>
>>>> But, um, is it seriously unrealistic to contemplate a panel of RGB LEDs,
>>>> say, 640 x 480, all by themselves, being pixels? Like a micro-jumbotron?
>>>> Frankly, I've been wondering about that ever since I started reading about
>>>> the various technologies that have been studied for making flat-screen
>>>> displays. Is it just that it would be so prohibitively expensive?
>>>>
>>>> Thanks,
>>>> Rich
>>>>
>>>Hi Rich,
>>>Actually, I think heat and power are the main reason. If you have a
>>>very bright LED, they tend to generate enough heat to need a little bit
>>>of heat sinking. As you try to get that many of them together, you
>>>don't have much room for the heat sink. At least the jumbotrons have
>>>room at the back for some fins, or even a little plumbing...
>>>
>>>Also, adding up all those 20mA LEDs, that's a considerable amount of power!
>>>
>>
>>LEDs are at least as efficient as CRTs and more efficient than a ccfl
>>backlight being pushed through lcd pixel polarizers and color filters.
>
>---
>Really?
>
>How about some numbers?

A ccfl is maybe 30% efficient, DC-to-light. An lcd pixel with color
filter can't theoretically be better than 16% (33% for the color
filter, 50% for the polarizer) and in real life is maybe half that at
best. So my Dell lcd screen is at best 5% efficient, not counting the
electronics. Probably less. And the 5% is when all pixels are full-on,
screen white, which few actually are; its dissipation does *not* go
down even when pixels are black... the light is just wasted.

I've seen the assumption of 5% panel transmission used for lcd's.
Multiply that by 30% for the ccfl, you get 1.5% overall.

Good LEDs are, what, 20% efficient? And when you dim an led pixel, it
uses proportionally less power, unlike the lcd. Zero when black.

I don't have numbers on CRTs, except to note that at equal brightness
they dissipate a *lot* more heat than an lcd screen.

>
>>So for a given brightness, an led panel should run cooler than a crt
>>or a color lcd of equal display area.
>
>---
>Really?
>

Really.

>How about some numbers?

See above.

John

.

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