Here is some text I found at the following link:
"
Light intensity Lab
"Light intensity is an indication of the strength of a light source. The SI unitof light intensity is the candela. The Onset HOBOs can be set to record using a variety ofdifferent units, but the most common choices are lumens per square foot or lumens persquare meter. A typical light intensity reading in a lighted room would be around 100lm/ft2(roughly 1000 lm/m2). When you move the sensor away from a point light source, you expect the lightintensity reading to decrease as you go. Specifically, light intensity should fall off as thesquare of the distance from the HOBO to the source.
For example, if you measure 2000lm/m2 with the logger 2 feet from a source, then you would expect to measure 500 lm/m2 when the logger is 4 feet from that source. If distance increases by a factor of two, then light intensity should decrease by a factor of four. The explanation for this relationship is simple. A point light source emits a given amount of power (we'll call this E for energy), which is spread out in all directions. The further from the source you go, the greater the surface area (SA) that the light source must illuminate. The power is evenly distributed across that surface area, so lightintensity at a distance r equals E/SA. Since the surface area of a sphere of radius r (theset of all points a distance r from the source) is proportional to the square of the radius(specifically, SA = 4r2), we get the equation:
Light intensity = k/r2(where k = E/4)
Thus light intensity falls off as the square of distance
Choader