P3D Re: Viewers and white LEDs

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Greg,

> "As for shade of white? The ones
> I tested seemed like about 6000
> to 6500 Kelvin. The color is slightly
> different in different directions,
> since the chip and the phosphor have
> slightly different directional
> characteristics."
 
I have some from DigiKey that I don't much like. They are a warm white.
Close to tungsten filament white. About $4 and not very bright compared
to my second choice. Also, the far field is very reddish near the edges.
But this may be outside the 20 degree circle. If so, it does not count.
The near field occupies the entire substrate so it looks like a 5mm
diameter source. You can see this with a 10k to 100k resistor in series
so that it just glows. Then use a magnifier or loop to examine the
emitter surface.

The others are a 15 or 20 degree cool white P/N CLP1509-UN. Nearly
daylight white but a little rich in the blue. Appears to me to be about
twice the total visible light of the DigiKey LEDs. They were about $2.50
from Currents Corp of America (425-702-9770, Redmond, WA). They were
formerly known as LED Power Inc. http://ledpower.com . They are in a
state of flux because of the recent buyout. They won't send old catalogs
and they do not yet have a new catalog. They do not like to sell small
quantities but my coworker managed to get 10 anyway.

I found these LEDs to be very bright and even in the far field, both to
the eye and in the physics sense of "brightness". The emitter appears as
a 3mm square within the lensed 5mm LED.

They have a new white LED product. About $5 each. I just ordered 10.
They call it Pirannah. Like the fish but I probably mispelled it. It has
a 70 degree viewing angle but can take over 40ma. Should have a lot of
potential as a source of white light. Surface mount with 4 contacts.
Only 2 are needed electrically but all 4 are needed for good heat
sinking. They are working on a 2 contact version. They are working on
lots of stuff. 

My recommendation for comparing LEDs is to choose a single viewing angle
that is common like 20 degrees and order from various vendors. Then you
stand a chance of comparing them. I suspect that the best one will
indicate the manufacturer that has the most control over the process.
Then choose whichever of their LEDs is most useful for your application.

I am looking for vendors from whom to buy samples for comparison. I am
collecting all the info that I can but only ordering 15 to 20 degree
LEDs. I designed the optics for this angle so swapping LEDs in the
microscope makes it easy to evaluate the performance. Other viewing
angles require different optics to capture and control all of the light
so it is very inconvenient to evaluate a variety of viewing angles.

Tom Hubin
thubin@xxxxxxxxx