When surface photometry is done in concentric annuli, there is a tendency toward distortion of the true profile because of the finite widths of the annuli. This distortion can be avoided by choosing an effective radius for the annulus such that the actual surface brightness at that radius is equal to the average surface brightness over the annulus. This effective radius depends on the shape of the radial profile, but its calculation can be performed in an iterative way. Formulas for the effective radius are given for some typical profile shapes.
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Ivan R. King (1988) studied this question.