# Apostilb

The apostilb is an obsolete unit of luminance. The SI unit of luminance is the candela per square metre (cd/m$^2$). In 1942 Parry Moon proposed to rename the apostilb the blondel, after the French physicist André Blondel. The symbol for the apostilb is asb.
The apostilb is defined in terms of another unit of luminance, the stilb (sb):
    1 asb = 1/π ⋅ 10$^{−4}$ sb
    π asb = 1 cd/m$^2$
Units of luminance :
| vte | vte | cd/m$^2$ (SI unit) ≡ nit ≡ lm/m$^2$/sr | stilb (sb) (CGS unit) ≡ cd/cm$^2$ | apostilb (asb) ≡ blondel | bril | skot (sk) | lambert (L) | foot-lambert (fL) = 1 ⁄ π cd/ft$^2$ |
| 1 cd/m$^2$ | = | 1 | 10$^{−4}$ | π ≈ 3.142 | 10$^7$ π ≈ 3.142×10$^7$ | 10$^3$ π ≈ 3.142×10$^3$ | 10$^{−4}$ π ≈ 3.142×10$^{−4}$ | 0.3048$^2$ π ≈ 0.2919 |
| 1 sb | = | 10$^4$ | 1 | 10$^4$ π ≈ 3.142×10$^4$ | 10$^{11}$ π ≈ 3.142×10$^{11}$ | 10$^7$ π ≈ 3.142×10$^7$ | π ≈ 3.142 | 30.48$^2$ π ≈ 2919 |
| 1 asb | = | 1 ⁄ π ≈ 0.3183 | 10$^{−4}$ ⁄ π ≈ 3.183×10$^{−5}$ | 1 | 10$^7$ | 10$^3$ | 10$^{−4}$ | 0.3048$^2$ ≈ 0.09290 |
| 1 bril | = | 10$^{−7}$ ⁄ π ≈ 3.183×10$^{−8}$ | 10$^{−11}$ ⁄ π ≈ 3.183×10$^{−12}$ | 10$^{−7}$ | 1 | 10$^{−4}$ | 10$^{−11}$ | 0.3048$^2$×10$^{−7}$ ≈ 9.290×10$^{−9}$ |
| 1 sk | = | 10$^{−3}$ ⁄ π ≈ 3.183×10$^{−4}$ | 10$^{−7}$ ⁄ π ≈ 3.183×10$^{−8}$ | 10$^{−3}$ | 10$^4$ | 1 | 10$^{−7}$ | 0.3048$^2$×10$^{−3}$ ≈ 9.290×10$^{−5}$ |
| 1 L | = | 10$^4$ ⁄ π ≈ 3183 | 1 ⁄ π ≈ 0.3183 | 10$^4$ | 10$^{11}$ | 10$^7$ | 1 | 0.3048$^2$×10$^4$ ≈ 929.0 |
| 1 fL | = | 1 ⁄ 0.3048$^2$ ⁄ π ≈ 3.426 | 1 ⁄ 30.48$^2$ ⁄ π ≈ 3.426×10$^{−4}$ | 1 ⁄ 0.3048$^2$ ≈ 10.76 | 10$^7$ ⁄ 0.3048$^2$ ≈ 1.076×10$^8$ | 10$^3$ ⁄ 0.3048$^2$ ≈ 1.076×10$^4$ | 10$^{−4}$ ⁄ 0.3048$^2$ ≈ 1.076×10$^{−3}$ | 1 |

## Table of SI photometry quantities for reference

SI photometry quantitiesvte :
| Quantity | Quantity | Unit | Unit | Dimension | Notes |
| Name | Symbol | Name | Symbol | Dimension | Notes |
| Luminous energy | Q$_v$ | lumen second | lm⋅s | T⋅J | The lumen second is sometimes called the talbot. |
| Luminous flux, luminous power | Φ$_v$ | lumen (= candela steradian) | lm (= cd⋅sr) | J | Luminous energy per unit time |
| Luminous intensity | I$_v$ | candela (= lumen per steradian) | cd (= lm/sr) | J | Luminous flux per unit solid angle |
| Luminance | L$_v$ | candela per square metre | cd/m$^2$ (= lm/(sr⋅m$^2$)) | L$^{−2}$⋅J | Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit. |
| Illuminance | E$_v$ | lux (= lumen per square metre) | lx (= lm/m$^2$) | L$^{−2}$⋅J | Luminous flux incident on a surface |
| Luminous exitance, luminous emittance | M$_v$ | lumen per square metre | lm/m$^2$ | L$^{−2}$⋅J | Luminous flux emitted from a surface |
| Luminous exposure | H$_v$ | lux second | lx⋅s | L$^{−2}$⋅T⋅J | Time-integrated illuminance |
| Luminous energy density | ω$_v$ | lumen second per cubic metre | lm⋅s/m$^3$ | L$^{−3}$⋅T⋅J | |
| Luminous efficacy (of radiation) | K | lumen per watt | lm/W | M$^{−1}$⋅L$^{−2}$⋅T$^3$⋅J | Ratio of luminous flux to radiant flux |
| Luminous efficacy (of a source) | η | lumen per watt | lm/W | M$^{−1}$⋅L$^{−2}$⋅T$^3$⋅J | Ratio of luminous flux to power consumption |
| Luminous efficiency, luminous coefficient | V | | | 1 | Luminous efficacy normalized by the maximum possible efficacy |
| See also: SIPhotometryRadiometry ||||||
* ↑ The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
* ↑ Standards organizations recommend that photometric quantities be denoted with a subscript "v" (for "visual") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
* 1 2 3 Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.