Question on Optics:

I've seen many threads where DIY lights use optics. My one question (totally noobish, I know) is how the degree rating for various optics are calculated. Is it that, say a 13* optic rated as light being spread between the two points of maximum dispersion
/
(ie: l) )
\

meaning: is that 13* the distance between the two outermost output of light, or is it like this...
/
(ie: l) ----
\

Where the 13* is the angle of dspersion off of the horizontal plane (in this case, the "----" dashes).

Any clarification of my deluded ideas would be greatly apprecited for any upcoming projects. Thanks! Also, moderators, if this thread fits anywhere else, feel free to move it...

I've used some of the optics that say 10 degree- and would appear that this is the hot spot...where most of the light is concentrated. At about 80 feet to my garage, I'd say I have an 8 foot diameter spot....just a rough guess. There is still a very large spill area- not like with a reflector...but it still lights up my whole yard.

Download this document for the answer to your question:
http://www.khatod.com/Documenti/kepl112706_xre_cree_test_report.pdf

It isn't really clear from that PDF if the angle specified is the full angle of the hotspot, or the angle formed by the outer edge of the hotspot and the angle's bisector (which would be half of the angle).

The measurement used to determine the outside ray of the beam angle is usually the half-maximum, the angle of the ray emanating from the LED where the light is half as bright as the brightest angle in the beam, usually the axis of the beam (0˚).

Sometimes the angle used to define the optic is a half angle and sometimes it's the full angle. If it'a a full angle measurement of the beam you should be able to find the acronym FWHM (Full-Width Half-Maximum) somewhere in the optic's documentation or on the product pace for the optic. When it's the half angle there's usually a hint. It won't say FWHM, it will say ±n˚ where n is a number. Another way to tell is that the angle given is impossiblly small for the optic's diameter and the LED's die size, like a 20mm dia optic for a single-die power LED claiming a 4˚ beam is definitely a half-angle.

Each company tends to use consistent nomenclature across their product line.