Light Luminance Converter
Convert luminance between cd/m², nits, foot-lamberts, stilbs, and other units. Enter a luminance value, then choose the From unit and To unit.
Convert Luminance Between Units
Light Luminance Formula
Where:
- \(L_{\mathrm{from}}\) — the entered luminance value in the selected From unit
- \(L_{\mathrm{to}}\) — the converted value in the selected To unit
- \(F_{\mathrm{from}}\) and \(F_{\mathrm{to}}\) — the number of candela per square meter (cd/m²) in one of each respective unit
The calculator first expresses the entered value in cd/m², then divides by the factor for the destination unit. A nit has the same value as one cd/m², so converting between those two units does not change the number.
Light Luminance – Calculation Example
Example: Convert the default value of 250 cd/m² to foot-lamberts (fL). The calculator uses 1 fL = 3.4262591 cd/m².
\[L_{\mathrm{fL}}=250\times\frac{1}{3.4262591}=72.965877\ \mathrm{fL}\]
Result: 250 cd/m² is approximately 72.965877 fL.
How to use the calculator
- Enter a nonnegative number in Luminance value. The default is 250.
- Choose the unit of that number in From unit. The default is candela per square meter (cd/m²).
- Choose the unit you need in To unit. The default is foot-lambert (fL).
- Read Converted Luminance. The result includes the selected destination unit and is displayed to no more than six decimal places.
You can reverse the selections to convert in the other direction. If both menus use the same unit, the numerical value stays the same.
What luminance means
Luminance describes how much luminous intensity a surface emits, transmits, or reflects in a particular direction per unit of projected area. Its SI unit is candela per square meter (cd/m²), commonly called a nit. Because direction and projected area matter, luminance is useful for describing displays, illuminated signs, and other surfaces viewed from a specified direction.
A larger luminance value means more light intensity per projected area in that direction; it does not, by itself, say how bright an object will look to a person. Perceived brightness also depends on surrounding light, contrast, adaptation, and viewing conditions. Unit conversion changes only the way a measured or specified luminance is written—it does not adjust for those conditions.
Luminance is not the same as illumination, which describes light arriving at a surface, or luminous intensity, which describes light in a particular direction without dividing by apparent area. Keeping these quantities separate is important when comparing a screen specification with a measurement of light falling on a desk or wall.
Foot-lamberts are often encountered in projected-image and cinema specifications, while nits are common in display specifications. Converting between them makes numerical comparisons possible, provided the specifications refer to comparable measurement conditions. For example, 250 nits and 250 cd/m² describe the same luminance, whereas 250 fL describes a substantially higher luminance.
Conversion factors and practical limits
The calculator uses cd/m² as an intermediate unit. The following factors show how many cd/m² correspond to one unit as implemented by this converter:
| Unit | cd/m² per unit |
|---|---|
| Candela per square meter (cd/m²) | 1 |
| Nit (nt) | 1 |
| Candela per square centimeter (cd/cm²) | 10,000 |
| Candela per square foot (cd/ft²) | 10.76391 |
| Candela per square inch (cd/in²) | 1550.0031 |
| Stilb (sb) | 10,000 |
| Apostilb (asb) | 0.3183 |
| Lambert | 3183.0988618 |
| Foot-lambert (fL) | 3.4262591 |
Some factors are rounded in the calculator, particularly the apostilb factor. Results can therefore differ slightly from conversions made with more precise constants. The displayed result is also rounded to six decimal places; a displayed zero after conversion can represent a very small, nonzero value.
Use a luminance measurement or specification as the input, not a value in lumens, lux, or watts. Those describe different quantities and cannot be converted to luminance from a unit choice alone. If you are starting with luminous flux and electrical power instead, the lumens to watts converter addresses that separate relationship.
Frequently Asked Questions
How do I convert foot-lamberts to nits by hand?
Multiply the foot-lambert value by 3.4262591. For example, 20 fL is approximately \(20\times3.4262591=68.525182\) nits. To go the other way, divide the nit value by 3.4262591. Keep enough digits during the calculation and round only the final answer, especially when comparing specifications.
What is a good luminance for a screen?
There is no single ideal value: it depends on ambient light, content, and how luminance was measured. An indoor SDR monitor might be set around 100–200 nits, while a brighter room may call for more. HDR peak-brightness figures describe highlights, not necessarily sustained full-screen brightness, so check the measurement conditions before comparing displays.
Can I convert lux to nits?
Not from lux alone. Lux measures light arriving at a surface; nits measure luminance leaving it in a viewing direction. For an ideal matte, diffusely reflecting surface, you can estimate \(L=E\rho/\pi\), where \(E\) is illuminance in lux and \(\rho\) is reflectance. Real surfaces may be glossy or unevenly lit, so a direct luminance measurement is more reliable.
Is a foot-lambert the same as a candela per square foot?
No. Both are luminance units, but 1 foot-lambert is approximately \(1/\pi\) candela per square foot. Consequently, 1 cd/ft² is approximately 3.1416 fL. The similar-sounding units are easy to confuse, and choosing the wrong one changes a conversion result by about a factor of \(\pi\).
Why might a converted luminance display as zero?
A very small positive result can display as zero because the converter rounds its output to six decimal places. It does not necessarily mean the original luminance was zero. For greater precision, perform the conversion with the listed unit factors and retain more decimal places. A negative luminance is not a meaningful input for this converter.
Can I compare a projector's luminance directly with a TV's nits?
You can convert the units, but the resulting numbers may not represent comparable tests. A projector's image luminance depends on the screen, its gain, image size, and measurement position; a TV specification may report peak output from only a small bright area. Compare measurements made under similar conditions, not just converted values.
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