Luminance
How to Convert cd/m² (Nits) to Foot-lamberts
To convert a measurement from candelas per square meter (cd/m²), also known as nits, to foot-lamberts (fL), you use the conversion factor that 1 fL is equal to 3.426 cd/m². Therefore, to convert from cd/m², you divide by 3.426.
Example:
A monitor has a brightness of 300 nits (300 cd/m²). Convert this to foot-lamberts.
300 cd/m² / 3.426 (cd/m²)/fL ≈ 87.56 fL
Answer: A luminance of 300 nits is equal to approximately 87.56 foot-lamberts.
The Brightness of Surfaces: Understanding Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light traveling in a given direction. In simpler terms, it describes the perceived brightness of a surface. It quantifies how much light is emitted from, passes through, or is reflected from a particular surface and falls within a given solid angle. Unlike illuminance (measured in lux), which quantifies the amount of light falling *onto* a surface, luminance quantifies the light *coming from* the surface. It is what characterizes how bright we perceive that surface to be.
For example, a movie screen in a dark theater has a certain luminance, which is the light coming from the projector and reflecting off the screen towards the audience's eyes. Two surfaces can have the same illuminance (the same amount of light hitting them), but have different luminance values if one is dark and absorbs most of the light while the other is white and reflects most of it. Luminance is a critical metric in display technology (for TVs, monitors, and smartphones), lighting design, and road safety. The SI unit for luminance is the candela per square meter (cd/m²), which is often referred to as a 'nit'. This converter helps translate between the nit and other historical or regional units of luminance.
Relevant Formulas and Concepts
- Definition of Luminance (Optics/Photometry): Luminance (Lᵥ) is defined as
Lᵥ = d²Φᵥ / (dΩ * dA * cos(θ)), where Φᵥ is the luminous flux, Ω is the solid angle, A is the area, and θ is the angle of emission. - Luminance of a Perfect Diffuser (Lambertian Surface): For a perfect diffusing surface, the luminance is related to the illuminance (Eᵥ) and reflectance (R) by
Lᵥ = (Eᵥ * R) / π.
A Deep Dive into Common Luminance Units
- Candela per square meter (cd/m²): The SI unit of luminance. It is the standard unit for all scientific and technical applications.
- Nit: An alternative, non-SI name for candela per square meter. The term 'nit' is very common in the display industry for specifying the brightness of screens. A typical computer monitor has a luminance of 200-300 nits, while HDR (High Dynamic Range) TVs can reach 1000 nits or more.
- Stilb (sb): An older CGS (centimeter-gram-second) unit of luminance, equal to one candela per square centimeter. 1 stilb = 10,000 cd/m². It is now largely obsolete.
- Lambert (L): Another obsolete CGS unit of luminance, equal to 1/π candela per square centimeter. Named after the Swiss mathematician Johann Heinrich Lambert.
- Foot-lambert (fL): An Imperial/US customary unit of luminance. One foot-lambert is equal to 1/π candela per square foot. It is still used in some industries in the United States, particularly in the motion picture and aviation industries.