Grow Lights – Intro Part 3: Light Intensity and Measurement

Welcome back, growers!

This is PART THREE in our series of blogs about Grow Lights. Check out PART ONE and PART TWO here.

So, in previous articles we have taken a look at what light is, and what colours (wavelengths) plants use and therefore need. Progressing on from that, in this blog we will talk about how we can measure the amount of light we are giving our plants, how much light we need to give them, and how we can judge what area we can expect a grow light to illuminate and effectively grow plants in.

How Do Humans and Plants Perceive Light Differently?

The most often used ways for us humans to measure the amount or brightness of light is in lumens or foot-candles. The measurements of lumens or foot-candles take into account the whole light spectrum that humans can see. i.e. from about 390nm to about 700nm, but our human eyes are far more sensitive to green light than light at the ends of the visible spectrum (red and blue): Here is the sensitivity of our eyes over the visible spectrum:

If you had 3 lights – one red, one green and one blue, and they all produced the same amount of light energy, the green light would have a higher lumen rating than the red or blue ones because that is how us humans would perceive it.

Plants do not perceive the brightness of light like our human eyes do; i.e. they do not “see” green light as brighter than red or blue. For this reason, lumens or foot-candles are inappropriate for measuring the light that we give our plants. As the grower’s saying goes: “Lumens are for humans“!

The measurement of the amount of light that we give to our plants is called PAR (Photo-synthetically Active Radiation). This is a measurement of the amount of photons that a plant can use to photosynthesise with in a given area in a certain length of time. It is measured in something called PPFD (Photosynthetic Photon Flux Density) and the unit of measurement  of PPFD is micromoles per square metre per second (umol/m2/s). It is measured between the wavelengths of 400nm to 700nm. There is no weighting given to one particular colour or wavelength range. All photons in the range 400nm to 700nm are counted the same:

Graph by

The graph shows us why lumens are the wrong thing to measure the light we are giving to our plants!

Our Li-Cor Radiometer / Photometer (pictured below) and its special Quantum sensor, measures PAR very accurately. It allows us to get a much more meaningful measurement of how much plant-usable light that a grow light is giving out. However, even PAR measurement is not perfect! The above chart shows the McCree curve spectrum which is the amount of photosynthesis response a plant has over the light spectrum. But we can accept that PAR is a far superior way of measuring plant usable light compared to measuring lumens.

What Is the Optimum Amount of Light That We Need to Give Our Plants?

A healthy plant leaf can withstand quite a lot of light but only for a short period of time. The ppfd could be 1500-2000 umol/m2/s for perhaps an hour or 2 (like the midday sun might provide at the equator) and no damage would occur. Some strains of plant will yield their maximum when given around 1500 umol/m2/s. Over this and there is risk of leaf bleaching and burn. Most high power grow lights get HOT. If you have a plant close enough to a hydroponics grow light that it is getting more than 1500 umol/m2/s then you will possibly find that the leaf temperature is too high for maximum photosynthesis anyway!

Around 1200 umol/m2/s is a good, safe maximum that we should expose our plants to for a 12 hour lights-on period. Even at this level, it is a good idea to keep an eye on leaf temperature.

Plant leaves can only really make good use of a certain amount of photons in any given 24 hour period. Plant leaf cells contain special little “factories” called chloroplasts which is  where all the photosynthesis happens. Just like any factory, they have a limited production rate. Chloroplasts can only capture light photons and use carbon dioxide and water to make sugar at a certain rate, then they need time to recover.

For this reason, a plant can only effectively use a certain amount of light per day.

If we add up all the light that a plant receives over a 24 hour period we arrive at a figure called the “Daily Light Integral”, or DLI

Rather than using the measurement of micromoles (as we do for the instantaneous measurement of light), we can use moles/m2/day to describe this measurement. Here is a graph of how well  typical plants can use the amount of light in a 24 hour period:

Notice how plants start to use extra light less effectively after a certain point. This point varies from plant species to species. It can also be affected by the health of the plant, the concentration of CO2 in the environment, as well as humidity and temperature. However, if we give our plants too much light then we could cause damage, plus we are starting to be wasteful with our electricity!

