If you do a google search you will find several references on the procedure. I can comment it is not very accurate, but if you are just looking for approximate readings you can use for comparisons of light conditions, it is a simple tool you can use.
Good online tool at Firstrays. http://www.firstrays.com/measurelight.htm Here is what I have a on record as the general procedure and it works OK for comparative readings. If you understand exposure compensation you can extend the range. Set the film speed at ASA 25 and the shutter speed at 1/60 second. Aim the camera at a white sheet of paper in the greenhouse. Get close enough so the meter records only the light reflected from the paper. Focus on the paper and adjust the f-stop until a correct exposure shows in the light meter of the camera. F-stops will equal approximate footcandles as follows: F-stop Foot-candles 2 100 2.8 200 4.0 400 5.6 800 8.0 1600 11 3200 16 6400 I have found lots of cheap light meters around that give more accurate readings than using your camera or a hand held light meter. This is similar to one of the inexpensive meters I have, you can find them for about $20 to $30 online. http://www.growerssupply.com/farm/supplies/ prod1;gs1_monitoring_meters;pg103400.html --Tom /----------------------------------------------------------------------- | Tom Hillson Orchid Grower Specializing | [EMAIL PROTECTED] In Paphs and Pleurothallids | http://www.orchids.iastate.edu ------------------------------------------------------------------------ |"There is always room for one more Orchid!!" On May 11, 2007, at 9:21 AM, [EMAIL PROTECTED] wrote: > Many years ago, there was a simple method to determine light > intensity in > foot-candles for plants in different greenhouse locations, using a > Weston > Master III light meter. Oldsters (like me) remember when these > light meters were > a mainstay in camera bags before cameras with built in light > meters. Does > anyone remember the simple formula for converting the numeric > reading of the > Weston meter to obtain an estimate of the foot-candles of light > reaching the > plants? > > As I remember, the method for greenhouses involved holding a sheet > of white > paper over the plants, taking a reflected light reading off the > paper, then > multiplying the numeric reading from the meter by a factor of ?? to > give a > good estimate of the foot-candles of light the plants were > receiving. The > method had the advantage of using a reflected light reading meter > rather than > requiring a direct light reading meter that was pointed toward the > light source. > The numeric scale on the Master III meter avoided having to use > the ASA film > speed, shutter speed, and f-stop to make a complicated conversion to > foot-candles required when using most of the other brands of > camera light meters. > > I used the method long ago when I built my first glass > greenhouse. With > time and experience, I knew which plants would receive the > appropriate amount of > light in various locations. Now, with a newly constructed > greenhouse using > a twin-wall polycarbonate cover, I have no idea what the light > levels are. My > old light meter still functions perfectly but I do not remember > the formula. > > > If anyone can refresh my memory on the procedure, it will be very much > appreciated. Thank you in advance. > > > > > > ************************************** See what's free at http:// > www.aol.com. > _______________________________________________ > the OrchidGuide Digest (OGD) > [email protected] > http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com _______________________________________________ the OrchidGuide Digest (OGD) [email protected] http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
