lamberts (L) to apostilbs (asb) conversion

1 L = 10000 asbasb → L
Formula
1 L = 10000 asb

Understanding Lamberts to Apostilbs Conversion

The lambert (L) is a CGS unit of luminance defined so that a perfectly diffusing surface emitting or reflecting 1 lumen per square centimetre has a luminance of 1 lambert. The apostilb (asb) is a photometric unit of luminance equal to one lumen per square metre of a perfect diffuser, or equivalently 1/π1/\pi candela per square metre. Both belong to the older brightness-measurement conventions still encountered in optics, illumination engineering, and historical instrument documentation.

Conversion Formula

1 L=10000 asb1\ \text{L} = 10000\ \text{asb}

To convert Lamberts to Apostilbs, multiply by this factor:

asb=L×10000\text{asb} = \text{L} \times 10000

Step-by-Step Example

Convert 25 Lamberts to Apostilbs.

asb=25×10000=250000 asb\text{asb} = 25 \times 10000 = 250000\ \text{asb}

How to Convert Lamberts to Apostilbs

Both units measure luminance, so conversion is a clean multiplication by a whole number.

  1. Record the luminance in lamberts: note the value in L.
  2. Multiply by 10,000: this scales from lumens per cm² to lumens per m².
  3. Calculate: for 25 L, compute 25×1000025 \times 10000.
  4. Report the result: the luminance is 250,000 asb.

lamberts to apostilbs conversion table

lamberts (L)apostilbs (asb)
00
110000
220000
330000
440000
550000
660000
770000
880000
990000
10100000
15150000
20200000
25250000
30300000
40400000
50500000
60600000
70700000
80800000
90900000
1001000000
1501500000
2002000000
2502500000
3003000000
4004000000
5005000000
6006000000
7007000000
8008000000
9009000000
100010000000
200020000000
300030000000
400040000000
500050000000
10000100000000
25000250000000
50000500000000
1000001000000000
2500002500000000
5000005000000000
100000010000000000

What is the Lambert?

The lambert is a non-SI unit of luminance, describing how much light is emitted or reflected from a surface toward an observer. It is named after the Swiss-German polymath Johann Heinrich Lambert and appears mainly in older optics, photometry, and lighting literature.

Definition

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

1 L=3183.10 cd/m21\ \text{L} = 3183.10\ \text{cd/m}^2

Equivalently, 1 L=1π cd/cm2=104π cd/m21\ \text{L} = \frac{1}{\pi}\ \text{cd/cm}^2 = \frac{10⁴{\pi}\ \text{cd/m}^2. The factor of π\pi enters because luminance in candela per unit area relates to the emitted lumens per unit area through the geometry of a Lambertian (cosine-law) emitter. Sub-multiples such as the millilambert (mL) and microlambert are common in practice.

Origin and History

The unit honors Johann Heinrich Lambert (1728–1777), whose 1760 work Photometria laid the mathematical foundations of photometry, including the cosine emission law of diffuse surfaces. The lambert itself was proposed in the early 20th century as photometry was being systematized, and it saw wide use before the SI candela-per-square-metre (nit) became standard.

Law and Notable Facts

The lambert is not part of the SI and is discouraged in modern metrology, but it survives in legacy instruments and texts. A key convenience of the lambert family is that a perfect diffuse reflector illuminated by one lux has a luminance of exactly 1π×10−4\tfrac{1}{\pi}\times10^{-4} lambert, tidily linking illuminance and luminance for ideal surfaces.

Real-World Examples and Conversions

  • A surface of 1 lambert equals about 3183 cd/m², roughly the brightness of a bright fluorescent lamp diffuser.
  • 1 millilambert = 3.18310 cd/m², close to comfortable indoor lighting levels.
  • The closely related foot-lambert (used for cinema screens) equals about 3.426 cd/m²; standard theater screen luminance of 14 fL corresponds to roughly 48 cd/m².
  • 1 lambert ≈ 929.03 foot-lamberts.

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 are in one lambert?

Exactly 10,000 apostilbs equal one lambert, a factor that comes directly from the square-centimetre to square-metre area ratio in their definitions.

How do I convert lamberts to apostilbs?

Multiply the lambert value by 10,000. So 3 L equals 30,000 asb.

Why is the factor exactly 10,000?

Because the lambert is based on lumens per square centimetre while the apostilb is based on lumens per square metre, and there are 10,000 cm² in one m².

Where are lamberts and apostilbs used?

They appear in classical photometry, older lighting and display engineering references, and instrument calibration documents predating the SI candela-per-square-metre standard.

How do I go back from apostilbs to lamberts?

Divide the apostilb value by 10,000, which is the same as multiplying by 0.0001.

People also convert

L to cd/m2L to ntL to sbL to fL

Complete lamberts conversion table

L
UnitResult
candelas per square metre (cd/m2)3183.099 cd/m2
nits (nt)3183.099 nt
stilbs (sb)0.3183099 sb
apostilbs (asb)10000 asb
foot-lamberts (fL)929.0304 fL

Luminance conversions