In tropical countries around the equator where many types of plant grow very well, plants can get up to 50-60 moles/m2/day. This is a good number to aim for (as a maximum) to give our plants

To convert this figure of 50 moles/m2/day to umol/m2/s we need to divide this number by the number of seconds in a day (86400) that the lights are on, and multiply it by a million (to get the number from moles to micromoles). Doing the maths, (50/86400) x 1000000 = 587. Rounding this up for simplicity we arrive at the figure of 500-600 umol/m2/s. But that number only applies if we a running our lights for all 24 hours of the day.

If we run our lights for less than 24 hours a day then we can still provide those  50-60 moles/m2 but we have less time to provide them in. This means that we can give our plants more light. For an 18 hours on / 6 hours off photoperiod we could give our plants about 700-800 umol/m2/s (as an absolute maximum). However, many growers in the know say that actually 500 umol/m2/s the right ppfd for veg.

Cuttings and seedlings probably only need about 100-150 umol/m2/s.

For a 12 hours on / 12 hours off photoperiod we can give our plants about 1000-1200 umol/m2/s. Beyond these numbers will almost certainly mean you are hitting the point of diminishing returns. And remember, your plants must be perfectly healthy, have plenty of CO2 around them, and have the correct humidity and temperature (particularly leaf temperature) to be able to use all that light!

In most cases, a number like 1000-1200 umol/m2/s  will only be achieved directly under the light (and a powerful one at that!). Out towards the sides the figure will usually drop off  by quite a lot – and this can depend on the footprint made by the reflector.  For a given grow light there is often an optimal compromise between intensity directly underneath it and getting an even light spread.

Going to the other end of the scale, plants need a minimum amount of light to even start photosynthesising. For any where near decent plant growth we really want to be giving somewhere in the region of 100 – 200 umol/m2/s

What Is The Inverse Square Law?

The light intensity from an indoor grow light has the unfortunate property of diminishing as you get further away from it. For example, a typical grower might set up a 600 Watt HPS grow light at a height of 18 inches above the top of their plants which are on a 12/12 photoperiod. His par meter might tell him that his plants are getting 600 umol/m2/s at the tops. However, if he then takes a reading lower down, say at 36 inches below the grow light, then he will probably get a reading of just 150 umol/m2/s!

This is because of something called the “inverse square law” which says that the brightness of a light goes down as a square of the distance from it. In the above example, the grower measured the light intensity at one distance and then again at 2 times that distance – 2 squared equals 4, so the light intensity at the lower distance will be 1/4 (a quarter) of that at the tops of the plants. If he moved even further away and measured the light intensity at 54 inches (3 times the distance), then he would probably get a reading of 1/9 (one ninth) of the first reading (because 3 squared = 3 x 3 = 9).

The light from the sun travels about 93 million miles before reaching us here on planet earth. If we go outside on a sunny day, whilst holding the sensor at chest level, our photometer might tell us we are getting 500 umols/m2/s. Now, if we place the sensor on the ground, then we will get the same reading of 500 umols/m2/s. This is because the sun’s light has already travelled so far that another metre will not make any discernible difference to the reading.

The inverse square law works for “point source” sources of light which are very small. If the light source is a large area, such as a panel of fluorescent tubes, then the drop off will be somewhat different.

Now we know that we can place our grow lights where the tops of the plants are getting, say, 600 umols/m2/s. We can also see how far down we get just, say, 150 umol/m2/s. We can measure the distance between these 2 points to get a “relative” depth penetration measurement. This will tell us a lot about how a grow light can maintain a decent intensity for plant growth compared to another similar grow light.

Now we’ve covered the basics of light intensity and depth penetration. Join us in the next part of the blog series, where we will go into the testing of grow-light footprint and space coverage.

See you soon, and happy growing! 🙂