[Laser] Re: Fresnel lenses and dispersal angle

[KY1K]

Hi Chris,

All ok, except......


>So, to your calculated dispersal, you have to ADD the 'inaccuracy factor', 
>which depends on the quality of your particular lens. I've noticed that 
>early fresnels, such as those in 1960s 'Overhead' transparency projectors 
>are not corrected for spherical aberration, in many cases. This would 
>exacerbate things further.

For those using lasers and LED's, spherical aberration is our friend, not 
an enemy........... The fact a fresnel or an early overhead projector lens 
is NOT corrected makes them desirable, not the other way around.

Spherical aberration is the uniformity of the focal length of the lens as 
the wavelength of the light is varied.

This is used to our benefit, because it is wavelength selective and tends 
to prevent light from undesired wavelengths from reaching the detector. 
Let's say your system is optimized for red. A blue or near IR photon will 
reach the detector in lower quantity because some of the off wavelength's 
will miss the detector due to the variable focal length of the lens for 
these different colors.

If your receiver is only interested in a small portion of the visible 
spectrum, you should strive for optics that are not corrected for spherical 
aberration, not the other way around.

An uncoated glass lens offers no correction for spherical aberration, and 
should be used rather than a coated lens.

Art