Lighting: Measuring Light Output with an Exposure Meter

OK, I am sure some of you have done this, But here is what I got.

1st, i'm a retired still photographer, and I have a lot of lighting (photographic) Experience. To get my readings I used a sekonic L-718 digimaster light meter that gave me a EV (exposure value) reading. This was a $250 meter when it was new, and in general very accurate. It is not a cheap $40 dollar garden center or camera shop meter. I then used the International Light Converter website. For general purposes, LUX and lumens are about the same thing except in their application. I am using a brand new Lumatek 600w (dimmable) ballast switched to 110% and a week old Hortilux 600w SuperHPS bulb in a Yield Master II 6" air-cooled reflector.

I first measured about 6" below the glass and got a reading of 15.4 or about 109k lumens. I then measured at my tops and got an average EV of 14.7-15.1 or 62.5k lumens to 89k lumens.

First I found it VERY interesting that I read 109k lumens off the bulb. The bulb is rated at 90k lumens so i checked the calibration on my meter and it was dead on. I wonder how they get the 90K lumens number? It has to be a number average from a different distance or different period in the bulbs life.

I am now excited to try it under different hoods, with different ballasts and with different wattages. If anyone else has has a similar experience PLEASE give your feedback.

Thanks!

Anyone got any feedback?

I was looking for a chart I saw. your about the same as the chart. It said something like there is unuseable light from bulb to 4" out. think it said 12-24" was best for the plants. or to get the best out of your light. I keep mine 8-10 off the tops.

Im not very experienced in this field but I use a light meter. I BELIEVE (not positive) that mine measure "foot candles" and at 55 ft candles of light you actually shut down photosynthesis. So if your plants are stretching you want the tops of your plants to be over 55 in order for lower bud sites to catch up. If you keep your plant around 50 foot candles it will grow with the most light intensity without shutting down photosynthesis

When I bought the light meter this is what I was told. So this info is based off the people I bought it from

I'm not sure how the manufactures rate their bulbs, but is can't be at a really close distance, because my 3,200 lumen T8s are measuring over 5,000 at the bulb.


Also, I'm a little confused as to what you're measuring. Well, I guess what I'm saying, is...why aren't you measuring 'lumens per square foot'? I think you're conversion is wrong, if I'm not mistaken. My 600 HPS gets similar readings, except that they convert to 6,250 and 8,900 lumens per square foot, not 62,500 and 89,000. The 90,000 lumen measurement of the bulb, is for it's total output, not the output in just one direction, so I think you might have things a little mixed up.

Ahhh, I see what you did wrong...the conversion of lux to lumens is a division is 10.17(or something close to that.lol), so you have an extra zero on the end of your number. Lumens is equal to footcandles, as a direct conversion. Lux, needs converted first.

stumps said:

I was looking for a chart I saw. your about the same as the chart. It said something like there is unuseable light from bulb to 4" out. think it said 12-24" was best for the plants. or to get the best out of your light. I keep mine 8-10 off the tops.

Click to expand...

I'd love to see that chart.


I have read somewhere that reccomended distances for bulbs are something like: 1000w 18-24", 600w 16-20", and 400w 12-16"

Generally I try to go as close as absolutely possiblr with out burning the tops. For HID lights, I believe that light falls off something like 50%/per ft. That is why your reflector can be so important. It focuses / concentrates your light. It is also why you need to have a larger wattage light for a bigger hood (ex. mondo xxxl). With an oversive hood, you will loose a lot of light actually inside of the hood as it bounces around before it actually reflects out, but with a large bulb (1000w) inside a large hood you can actually have a larger effective footprint than a smaller hood where you have to have the light higher from your tops for the same coverage area.

I have a super spreader diffuser that I used with my old 1000w to reduce the hotspot. I removed it last week when I blew out the ignitor in my old ballast and the bulb that went along with it. I switched to the 600w system and removed the superspreader. I am going to reinstall it and meter again for hot spots and hopefully see if there are any large differences.

Interestingly the brightest readings I am currently getting are where the edge of the reflective surface meets the glass and not directly under the bulb as logic would say.

If you have a light meter, that eliminates the guessing game. You should have your light set so that you have a minumum of 3,000 lumens/footcandles at the outermost plants(at the tops), and the higher the number, the better.(the sun puts out 10,000 max)

searched up and down. Can't find it.

I have the light chart no problem,

Also Lumens is a foot candle measurment which means ammount of light 1 foot away from source

I think thats the one. if not it's close.