What does sr stand for here? I think it should describe the unit of solid angle? Does the formula imply that lm is equivalent to W?
andrewdcampbell
sr is "steradian", the SI unit of solid angle. It is the 3D analog of the radian. The slide is only saying that the radiometric units are W/sr while the photometric units are lm/sr - they are not equivalent.
andrewdcampbell
I don't think the picture is very helpful in demonstrating radiant intensity. It is best visualized as the power Φ [W] from a point source directed along the center of a vanishingly small cone encompassing a solid angle of Ω [sr].
julialuo
Is it possible for flux to be defined in terms of ω? So different parts of the unit sphere surrounding the light source could get different values of radiant flux? In this case, the intensity would also possibly depend on ω, so it would be important to have a standard definition of where a certain steradian value would refer to (connects to my comment on the next slide).
wangcynthia
Does the omega in the equation denote a direction vector as in slide 18?
VioIchigo
@wangcynthia yes, it should be the same. As mentioned in the class, it is a "particular direction in the world" and look at the first answer here: https://physics.stackexchange.com/questions/346817/how-to-interpret-the-radiant-intensity-equation ,it is indeed a unit vector
[deleted]
What does sr stand for here? I think it should describe the unit of solid angle? Does the formula imply that lm is equivalent to W?
sr is "steradian", the SI unit of solid angle. It is the 3D analog of the radian. The slide is only saying that the radiometric units are W/sr while the photometric units are lm/sr - they are not equivalent.
I don't think the picture is very helpful in demonstrating radiant intensity. It is best visualized as the power Φ [W] from a point source directed along the center of a vanishingly small cone encompassing a solid angle of Ω [sr].
Is it possible for flux to be defined in terms of ω? So different parts of the unit sphere surrounding the light source could get different values of radiant flux? In this case, the intensity would also possibly depend on ω, so it would be important to have a standard definition of where a certain steradian value would refer to (connects to my comment on the next slide).
Does the omega in the equation denote a direction vector as in slide 18?
@wangcynthia yes, it should be the same. As mentioned in the class, it is a "particular direction in the world" and look at the first answer here: https://physics.stackexchange.com/questions/346817/how-to-interpret-the-radiant-intensity-equation ,it is indeed a unit vector