(Jay) wrote in message . com...
(Jeff Peterson) wrote in message om...

In order to get the wide angle coverage you need, you can either use a
large number of LEDs, each with a narrow beam or you can use the same
number of LEDs each with a broad light beam. When you grind the front
off the LEDs the forward-directed brightness (officially called the
luminous intensity [in milli-candella, aka mcd] ) will be decreased
because the light is no longer directed forward but is spread out
instead. The total amount of light emitted (officially called the
luminous flux [in lumens, aka lm]) is not affected by grindind off the
front. This means you need just as many ground-off LEDs as you do
intact ones. The advantage of grinding them off is that you dont need
to go to all the trouble of pointing them every which way. As far as
scratches go, yes, they do diffuse the light output, and that's just
what you want.

You save having to do as much pointing but you have to grind and
POLISH each one. The scratches cause a loss in the total flux. Look
at what you have to do to prepare the ends of fiber for installation
of connectors, and thats even for the longer wavelengths in IR.
you polish optical fiber so one end will press tightly up against the
next and transmit the light with little reflection. effectively you
get a direct glass to glass connection, at 1.3 microns, if the ends
are polished well. in our application the LED plastic case terminates
and the light continues in air. there will be about a 4 percent
reflection here. unless we were so fussy as to anti-reflection-coat
the plastic we are stuck with this 4% reflection.
cant fix this by polishing. a scratched up surface diffuses the
transmitted light to a wide pattern but doesnt reduce the flux.

by the way, painting the body white helps increase output by giving
that light reflected back into the plasitc a second cahnce to get out.



i noticed this coincidence: all these devices seem to use about the
same current, 20 ma. so very likely they have similar flux. i think
the more expensive ones are the ones that have the LED chip located
very close to the focus of the hemispherical lens....these have the
narrow beam that gives them high mcd numbers. if you are grinding the
dome off, it doesnt make sense to pay extra for this.

They use the same current because its the same little chunck of Si.
The cost of molding the lens doesn't vary much depending on the angle.
What you're paying all the money for as I understand it is:
1) Pick of the litter, the semi-conductor die vary in efficiency and
are binned. You pay more for the really good one, and less for the
others.
i am no expert on this, but i think the selection is to find the ones
with the narrow (and therefore bright) beams. we want braod beams so
would should
buy the cheaper LEDs