Light & Perception

Terms used for light and the measurement of light properties distinguish between those that refer to physical entities and those that refer to human perception.

• Luminance and Illuminance (Physical Entities) - Luminance and illuminance are both physical properties of light that can be measured with appropriate instruments. The measurements are weighted for the sensitivities of the human visual system. The units of measure are candelas per square meter (cd/m²).
  • Luminance – light that is emitted by a source. A computer monitor generates its own light and, therefore, is termed “luminous,” “self-luminous,” or “emissive.”
  • Illuminance – light that is reflected from an object. A book on a table is illuminated by a light source. That is, the book does not generate light, but reflects light generated by a luminous source.
• Brightness and Lightness (Perception) – The human visual system does not "measure" the amount of light on the retina. Instead, the signals sent to the brain indicate "relative" amounts of light by comparing light from different areas or objects in the visual scene or changes in the amount of light in the environment. Terms used for the perception of light are:
  • Brightness – the perception of light from a self-luminous source. A computer monitor has brightness, as does a candle or light bulb.
  • Lightness – the perception of reflected light. A white piece of paper is light; the ink on a newspaper is dark. Paint colors differ in lightness or darkness.