candelas per square metre (cd/m2) to apostilbs (asb) conversion

1 cd/m2 = 3.141593 asbasb → cd/m2
Formula
1 cd/m2 = 3.141593 asb

Understanding candelas per square metre to apostilbs Conversion

The candela per square metre (cd/m2), the SI unit of luminance, measures the luminous intensity emitted or reflected per unit area and is the standard figure for display brightness and lighting design. The apostilb (asb) is an older photometric unit of luminance from the CGS-adjacent tradition, defined so that a perfectly diffusing surface reflecting one lux appears at one apostilb. Because the apostilb bakes in a factor of π, converting from cd/m2 involves multiplying by pi.

Conversion Formula

1 cd/m2=3.141593 asb1\ \text{cd/m2} = 3.141593\ \text{asb}

To convert candelas per square metre to apostilbs, multiply by this factor:

asb=cd/m2×3.141593\text{asb} = \text{cd/m2} \times 3.141593

Step-by-Step Example

Convert 25 candelas per square metre to apostilbs.

asb=25×3.141593=78.5398 asb\text{asb} = 25 \times 3.141593 = 78.5398\ \text{asb}

How to Convert candelas per square metre to apostilbs

Translate an SI luminance into the older apostilb unit with these steps.

  1. Note the luminance: Start with the value in cd/m2, for example 25.
  2. Multiply by π: Use 3.141593, the number of apostilbs in one cd/m2.
  3. Compute: 25×3.141593=78.539825 \times 3.141593 = 78.5398.
  4. State the result: 25 cd/m2 equal about 78.5398 apostilbs.

candelas per square metre to apostilbs conversion table

candelas per square metre (cd/m2)apostilbs (asb)
00
13.141593
26.283185
39.424778
412.56637
515.70796
618.84956
721.99115
825.13274
928.27433
1031.41593
1547.12389
2062.83185
2578.53982
3094.24778
40125.6637
50157.0796
60188.4956
70219.9115
80251.3274
90282.7433
100314.1593
150471.2389
200628.3185
250785.3982
300942.4778
4001256.637
5001570.796
6001884.956
7002199.115
8002513.274
9002827.433
10003141.593
20006283.185
30009424.778
400012566.37
500015707.96
1000031415.93
2500078539.82
50000157079.6
100000314159.3
250000785398.2
5000001570796
10000003141593

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 apostilb?

The apostilb is an obsolete unit of luminance once used to describe the brightness of diffusely reflecting or emitting surfaces. It belongs to a family of "1/π" luminance units designed to simplify calculations for perfectly diffuse (Lambertian) surfaces.

Definition

One apostilb is defined as one lumen per square metre emitted from a perfectly diffusing surface, which equals 1/π candela per square metre.

1 asb=0.318310 cd/m21\ \text{asb} = 0.318310\ \text{cd/m}^2

Exactly, 1 asb=1π cd/m2=10−4 lambert1\ \text{asb} = \frac{1}{\pi}\ \text{cd/m}^2 = 10⁻⁴\ \text{lambert}.

Origin and History

The apostilb was part of an early-20th-century set of photometric units (alongside the lambert and foot-lambert) built around the factor 1/π so that a Lambertian surface illuminated to a given number of lux would have a numerically equal luminance. The prefix "apo-" distinguished it from the stilb.

Law and Notable Facts

The apostilb is not an SI unit and is now essentially obsolete, replaced by the candela per square metre. Its defining convenience was that a perfect diffuser receiving an illuminance of one lux has a luminance of exactly one apostilb.

Real-World Examples and Conversions

  • A white matte surface at 1 lux illuminance: about 1 apostilb.
  • 1 asb=1π cd/m2≈0.318310 cd/m21\ \text{asb} = \frac{1}{\pi}\ \text{cd/m}^2 \approx 0.318310\ \text{cd/m}^2.
  • 1 cd/m2=π apostilbs≈3.14159 asb1\ \text{cd/m}^2 = \pi\ \text{apostilbs} \approx 3.14159\ \text{asb}.
  • 104 apostilbs=1 lambert≈3183.10 cd/m210⁴\ \text{apostilbs} = 1\ \text{lambert} \approx 3183.10\ \text{cd/m}^2.

Frequently Asked Questions

How many apostilbs equal one candela per square metre?

One cd/m2 equals π apostilbs, about 3.141593 asb. The factor is exactly pi by the apostilb's definition.

How do I convert apostilbs back to candelas per square metre?

Multiply the apostilb value by 0.3183099 (which is 1/π). So 3.141593 asb return to 1 cd/m2.

Why does π appear in this conversion?

The apostilb was defined for a Lambertian (perfectly diffusing) surface, whose luminance in cd/m2 relates to illuminance by a factor of π; that geometry embeds pi in the unit itself.

Is the apostilb still used today?

Rarely. It survives mainly in older lighting and vision-science literature; modern display and lighting work uses the candela per square metre (nit) instead.

What is 25 cd/m2 in apostilbs?

It is about 78.5398 asb, a dim-to-moderate luminance typical of a subdued indoor surface or a low display setting.

People also convert

cd/m2 to ntcd/m2 to sbcd/m2 to Lcd/m2 to fL

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