The Basic Concepts of Lighting
Understanding the Physics and Measurement of Light
1. The Nature of Light & Visible Light
The Nature of Light: Light refers to a part of the radiant energy emitted by a light source that can produce vision, often called "visible light". From a physics perspective, light is a kind of electromagnetic radiation energy (electromagnetic wave). Various electromagnetic waves spread out according to their wavelengths, forming the electromagnetic spectrum.
Visible Light: Visible light accounts for a small fraction of electromagnetic waves, with wavelengths ranging from 380 ~ 780 nm (1 nm = 10-9 m). Different wavelengths give people different color perceptions. As the wavelength increases from 380nm to 780nm, the color transitions from purple to blue, cyan, green, yellow, orange, and red. All visible light waves mixed together form daylight (white light).
The visual response of the human eye to light is the result of adapting to the light transmission effect of the Earth's atmosphere during human evolution. Wavelengths greater than 1400nm are absorbed by water vapor and carbon dioxide, while those smaller than 290nm are absorbed by the ozone layer. The wavelengths that reach the Earth's surface perfectly match visible light.
2. Core Photometric Quantities
The effect of lighting is ultimately assessed by the human eye. Therefore, it is not enough to describe light sources only with energy parameters; we must introduce light quantity parameters based on human vision.
Luminous Flux (Φ)
The energy that a light source radiates into the surrounding space and causes vision per unit time. It is an essential data point for lighting design.
Optical Efficiency = lm/W
Because the human eye's sensitivity varies (most sensitive to 555nm yellow-green light), 1W of 555nm monochromatic radiation equals 680 lm. Electric light sources are often evaluated by optical apparent efficiency: lumens produced per 1W of electrical power.
Luminous Intensity (I)
The luminous flux radiated by the light source in a unit solid angle in a specific direction. It shows how flux is distributed in space (e.g., adding a lampshade changes this distribution).
Formula: Iθ = Φθ / ωθ
Φθ = Luminous flux in solid angle
ω = Solid angle (steradian, sr). For a whole sphere: ω = 4πr2 / r2 = 4π (sr).
Illuminance (E)
Indicates the intensity of light falling on a surface. It is the ratio of the luminous flux projected onto the illuminated surface to the area of that surface.
Vertical surface: E = Φ / S
Angled surface: E = Iθ cosθ / r2
Reference: 1m from a 40W bulb ≈ 30 lx. Outdoor sunny noon ≈ 8×104 ~ 12×104 lx. Full moon ≈ 0.2 lx.
Brightness / Luminance (L)
Indicates how bright an object appears to the human eye. It is the luminous intensity per unit projected surface area of the luminous (or reflecting) body in the direction of the line of sight.
Formula: L = Iθ / (S cosθ)
A white object reflects light more strongly than a black object, appearing brighter even if both receive the exact same illuminance. If the brightness is too high, it causes glare.
- Luminous Flux (lm): Indicates the total amount of light emitted by the luminous body.
- Luminous Intensity (cd): The luminous flux density emitted in a certain direction, indicating the spatial distribution.
- Illuminance (lx): The luminous flux density accepted by a surface, identifying how well-lit the surface is.
- Brightness (cd/m2): The luminous intensity per unit surface area, indicating how bright an object actually appears to the eye.
