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

1 asb = 0.3183099 cd/m2cd/m2 → asb
Formula
1 asb = 0.3183099 cd/m2

Understanding apostilbs to candelas per square metre Conversion

The apostilb (asb) is a photometric unit of luminance defined so that a perfectly diffusing surface emitting one lux has a luminance of one apostilb; it equals 1/π candela per square metre. The candela per square metre (cd/m², also called the nit) is the SI unit of luminance and the standard for specifying display and screen brightness. This conversion links the older apostilb, common in illumination engineering, to the SI unit used across modern optics and display specifications.

Conversion Formula

1 asb=0.318310 cd/m21\ \text{asb} = 0.318310\ \text{cd/m2}

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

cd/m2=asb×0.3183099\text{cd/m2} = \text{asb} \times 0.3183099

Step-by-Step Example

Convert 25 apostilbs to candelas per square metre.

cd/m2=25×0.3183099=7.95775 cd/m2\text{cd/m2} = 25 \times 0.3183099 = 7.95775\ \text{cd/m2}

How to Convert apostilbs to candelas per square metre

Convert legacy apostilb luminance values into SI nits (cd/m²) in a few steps.

  1. Take the apostilb value: Start with the luminance, for example 25 asb.
  2. Multiply by 0.3183099: This factor equals 1/π, the apostilb-to-cd/m² ratio.
  3. Read the result: 25 × 0.3183099 = 7.95775 cd/m².
  4. Reverse if needed: Multiply cd/m² by 3.141593 (π) to return to apostilbs.

apostilbs to candelas per square metre conversion table

apostilbs (asb)candelas per square metre (cd/m2)
00
10.3183099
20.6366198
30.9549297
41.27324
51.591549
61.909859
72.228169
82.546479
92.864789
103.183099
154.774648
206.366198
257.957747
309.549297
4012.7324
5015.91549
6019.09859
7022.28169
8025.46479
9028.64789
10031.83099
15047.74648
20063.66198
25079.57747
30095.49297
400127.324
500159.1549
600190.9859
700222.8169
800254.6479
900286.4789
1000318.3099
2000636.6198
3000954.9297
40001273.24
50001591.549
100003183.099
250007957.747
5000015915.49
10000031830.99
25000079577.47
500000159154.9
1000000318309.9

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.

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 apostilb?

One apostilb equals 0.3183099 cd/m², which is exactly 1/π, because the apostilb is defined with a factor of π relative to the SI luminance unit.

How do I convert apostilbs to candelas per square metre?

Multiply the apostilb value by 0.3183099. For example, 10 apostilbs equals 10 × 0.3183099 = 3.183099 cd/m².

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

Multiply cd/m² by 3.141593 (π). So 5 cd/m² equals about 15.71 apostilbs.

Why does the factor 1/π appear?

The apostilb is an "apparent" luminance unit tied to a perfectly diffusing (Lambertian) surface, and integrating a Lambertian emitter over the hemisphere introduces the factor of π.

Where is this conversion useful?

It lets lighting engineers translate legacy apostilb luminance figures into cd/m² (nits), the unit used for display brightness and modern photometric standards.

People also convert

asb to ntasb to sbasb to Lasb to fL

Complete apostilbs conversion table

asb
UnitResult
candelas per square metre (cd/m2)0.3183099 cd/m2
nits (nt)0.3183099 nt
stilbs (sb)0.00003183099 sb
lamberts (L)0.0001 L
foot-lamberts (fL)0.09290304 fL

Luminance conversions