Re: Luminance - Illuminance

Douglas, I was actually thinking of lamp, with a point source at the focal point of a lens with 10cm diameter, such that the lamp emits a beam (of nearly parallel rays) with 10cm diameter. In the "ideal" case, the beam should project a circle of 10cm diameter, at any distance. To account for the imperfectness, I assumed some spread of the beam, such that the beam has a diameter of 10cm when it exits the lamp, and increases by 2cm per 1m distance from the front ot the lamp. Wouldn't, under this assumption, the beam project a 12cm circle at 1m distance from the lamp, and a 14cm circle at 2m distance from the lamp? Please help me to understand what's wrong with this consideration, and what am I missing?

Thanks,
Gerhard

if you have a perfec parabolic reflector, and a puntual lamp (like a arc lamp) you achieve a perfect cilindrical light beam.

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