> > What are DO-elements?
>
Diffractive Optical Elements.
They may look like Fresnel lenses, but optically they are quite different.
A Fresnel lens looks like a very coarse version of a similar DOE. Both the
Fresnel lens and the DOE look like the equivalent thick lens that has been
compressed into a series o rings. In the case of the Fresnel lens, each
individual ring is indeed a section of the original lens and completely
independent of the neighboring rings. The width and thickness of each ring
does not matter, only the cross section of each ring.
In the case of a DOE, this is reversed - the width of each ring is most
important. The spacing between rings is such that it acts like a
diffraction grating, directing the light in a particular direction. The
required ring spacing can be computed from the diffraction grating. The
profile of each ring affects is the efficiency of the grating, i.e., the
profile acts the same as blazing a classical grating and determines how much
of the light is diffracted into the various diffraction orders.
I expect that the big problem with the 400 mm DOE lens will be scattering
of light into the wrong diffraction orders. I will be very interested in
seeing how well this lens performs when there is a bright light source in or
nearly in the FOV. Canon's use of two DOE almost in contact with each other
may be an attempt to reduce this problem.
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