TABLE OF CONTENTS A collimator is simply a device which produces an image at infinity. This is done by placing the object at the focal point of the optical system. The collimator is a common laboratory device, useful when a system designed to be used with a very distant object is tested. A collimated beam of light is one in which a small source is imaged at infinity. A common misconception is that all the light rays in a collimated beam are parallel to each other and that the beam does not expand. This is of course incorrect; the beam does spread, and the angle by which it spreads is equal to the size of the source divided by the effective focal length of the collimating lens. The rays from a single geometric point in the object are indeed parallel, but bear in mind that a true geometric point has dimensions of zero by zero, and an area of zero emits no energy. Even a perfect laser source beam will spread because of diffraction. A laboratory collimator is typically a well-corrected lens, such as an achromatic doublet, an apochromatic triplet, or a parabolic mirror, with an illuminated target placed at its focus. A collimated beam is often used in applications where a minimum beam spread is desired.
