# Near point source to beam

Is it possible to use a system of reflections to direct most (if not all) of the light from a small luminous sphere to produce a thin beam that can focus to infinity?

Andy Resnick
No. For one thing, the radiance is conserved- the size of the object multiplied by the divergence of the light- is constant in a lossless optical system. You can make a well-collimated beam from a small source, but the beam diameter will be much larger than the source.

Thanks. Could a point source be focused to infinity with the help of a parabolic mirror and maybe some lenses?

Andy Resnick
Sure- that's called 'collimation'. Put the source at the rear focal point.

K^2
You'll always have some divergence in the beam, which is going to go like ratio of wavelength to mirror's diameter.

Andy Resnick
You'll always have some divergence in the beam, which is going to go like ratio of wavelength to mirror's diameter.
Actually, the beam divergence is more related to the size of the source with respect to the numerical aperture of the lens, than the wavelength (assuming a perfect lens).

Thanks everyone.

With your permission, may I revive for a short while this topic?
So, I tried to collimate the UV beam from Deuterium lamp with glowing area of 0.5 mm diameter, using plano-convex fused silica lens having 75 mm focal length and about 12 mm clear aperture.
The lens plane surface was facing the lamp.
Also, I had a possibility to vary the distance between the lens and the lamp.
The UV beam diameter was evaluated by viewing the phosphorescent spot size on a special phosphor screen.
At first sight everything looks OK, doesn't it?
Nevertheless, I am a little disappointed because I still could not achieve a more or less constant spot diameter (at least up to 10%) within the range of several centimeters from the lens. Moreover, with increasing the distance from the lens the spot edges became more diffused.

Andy Resnick