Every point of the specimen is represented by an Airy diffraction pattern in the focused image plane of the microscope. It follows that the resolving power of an objective lens can be determined by examining the size of the Airy pattern formed by that lens. The Airy pattern radius is governed by the wavelength of illumination and the combined numerical apertures of both the objective and condenser.

When the specimen is illuminated by a large-angle cone of light, or for self-luminous objects, the light rays forming adjacent Airy patterns are incoherent and do not interfere with each other. This makes it possible to determine the minimum separation distance that can be resolved with a particular objective by examining the total intensity distribution of closely spaced, or overlapping, Airy patterns in the intermediate image plane. In the case of Airy patterns generated by coherent light waves, adjacent diffraction patterns would interfere with each other, increasing the minimum separation distance necessary to resolve the individual patterns.

The peak-to-peak distance (**Separation Distance, D**) between adjacent intensity distribution curves equals that between corresponding Airy diffraction patterns. If the Airy pattern radius is defined as the distance r, and when D is greater than r, the sum of intensities of the pair of Airy patterns clearly shows two peaks. In the case where the separation distance (D) equals the Airy pattern radius (r), two overlapping peaks are observed. This condition is known as the **Rayleigh criterion**, which is the minimum distance between Airy patterns that can be separately resolved. When the separation distance is less than the Airy pattern radius (not illustrated in the tutorial), the intensity distributions merge into a single peak and they are said not to be resolved.

**Contributing Authors**

**Rudi Rottenfusser** - Zeiss Microscopy Consultant, 46 Landfall, Falmouth, Massachusetts, 02540.

**Tiehu Jiang** and **Michael W. Davidson** - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.