How does a microscope get light to the specimen? What do I need to know about illuminators?

The lowest-powered microscopes may not need any light other than that provided from room lighting or the sky. However, the microscope may be so close to the specimen that the instrument may shade it. Or you may want light to go through the specimen to see interesting internal details. Because of these and other reasons, most microscopes have built-in lights or mirrors called illuminators. 

A microscope gets its light from the specimen in two ways. Reflection is the most basic and it’s just what it says: existing light or light from an illuminator above the specimen is reflected off it and up to the microscope’s objective. Reflected light is great for seeing surface details in relatively large specimens and is frequently used for low-power microscopy and stereomicroscopic work. Transmitted light (sometimes called transillumination) shines light through the specimen. It’s frequently used for transparent or translucent objects, commonly found in prepared biological specimens (e.g., slides), or with thin sections of otherwise opaque materials such as mineral specimens. Transmitted light usually gets to the specimen from below the stage by means of an illuminator. It's the most frequently used lighting for compound, high-power microscopy.

The simplest illuminator is a pivoted mirror to beam external light to the microscope. It’s used to direct room light, lamp light, or skylight from below the scope’s stage up through the specimen as transmitted light. Mirror illuminators most often have flat and concave sides. The flat side simply reflects light and gives a sharper image. The concave side concentrates the light and provides brighter illumination. 

It can be difficult to direct the light with a mirror illuminator or it may not be bright enough to do the job. Illuminators with their own lamps, bulbs, or diodes don’t have these problems and can be battery-powered or run on AC power. These lights can be used for either reflected or transmitted light. Sometimes both will be used at the same time on a microscope. As artificial light can be hot, so caution is needed not to overheat delicate specimens. Use of heat absorbing diffusers or variable-intensity illuminators may help here.

For the highest-powered microscopes, a condenser lens is an essential feature with their transmitted light illuminators (either mirror or lamp-type). These lenses are located above the light and underneath the stage and act to concentrate the light efficiently through the specimen and into the microscope’s objective lens.

Updated 12/18/13