help me understand this about PAR readings

so I've noticed while browsing par readings from commercial lights that the far edge reading is always higher when the light is further away. For example on a 4x4 the corner edge will read 64 when 18" from canopy and 106 when 24" from canopy.

I have drawn a picture if it helps. Considering physics (the inverse square law) and that the X distance is bigger than the Y distance it should be the other way around shouldnt it ?

Are these readings like this because the light is hitting the sensor with a smaller angle when closer (18") (more parallel) thus fooling the sensor ? I don't get it

I would think the outside limit is closer to the COB under 18" and the outside reading is lower because it is not being hit directly with any light density. Your footprint decreases as you drop the light down.

I would think it is actually a bit of both. The ops diagram is certainly a valid point...... Hmmm I don't think it makes up for 100% of the increase but the angle could slightly skew the results.

can someone with a meter do a test and hold the sensor perpendicular to the light source in a scenario like this ...see if that matters ?

davillains said:

can someone with a meter do a test and hold the sensor perpendicular to the light source in a scenario like this ...see if that matters ?

Click to expand...

So what needs to be fine is the meter needs to maintain the same angle to the light to give accurate readings when comparing mounting hieghts.....

its because leds radiate more light along the "X" angle than they do along the "Y" angle

A good light/quantum sensor should have cosine correction built in to correct as much as possible for the angle of incidence of the light. That's done with the diffuse white "bit" on top.