Luminance

Luminance is a measure of the perceived brightness of light reaching the human eye, considering both the amount of light emitted, transmitted, or reflected from a surface and the human eye’s sensitivity to different wavelengths of light. In simpler terms, luminance quantifies how bright a surface appears to the human eye under given conditions.

    • Luminance focuses on luminous intensity as experienced by an observer. While luminous intensity refers to the amount of light emitted in a specific direction by a source, luminance measures how bright a surface appears based on both the light it reflects or emits and the observer’s perspective.
    • For example, imagine a lamp shining in a dark room. While the lamp emits a certain amount of total light (luminous flux), the actual brightness (luminance) of a wall the light falls on depends on several factors:
      • Reflectivity: The wall reflects only a portion of the light that hits it. The amount of light reflected affects the overall luminance the human eye perceives.
      • Spectral Sensitivity: Human eyes are more sensitive to green light than to blue or red light. Therefore, even if two surfaces receive the same light, a green surface will appear brighter than a blue one because of the eye’s higher sensitivity to green wavelengths.

  • Measuring luminance helps us understand real-world scenarios:

    • Moonlight: While not very luminous (doesn’t emit much light), moonlight creates a certain luminance on  sand in a desert, allowing us to see our surroundings.
    • Road safety: Streetlights need specific luminance levels to ensure safe visibility for drivers, considering both the total light emitted and road reflectivity.
    • Book reading: The luminance of a book under a lamp determines how comfortable and clear the text appears for your eyes.

Remember, luminance is a measurable quantity, meaning we can use instruments to analyse the brightness of various objects and light sources.

ABOUT LUMINOSITY & LUMINANCE
  • Luminosity signifies the total amount of visible light emitted by a source, measured in watts or lumens. It’s like the raw power output of light, similar to an engine’s horsepower.
  • Luminance, on the other hand, tells us about the perceived brightness of that light when it reaches our eyes. It considers multiple factors:
    • Luminosity of the source
    • Reflection of the surface receiving the light
    • Human eye’s sensitivity to different wavelengths
  • Think of it this way: a powerful light source (high luminosity) might shine on a dark surface (low reflection), resulting in a lower perceived brightness (lower luminance).
  • Conversely, a weaker light source shining on a highly reflective surface could still lead to a higher perceived brightness due to the reflection boosting the received light.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References