Luminance Converter

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes how much light a surface emits or reflects toward an observer, measured in candelas per square meter (cd/m²). Luminance is the property of light that determines how bright a surface appears to the human eye, making it one of the most important quantities in lighting design, display technology, and visual ergonomics.

Unlike illuminance, which measures how much light falls on a surface, luminance measures how much light leaves a surface in a particular direction. This distinction matters because two surfaces receiving the same illuminance can have very different luminances—one might be matte and diffuse the light evenly, while the other might be glossy and concentrate light into a bright reflection. Luminance captures this directional quality of light.

The SI unit of luminance is the candela per square meter (cd/m²), historically also called the nit. In the display industry, nits have become the de facto standard unit for specifying screen brightness. A typical laptop screen produces 250–400 nits, a smartphone 500–1,200 nits, and a high-end HDR television can exceed 2,000 nits. In contrast, the Sun has a luminance of approximately 1.6 billion cd/m², illustrating why staring directly at it is so dangerous.

Several legacy and specialized luminance units exist alongside the SI standard. The foot-lambert is used in the American film and projection industry, the stilb and lambert are CGS units used in some European and older scientific contexts, and the apostilb (also called the blondel) was historically used in France. This converter handles all of these units, providing instant conversion between them.

All luminance unit conversions are based on their relationship to the SI unit, candela per square meter (cd/m²). Each unit has a defined conversion factor relative to cd/m². The conversion process involves two steps: first, convert the input value to cd/m², then convert from cd/m² to the target unit.

The key conversion factors are: 1 nit = 1 cd/m² (they are identical), 1 foot-lambert = 3.426259 cd/m², 1 stilb = 10,000 cd/m², 1 apostilb = 1/π cd/m² ≈ 0.3183 cd/m², and 1 lambert = 10,000/π cd/m² ≈ 3,183 cd/m². Additional conversions include the millilambert and candela per square foot and inch.

The general formula is: Result = (Input Value × From-Factor) ÷ To-Factor, where each factor represents how many cd/m² correspond to one unit of that type. This two-step process ensures accurate conversions regardless of which source and target units you choose.

The relationship between luminance units often involves powers of π because the lambert family of units is based on the uniform emission from a Lambertian surface, which emits light uniformly in all directions according to Lambert's cosine law. The mathematical derivation of these factors traces back to the fundamental definitions of solid angle and luminous intensity.

This luminance converter provides a comprehensive conversion tool with all major luminance units:

1. Enter the Value: Type the luminance value into the input field. You can enter any positive number, including very small values for dim sources or very large values for bright sources.
2. Select the From Unit: Choose the unit you are converting from. The dropdown lists all available units including cd/m² (nits), foot-lamberts, stilbs, apostilbs, lamberts, millilamberts, cd/ft², and cd/in².
3. Select the To Unit: Choose your desired output unit from the second dropdown. Use the swap button (the double-arrow icon) to quickly reverse the from and to selections.
4. Read the Result: The main display shows the converted value. Below, the "All units" panel shows your input value expressed in every available unit simultaneously, making it easy to compare across different systems.

The converter provides both a direct conversion to your chosen target unit and a comprehensive panel showing your input value in all available luminance units. This simultaneous display is particularly useful when you need to understand how a luminance value translates across different measurement systems.

For context, typical luminance values range from about 0.001 cd/m² (starlight) to over 1,000,000 cd/m² (direct sunlight). Common references include: a candle flame at about 10,000 cd/m², a typical computer monitor at 200–500 cd/m², and a clear blue sky at approximately 8,000 cd/m². These reference points help you judge whether a converted value is reasonable.

The Quick Reference panel shows the four most important conversion relationships: 1 nit = 1 cd/m², 1 foot-lambert ≈ 3.426 cd/m², 1 stilb = 10,000 cd/m², and 1 lambert ≈ 3,183 cd/m². These relationships form the basis for all conversions performed by the calculator.

Display technology and screen brightness is the most common modern application of luminance measurement. Television, monitor, and smartphone manufacturers specify peak brightness in nits (cd/m²). Higher nit values indicate brighter screens that are more visible in direct sunlight and better reproduce HDR content. Professional color grading studios calibrate monitors to specific nit levels.

Architectural lighting design uses luminance to evaluate visual comfort and ergonomics. The ratio between a task area and its surroundings (the luminance contrast ratio) determines eye strain and readability. Standards like EN 12464-1 specify minimum luminance values for workspaces, typically 300–500 cd/m² for office tasks.

Film and projection relies on foot-lambert measurements for cinema screen brightness. The Society of Motion Picture and Television Engineers (SMPTE) specifies that cinema screens should maintain 14 foot-lamberts (approximately 48 cd/m²) for standard projection. This standard ensures consistent viewing experiences across different theaters.

Automotive and aviation instrument panels and heads-up displays are designed with specific luminance values for readability under varying ambient conditions. Night-driving displays must be dim enough to preserve night vision while remaining readable, requiring luminance control over several orders of magnitude.