Photometry and illuminance

Photometry measures the effect of light on the eye. Luminous flux Φ (unit: lumen, lm) is the radiant power of the source as sensed by the eye. Luminous intensity I is the flux of a source per unit solid angle in a given direction: I = Φ/Ω; its unit, the candela (cd), is one of the SI base units (current definition: the luminous intensity in a direction in which monochromatic radiation of frequency 540·10¹² Hz has a radiant intensity of 1/683 W/sr; this is a wavelength of ≈ 555 nm, the green light to which the eye is most sensitive). Solid angle Ω = S₀/R² (steradian, sr); the whole space is 4π sr, so a point source's total flux is Φ = 4π·I. Illuminance E = Φ/S (unit: lux, 1 lx = 1 lm/m²). At distance R from a point source, E = I/R² if the light falls perpendicular to the surface and E = I·cosφ/R² if the ray makes angle φ with the surface normal; illuminances of several sources add. Luminance B = I/S (cd/m²) is the luminous intensity per unit area of the emitting surface. Illuminance falls with distance by the inverse-square law.