Luminance Converter
Convert nits, candelas per square metre, foot-lamberts and lamberts — the brightness a surface sends back — where the factor of π is real physics, not a rounding.
Luminance Converter
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What this result does not account for
- Luminance and illuminance are different quantities; the diffuser bridge is an estimate for reflective surfaces only.
- The Lambertian assumption fails for glossy, textured or strongly directional materials.
- Display nit ratings are usually peak, often over a small window, and rarely sustained full-screen.
- Self-luminous screens have no associated illuminance, so no reflectance relation applies to them.
In short: Luminance is light leaving a surface, measured in nits, which are candelas per square metre. A foot-lambert is 3.4262591 nits and a lambert is 3,183.0989. Those numbers contain a π, because the lambert family is defined so a perfect diffuser under one footcandle shows one foot-lambert. Luminance is not illuminance: nits describe a screen, lux describe a desk.
Formula
Each factor is the number of nits, that is candelas per square metre, in one of that unit. The lambert, foot-lambert, apostilb and skot all carry a factor of 1/π by construction, so that a perfect diffuser under one unit of illuminance shows one unit of luminance.
Worked Example
- Identify the source factor. One nit is 1 cd/m².
- Convert to the SI unit. 300 × 1 = 300 cd/m².
- Identify the target factor. One foot-lambert is 3.4262591 cd/m².
- Divide. 300 ÷ 3.4262591 = 87.559 fL.
- Check the π. 3.4262591 is 10.763910417 divided by π — the footcandle over π, exactly.
Why cinema uses foot-lamberts. Projection standards call for about 14 foot-lamberts on screen for film, roughly 48 nits. The imperial unit survives because of the π: in foot units, a perfectly diffuse screen lit to N footcandles has a luminance of N foot-lamberts times its reflectance, with no π to carry. In metric the same relationship needs dividing by π, which is why the convenience of the older unit kept it alive in the industry.
Strengths & Limits Of This Model
Where this engine is strong
- Foot-lambert derived as footcandle over π, not copied from a table.
- Explains where the π comes from instead of leaving it unexplained.
- Keeps luminance strictly distinct from illuminance and intensity.
- Covers the legacy CGS units found in older optical literature.
Where it stops
- Cannot convert to illuminance without a reflectance, and never for a display.
- Does not model viewing angle or non-Lambertian surface behaviour.
Practical Use Cases
Display and screen specification
Monitors, phones and televisions are rated in nits. Typical laptops sit near 300, while sunlight-readable and HDR panels reach well beyond 1,000.
Cinema and projection
Screen luminance is specified in foot-lamberts, with around 14 fL the long-standing target for film presentation.
Lighting quality and glare assessment
Luminance ratios between a task and its surroundings drive visual comfort far more than raw illuminance — pair with the Illuminance Converter for the light arriving side of the same scheme.
Legacy photometric literature
Stilbs, lamberts, apostilbs and skots appear throughout older optical texts and need translating into candelas per square metre.
Methodology & Editorial Standards
All conversions route through the candela per square metre, also called the nit, which is the SI unit of luminance. The candela itself is an SI base unit, defined by fixing the luminous efficacy of 540 THz radiation at 683 lumens per watt. The lambert family is derived rather than assumed: the foot-lambert is exactly one footcandle divided by π, giving 3.4262590996353905 nits, and the lambert is 10,000 divided by π. That π comes from integrating a cosine emission distribution over the hemisphere and is exact for an ideal Lambertian diffuser. The engine keeps luminance strictly separate from illuminance and offers the diffuser relation only as a labelled estimate, noting explicitly that it does not apply to self-luminous displays, which emit rather than reflect and therefore have no associated illuminance at all. All conversion factors are exact by definition under the International System of Units, or exact by international agreement where the unit is defined by treaty. Values are held at full IEEE-754 double precision internally and rounded only for display, so chained conversions do not accumulate drift.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Luminance Converter — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is a nit?
One candela per square metre, the SI unit of luminance. It measures how much light a surface sends toward your eye, which is what you perceive as brightness. The name comes from the Latin nitere, to shine. It is not an official SI name but is universal in the display industry.
How many nits are in a foot-lambert?
Exactly 3.4262590996353905 nits. The number is one footcandle divided by π: 10.763910417 ÷ π. It looks arbitrary only until you know the lambert family was constructed so that a perfect diffuser lit by one footcandle has a luminance of exactly one foot-lambert.
What is the difference between luminance and illuminance?
Illuminance, in lux or footcandles, is light arriving on a surface. Luminance, in nits, is light leaving it toward the observer. Two walls under identical illuminance have very different luminance if one is white and one is dark, which is exactly why a room can hit its lux target and still feel gloomy.
Why is there a π in these units?
Because a perfectly diffuse surface emits with a cosine distribution over the hemisphere, and integrating that gives π rather than 1. The lambert, foot-lambert, apostilb and skot were all defined to absorb that π, so that in those units luminance equals illuminance times reflectance with no correction factor.
How many nits does a display need?
Around 250 to 350 nits suits ordinary indoor use, 400 to 600 helps in bright rooms, and sunlight-readable or HDR panels run 1,000 and above. Cinema projection, by contrast, targets only about 48 nits, because the room is dark and the eye adapts.
Can I convert nits to lux?
Not directly. They are different quantities. For a perfectly diffuse reflecting surface you can relate them with L = ρE/π, where ρ is reflectance, but that requires knowing the surface. For a self-luminous display no conversion exists at all, because the screen emits its own light rather than reflecting any.
Is a candela the same as a nit?
No. A candela is luminous intensity, light in a given direction, and is an SI base unit. A nit is a candela per square metre, so it accounts for the area of the emitting surface. A small intense source and a large dim one can have the same candela rating but very different luminance.
What is a stilb?
The CGS unit of luminance, one candela per square centimetre, equal to 10,000 nits. It appears in older optical literature alongside the lambert and apostilb, all of which have been superseded by the candela per square metre in modern practice.
Why does cinema still use foot-lamberts?
Because of the π. In imperial photometric units the relationship between screen illuminance and screen luminance loses its π entirely, so a diffuse screen lit to N footcandles reads N foot-lamberts times reflectance. That arithmetic convenience kept the unit embedded in projection standards long after metric took over elsewhere.
Does luminance depend on viewing angle?
For a perfectly Lambertian surface, no — that is the defining property, and it looks equally bright from every direction. Real surfaces are not ideal: glossy materials and many display panels are noticeably brighter head-on than off-axis, which is why display specifications quote a viewing angle alongside the nit figure.