RE: filmscanners: LED Illumination for Film Scanners

[Raphael Bustin <rafeb@channel1.com>]

On Fri, 22 Jun 2001, Austin Franklin wrote:

> 
> My guess is you are not an electrical engineer, or you would know that LEDs
> do have a life span.  Because you haven't heard of them burning out, doesn't
> mean they don't burn out.  In fact, their typical MTBF is rated for 1000
> hours.  Incandescent light bulbs are rated for 1000 hours.  Aside from
> having written and reviewed quite a few MTBF and MTTR studies on designs
> that included LEDS, I recently replaced 4 of the 6 LEDs in my
> radio/CD/Cassette in my 1989 Range Rover, so I DO know they do burn out.


1000 hours MTBF can't be right, Austin.

Optocouplers and fiberoptics are ubiquitous these 
days, and that figure would imply a rate of failure
that's totally out of line with reality.  Hell, 
the entire telecom industry is built on fiberoptics 
these days, and the transmitters are always LEDs 
of some sort.

Virtually every single (modern) mechanical 
actuator these days is either stepper driven 
or DC-servo driven, and the latter type usually 
use quadrature-encoded OPTICAL position sensors.

(Eg., any inkjet printer made by Epson, Lexmark, 
or HP uses optical encoding for the carriage 
position.)  Just one example from among tens 
of thousands, and maybe a few hundred that 
I could cite.  The "sender" in the optocoupler 
is a LED (two of 'em, for quadrature encoding.)

I'd believe 100,000 hours MTBF, maybe, but 
clearly it'll be a function of several 
variables -- mostly the operating current.



rafe b.