Distance from lights and DLI

I wanted to open up a thread to address a very common misconception. Mainly the misconception that putting lights closer to plants means the plants get a higher DLI.

Let’s first establish some baselines. The light intensity of a point like light source will drop as a function of the square of the distance to it. This is because as you move away from it, the light becomes distributed into a wider area.

However, when you have an array of point like light sources (think a series of LEDs), the dispersion of the sources starts adding to the light intensity coming from others. Closer to the light source you get much more intense peaks and valleys, but the total light integral is the same as a short distance further away. A big area of canopy 5 feet or 10 feet away from an array of point like light sources will receive the same light integral, but there will be more hot spots in the area at 5 feet. You can see that in the horrible drawing I have made below. (the graphs to the right are what you would expect from measuring the PPFD).

This is why a top closer to the lights can burn, because it can be caught in a peak that is a substantially higher PPFD than its surroundings.

As the distance becomes larger, you lose those intensity peaks and the distribution becomes more homogeneous as you start getting light contributions from multiple sources into the same area. So you have a much smoother profile than close to the light.

If you move too far you will start losing light to the sides (because the photons will have spread beyond your area of canopy). However, moving closer to the light will never give you a higher DLI than beyond the point where the lighting is contained within your canopy. All you will get is higher PPDF hotspots.

To sum it up, you would want the lights to be as far away as possible as you can to get the smoothest PPFD distribution at the minimal possible side lighting loss. The lenses of the lights also matter, more collimated sources will spread less which means distance will matter less as you move further out.

I just hope this can be useful to some, as it’s a very common misconception.


Thanks Daniel, that is a common misconception. We always design facilities for equal ppfd which is difficult to achieve with room geometry. On tours people still ask why we would put lights so close to the wall, instead of centered over the rolling tables. Simple we used dialux, calculux, and in room readings to produce equal light levels across the entire room.

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Yes, a consequence of light being emitted in a cone shape from the point light sources is that optimal light distribution at the optimal distance requires off-center placement for the most homogeneous distribution.

This is also why when calculating yields per area, you need to account for the area of the entire room instead of just the area of canopy, because side lighting effects can be very important.

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Funny you mention side lighting. Several people in our organization comment that the front and back of the rooms always have better lower/side bud development. Well yes, these spots are the only areas in the room that aren’t canopy and get significant side lighting from the small walkway area.

Very interesting topic. Not surprising this is flying in the face of commonly accepted cannabis practices of getting the lights as low as possible.

I was just talking to someone yesterday who was considering lowering their lights because they weren’t happy with their perceived “canopy penetration”. They don’t use any type of light meters or anything, they’re “old school” and just going by intuition. We’ll see how it works out for them.

Here are some relevant definitions. I’ve learned there can be a lot of acronyms and different metrics that deal with lighting.

PAR refers to the spectral range of radiation that is utilized by plants to drive photosynthesis. This range is 400-700nm however recent research suggests that plant utilize light outside this range causing many companies to use an extended PAR range of 380-780nm.

PPFD is the total number of photons in the PAR range that hit a square meter per second from a light source measured in μmol/m2s (alternatively written μmol m-2 s-1). It determines the amount of PAR light reaching the crop canopy from a light source by using a quantum sensor that measures PPFD such as the Apogee SQ-520. These sensors measure the amount of PAR light reaching at a certain point each second and use this reading to approximate for a square meter. This approximation works well for a fully homogenous light source like sunlight however for artificial lights, many readings should be taken over a square meter to get an accurate average PPFD measurement as results will be at a maximum directly under the light then will decrease as the sensor is moved away

DLI (Daily Light Integral) is the number of photons in PAR range that are delivered to a square meter over the course of a day . It is often calculated by measuring the PPFD throughout the day then using this data to estimate the DLI. This is an important metric to know as different types of plants require varied DLI for optimum growth.

@UneekkUserName I haven’t heard of dialux or calculux. I’ll look into them. Thanks for mentioning.


When measuring PPFD it is also always important to measure PPFD across a grid of locations at constant height, to get a sense of the homogeneity of PPFD. It is important to note that we’re interested about the DLI of significant areas, not of single points.

To better measure DLI I would suggest using bars, that have multiple PPFD sensors and can help you better sense the average DLI. Apogee sells bars with 10 sensors that you can use to actually get a much better sense of the mean DLI (MQ-301X: Line Quantum with 10 Sensors and Handheld Meter - Apogee Instruments, Inc.).

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Hey Dr. Daniel,

I am getting ready to purchase a light meter and I’m considering the one you have linked, the MQ-301X.

I noticed that it measures PAR and not the new “ePAR” spectrum which I have gathered measures further into the red spectrum.

Do you think this is significant?

I thought with the “new” world of LEDs and the ability to change spectrums it might be necessary to have the ePAR measurment, but I’m not sure.

I’m wondering why The flowers at the top smell worse than the flowers lower down. Their profiles and popularity are very different

I have some experience with that. The details of the grow are very important but here are some basics.

There is a maximum amount of light (PPFD) a plant can withstand before quality drops, this typically is not the issue. The micro climate at the top of canopy is typically hotter and maybe drier, but certainly higher vpd. The upper canopy stomata close some and transpiration drops. This is the plants cooling, transpiration, so the upper canopy gets hotter without as much evaporative cooling. That is my theory and here are some things that can help with upper canopy vpd but can hurt other aspects of the grow.

Lower light levels
Reduce room overall temp (vpd control)
Raise room overall humidity (vpd control)
Irrigate more