The Rayleigh Range is the distance from the beam waist (smallest focused spot) to the point where the waist has increased by a factor of √2.  Images focused within one Rayleigh Range of the waist appear to be in focus.  When working with gaussian beams (often an excellent model for a laser beam), the depth of focus is quantified by the Rayleigh Range.  In order for images to remain in focus, reflecting filters (dichroic beamsplitters, for example) need to be flat enough to cause the reflected beam to shift less than one Rayleigh Range.

Where R is the radius of curvature of the filter, D is the diameter of a laser beam incident on the filter, and λ is the wavelength of interest. Semrock specifies the flatness of dichroics by listing the nominal radius of curvature and also the waist diameter of the beam that will maintain less than one Rayleigh Range of focal shift.