Question regarding light meter readings (Sun vs. Florescent) - Orchid Board

Paul Mc · #1 02-01-2011, 08:30 AM

I just bought a light meter, and I'm not understanding some things. There is a switch between Sun/HID and Florescent. While I do understand that even Florescent lighting can affect plant growth (i.e. leaving it on all night), I don't understand why there are two settings. Is the footcandle reading for the Florescent supposed to be read exactly the same as the Sun/HID setting? Any calculations to change the Florescent reading to the Sun/HID?

Please be in mind that I am technically and terminology wise challenged, so dumb down your answers for me, lol...

Ray · #2 02-01-2011, 10:56 AM

It probably has to do with the sensitivity of the sensor to specific wavelengths of light, so they are trying to provide some compensation.

For example, a typical metal halide HID light puts out about 4x-5x the amount of red as it does blue (in the chlorophyll absorbing regions), while direct, noontime sunlight has only about a 10%-20% boost in the red over the blue intensity.

A 6400°K fluorescent bulb is mean to to replicate the spectrum of the direct sun plus the back-scatter of the blue sky, so has a spectrum that is about 15%-20% stronger in the blue area than the red - which by the way, most closely replicates the relative absorption by chlorophyll.

If my logic in this is correct, then the color temperature of the fluorescent bulb would have an effect, suggesting that there should be different "fluorescent" settings.

Paul Mc · #3 02-17-2011, 08:47 AM

Ah, that makes perfect sense to me! Thank you again Ray!

susiep · #4 02-17-2011, 10:21 AM

Thanks, Ray! I have a meter like yours Paul and I always wondered why we needed that setting. Now I know!

Connie Star · #5 02-17-2011, 10:56 PM

Susie P and Paul Mc, do you get different readings from the same light source if you flip the switch?

susiep · #6 02-18-2011, 11:39 AM

Yes, Connie. If it measures 5,000 in light shade on the "Sun" setting and I flip the switch, it is off the top of the measurement range on the "Fluorescent" setting.

Connie Star · #7 02-19-2011, 09:02 PM

Quote:

Originally Posted by susiep

Yes, Connie. If it measures 5,000 in light shade on the "Sun" setting and I flip the switch, it is off the top of the measurement range on the "Fluorescent" setting.

Hmmm. It sounds like it has some way of reading the wavelengths.