The apostilb (asb) is an obsolete unit of luminance (luminous intensity per unit area) in the centimeter-gram-second (CGS) system . It was used to measure the brightness of a surface that emits or reflects light .
1. Formal definition of Apostilb
1 Apostilb (asb) is defined as:
In SI units the conversion is:
whereby:
- cd/m² (candela per square meter) is the current SI unit for luminance.
- 1 Apostilb ≈ 1/π Lambert , another obsolete unit for luminance.
2. Use of Apostilb
📌 Historical measurements of luminance → Formerly used in optics and lighting technology.
📌 Description of the brightness of surfaces → For example in the study of reflective materials .
📌 Replaced by the SI unit cd/m² → Rarely used today.
3. Conversion from Apostilb to other luminance units
| Unit | Ratio |
|---|---|
| 1 asb | 0.3183 cd/m² |
| 1 asb | 1/π Lambert |
| 1 cd/m² | 3.14 asb |
| 1 Lambert | 3.14 cd/m² |
4. Difference between Apostilb and Candela per m² (cd/m²)
| Characteristic | Apostilb (asb) | Candela per square meter (cd/m²) |
|---|---|---|
| System | CGS (non-SI) | SI (International) |
| Usage | Historical | Modern standard |
| Relation | 1 asb = 0.3183 cd/m² | 1 cd/m² = 3.14 asb |
💡 In short:
The apostilb (asb) was an formerly used unit for luminance , but has been replaced by the SI unit cd/m² . 1 asb = 0.3183 cd/m² , and although the term is still sometimes found in old literature, it is no longer used in modern lighting technology.