lightgathering

Light gathering refers to the ability of an optical system to collect light from a source and deliver it to a detector. It is crucial for imaging faint objects and short exposure times. The light gathering power (LGP) is proportional to the area of the entrance aperture; for a circular aperture with diameter D, collecting area A = π(D/2)^2, so LGP scales as D^2. A common reference is the human eye; LGP can be expressed as the ratio of telescope aperture area to the pupil area of the eye, or more generally as the telescope's collecting area relative to a reference detector. In practice, LGP is reduced by central obstructions (secondary mirrors), transmission losses, scattering, and coating reflectivity.

In astronomy, larger apertures provide greater light gathering, enabling detection of fainter objects and higher signal-to-noise ratios, although diffraction sets a limit on resolution. The f-number (f/#) affects light per unit area on the detector; lower f-number (faster) lenses gather more light per unit time, beneficial for bright exposures. In photography and imaging, exposure time and sensor sensitivity interact with LGP to determine image brightness.

Etendue is a related concept describing how light is conserved through an optical system; it combines area and accepted light angles. Effective light gathering is hence influenced by aperture, optical design, coatings, alignment, and contamination.