Early microscopes suffered from inconsistent lighting of the object to be examined, which made observation and especially photography difficult. The cause is diffusion of light (middle) from a diffuse light source. Optical collimation (top) passes an beam of light through a small aperture (A) and into a plano-convex lens (L) that bends the component beams proportionately according to their angle, so that they pass out of the lens all moving in the same direction.

Other forms of collimation may use a polarizing filter (bottom) that passes only those beams moving in parallel onto the object to be illuminated.

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