Problem graphing Irradiance and distance experiment

[SuchBants]

Homework Statement

Used a solar cell and a ray box, altering the distance from the solar cell to the light source in 10cm increments.
Did the usual high school experimental controls and repeats.
Assuming that the voltage is proportional to the irradiance, I am meant to graph the results.

Homework Equations

[/B]
I₁d₁²=I₂d₂²

I = k / d^2

The Attempt at a Solution

I graphed the results:
http://imgur.com/a/Pc70V[/B]

But as you can see, the inverse square graph isn't a straight line and I don't know why. Could it be my results or am I graphing it wrong?

 

[berkeman]

Homework Statement

Used a solar cell and a ray box, altering the distance from the solar cell to the light source in 10cm increments.
Did the usual high school experimental controls and repeats.
Assuming that the voltage is proportional to the irradiance, I am meant to graph the results.

Homework Equations

[/B]
I₁d₁²=I₂d₂²

I = k / d^2

The Attempt at a Solution

I graphed the results:
http://imgur.com/a/Pc70V[/B]

But as you can see, the inverse square graph isn't a straight line and I don't know why. Could it be my results or am I graphing it wrong?

What did you get when you turned the light source off?

 

[SuchBants]

What did you get when you turned the light source off?

I removed as much background light as I could so the base reading was 0V, I checked that it returned to 0V between each distance when the light source was off

 

[berkeman]

I removed as much background light as I could so the base reading was 0V, I checked that it returned to 0V between each distance when the light source was off

Can you say more about your "light box"? How well were reflections attenuated?

 

[SuchBants]

Can you say more about your "light box"? How well were reflections attenuated?

I don't know, it's just an average high school ray box with no filter or slits attached. So it's not a point light source

 

[berkeman]

I don't know, it's just an average high school ray box with no filter or slits attached. So it's not a point light source

Can you post a picture or a drawing? Can you see how there might be multiple paths that the source light can take to get to your detector (via reflections)? Would something like that lead to less than the ideal drop-off as you got farther away? If so, how?