# Light through a window

1. Mar 21, 2011

### bobobano

1. The problem statement, all variables and given/known data
A lamp producing 1600 lumens is 16" away from a window of 0.015m^2 on a wall 0.14m^2, what is the amount of light through the window.

2. Relevant equations
lux = lumens/m^2

3. The attempt at a solution
Since there's no material in the window, shouldn't it be the full 1600 lumens seen inside?

2. Mar 21, 2011

### Staff: Mentor

Welcome to the PF.

I think the point of the question is that the light source is isotropic, so only a fraction of the total light goes through the area of the window. How can you calculate the fraction of the total area that the window represents (hint -- use the distance to the window for something...)

3. Mar 21, 2011

### bobobano

am I correct in thinking that if I multiply the lux over the area, from the lamp to the window, by the window's area I'll get the lumens through the window?

Light through window = (light from lamp)/(pi*distance2)*(window surface area)

4. Mar 21, 2011

### Staff: Mentor

It is some area ratio, but on re-reading the question, maybe the light source is not isotropic? They mention a wall and a window, so I'm not sure whether to ratio the area of the window to the wall or to an isotropic sphere. Is there a picture that goes with the question, or else are you able to understand the question well enough to answer it now?

5. Mar 21, 2011

### bobobano

I dropped him an e-mail and he says to consider it as a sphere.

6. Mar 21, 2011

### Staff: Mentor

Okay, so you know what to ratio then?

7. Mar 21, 2011

yea.