Using a Photographic Exposure Meter as a Laser Power Meter
In the old days, before dinosaurs and even before digital cameras, photographers carried around exposure meters. They are compact and handy, and probably now gathering dust in the forgotten corner of a closet somewhere.
These are the type that are aimed either at the scene to be photographed or in the direction of the illumination. There have been many types manufactured over the years and all *are* basically measuring light intensity one way or another. However, measuring the power in a laser beam is not quite the same thing. In particular, the reading should be independent of the spot size to the greatest extent possible. And, of course, there are those trivial issues of wavelength. :)
As a test, I tried using an old General Electric exposure meter based on a selenium cell that requires no batteries. This thing is so old that there isn't even a model number. The readout is in Foot Candles. For measuring laser power, some arbitrary conversion factor would be needed for each wavelength. There is a frosted plate over the actual sensor and the response doesn't matter much where the laser is aimed. Similar exposure meters that have a fly's eye or honeycomb lens in front of the sensor are very sensitive to spot position and useless without further work.
However, the spot size definitely affects the sensitivity, possibly by more than 2:1. So, unless you can standardize on spot size, any readings would be quite questionable. For example, shining the laser on an oblique angle produces a much *higher* reading, as does reflecting the beam from a mirror 5 feet away (due to the larger spot, even though there is a loss hitting the mirror).
