does anyone use a lux meter to gauge how close they have their lights to the plants

Hi
I have a question for all you knowledgeable chaps and chapesses..
I am trying to work out the ideal lux to the top of the plant

I grow in a 60 x 60cm tent using 168 watts of T5 ( 12 x 14 watt ) their specs say the give out a total 14,000- 15,000 lumens depending on whether i use red or blue (blue is slightly weeker ) or 3600 per square foot or 90 ish lumes per watt

I have grown quite a few ladies in my time using hps/mh , cfl and even had a quick go with led
but for the last 18 months or so i been trying various auto strains in my t5 tent, atm I have an 8 week old biodeisel mass and a 2 week old la Musa.... and I smokin think different.... it's nice a definate recomend ( if I'm allowed)

I have read lots of journals but none have the answer i was looking for ......so here I am

Why ???... well.. 19 day old La Musa doesnt like any more than 20,000 lux (1900 ish lumens) ...
I notice that at about 1 week old I moved her closer to the lights (approx 25,000 lux ) and the individual segments on each leaf had narrowed ..... i moved her back to below 20,000 lux and the next segments where wide again .... as she grew a inch or 2 I could see it happening again ....

20,000 lux is considered (by wikipedia) to be daylight and direct sunlight is 35,000-150,000 lux
so I thought maybe its because the light has a better "par" than the sun ...like led is supposed to
I also thought it maybe because she is young and as she got older she would handle more lux ... but new growth is always young so this may not be the case either

so I need someone elses results using a lux meter to compare ....

BTW 20,000 lux is about 9 inches distance from lights to top of plant
25,000 6 inches
30,000 3 inches
SO ... my next question ..... is my lux meter broken or is that inverse square law or radiation a pile of bullsh....
though on reflection....hehe... i think it doesnt apply to a light tube as apposed to a light globe

I hope atleast some of this makes sense to someone ....it does to me... honest..

anyways thankyou for reading, and thanks in advance for any helpful comments...

aww.... I'm gutted no one replied ....that means I'm the only geek out there with lux meter ...

inverse square law applies to a point source of light. Relative to a bank of fluoro tubes, the lux is not dropping that fast cause your still within the area of the point source and are getting light from the sides. try measuring straight down on the edge of the t5 light. am I making sense ? hard to explain.

dont mind me

PurpleBuz said:

inverse square law applies to a point source of light. Relative to a bank of fluoro tubes, the lux is not dropping that fast cause your still within the area of the point source and are getting light from the sides. try measuring straight down on the edge of the t5 light. am I making sense ? hard to explain.

Click to expand...

I know what you mean and i agree .... it also has to do with reflectors and reflective surfaces ....
ie the law is best applied to the sun...it does not wear a hat (or reflector) and does not have any side to reflect light back ..

cheers for the input PurpleBuz

and to moggggys.....

"dont need to get complicated"

well.... lets try eh!!..... so using a T5 then you think i should be 1 inch away.... but I already said that 6 inches away was harming growth and the plant preferred 9 inches.. if i was using 1 x T5 your statement would probably be correct....but I use 12 x t5 (basicly a wall to wall light arrary) ... and this does not mean that as I have 12 lights I should be 12 inches away ...

to be honest its the same stuff I've read many times b4 ...... but it doesn't take into account that too much light is just as harmfull as not enough


thanks anyway

Bump ;?D
I just got my lux meter and I'm interested in how I should best use this - that is it sounds pretty straight forward, but what seems like and what is are often two stones of a different feather

You need to use a PAR meter, especially with all of the different types of lighting around.
and do umole/m2 measurements.

PB, thanks but since I'm stuck with what I've bought, and am using standard MH and HPS, I'm just going to have to figure out what will give me a ball park measurement to work with. I don't really understand the hard science, just a vague idea that I want an optimal amount of light exposure and then figure out the best way to use the lighting I've got. In fact, if this meter hadn't been on clearance I don't think I'd have bought it but figured it might be a handy tool to diagnose possible issues that might come up.

Go to the led section. A few people use par meters. Lumens is for humans not plants