Re: filmscanners: LED Illumination for Film Scanners

[Arthur Entlich <artistic@ampsc.com>]

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.

Didn't the word humility crop up in a message I sent you about a year 
ago? ;-)

Also, I'm guessing LEDs made in 1987-88, when your Range Rover was 
likely put together, were not the same ones used today, about 12 years later


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

I never met a Stanley parts catalogue I couldn't trust ;-) ;-)


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

Austin, this is why it sometimes is helpful to engage your obviously 
very capable brain rather than relying totally on written "fact".  I'm 
sure given a little thought, you'd have recognized the silliness
  of the 1000 hr number, considering how often we replace incandescent 
household light bulbs.  (then again, lately companies like GE seem to be 
happy with 100 hr bulbs or ones that just fail on power up, by that's 
another matter).  Can you imagine what problems in equipment failures 
would exist if LEDs lasted on average 1000 hr only?  I think Rafe gave 
some good examples of the huge number of devices we use which rely upon 
LEDs to sense locations, positioning, actuation or switching (how about 
most computer mouses, for instance).

Tell you one thing, I'm not buying any Stanley LEDs! (they must have 
gotten a great buy on these!) ;-)

> 
> It very clearly says "1000".  The Lumex web site says their Life Test is
> 10,000 hours, which sounds a lot better.
> 

I also expect, like incandescents and other illumination sources, LEDs 
can be built to different specs and be run under different electronic 
designs.  For instance, I know that with the halogen bulbs used in 
projectors, a "25 hr" bulb will last only 5-11 hours run at 125-130 
volts, will last 11-19 hours run at 120-125 volts, they last upwards of 
19-31 hrs run at 115-120 and if you use a dimmer to bring them down to 
about 105-110, they will last well over 50 hours, or nearly one order of 
magnitude from running at 125-130 volts.

> Your scanner should have a cited MTBF and MTTR, what are they?

Yes, that would be interesting, if they consider the light source 
separately, and not the whole unit.

Art