Why do we need a Pinhole in a Confocal Microscope?

Dr. Rolf T. Borlinghaus Kontakt, Leica Microsystems GmbH

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Whenever we use a confocal microscope and when we will have to discuss its parameters and features, inevitably the pinhole will be talked about and how its size effects the resulting images. This brief introductory text explains the meaning of the pinhole and is meant for readers who do not want to spend a lot of time studying the detailed theory of confocal microscopy, but still want to have an idea of how the instrument works and which limitations it has.

What is a Pinhole?

Optical lenses are essentially characterized by two parameters: the curvature of the lens' surface and the diameter. The curvature determines in which directions the rays are refracted and the diameter defines how many rays will contribute to the resulting image. In the simplest case, the beam diameter is given by the lens' edge. However, in many cases though, the beam diameter is controlled by a separate diaphragm. Iris-diaphragms, which are composed of a number of lamellae, are commonly used. The diameter is changed by mechanical adjustment of that lamella, resulting in a stepless control of the resulting diameter. The term iris-diaphragm refers to the function of the iris in the human eye. It is named after Iris, the ancient Greek goddess of the rainbow.

Very small diaphragms (apertures) are tricky or impossible to make with this lamella-technique. The simplest method for generating a very tiny aperture is quite trivial: take a piece of cardboard or aluminum foil and prick a tiny hole with a sewing needle. That hole can be called then a "pinhole". With such a pinhole you can make a camera obscura yourself - without any lenses .

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