[Laser] laser stuff
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[email protected]
Mon, 17 Nov 2003 18:27:22 EST
Andy
Ok. Try this (and don't roll your eyes like a kid in science class, I'm a
little rusty as a teacher)
The shift in the focal length is caused by the same thing that caused colors
to separate when light passes through a prism (dispersion) . It is the change
in the index of refraction as the frequency of light changes. Now, if you do
not have the dispersion characteristics of the glass for the lens, a formula
won't help you.
(The process is simple, divide half the diameter of the lens by the focal
length of the lens for one frequency of light, say green. That will give you the
tangent of the angle - call it m - that a ray of light traveling parallel to
the axis of the lens will bend toward the axis if it strikes the lens at its
outer edge. You could use any point on the lens except its center, but the
outer edge has the biggest angle. Using Snell's law, the sine angle m and the
ratio of the index of refraction at green and the index of refraction of the
desired color, red, or near IF, or whatever - will give you the sine of the
angle of the new color - call that angle n. Then the half diameter of the lens
divided by the tangent of n will give you the new focal length. Now wasn't that
simple.)
If you have some colored light sources, say blue, green, and red, you can
find the approximate focal length of each down in your basement laboratory. That
should allow you to estimate a new one in IR - just interpolate using the
wavelengths of the knowns to the get the focal length for the new wavelength (780
nm). It is a lot easier with a camera that is sensitive to that light. A
webcam should be good enough.
James
The lenses I'm using are 100mm diameter with a 200mm focal length
uncoated DCV glass. Any idea how much different the focal length is
between 660 and 780nm on such a thing?
Andy
K0SM