<Tr> <Td> Spectral intensity </Td> <Td> or </Td> <Td> watt per steradian per hertz or watt per steradian per metre </Td> <Td> W ⋅ sr ⋅ Hz or W ⋅ sr ⋅ m </Td> <Td> M ⋅ L ⋅ T or M ⋅ L ⋅ T </Td> <Td> Radiant intensity per unit frequency or wavelength . The latter is commonly measured in W ⋅ sr ⋅ nm . This is a directional quantity . </Td> </Tr> <Tr> <Td> Radiance </Td> <Td> </Td> <Td> watt per steradian per square metre </Td> <Td> W ⋅ sr ⋅ m </Td> <Td> M ⋅ T </Td> <Td> Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area . This is a directional quantity . This is sometimes also confusingly called "intensity". </Td> </Tr> <Tr> <Td> Spectral radiance </Td> <Td> or </Td> <Td> watt per steradian per square metre per hertz or watt per steradian per square metre, per metre </Td> <Td> W ⋅ sr ⋅ m ⋅ Hz or W ⋅ sr ⋅ m </Td> <Td> M ⋅ T or M ⋅ L ⋅ T </Td> <Td> Radiance of a surface per unit frequency or wavelength . The latter is commonly measured in W ⋅ sr ⋅ m ⋅ nm . This is a directional quantity . This is sometimes also confusingly called "spectral intensity". </Td> </Tr> <Tr> <Td> Irradiance Flux density </Td> <Td> </Td> <Td> watt per square metre </Td> <Td> W / m </Td> <Td> M ⋅ T </Td> <Td> Radiant flux received by a surface per unit area . This is sometimes also confusingly called "intensity". </Td> </Tr>

Who discovered that light travels in a straight line