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

1 cd/m2 = 0.2918635 fLfL → cd/m2
Formula
1 cd/m2 = 0.2918635 fL

Understanding candelas per square metre to foot-lamberts Conversion

The candela per square metre (cd/m2), also called the nit, is the SI unit of luminance used for screen brightness and lighting specifications. The foot-lambert (fL) is the imperial luminance unit still common in North American cinema, projection, and display calibration, where projected image brightness is often specified in foot-lamberts. Converting cd/m2 to foot-lamberts is essential when matching international nit ratings to US projection standards.

Conversion Formula

1 cd/m2=0.2918635 fL1\ \text{cd/m2} = 0.2918635\ \text{fL}

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

fL=cd/m2×0.2918635\text{fL} = \text{cd/m2} \times 0.2918635

Step-by-Step Example

Convert 25 candelas per square metre to foot-lamberts.

fL=25×0.2918635=7.296588 fL\text{fL} = 25 \times 0.2918635 = 7.296588\ \text{fL}

How to Convert candelas per square metre to foot-lamberts

Match an SI nit value to the imperial foot-lambert with these steps.

  1. Note the luminance: Start with the value in cd/m2, for example 25.
  2. Multiply by 0.2918635: This is the number of foot-lamberts in one cd/m2.
  3. Compute: 25×0.2918635=7.29658825 \times 0.2918635 = 7.296588.
  4. State the result: 25 cd/m2 equal about 7.297 foot-lamberts.

candelas per square metre to foot-lamberts conversion table

candelas per square metre (cd/m2)foot-lamberts (fL)
00
10.2918635
20.583727
30.8755905
41.167454
51.459318
61.751181
72.043045
82.334908
92.626772
102.918635
154.377953
205.83727
257.296588
308.755905
4011.67454
5014.59318
6017.51181
7020.43045
8023.34908
9026.26772
10029.18635
15043.77953
20058.3727
25072.96588
30087.55905
400116.7454
500145.9318
600175.1181
700204.3045
800233.4908
900262.6772
1000291.8635
2000583.727
3000875.5905
40001167.454
50001459.318
100002918.635
250007296.588
5000014593.18
10000029186.35
25000072965.88
500000145931.8
1000000291863.5

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}.

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.

Frequently Asked Questions

How many foot-lamberts are in one candela per square metre?

One cd/m2 equals about 0.2918635 foot-lambert. Roughly, 1 fL is about 3.426 cd/m2.

How do I convert foot-lamberts back to candelas per square metre?

Multiply the foot-lambert value by 3.426259. So 0.2918635 fL return to 1 cd/m2.

Why do cinemas and projectors use foot-lamberts?

North American projection standards (such as SMPTE recommendations) historically specify screen luminance in foot-lamberts, so calibrators convert nit measurements to fL to verify targets like 14 fL for film.

What is 25 cd/m2 in foot-lamberts?

It is about 7.297 fL, roughly half the classic 14 fL cinema target, indicating a relatively dim screen setting.

Is a foot-lambert the same as a lambert?

No. A foot-lambert is based on the foot and is much smaller; the (full) lambert is a CGS-based unit equal to 1/π candela per square centimetre and thousands of times larger.

People also convert

cd/m2 to ntcd/m2 to sbcd/m2 to asbcd/m2 to L

Complete candelas per square metre conversion table

cd/m2
UnitResult
nits (nt)1 nt
stilbs (sb)0.0001 sb
apostilbs (asb)3.141593 asb
lamberts (L)0.0003141593 L
foot-lamberts (fL)0.2918635 fL

Luminance conversions