Understanding Intensity in Laser Scanning

1. A new laser scanning reference text by Shan and Toth appears to be a must read.
2. Intensity is often misused in the laser scanning industry.

3. There are a number of factors that affect the intensity of the return signal.

I have just received what appears to be an excellent laser scanning technical resource, Topographic Laser Ranging and Scanning, by Jie Shan and Charles Toth. I mentioned this a few weeks ago. More on this once I have a chance to review.

One term that I could not find discussed in much detail was intensity, so I checked with my ASTM E57.04 colleague Kevin Ackley from Course Six, Inc. he provided me with the following insights:

The term intensity, I think, is frequently misused. In radiometry (measurement of light) it means irradiance, which is optical power per unit area (watts per square meter). The imprecise definition, which most people are using is : how much light (watts) did the instrument receive.

There are a number of factors that affect ” intensity”:

1.  How much light was sent out?
2. Range to the target
3. Attenuation of beam by atmosphere
4. Reflectance of target (at the appropriate wavelength)
5. Incident angle of light on the target surface
6. Size of receiver aperture
7. Efficiency of receiver optics at focusing the return light onto receiver sensor
8. Specularity of the target surface
   

An instrument manufacturer would want to compensate for some of these effects so that the  “intensity” number is more meaningful. Items 1, 6, and 7 are the easiest – they are internal to the instrument. Item 2 should be known after the range calculation is complete. But items 3, 4, 5, and 8 are dependant on conditions remote from the instrument. My guess is that most instruments would compensate for 1, 2, 6, and 7. The remaining factors are what the “intensity” is really characterizing.

Side note: Surprisingly (to me), the angle of incidence of the transmitted beam onto the target surface does effect the return signal strength (item 5), This assumes a monostatic configuration (co-located transmit and receive). The return power is proportional to cosine of incident angle. So as the surface normal of the target tilts away from the direction of the incoming light the return power is reduced.

Thanks Kevin. That helps to understand why intensity is not all that valuable an attribute.