F

#### frank

- Jan 1, 1970

- 0

What EXACTLY is steradian ? I mean the EXACT definition.

I have the following dillema:

I am trying to figure out how many lx (illumunance units) does an area

"A" receive from a distance "d" away from a ligth source (more

precisily a LED).

The LED's luminous intensity is defined as 3 cd (candelas, luminous

intensity units).

A candelas is be defined as : [luminous flux]/ steradian.

Luminous flux is defined illuminance/area; lx/m^2.

http://www.convertit.com

I understand everything mentionned above EXCEPT for the following:

From what i remember of my calculus days, a steradian is suppose to be

a solid angle whose lengthwise cross-section makes an angle of 1 rad.

Hence, a steradian is suppose to resemble a cone of infinite

length/volume. It suppose to represent a percentage of space if one

perceive space as an expanding sphere starting from a point (in this

case, the ligth source), rather than a volume of finite dimenions.

This being said, I do not see how a ligth source's luminous intensity

can be constant at all distance from it -- this is what is interpreted

if one assumes a steradian to be a solid angle as described above.

I.e

Luminous intensity: 3 [lx/m^2]/steradian

Therefore, wherever the person is within the steradian, the LED should

seem just as brigth becaue the luminous flux remains the same. Doesn't

make sense.

However, if a steradian happened to be a cone of fixed volume, whose

lengthwise cross section makes 1 rad angle, it would make more sense.

Let's assume that the cone's sides are long by 1m.

Because in this case, the 3 [lx/m^2]/steradian would imply that at 1m

distance from the ligth source, the luminous flux is 3 lx/m^2. At 2m

distance the luminous flux would be 4 times less (i think), and so on.

Hence the closer one is, the brigther the ligth would appear. Now this

would make sense.

So getting back to the original question: What EXACTLY is steradian ?

Any help would be much appreciated.

Thx

-Frank