Understanding apostilbs to lamberts Conversion
The apostilb (asb) is a metric luminance unit equal to 1/π cd/m², built on a perfectly diffusing surface. The lambert (L) is a larger CGS luminance unit equal to 1/π candela per square centimetre, or 10,000 apostilbs. Because both share the 1/π diffuse-luminance basis but differ by the metre-to-centimetre area scaling, converting apostilbs to lamberts is a clean factor of ten-thousandths.
Conversion Formula
To convert apostilbs to lamberts, multiply by this factor:
Step-by-Step Example
Convert 25 apostilbs to lamberts.
How to Convert apostilbs to lamberts
Convert metric apostilb luminance to CGS lamberts with a simple factor of ten-thousandths.
- Take the apostilb value: Begin with your luminance, for example 25 asb.
- Multiply by 0.0001: Equivalent to dividing by 10,000, the apostilb-to-lambert factor.
- Read the result: 25 × 0.0001 = 0.0025 lamberts.
- Reverse if needed: Multiply lamberts by 10000 to return to apostilbs.
apostilbs to lamberts conversion table
| apostilbs (asb) | lamberts (L) |
|---|---|
| 0 | 0 |
| 1 | 0.0001 |
| 2 | 0.0002 |
| 3 | 0.0003 |
| 4 | 0.0004 |
| 5 | 0.0005 |
| 6 | 0.0006 |
| 7 | 0.0007 |
| 8 | 0.0008 |
| 9 | 0.0009 |
| 10 | 0.001 |
| 15 | 0.0015 |
| 20 | 0.002 |
| 25 | 0.0025 |
| 30 | 0.003 |
| 40 | 0.004 |
| 50 | 0.005 |
| 60 | 0.006 |
| 70 | 0.007 |
| 80 | 0.008 |
| 90 | 0.009 |
| 100 | 0.01 |
| 150 | 0.015 |
| 200 | 0.02 |
| 250 | 0.025 |
| 300 | 0.03 |
| 400 | 0.04 |
| 500 | 0.05 |
| 600 | 0.06 |
| 700 | 0.07 |
| 800 | 0.08 |
| 900 | 0.09 |
| 1000 | 0.1 |
| 2000 | 0.2 |
| 3000 | 0.3 |
| 4000 | 0.4 |
| 5000 | 0.5 |
| 10000 | 1 |
| 25000 | 2.5 |
| 50000 | 5 |
| 100000 | 10 |
| 250000 | 25 |
| 500000 | 50 |
| 1000000 | 100 |
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.
Exactly, .
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.
- .
- .
- .
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.
Equivalently, . The factor of 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 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.
Frequently Asked Questions
How many lamberts are in one apostilb?
One apostilb equals 0.0001 lamberts, because a lambert is defined per square centimetre while an apostilb is per square metre, a 10,000-fold difference.
How do I convert apostilbs to lamberts?
Multiply the apostilb value by 0.0001 (or divide by 10,000). For example, 5,000 apostilbs equals 5,000 × 0.0001 = 0.5 lamberts.
How do I convert lamberts back to apostilbs?
Multiply lamberts by 10000. So 0.2 lamberts equals 2,000 apostilbs.
Why is the conversion an exact power of ten?
Both units use the same 1/π Lambertian definition, so their ratio comes purely from the square-metre to square-centimetre area factor of 10,000.
What is the lambert used for?
The lambert is a CGS luminance unit found in older optics and astronomy literature; the millilambert is often more practical for everyday luminance levels.
People also convert
Complete apostilbs conversion table
| Unit | Result |
|---|---|
| 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 |