Luminance Converter

Luminance is the photometric quantity that describes how bright a surface appears when viewed from a specific direction. It is defined as the luminous intensity emitted or reflected from a surface per unit projected area in that direction: L = d²Φ / (dA · cos θ · dΩ). The SI unit is candela per square meter (cd/m²), universally known in the display industry as the nit. Unlike illuminance (which describes light falling on a surface), luminance describes what reaches your eyes from a surface — making it the definitive measure of perceived brightness for displays, luminaires, windows, and any extended light source.

In the display and consumer electronics industry, nits (cd/m²) are the standard brightness specification for smartphones, monitors, televisions, and projector screens. A typical office monitor operates at 250–350 nits; HDR (High Dynamic Range) displays reach 1000–4000 nits peak brightness; outdoor digital billboards may exceed 10,000 nits for daytime readability in direct sunlight. The VESA DisplayHDR certification levels (400, 600, 1000, 1400 nits) define performance tiers that consumers and designers reference when comparing displays.

In cinema and professional video, the traditional unit is the foot-lambert (fL). The Society of Motion Picture and Television Engineers (SMPTE) specifies cinema screen brightness at 16 fL (≈55 nits) at the center of the screen. HDR cinema standards (Dolby Vision Cinema, laser projection) push to 108 nits or higher. Cinematographers and projectionists routinely convert between nits and fL when configuring projection systems or comparing specifications from manufacturers using different unit conventions.

The stilb (sb) is the CGS unit of luminance: 1 sb = 1 cd/cm² = 10,000 cd/m². It appears in older photometric literature and some physics texts. The lambert (L) and millilambert (mL) are related CGS units based on the perfectly diffuse (Lambertian) emitter concept: 1 L = 3183.099 cd/m². These units were standard in US and European photometry before SI adoption and still appear in military optical specifications, legacy documentation, and some US medical device standards.

The apostilb (asb) and blondel are equivalent units (1 asb = 1 blondel = 1/π cd/m² ≈ 0.3183 cd/m²), used in European photometric standards. They represent the luminance of a perfectly diffuse surface emitting 1 lumen per square meter. The skot (1 skot = 10⁻³ apostilb) was used for measuring very low luminance levels relevant to scotopic (dark-adapted) vision. The bril is an extremely small unit (1 bril = 10⁻⁷ lambert ≈ 3.183 × 10⁻⁸ cd/m²) proposed for ultra-low luminance measurement near the threshold of human vision.

In automotive and road lighting engineering, luminance in cd/m² governs road surface brightness requirements in standards such as CIE 115 and ANSI/IES RP-8. Tunnel lighting design requires calculating both entrance zone luminance (matching outdoor luminance to avoid the "black hole" effect) and interior zone luminance for safe driving visibility. Road marking retroreflectivity is also measured in mcd/m²/lux — a related luminance coefficient.

In architectural and workplace lighting, luminance ratios between task areas, surrounding areas, and backgrounds affect visual comfort and glare. CIBSE and IES guidelines specify maximum luminance values for luminaire surfaces visible within the field of view to prevent discomfort glare. Office lighting designs typically aim for screen luminance below 200 cd/m² to prevent veiling reflections on monitor screens in the workspace.

In medical imaging and ophthalmic instruments, the luminance of retinal stimuli and diagnostic display monitors is tightly controlled. DICOM GSDF (Grayscale Standard Display Function) calibration requires monitors to meet specific luminance response curves from minimum (L_min) to maximum (L_max) luminance in cd/m². Ophthalmic perimetry (visual field testing) stimuli are specified in apostilbs — the Goldmann perimeter uses apostilbs as the standard unit for stimulus luminance.

This luminance converter supports all 19 units: candela/m² (nit), candela/cm², candela/ft², candela/in², kilocandela/m², stilb, lm/m²/sr, lm/cm²/sr, lm/ft²/sr, W/cm²/sr at 555 nm, millinit, lambert, millilambert, foot-lambert, apostilb, blondel, bril, and skot — instant, precise to 12 digits, completely free.