Leison Posted March 13, 2005 Share Posted March 13, 2005 unit problem of ILLUMINACE illuminance(E)= intensity (I) ------------------ radius®^2 also E= flux --------- area the first formula gives unit of E as cd/m^2 second gives lm/m^2 it seems that cd=lm but cd=lm/steradian mistake. where? Link to comment Share on other sites More sharing options...
swansont Posted March 13, 2005 Share Posted March 13, 2005 This stuff always confuses me a little, since I rarely use these units, but the units aren't interchangeable. cd is for uniform light emission in all directions, and lm is for light sent in a particular direction. So the cd has to define the light in terms of a solid angle of a sphere. From http://www.electro-optical.com/whitepapers/candela.htm: In plain English: The flux from a light source is equal to the intensity in candela multiplied by the solid angle over which the light is emitted, taking account of the varying intensity in different directions. And since they are units for perceived brightness, the response of the eye has to be taken into account - things are defined at a particular wavelength. Link to comment Share on other sites More sharing options...
Leison Posted March 15, 2005 Author Share Posted March 15, 2005 swansont i think u didnt understand my question i know the lm and cd but where i got confused is when i tried to equate the two formulas in units. Link to comment Share on other sites More sharing options...
swansont Posted March 15, 2005 Share Posted March 15, 2005 swansont i think u didnt understand my question i know the lm and cd but where i got confused is when i tried to equate the two formulas in units. They aren't defined in such a way that you can do that. Just like torque is N-m and energy is Joules, which is also a N-m, but torque and energy aren't the same thing. Link to comment Share on other sites More sharing options...
Leison Posted March 16, 2005 Author Share Posted March 16, 2005 i think u can't compare my problem with torque-energy because the cases are different: in torque-energy although units are same, they are different but in intensity-flux both units as well as the meanings are different but the equations shows them equal Link to comment Share on other sites More sharing options...
The Rebel Posted March 16, 2005 Share Posted March 16, 2005 the equations shows them equal I can't see this. The flux is emitted in all directions like a light bulb. This flux acts onto a surface area to give luminance. Its, if you like, a measure of how much light is received from a radial source. The intensity is more linear in that it looks at how much "energy" there is at a given distance away. In this case the r refers to the distance from the point source. The formula should actually be I = L / {4 * pi * d^2}. The 4pi indicating where the steradian is. The formulas are used in different concepts, the 1st is as it is received on to a surface, the 2nd is how the source is perceived at a certain distance. Chalk and Cheese clearly even is it is both about light. Link to comment Share on other sites More sharing options...
Leison Posted March 18, 2005 Author Share Posted March 18, 2005 i got it .thanks for all who've posted Link to comment Share on other sites More sharing options...
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