Light Meters for Dummies

[WaterWitchin]

I just don't get it. I've had this light meter for several years. The grandkids love to play with it. I'm still not sure how to use it.



It's a basic HydroFarm light meter. Here's what the panel looks like...



It's hard to see, but the switch on right is simply ON/OFF. I get that part.

The other switch to the left has three settings:

0-199 200-1999 2000-5000
x1 X10 x100

I sit in same spot on my desk, no light directly hitting it, about two feet from windows:

Put it on the 0-199 (x1) it reads 80
Put it on the 200-1999 (x10) it reads 8.0
Put in on the 2000-5000 (x100) it reads .8

Put it on a windowsill with sun shining in window:

Put it on the 0-199 (x1) it reads 1 .
Put it on the 200-1999 (x10) it reads 106.3
Put in on the 2000-5000 (x100) it reads 10.62

It's a meter for footcandles. What in the heck are these three switches for, and how do I figure out what my footcandles are in a specific place?

Bought it as a fun toy to play with for the orchids. It's not fun. Possibly funny, since I don't get it. Time to figure out how it works. I know there must be a simple answer, but I'm too simple to get it. Please help. Draw me a picture if necessary ya techies.

PS Please don't go into lux and lumens and kilowatts... I'll pass out from the stress.

[Ray]

Originally Posted by WaterWitchin

The other switch to the left has three settings:

0-199 200-1999 2000-5000
x1 X10 x100

I sit in same spot on my desk, no light directly hitting it, about two feet from windows:

Put it on the 0-199 (x1) it reads 80
Put it on the 200-1999 (x10) it reads 8.0
Put in on the 2000-5000 (x100) it reads .8

Put it on a windowsill with sun shining in window:

Put it on the 0-199 (x1) it reads 1 .
Put it on the 200-1999 (x10) it reads 106.3
Put in on the 2000-5000 (x100) it reads 10.62

The three-selector switch is the scale- the first is for 0-199 fc, second for 200-1999, and you multiply the displayed number by 10, and the third for 2000-5000 fc and you multiply the displayed number by 100.

Your “80, 8, & 0.8” is a great example, as 80=8x10=0.8x100.

The reason the sunny window doesn’t follow that pattern for the lowest setting is that it’s outside the range - the other two tell you you have 1063 fc and 1062 fc, respectively, which is well beyond the 0-199 range of the lowest scale.

[WaterWitchin]

I figured if I whined you'd chime in Bill. So I add the appropriate number of zeros to each... none, one zero, or two, and it gives me footcandles. Right?

And if so... when I'm at the bottom (the 1-199)... why even have that option? Wouldn't it be easier to just have the x100 and move forward? Who wants to measure 1-199 footcandles in the first place? Or even the next level? Just a bells and whistles thing so you feel like you've gotten something for your $18?

And I'm feeling kind of de je vu-ish... like maybe you've explained this to me before, and I just didn't get it the first go-round. Regardless, thank you.

[estación seca]

Some people want to measure low light levels. This meter has a wide range, which is a selling point.

[Ray]

Originally Posted by WaterWitchin

I figured if I whined you'd chime in Bill. So I add the appropriate number of zeros to each... none, one zero, or two, and it gives me footcandles. Right?

Correct.

And if so... when I'm at the bottom (the 1-199)... why even have that option? Wouldn't it be easier to just have the x100 and move forward? Who wants to measure 1-199 footcandles in the first place? Or even the next level? Just a bells and whistles thing so you feel like you've gotten something for your $18?

The photocell has a non-linear output, and theoretically more precise in the meter’s output if in the correct scale. They don’t make that meter any longer, so I don’t know the specs, but for the sake of argument, let’s say the accuracy is +/- 5%. In the lowest scale, that would mean your reading could be as much as 10 fc off. In that highest scale, it could be as much as 250 fc off, so if used for really low light levels, your potential error is greater than the range.

It’s a cheap, unreliable meter. Let the grandkids play with it.

[WaterWitchin]

Okay, it all makes sense now. And yes, it's cheap. And of course the grandkids get to play with it! But for a difference of 250+/- footcandles, it serves its purpose for me very well, now that I know for sure how to use it.

I'm sure now it's as accurate as the ol' hand creating no shadow, a fuzzy shadow, or sharp shadow. That's all I need. When I have a bench of flowers sitting at a window, it's nice to be able to get an idea of the light the ones farthest away from the window are receiving.

If I needed more accuracy, I'd be buying a scale like the one I use for weighing ingredients for sourdough bread. Now that's important stuff!

Gracias for the lesson.

---------- Post added at 07:28 AM ---------- Previous post was at 07:24 AM ----------

PS What in the world would someone be wanting to measure really low light for?

[DirtyCoconuts]

for plants that need less than a certain amount or they burn?

id imagine that for real low light plants (or animals) the fear is too much light

[estación seca]

Photography?

[WaterWitchin]

Wouldn’t someone into photography have a better camera and/or light meter? Are there any plants that require light levels that low? Not arguing, just asking the question because it makes me curious.

[estación seca]

Installers of spy cameras? Algae growers (yes, there are algae enthusiasts)? Keepers of nocturnal animals?