The Scanning Optical Microscope

Instead of forming an entire image at one instant, a scanning optical microscope (SOM) scans a beam of light across specimen in a regular pattern, or raster, and forms its image point by point. A point of light is focused to a diffraction-limited spot on the sample by the objective lens. An image is built up point by point by a detector, either below the slide (if the microscope is exclusively an SOM) or below the condenser (which is more practical if the microscope is also used for conventional imaging). SOMs have been used for about 50 years, and the image they produce is formally and in practice equivalent to the image given by a conventional microscope. Why, then, use a scanning microscope? The most common reason is that image processing and analysis are facilitated by having the picture broken up into a series of points (pixels); before the advent of digital cameras, the SOM was the simplest way to achieve this. Scanning also facilitates contrast management when dealing with samples that have either very high or very low contrast. However, what made the SOM an essential part of cell biology was the simple modification that introduced the confocal imaging technique. The transformation from SOM to CSM (confocal scanning microscope) is summarized in Figure 5.1.