Trying to understand light measurements

Hi all,

I have spent the past couple of weeks learning about lighting measurements. I am beginning to get a feel for things but still have a long way to go. I understand a lot of it is referenced to human perception of light intensities and the luminous efficiency function. So we get units of lumens, lux, candela and so on.

Here is the report for my lights, maybe someone can help me understand the numbers and where they come from:

The electrical parameters make perfect sense to me. And the Efficacy does too when I multiply that value 46.13 lm/w X the power 112.4 watts it equals the Flux 5185.12 lm.

Is the Flux an absolute value? Like the total output of the fixture and not related to distance and/or area? This is where I am having trouble grasping things. Also, what is the Fe stand for?



@dbrn32 can maybe shed some “light” on this :heart_eyes:


Hi @AxisCat For cannabis, you want to look at PPFD and figure out your DLI
There is a good thread on DLI (daily light integral)

If there’s anything I can help with just tag me put a @spankyjr1 and I’ll get notified
Good luck


Are you trying to convert from Lux / CP or something? Shane from MIGRO did a good video on that, testing lower end lux meters and showing some conversion formulas.

In my opinion, given that light is the #1 most critical factor for these plants, a measuring device is not an optional piece of equipment, it is a must-have. If you have an iPhone, the Photone app has been tested to be very close to the Apogee MQ-500.


Photone app works well. I have the beta version, good tool. :+1:


@StonedCold13 , @spankyjr1 , @CurrDogg420 , @RicWrench

Hi all. Thanks for the replies. Yes, I understand that cannabis sees differently than people and hence the different units of measure and spectrum. PAR spectrum measured in PPFD umoles/m^2/s

I have seen Photone app and a u tube video from MIGRO where he shows his conversion for various Lux meters using his true par meter.

At this point I am just trying to understand the basics of how light is measured. Lumens and lux are measurements of light adjusted with the Luminosity Function, which is a subset of the PAR radiation spectrum from 400-700nm (which we are really interested in) which itself is a subset of the entire electromagnetic radiation spectrum.

What I am working on is using a multichannel color sensor that reads out power levels at various wavelengths between 400-700nm sort of like a simple spectrometer, with the eventual goal of converting these measurements to PPFD using the measured spectral power distribution (SPD) of different light sources. And even into the ultraviolet and near infrared ranges.

I am just really digging deep into the properties of light.


Have you viewed Dr Bruce Bugee video’s he goes into great detail about lighting this is only one of his

I do not have an Iphone. I have Samsung Galaxy s21 and i did download the app
photone. It seems to be working. I have used a few times. What fun this is
I feel like a cheating scientist :confused:


This guy programmed an esp32 to read for dli using tsl2591 trust me he knows how light and plants work

If he’s asking this question it’s for other reasons, let’s not slow him down



@axis umols/meter squared

Once you figured out how to do this you’ll created your own apogee quantum sensor

I’d steal their tech…

Also check out light scout


Not for nothin, but spectral radioscopy is probably a bigger topic than one might be able to find answers for on a weed forum.

It sounds like a cool project. Apogee makes a spectral radiometer, but they ain’t giving it away cheap.

My sense of the situation is, the quality of your results will be a function of the quality of your sensor. Math isn’t going to be able to fix a crummy reading. Garbage in, garbage out, as the saying goes.

Perhaps if you had an Apogee to compare against, you could then train a neural network to correct the results.


Thanks for the link… These guys are brilliant and in a whole different ballgame then I will ever play… It just makes me want to learn more.


Hi Bob,

Thanks for the hat tip!

Hi @CurrDogg420,

I agree with you 100% on needing good sensors. And at some point I will be laying down some serious money on a real instrument to measure. I want to learn more about this subject before I select something.

I am using an AMS AS7262 6 channel sensor now. Cost around $18. It is a little limited on either of the spectrum though:

My early tests have been encouraging. This is how it measured a tungsten (standard clear incandescent) bulb, something that has a well know spectral response, it is the red dashed line in my graph and tracts pretty well with the sensor.

This same company just came out with an eleven channel sensor which I should receive sometime this week. It has an extended response on either end of the spectrum as well as an infrared sensor. Again about 20 bucks.


At some point I will for sure compare my results to the photone app. I also have the s21 phone…


@AxisCat, I found a couple articles which may answer some of your questions. I haven’t found any direct conversions between Lm/LUX to PPF / PPFD, for a specific wavelength. Everything I’ve found wants an overall color temperature, which is basically the graph you posted.

Hopefully there’s something useful to glean from these.

[EDIT] Color temp is a concept from astrophysics - think “red shift”. Maybe that’s an avenue.

Perhaps! But think what Einstein would have accomplished if he had had a computer. No wasting time on doing the calculations.

Not that this is on that level…

1 Like

This is a simple question that deserves a simple answer. It should be, but that isn’t what you’re always getting. Lm or ppf should be total flux.

The spectral distribution graph is not an absolute value. It’s just showing relationships from one wavelength or color grouping to the next. It could ve shown as 0-1, 0-100, or even 0-1000 and still means exactly the same.

I disagree that you should care about ppfd as measured with any device. Not unless you’re going to measure every 3-4" across your space and average all readings. The testing i don’t really get into. I just try to buy products that i feel are provided with accurate and reliable performance specs.


I am actually very far along with the math, like @Fiz said the power of modern computing is incredible. But trust me I am no Einstein, barely average when it comes to math.

I have created a spreadsheet that runs all the calculation on each wavelength between 400-700nm. I am still rechecking my calculations but at this point I don’t see any errors in it.

I do an initial calibration of the 6 channel color sensor using a lux meter as my yard stick… that is all I have and when using light spectra heavy in the greens which is what the lux meter is really designed to measure I feel it does a decent job.

Once the initial calibration is made I can toss the lux meter and all the measurements are derived from the 6 channels on the sensor: ppfd, watts, lux

1 Like

Thanks for the reply, after doing a lil more research I have concluded it is an absolute value for all practical purposes. Some number of watts in to the light fixture is going to equal some number of lumens or PPFD out. The integrating sphere helped me understand where this lumen measurement comes from and how it relates to lux. It just counts the total number of photons.

This is all new to me and I am still in the courting stage. My problem is I am using three identical lights with poorly published specifications. I bought these before I really knew anything. So I am flying a little blind. Besides I love a good spreadsheet.

Does it help me grow better smoke? Doubtful with my current experience level. This is just one of the many variables I am trying to understand.