Luminance is the photometric quantity that quantifies the luminous intensity emitted or reflected by a surface in a given direction, relative to the apparent area of that surface seen from that direction. It characterizes what the eye perceives as the “brightness” of a surface. Unit: candela per square meter (cd/m²).
Discover FizziQ
How to measure it in class
With the FizziQ app, the light meter can help explore the concepts of luminance and illuminance.
Steps:
- Understand the difference: illuminance (light meter) measures received light, luminance measures emitted/reflected light
- Illuminate different surfaces (white, black, shiny, matte) with the same source
- Measure the reflected illuminance with FizziQ
- Observe that shiny surfaces have a more directional luminance
- Compare the luminance of a screen (emitter) and an illuminated sheet of paper (reflector)
Scientific activities on this topic
Possible extensions with FizziQ: comparison of reflective surfaces, measurement of the reflectance factor of a surface, and study of glare.
Learn more
Luminance vs illuminance:
| Quantity | What it measures | Unit | Direction |
|---|---|---|---|
| Illuminance E | Received light | lux (lm/m²) | Receiving surface |
| Luminance L | Emitted/seen light | cd/m² | Direction of observation |
Visual perception:
Luminance is the quantity closest to what the eye perceives. Two surfaces with the same luminance appear equally bright, regardless of their distance.
Typical values:
| Source | Luminance (cd/m²) |
|---|---|
| Sun | 1.6 × 10⁹ |
| Incandescent lamp | 10⁷ |
| Computer screen | 200-400 |
| White paper under office lighting | 100 |
| Moonless night sky | 10⁻³ |
Lambertian surface:
A perfectly diffusing (matte) surface has the same luminance in all directions. Its luminance is L = ρ × E / π where ρ is the reflectance factor and E is the illuminance.
Formula
Definition: L = dI / (dS × cos θ)
where:
- L: luminance (cd/m²)
- dI: luminous intensity emitted in the considered direction (cd)
- dS: surface element (m²)
- θ: angle between the normal and the direction of observation
For a Lambertian surface: L = ρ × E / π
Application examples
- Glare depends on the luminance of the source, not the illuminance
- Screens are adjusted for a comfortable luminance (200-300 cd/m²)
- A well-lit white wall can have the same luminance as a screen
- Reflective road signs have a high luminance in the direction of the headlights
FAQ
Q: Do the Moon and the Sun have the same luminance? A: No. The solar disk is much brighter than the lunar disk: the Sun’s luminance is about 100,000 times (10⁵) that of the Moon, whereas their apparent diameters are similar.
Q: Does FizziQ measure luminance? A: No, the light meter measures illuminance. To measure luminance, you would need a luminance meter pointed at the studied surface.
Q: Why does a white surface appear brighter than a black one? A: The white surface reflects more light (high reflectance factor ρ), so its luminance is greater at equal illuminance.
Q: Do lighting standards use luminance? A: Yes, to limit glare. The luminance of visible sources (luminaires, windows) is limited more than the illuminance of work surfaces.
Related concepts
Illuminance - Luxmeter