Re: LED intensity/angle mcd?

On 17 May 2006 21:05:34 -0700, the renowned wrongaddress@xxxxxxx
wrote:

Looking at the specs for 3 red LEDs, I'm wondering which would be the
better choice for maximum brightness.

Allied part number 670-1257 which claims to be a 400mcd device with a
viewing angle of 60 degrees at 2x theta. Cost is 72 cents.

A second part 670-1224 claims to be a 1800mcd device with a viewing
angle of 30 degrees at 2x theta. Cost is 50 cents.

A third device is 670-1226 claims to be a 1200mcd device with viewing
angle of 30 degrees at 2x theta. Cost is 31 cents.

I'm not sure what 2x theta means, but I think if the angle is twice as
wide, the intensity should be 1/4 as much, so that a 1800mcd device at
30 degrees would be the same as a 450mcd device at 60 degrees?

Why would the the 400mcd device sell for 50% more than the 1800 mcd
device which has more total output?

Would the more expensive device be the better choice?

-Bill

I suggest you view all the data sheets side by side and try to
understand all the differences in detail.

The first one (670-1257) is a 10mm diameter LED with a wide viewing
angle. The wide angle means that the light is spread over a wider area
so it appears not to be as bright on-axis, but can be seen over a
wider angle. Too far off-axis and the others may be difficult to tell
whether they are on or off, depending on ambient lighting conditions.
It also has a red *diffused* lens, which means that the light appears
to come from the surface of the dome rather than through the 'water
clear' lens that the others have. It's also tinted red, which affects
how ambient light is reflected or absorbed. Chances are good it has
the exact same type of 660nm color die in it as 670-1226, just a
different molding.

The main difference between the second two is the color of the emitted
light. They are both 5mm diameter LEDs. The 670-1224 emits 636nm
wavelength light, which is more of an orange-red. The 670-1226 emits
longer wavelength 660nm light. Both are water-clear epoxy lens. They
are narrow angle (30°), which means they look much brighter when you
look almost directly into them, as opposed to much off the axis. Most
manufacturers provide a graph of brightness versus viewing angle. The
different dies have slightly different forward voltages as well
(higher typical forward voltage by 300mV for the 670-1224, which emits
shorter wavelength light).

BTW, please get into the habit of referring to such parts by the
manufacturer's (Lumex, in this case) part number rather than the
distributor catalog number. You will find it infinitely easier to deal
with the manufacturer's data sheets and when you change distributors
or shop around you won't have to translate everything if you stay with
the same manufacturer.

That said, why not buy all three, power them up at a sensible current
(in series is good for side-by-side comparisons) and look at the
differences for yourself?


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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