RE: filmscanners: LED Illumination for Film Scanners

[Clark Guy <guy.clark@sbt.siemens.com>]

Hi, all!

<I'm only going by what the catalog says, and I didn't write it.  Stanley
LED
catalog, p. 24:

"Operating Life JIS C 7035 Ta=25C, IF=Max, t=1000Hrs.">

I'm a little behind in my reading of this list, but I thought I'd respond to
this one.

One thing that hasn't been mentioned (that I noticed) is that this spec from
Stanley gives the operating life as 1000 hrs at IF=max.  Usually, we power
an LED with something on the order of 20mA current.  This is the TYPICAL
forward current.  LEDs can be made to run somewhat brighter by pushing more
current through them.  This IF=max probably means a forward current of over
50mA.  Beyond that, the LEDs in my experience undergo a wavelength shift
(color shift) and begin to get dim after a short period of time. (I've
designed and build many different LED strobe devices for previous
employers).  

One can get around this by pulsing the LED.  I've pushed over 500mA through
an HP ultra bright LED by pulsing it at on the order of one microsecond with
a repitition rate of 1kHz or so and seen no degradation of the LED over the
lifetime of the device (several years).  YOu just don't want the LED to heat
up too much, or it dies quickly.

Anyway, Stanley was simply giving a conservative estimate of the longevity
of their LED when powered by the highest allowable current.  Of course, at a
sane current drive, they will last for hundreds of thousands of hours, at
least!!!

On a historical note, back in the late '80s and early '90s, blue LEDs were
very dim.  They were made from Silicon Carbide, and put out less than
100mCandela while good red or green LEDs put out in excess of 1000mC.  I
used a particular HP red diode that put out 3-4 Candela! from a T-1 package
(small size).  In the mid '90s I saw an example of a (then) US$50 blue
ultribright diode.  It put out at least one full Candela of power, but it
was too expensive for my medical diagnostic device application.   This
weekend, I went to the drugstore to buy some film , and found a
blueish-white LED flashlight for sale for ~$8.00  It is blinding in it's
intensity!!!  Clearly the state of the art is moving forward at quite a
rapid pace!

Hope this helps!!

Guy Clark


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