Using a lens instead of a mirror for collimating light in display holography
When recording display holograms, the traditional method of collimating (making parallel) a diverging light beam is to use a parabolic mirror. This is a good method for the least light loss and the most perfect collimation, but the mirror needs to be an off-axis parabolic mirror, making it expensive. Otherwise an on-axis mirror can be use in an off-axis mode, if aberrations are tolerable. The longer the focal length of the mirror, the less off-axis it needs to be used and the lower the aberrations. But the longer the focal length, the more room the structure will occupy on a recording table, and the higher the risk of vibrations – the enemy of holography. So choosing the right mirror is a trade-off.
An option usually not considered is to use a collimating lens, in which case the system is on-axis and can occupy less space. It turns out that a plano-convex lens is almost the ideal shape for collimation. The compact set-up also results in a lower level of vibrations in the set-up.