DLI Part 2: Measuring DLI at Home – House Plant Journal

DLI Part 2: Measuring DLI at Home

The Daily Light Integral (DLI) is the number of PAR photons that land on a place in one day. We can get a reasonable approximation by measuring the foot-candle reading at one spot every hour throughout the day.

DLI is like the temperature – each day will be different but you want to get an average based on the time of year, which will ultimately give you a quantitative/better answer to the question “how much light is my plant getting?” At the end of this article, I’ll include a link that will address the question “how much light does THIS plant need?” – and I’ll throw in my two cents to help houseplant owners make sense of the numbers!

Method

This is essentially what we’re doing:

Technically, I would need to measure the light right here EVERY HOUR to get DLI.
Technically, I would need to measure the light right here EVERY HOUR to get DLI.

So now, here’s the actual work

On a clear day, I took foot-candle measurements at the spot where my Peace Lily (Spathiphyllum) lives.

Here are the readings (foot-candles vs. hour of the day):

These are the readings from the position of my Peace Lily, next to a west-facing window on September 22 on a clear day. In the morning until around 2pm (14h), the light intensity hovers around 150 foot-candles and gets higher as the sun comes closer into view. Once the sun does come into view - between 3 and 4pm - the light reading shoots up to 2000 foot-candles - note that the intensity is much lower compared to outdoors because the window greatly attenuates the strength AND, the sun’s rays strike at an angle past noon. As the sun dips behind some buildings, the intensity drops until sunset.
These are the readings from the position of my Peace Lily, next to a west-facing window on September 22 on a clear day. In the morning until around 2pm (14h), the light intensity hovers around 150 foot-candles and gets higher as the sun comes closer into view. Once the sun does come into view – between 3 and 4pm – the light reading shoots up to 2000 foot-candles – note that the intensity is much lower compared to outdoors because the window greatly attenuates the strength AND, the sun’s rays strike at an angle past noon. As the sun dips behind some buildings, the intensity drops until sunset.
Here’s a map of the sun’s movement for September 22 (the day I took the measurements) in relation to the plant’s view. From the previous table, now we can see why the average light intensity shoots up during the 15th (3pm) and 16th (4pm) hours. Note: the outside buildings aren’t visible due to the camera exposure.  Solar map generated from:  https://www.sunearthtools.com/dp/tools/pos_sun.php
Here’s a map of the sun’s movement for September 22 (the day I took the measurements) in relation to the plant’s view. From the previous table, now we can see why the average light intensity shoots up during the 15th (3pm) and 16th (4pm) hours. Note: the outside buildings aren’t visible due to the camera exposure.Solar map generated from: https://www.sunearthtools.com/dp/tools/pos_sun.php

Great! So what is the DLI from this? Let’s calculate…

Here’s the raw data:

Foot-candle readings at each hour of the day. I wasn’t home to measure EVERY hour - only the ones in red were actual measurements. For the rest, I made an educated guess.
Foot-candle readings at each hour of the day. I wasn’t home to measure EVERY hour – only the ones in red were actual measurements. For the rest, I made an educated guess.

Take the average intensity over the whole day: (add up all the numbers and divide by 24)

248 foot-candles

Convert foot-candles to PPFD: the conversion factor for sunlight is 0.20 so let’s multiply 248 x 0.20 to get…

49.625 μmol/(m²⋅s)

FYI: μmol/(m²⋅s) reads as “micromoles of PAR photons per square meter per second”

But to get DLI, we want to know how many moles of PAR photons we’ll get in a whole day. This can be done by converting micromoles to moles (divide by 1,000,000) and seconds to day (multiply by 86,400 seconds in a day). Effectively multiply by 0.0864, which gives us:

4.29 mol/(m²⋅d)

So we can say that on a clear day on September 22 in Toronto SPECIFICALLY at the spot where my Peace Lily lives, that plant gets 4.29 moles of PAR photons for that day. I purposely chose to do the measurement near the equinox to get a median DLI, knowing that it would be higher in the summer and lower in the winter.

Now here’s the fun part, looking up what plants would do well at that spot:

My Peace Lily (Spathiphyllum) is happy in the “Good quality” range of DLI with 4.29 mol/(m²⋅d)  Check out the full list here:  https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf
My Peace Lily (Spathiphyllum) is happy in the “Good quality” range of DLI with 4.29 mol/(m²⋅d)Check out the full list here: https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf

A caveat to using this chart is that these measurements were taken at a greenhouse, which means the light sensor would be pointed straight up and the ceiling has diffusion material, which spreads the sun’s light evenly all around. So even though my DLI reading is in the “Good quality” range for Spathiphyllum, I shouldn’t expect growth exactly like at the greenhouse. A window, even a very large one, is still surrounded by other walls and a ceiling, which are opaque.

On the other hand, the aesthetic standards are for COMMERCIAL growers where they have time constraints (i.e. get the plants to grow to sellable size as fast as possible). As casual plant parents, we do not need to get this level of growth to be HAPPY with our plants.

But what if I put a plant some distance from a small north-facing window? You might measure something like this:

light readings - small north window.png

The DLI from this place would be: 0.69 mol/(m²⋅d); well below the “Minimum acceptable quality” for the even lowest light plants.

So here’s where grow lights come to the rescue…here’s a handy set of TABLES!

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My trusty light meter: https://amzn.to/317tUqu

PAR meter: https://amzn.to/2A3j88Q

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5 responses to “DLI Part 2: Measuring DLI at Home”

  1. You can google “DLI calculator”, plug in your PPFD measurement and how many hours per day your light is on, and you get the DLI. Of course, if you have windows with substantial light coming in, that complicates the calculation. This gives me a minimum DLI, and I can assume that I’m getting more through the windows.

  2. Hi Darryl, I have been trying to figure my indoor lighting scientifically hence was doing some reading up about light meter, PAR and daily light integral (DLI) values. Trying to figure out what is the PAR range and DLI values for cactus, Commiphora, Boswellia, euphorbias and pachypodium?

    My Urceri light meter shows me 4000-foot candles
    1. Applied the conversion factor 0.17 (white LED) = 680
    2. Converted it to moles/(m²⋅d) = 68 moles/(m²⋅d)
    3. 68 moles/(m²⋅d) / 86400sec in a day = 5.8752 moles/day
    I don’t think the values look right, and please enlighten!

    Please share with me if you have a chart or your personal findings on what’s a good range for cactus, Commiphora, Boswellia, Euphorbias. Thank you so much!

    • Hello Sen – thanks for reaching out. What did you do in step 2? In step 1, the conversion factor gets you 680 for PPFD, which is micromoles per second. You can’t go from PPFD to DLI without specifying the DURATION of that light, as in, how long you leave the grow light on. So if you left it on for 12 hours, the DLI becomes 680 x 60 (sec/min) x 60 (min/hr) divided by 1,000,000 (micromol/mol) x 12 hours = 29.4 mol/d –> this would be fine for all the plants you mentioned and definitely too much for lower light, tropical foliage plants.

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