Properly using a light meter

[Decisive]

Hey guys, yesterday I picked up one of these hydrofarm digital light meters:



I have a vertical room with 8 1000w lights and have been having a whole bunch of trouble getting the light distance/ number of lights on for new clones right. In an attempt to do so i purchased the above light meter and am quite puzzled with the readings. I have 2 of the 1000w HPS lights on and the clones are about 3 feet in a diagonal away from the lights here are my readings:

at the top of the clones with both lights on (about 3 feet from lights in a diagonal) = 600-800 foot candles

In my backyard where the sunlight is pretty intense (direct sunlight) = 7200-8500 foot candles

In my backyard in the shade (pretty dim to the eye no direct light) = 300-400 foot candles

These readings really confuse me because the room is really bright with the 2 HPS lights on whereas the readings in the shade outside where it is quite dim give similar readings. How can something so bright give similar readings to something so dim. I notice in the pdf I linked above that there is a graph with relative sensitivity % vs wavelength. Is the fact that I am measuring daylight going to affect the footcandle readings vs measuring the orange light of HPS.

In other words is the difference in spectra affecting footcandle readings where the meter is not as sensitive to the wavelengths the HPS emits the most light at? Its the only explanation I can think of, was wondering if anyone else has any other explanations.

 

[Seamaiden]

You need GanjaGardener to see this thread. But what you also need to understand is that footcandles is another term for lumens, lumens only speak to what the human eye perceives, and sunlight is much, much stronger than man-made light in many, if not most, cases. We are often fooled by our perceptions, too, so while you may perceive the shade to be darker than the HPS lighting, the plants may not.

Also, go look up PAR (Photosynthetically Active/Available Radiation, can't remember what the A is for).

 

[Papa]

But what you also need to understand is that footcandles is another term for lumens, lumens only speak to what the human eye perceives, and sunlight is much, much stronger than man-made light in many, if not most, cases.

actually sm, footcandles is not another term for lumens.

Lumens are the unit of measurement used to quantify the total amount of visible light emitted by a source.

Footcandles and lux are units of measurement to quantify the amount of light at a specific point.

i'm trying to think of an analogy for you . . . but they all suck.

Decisive, your readings make sense. our eyes are actually really terrible at attempting to measure the amount of light. our pupils adjusting make them ever variable. and actually, our eyes don't see footcandles (or amount of light), they see brightness - or the light reflecting off a surface.

for instance, imagine two identical rooms, each with four downlights. but one room is painted black and the other white . . . the white room will seem to have much more light, it may in fact, seem bright. the black room will seem dark. but if you put your light meter in the center of each room the readings will be about the same [reflected light off the white walls may give you a bit more light in the white room].

now, if you were to sit in a dark closet for an hour before entering the black room, it would seem bright enough to walk around in safely. but if you were to sit in direct sunlight at the beach for an hour before entering the white room, until your pupils adjusted, it would seem so dark that you might not feel safe walking around in the room.

your readings seem correct. i'm in socal, and direct summer sun is about 10,000 footcandles, but i can find dark shade behind the garage that's only 300 footcandles.



Papa

 

[Seamaiden]

Papa, everything I've read and come to understand about footcandles versus lumens versus lux demonstrates that they're still measuring the same thing--perceivable light.

A lumen = the amount of light 1 footcandle on 1'square of area.

I wonder if what you're driving at is radiance and illuminance, which might be more analagous (even though they really seem to be measuring the same thing, just using different metrics).

However, I'm stopping at this point because this is an area of particular expertise for GanjaGardener.