foot-lamberts (fL) to candelas per square metre (cd/m2) conversion

1 fL = 3.426259 cd/m2cd/m2 → fL
Formula
1 fL = 3.426259 cd/m2

Understanding foot-lamberts to candelas per square metre Conversion

A foot-lambert (fL) is a US customary unit of luminance widely used to specify cinema and projection screen brightness. The candela per square metre (cd/m², also called the nit) is the SI unit of luminance and the standard for displays, televisions, and lighting worldwide. Converting foot-lamberts to cd/m² is essential when translating US projection specifications into the SI values used on international display datasheets.

Conversion Formula

1 fL=3.42626 cd/m21\ \text{fL} = 3.42626\ \text{cd/m2}

To convert foot-lamberts to candelas per square metre, multiply by this factor:

cd/m2=fL×3.426259\text{cd/m2} = \text{fL} \times 3.426259

Step-by-Step Example

Convert 25 foot-lamberts to candelas per square metre.

cd/m2=25×3.426259=85.6565 cd/m2\text{cd/m2} = 25 \times 3.426259 = 85.6565\ \text{cd/m2}

How to Convert foot-lamberts to candelas per square metre

Translate a US luminance figure into the SI nit scale with a single multiplication.

  1. Note the factor: One foot-lambert equals 3.426259 cd/m².
  2. Take your luminance in foot-lamberts: Begin with the value to convert, such as 25 fL.
  3. Multiply: Multiply the foot-lambert value by 3.426259.
  4. Read the result: 25×3.426259=85.656525 \times 3.426259 = 85.6565 cd/m².

foot-lamberts to candelas per square metre conversion table

foot-lamberts (fL)candelas per square metre (cd/m2)
00
13.426259
26.852518
310.27878
413.70504
517.1313
620.55755
723.98381
827.41007
930.83633
1034.26259
1551.39389
2068.52518
2585.65648
30102.7878
40137.0504
50171.313
60205.5755
70239.8381
80274.1007
90308.3633
100342.6259
150513.9389
200685.2518
250856.5648
3001027.878
4001370.504
5001713.13
6002055.755
7002398.381
8002741.007
9003083.633
10003426.259
20006852.518
300010278.78
400013705.04
500017131.3
1000034262.59
2500085656.48
50000171313
100000342625.9
250000856564.8
5000001713130
10000003426259

What is the Foot-Lambert?

The foot-lambert is a non-SI unit of luminance, expressing the brightness of a surface as seen by an observer. It remains widely used in the cinema, projection, and display industries in the United States, where screen brightness is often specified in foot-lamberts.

Definition

The foot-lambert is defined so that a perfectly diffusing (Lambertian) surface emitting or reflecting a total luminous flux of one lumen per square foot has a luminance of one foot-lambert.

1 fL=3.42626 cd/m21\ \text{fL} = 3.42626\ \text{cd/m}^2

Equivalently, 1 fL=1π cd/ft21\ \text{fL} = \frac{1}{\pi}\ \text{cd/ft}^2. As with the lambert, the factor of π\pi arises from the cosine emission geometry of a Lambertian source. Because one square foot equals 0.09290304 m20.09290304\ \text{m}^2, the conversion follows directly: 1π×0.09290304 cd/m2=3.42626 cd/m2\frac{1}{\pi \times 0.09290304}\ \text{cd/m}^2 = 3.42626\ \text{cd/m}^2.

Origin and History

The foot-lambert is the imperial-unit counterpart of the lambert, both descending from Johann Heinrich Lambert's foundational photometry. It became entrenched in mid-20th-century American engineering practice, particularly in cinema, where SMPTE standards long specified projected picture brightness in foot-lamberts.

Law and Notable Facts

The foot-lambert is not an SI unit, but it persists in professional standards. SMPTE recommends an open-gate (unmodulated) screen luminance of 14 fL (≈ 48 cd/m²) for film projection and 16 fL peak white for digital cinema in a dark theater. HDR home displays, by contrast, target hundreds to over a thousand cd/m².

Real-World Examples and Conversions

  • 14 foot-lamberts (SMPTE film reference) ≈ 47.97 cd/m².
  • 1 foot-lambert ≈ 3.42626 cd/m² ≈ 0.001076 lambert.
  • A typical office display of 250 cd/m² is about 73 foot-lamberts.
  • 1 lambert ≈ 929.03 foot-lamberts.

What is the candela per square metre?

The candela per square metre is the SI unit of luminance, measuring the luminous intensity emitted or reflected from a surface per unit area in a given direction. It describes how bright a surface appears to the human eye and is the standard used to specify display and screen brightness.

Definition

Luminance is luminous intensity per unit projected area. One candela per square metre is the luminance of a surface emitting one candela of luminous intensity from each square metre of its projected area.

1 cd/m2=1 cd/m21\ \text{cd/m}^2 = 1\ \text{cd/m}^2

As a derived SI unit it can be written 1 cd/m2=1 lm/(m2⋅sr)1\ \text{cd/m}^2 = 1\ \text{lm}/(\text{m}^2 \cdot \text{sr}), since the candela equals one lumen per steradian.

Origin and History

The candela became one of the seven SI base units in 1948 (as the "new candle"), and luminance in candela per square metre followed as the coherent derived unit. It replaced a patchwork of earlier photometric units such as the stilb, lambert, and foot-lambert.

Law and Notable Facts

The candela per square metre is the internationally sanctioned SI unit of luminance and is colloquially known as the "nit." Its value ties to the modern definition of the candela, fixed by the luminous efficacy of monochromatic 540 THz radiation at 683 lm/W.

Real-World Examples and Conversions

  • A typical computer monitor: 200–350 cd/m².
  • A smartphone in bright sunlight mode: 800–2000 cd/m².
  • Clear daytime sky: several thousand cd/m².
  • 1 cd/m2=1 nit=10−4 stilb1\ \text{cd/m}^2 = 1\ \text{nit} = 10⁻⁴\ \text{stilb}.

Frequently Asked Questions

How many candelas per square metre are in one foot-lambert?

One foot-lambert equals about 3.42626 cd/m², so a value in foot-lamberts is roughly one-third of the corresponding nit figure inverted.

Is candela per square metre the same as a nit?

Yes. The nit is simply another name for the candela per square metre, so 1 fL is also about 3.42626 nits.

Why does the film industry use foot-lamberts?

The SMPTE cinema standard specifies a reference white luminance of 16 fL (about 55 cd/m²) for film projection, so US cinema equipment is calibrated in foot-lamberts.

How do I convert 16 foot-lamberts to cd/m²?

Multiply 16 by 3.426259 to get about 54.82 cd/m², the SI equivalent of the SMPTE cinema white level.

Which unit should modern display specs use?

Candela per square metre (nit) is the SI standard and is preferred for TVs, monitors, and phones; foot-lamberts remain mainly in legacy US projection contexts.

People also convert

fL to ntfL to sbfL to asbfL to L

Complete foot-lamberts conversion table

fL
UnitResult
candelas per square metre (cd/m2)3.426259 cd/m2
nits (nt)3.426259 nt
stilbs (sb)0.0003426259 sb
apostilbs (asb)10.76391 asb
lamberts (L)0.001076391 L

Luminance conversions