The Basic Concepts of Lighting
Understanding the physics and measurement of light — from electromagnetic waves to the four core photometric quantities.
The Nature of Light & Visible Light
Light is the part of radiant energy emitted by a source that produces vision, often called visible light. Physically, light is electromagnetic radiation; different electromagnetic waves spread out by wavelength and together form the electromagnetic spectrum.
Visible light accounts for a small fraction of the spectrum, with wavelengths from roughly 380 to 780 nm. As wavelength increases, perceived colour shifts from violet through blue, cyan, green, yellow, orange to red; all visible wavelengths mixed together form white (daylight) light.
The human eye’s response to light evolved in Earth’s atmosphere: wavelengths above about 1400 nm are absorbed by water vapour and carbon dioxide, and wavelengths below about 290 nm by the ozone layer. The range that reaches the surface closely matches the visible band.
Core Photometric Quantities
Four quantities describe light from the perspective of human vision and lighting design.
Luminous Flux (Φ)
The energy a light source radiates into the surrounding space per unit time that produces vision — the essential starting point for lighting design. Because the human eye is most sensitive to 555 nm yellow-green light, 1 W of monochromatic radiation at 555 nm equals 680 lm. Electric sources are usually compared by their luminous efficacy: lumens produced per watt of electrical power.
Luminous Intensity (I)
The luminous flux radiated by a source per unit solid angle in a given direction. It describes how flux is distributed in space — for example, adding a shade changes this distribution. For a whole sphere the solid angle is 4π steradians.
Illuminance (E)
The intensity of light falling on a surface — the ratio of luminous flux projected onto the surface to the surface area. Reference values: about 30 lx at 1 m from a 40 W bulb; roughly 8×10⁴ to 12×10⁴ lx under sunny outdoor noon; about 0.2 lx under a full moon.
Brightness / Luminance (L)
How bright an object appears to the eye — the luminous intensity per unit projected area of the luminous or reflecting body in the direction of view. A white object reflects more light than a black object and appears brighter even at identical illuminance. When luminance is too high, it causes glare.
