I get the feeling that we are talking in circles here. :)

Let's say you have the LED at the center of an actual 30 foot radius
spherical shell. If you were also at the center and took a look
around, you would see some kind of a light pattern on the inside of
the spherical shell. There would most likely be an area of high
brightness within which there would be several local maxima, and then
the edges of this area would fade to lower and lower levels of
brightness, perhaps with some local maxima and minima along the way
(sidelobes and nulls).

You said that the pattern does not have symmetry around the radius of
the sphere (axis of radiation from the LED). This means that the
"bright spot" on the inside of the shell would be an ellipsoid rather
than a circle.

Now, can you describe what part of the inside of the sphere you need
to consider for your light intensity measurement? In general, this
area of interest will NOT be the same as the "bright spot" from the
LED, especially since the actual bright spot will vary somewhat from
LED to LED and especially across different models of LEDs. The area of
interest would usually be dictated by your application and NOT the
device being used to do the illumination.

It sounds like you are saying that you want to find the minimum
illumination level that occurs within the main lobe of the LED, where
you are defining the boundaries of the main lobe by the half-power
beamwidths (which are different in the two angular coordinates since
it is not a cone). Is this what you want? If so, beware that this may
not actually be spec'd very well. I'd suspect that at 30 feet it would
actually vary a fair amount within the spot due to imperfections in
the LED lens.

Sean


On Sat, Apr 19, 2008 at 11:02 PM, David VanHorn <microbrix@gmail.com> wrote:
> > If you want to guarantee a minimum illumination intensity on the
>  > surface of the sphere, then clearly you need to specify some area of
>  > the sphere as a region of interest because the parts of the sphere
>  > directly behind the LED will not be lit much at all.
>
>  That's why I was referring to the cone. Figure a cone whose apex is at the LED.
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