How to calculate light intensity without a light meter

Discussions about fluorescent, LED and other types of grow lighting for Venus Flytraps and other plants
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Leilani Kimiko

 
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How to calculate light intensity without a light meter

by Leilani Kimiko » Sun May 26, 2013 4:41 pm

It's easy to calculate light intensity using only an old single lens reflex (SLR) type camera with a built in light meter and the online calculator at this link:
http://www.firstrays.com/measurelight.htm

You only need three things: your camera, a piece of white paper, and the calculator.

Simply put the paper in the location you wish to measure, facing it as close to perpendicular to the light source as possible, and point the camera at it so it fills the frame completely - trying to shoot as close to perpendicular to the paper as possible without creating a shadow. Adjust the aperture so that you get a decent shutter speed, and then use the ISO setting and those values in the calculator to determine the foot-candles of light intensity. (Note that these numbers are not exact, but certainly give a "ballpark" figure to work with.)

For example:

ISO = 100
Shutter Speed = 1/1000 second
(Ignore all the extra periods between columns, I just needed them as spacers.)

f stop......fc.......lux......lumens/sq. ft.
f/1.4......125....1345........125
f/2........250.....2690........250
f/2.8 .....500.....5380........500
f/4.......1000....10760......1000
f/5.6.....2000....21520......2000
f/8.......4000....43040......4000
f/11......8000....86080......8000
f/16....16000...172160.....16000

I made a graph of the data so I can read intensity right from the graph without having to be online when I made a measurement.
I used this a lot before I bought an actual light meter.

I posted this in my blog also.
Last edited by Leilani Kimiko on Wed May 29, 2013 8:47 pm, edited 1 time in total.

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Veronis
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Re: How to calclate light intensity without a light meter

by Veronis » Sun May 26, 2013 6:19 pm

Awesome.

+sticky

Here's an old one specifically for carnivorous plants - same method: http://www.sarracenia.com/faq/faq3400.html

Keep in mind he's using lights equivalent to probably fluorescent T12's or T8's; this article is many years old.

100% "Barry Factor" = 500 lumens per square foot (1 foot candle = 1 lumen per square foot).

Barry's wife's plants get 350 lumens per square foot.

To put it in perspective, a standard T5HO setup can output somewhere around 1200-2000 lumens per square foot (2-bulb T5
s on the lower end, 4-bulb T5's on the upper end), but even 1000 or so should get you some very good growth.

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cyph3r_gfy
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Re: How to calculate light intensity without a light meter

by cyph3r_gfy » Thu May 30, 2013 2:19 pm

great data you two, thank you!

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Re: How to calculate light intensity without a light meter

by zendog » Sun Jan 12, 2014 4:36 pm

This is great info for those using fluorescents, but I'm wondering if there is an equivalent way of measuring LED lights with different colors and determining appropriate lumens/sqft. For instance, I'm trying out these for a small setup:
http://www.ebay.com/itm/171202640168

I use my Nikon to take shots of the light and then look at the shots in iPhoto, I can look at exposure of the different Red,Green and Blue levels in the histogram. In pure white light the red, green and blue would show similarly in a histogram, but the LED has different color diodes so the colors aren't symetrical at all. So if I just look to see full exposure of the blues, which are the highest total in these lights, I think I'm at around 400-500 lumens per bulb, but if I want to bring it up so that the red is fully exposed (and blue is now overexposed), I'm down around 250 lumens, maybe less. But of course, the idea of the LEDs is that you are using light that is best suited to photosynthesis, so not creating lumens that are "wasted". If I use the Barry method in the blog and just go off total exposure letting the camera autoexpose, I get around 60% with one bulb and 100% if I use 2 bulbs to illuminate the same area. So not exactly the same results but in the ball park. Right now I'm thinking that I'll need to overlap the two bulbs to make sure I am getting enough light on the plants, but am also wondering if the specific wavelengths of light approach of the LEDs mean that each one is providing enough light... I also realize that different manufacturers of LED lights seem to mix their colors differently so it would be hard to have a definitive rule.

Anyway, I'm sure the proof will be in the growing once our plants come out of dormancy, but thought I'd raise this question to see if anyone had insight.

Thanks,
Walter

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Leilani Kimiko

 
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Re: How to calculate light intensity without a light meter

by Leilani Kimiko » Sun Jan 12, 2014 6:02 pm

Hi,
I can't help you with your intensity measurements using LEDs vs fluorescent lights, but I do have an opinion about the wavelengths involved. Chlorophyll has an absorption spectrum with wide bands because of the size and type of molecule and it's location in leaves. There are wavelengths at which it absorbs best, but it doesn't absorb 'only' at those wavelengths. LEDs have very narrow wavelength spikes so you are correct to question the amount of light you can get from them. While the chlorophyll can be absorbing all that LED light, it could also be absorbing nearby wavelengths as well. Therefore, by using a light with a broader spectrum, more like real sunlight, you can maximize chlorophyll absorption more easily and with less intensity of light. Fluorescent lights have broad spectra. Arc lights have some broadening of their emission peaks due to how they are made, but mostly they have narrow peaks. So, I feel fluorescent lights are better for a broader spectrum, but LEDs are best for electrical efficiency. There's always a trade off. I've included a pic of chlorophyll's absorption spectra.
Attachments
Absorption Spectra of Chlorophyll a & b.jpg
Chlorophyll A and B absorption spectra
Absorption Spectra of Chlorophyll a & b.jpg (31.98 KiB) Viewed 9493 times

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Re: How to calculate light intensity without a light meter

by HeliamphoraWalnut » Tue Sep 29, 2015 3:58 am

Wow, sticky this! Very interesting info!
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Re: How to calculate light intensity without a light meter

by roarke » Tue Sep 29, 2015 9:51 am

Page down !

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Re: How to calculate light intensity without a light meter

by alstonamos » Mon Apr 11, 2016 11:47 am

somewhere around 1200-2000 lumens per square foot (2-bulb T5
s on the lower end, 4-bulb T5's on the upper end), but even 1000 or so should get you some very good growth.

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Re: How to calculate light intensity without a light meter

by davvekavve » Mon Apr 11, 2016 9:00 pm

Hello! I use a app on AppStore for it, think it's called light meter or Lux.


Skickat från min iPhone med Tapatalk

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Re: How to calculate light intensity without a light meter

by Aozora » Sat Oct 01, 2016 6:11 pm

i just downloaded the ''light meter'' app from the play store, just open the app and place it on the surface you want to measure and it will tell you the light intensity and color temperature.

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Re: How to calculate light intensity without a light meter

by roarke » Mon Oct 03, 2016 10:21 pm

davvekavve wrote:Hello! I use a app on AppStore for it, think it's called light meter or Lux.


Skickat från min iPhone med Tapatalk


By who ?
i just downloaded the ''light meter'' app from the play store, just open the app and place it on the surface you want to measure and it will tell you the light intensity and color temperature.


By who ?

The original link from this topic can be found at archive.org

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https://web.archive.org/web/20130129134715/http://www.firstrays.com/measurelight.htm

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Re: How to calculate light intensity without a light meter

by Chesser » Sun Dec 30, 2018 6:52 pm

Thanks this was really helpful and I’ll definitely be using it again.
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