Re: LED Apparent Brightness

On Tue, 10 May 2005 20:37:46 GMT, Rich Grise <richgrise@xxxxxxxxxxx>
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?

---
With a white backlight running CW and three planes of filters in front
of it, all you have to do is attenuate what you don't want to come
through. The other way, with three LEDs per pixel, you've got to send
enough current into each of the LEDs comprising the triad to get it to
radiate just exactly the right hue at just exactly the right time.

That means that, in order to push current through each LED in the
triad in order to get the pixel to radiate the proper color, turning
the LEDs in that triad on for short periods of time would be
necessary. Because of that, the array would radiate. That's not a
good thing, and using nano (or pico) amps to do do the color selecton
through attenution seems to make a lot more sense.

--
John Fields
Professional Circuit Designer
.

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