This guide will show you how to use an inexpensive LUX meter to check if the lighting in your grow is adequate. This is also a great resource for figuring out how high to hang your lights when dealing with multiple lights that have an overlay effect in the room.
The hallmark study on relating to grow room lighting is Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions (Chandra, Suman et al.)
I will quickly summarize the findings of this study. The graph below shows net photosynthesis at varying temperatures levels and PPFD. As you can expect there is a positive relationship between growth and amount of light. In this study the highest levels of photosynthesis are achieved at 30 degrees celsius and 1500 Photon Flux Density (PFD).
In Simple terms:
PPFD – is a measure of the amount (or more correctly density) of photons hitting your canopy. It is expressed in micromoles per second per meter squared (μmol/s/m2).
Lux – Is a measure of lumens per meter squared.
Based on the article above and my findings with growing cannabis PPFD should be between 800-1500 for a flowering plant with CO2 supplementation.
| Plant Stage | PPFD | Lux |
| Seedlings, Clones (18 hour) | Under 100 PPFD | 10,000 |
| Mild Vegetative (18 hour) | 100 PPFD – 500 PPFD | 10,000 – 35,000 |
| Agressive Vegetative (18 hour) | 400 PPFD to 600 PPFD | 28,000 – 42,000 |
| Flower (12 hour) | 500 PPFD to 900 PPFD | 34,500 – 62,000 |
| Flower w/ CO2 (12 hour) | 700 PPFD to 1500 PPFD | 48,000 – 100k+ |
The reason for converting from PPFD to the less accurate Lux is that PPFD meters are expensive (a proper quantum sensor can cost over a thousand dollars). A Lux meter (which measure lumens) is cheap and can be used in place of a PPFD meter when measuring the output of full spectrum LED boards. It is not 100% accurate as the conversion is an approximation but it is more than enough for planning out your grow room light spacing and deciding how high to hang your lights.
Note that above 1200 PPFD or 85,000 Lux the risk of light bleaching is high. If you notice any flowers starting to turn white it is a good indication that you are giving them too much light.
Also note that these numbers are just approximations. Genetic and environmental factors will play a large role. My numbers are based on trial and error in a real world cannabis DWC grow. With soil growing I would recommend reducing these numbers by a factor of 25%.
Overview
Measuring with a Lux Meter
My recommended device for measuring Lux is the Dr. Meter Lux Meter. While there are smartphone apps you can use I have found them to be unreliable at best. There are a number of Lux meters on Amazon but the Dr. Meter had the best reviews and their pH meter’s are a good mid range product so that is what I went with. I really like that the sensor is separate from the digital readout. This makes it much easier to use.
Making a Lux Map
I recommend taking readings throughout different parts of the room and plotting them out to see if there are any anomalies in your room. If you use modular trellis nets like I do taking a reading at all 4 corners and one in the centre is makes it easy to divide up your room.
Increasing Light Brightness Vs Decreasing Hanging Height
There are two ways to increase LUX if it is not high enough.
- Increase LED Board intensity via dimmer
- Hang lights closer to plants
The major question here is heat. If you need to add heat to your room I find it easier to crank the brightness and keep the lights spaced out more, rather than adding a heater to the room. Always check a VPD chart when dialling in your temperature and humidity.
Rule of Thumb for Hanging Height
To make things easy for new growers (especially if running a single LED panel) I have created a chart that has basic hanging height guidelines. Use this as a guide and if you notice any signs of bleaching or light stress move the lights further away.
If you are running multiples LED lights in a room I always recommend spending the money to pickup a lux meter.
