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The steradian (symbol: sr) or square radian [1] [2] is the unit of solid angle in the International System of Units (SI). It is used in three dimensional geometry , and is analogous to the radian , which quantifies planar angles .
The magnitude of an object's solid angle in steradians is equal to the area of the segment of a unit sphere, centered at the apex, that the object covers. Giving the area of a segment of a unit sphere in steradians is analogous to giving the length of an arc of a unit circle in radians. Just as the magnitude of a plane angle in radians at the ...
The SI unit of radiant intensity is the watt per steradian (W/sr), while that of spectral intensity in frequency is the watt per steradian per hertz (W·sr −1 ·Hz −1) and that of spectral intensity in wavelength is the watt per steradian per metre (W·sr −1 ·m −1)—commonly the watt per steradian per nanometre (W·sr −1 ·nm −1).
1: intensive, scalar Reluctance: resistance to the flow of magnetic flux H −1: L −2 M −1 T 2 I 2: scalar Solid angle: Ω: Ratio of area on a sphere to its radius squared steradian (sr) ∠ 2: Specific energy: Energy density per unit mass J⋅kg −1: L 2 T −2: intensive Specific heat capacity: c: Heat capacity per unit mass
The solar flux unit is a unit of spectral irradiance equal to 10 −22 W⋅m −2 ⋅Hz −1 (100 yW⋅m −2 ⋅Hz −1). The nox (nx) is a unit of illuminance equal to 1 millilux ( 1 mlx ). The nit (nt) is a unit of luminance equal to one candela per metre squared ( 1 cd⋅m −2 ).
The 26th General Conference on Weights and Measures (CGPM) redefined the candela in 2018. [10] [11] The new definition, which took effect on 20 May 2019, is: The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, [a] K cd, to be 683 when expressed in the unit lm W −1, which is equal to cd sr W −1 ...
The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.4 × 10 14 hertz and that has a radiant intensity in that direction of 1 / 683 watt per steradian. cd: SI: Physics: Relational: radian [n 4] rad: plane angle: m/m: 1 SI: Physics: Relational: steradian: sr: solid angle: m 2 /m 2: 1 ...
Analogous to one degree being equal to π / 180 radians, a square degree is equal to ( π / 180 ) 2 steradians (sr), or about 1 / 3283 sr or about 3.046 × 10 −4 sr. The whole sphere has a solid angle of 4 π sr which is approximately 41 253 deg 2 :