How is the difference between an extended source and a point source defined?

The distinction between an extended source and a point source primarily hinges on angular subtense, which is a measure of the angle that the source subtends at the observer's eye. A point source is assumed to be very small, resulting in minimal angular size; it appears to emit light from a single point. In contrast, an extended source has a larger physical size and occupies a greater angular subtense in the observer's field of view.

Understanding this relationship is essential for laser safety, particularly in applications involving laser beams. The angular subtense influences how light interacts with the eye and contributes to the perception of intensity and risk associated with exposure. Larger extended sources can pose different safety hazards compared to small point sources since they may produce more scattered light and have different characteristics in how they project light over distances.

Other factors, such as the size of the source itself, power level, or distance from the observer, may impact safety considerations but do not directly define the difference between point and extended sources in terms of their basic definition.