This post is kind of self-serving. I try to help with lighting questions on the r/Hydroponic subreddit on Reddit, and to go through the process of selecting lights of the right configuration and intensity takes a lot of words and kills a lot of innocent bytes. So, if I do it here, I can just point to the blog for all that.
It’s not new information for many, and it’s nothing mysterious or revolutionary, but it does get into lighting issues that are not always intuitive. It will be a step-by-step approach to answering questions like:
What kind of light should I buy to grow my _____?
Can I use this light to grow _________?
How closely should I mount my light to the plants?
How many hours should I run my lights?
How much light does this plant need?
The choices can be daunting. But the most important question is about a light’s ability to provide for all the plants in the growing area.
Our discussion here is strictly about intensity and coverage. It is not a discussion of differences among lighting technologies or differences in things like “blurple”, so-called full-spectrum or new developments in LED lighting. It will use LED lights for the examples, but so long as you have data on any light, you can apply the steps to evaluate it.
First, some tools.
I usually work backwards, from the light that is needed for the plants and then on to specific light specifications. So, we need some information on what light plants need. When we talk about what plants need, we mean the required minimum numerical Daily Light Integral, also called Daily Light Interval. Daily light interval is a number derived from light intensity and hours of operation.
DLI is the amount of photosynthetically active radiation (PAR) that reaches the plants. It is expressed in moles of light per square meter per day. Don’t worry about the formula or that it refers to a square meter. That’s just because we need a standard. You don’t have to figure out what part of a square meter you’re lighting or anything. Think of it as simply the amount of light that a grow light is projecting onto a point on the growing surface.
This is a DLI chart developed by LEDTronic and displayed on their website at https://www.ledtonic.com/blogs/guides/dli-daily-light-integral-chart-understand-your-plants-ppfd-photoperiod-requirements
Note the units. The columns are PPFD values. Hours are down the left side. The intersections of hours and PPFD values are DLI values. One very useful thing to note is that these relationships are linear. That means that you can add, subtract or multiply PPFD, hours or DLI. Doubling the PPFD doubles the DLI. Doubling the hours doubles the DLI. Multiplying the DLI changes the PPFD or hours by the same factor. It’s very convenient. We could really recreate the entire table from the single one-hour value for the DLI for a PPFD of 1, found under the “1” column.
The metric for intensity is PPFD. PPFD means Photosynthetic Photon Flux Density. It’s the way we measure the PAR light that reaches the plants. At all times, we are talking about PAR light.
PAR is a concept developed in the 1960’s by the late Keith McCree. He discovered the wavelengths of light that he found seemed to be used by plants for their various functions. This is the curve he developed. You will also see here a curve labeled Lumens. That illustrates why human eyes are poor instruments for judging light and why common light meters, or LUX meters, are not reliable for growing. They literally don’t know what they’re looking at. The human eye judges light largely by the intensity under the Lumens curve. But the plants “see” and use that whole range from 400 nm to 700 nm.
Or that was what McCree thought and what everyone else thought for a long time. But McCree didn’t have single-color LED’s. They hadn’t been invented. So, he had to use other light sources and filters, and some wavelengths outside the old PAR range are low intensity but have a high effect on some plant functions. One day in the not so distant future, you will likely see PAR redefined to include those regions and the curve will change.
But all that is just orientation to the real thing we want to know, which is how to know what PPFD value to use which, along with the shape of the light, defines which lights are candidates for our use.
On the same LEDTronic page we find two tables of recommended DLI values for various plants and classes of plants. The name at the top of the two lists are the researchers who concluded the required DLI values. Where a DLI value has a “+”, it means that is a minimum recommendation.
So, we know how to find DLI and more or less what plants need. Now we need lihts that can satisfy those needs. What lights, at what distance for how many hours are appropriate and reasonable?
Let’s imagine we want to grow lettuce in one area and tomatoes in another. The chart suggests DLI values of 14 to 16 for lettuce and 22 to 30 for tomatoes. Not surprising. We already know lettuce has lesser needs than fruiting plants.
We now go back to the DLI table and find the DLI numbers of interest.
Here, I’ve circled approximately the regions where DLI values comply with the lettuce recommendations.
We find useful minimum combinations of time and PPFD from 16 hours at 250 PPFD to 4 hours at 900. If we want to run lights at the more usual periods of 12 to 14 hours, we find we will need PPFD values of around 300.
Let’s stop there for a while and explore some other issues. We know now what it would take to satisfy a lettuce plant. If we could put light intensity of 300 PPFD onto the plant for about 12 hours, we would be good. But it is not in the nature of the kinds of lights we usually use that they light our whole growing area the same, giving each plant equal light. So, we would like to know more about the lights.
I’m going to use some lights that could be used in our lettuce growing operation in various ways. We will be depending on data from the manufacturer to tell us about the lights’ intensities at different distances and positions within the growing area.
I had better point out here that in school, you may have seen the “inverse square law” of light. That law says that light decreases according to the inverse square of the distance. In other words, double the distance, and the light at that distance is only one quarter of the intensity it was at the closer distance. Such a nice rule. So useful. But not really. That law applies to point sources of light, light emanating from a single point and radiating in all directions. It’s perfectly good for a photographer using a single strobe to light a room or do a portrait. He knows how the light from his source, which is very nearly a point source, will behave. But our lights are arrays of LED’s in various configurations. The law doesn’t work so well for us.
Every light source has a “beam angle” and a “field angle.” Sometimes, you will find a grow light maker stating the beam angle.
The beam angle is the angle that covers the target with at light of at least 50% of the intensity at the center. The “field angle” extends that to the point where intensity drops to 10%. Beam angle is what we are most interested in, because it’s where the real light is. The rest of the area, the “spill angle,” is not useless, but it’s usually less than we want.
But wait. Our lights are not point sources. They are, in fact arrays of a lot of point source LED’s. So there may be hundreds of LED beam angles overlapping in various ways, adding to each other’s intensity or fading off at the ends and edges of the target area. The most intense light will be at the point directly under the center of any grow light. And when a manufacturer gives you a PPFD value for a given distance, unless they say otherwise, they mean right under the center.
So, when we see a maker who provides data in the following form, we know it’s for the area under the center of the light.
The light does not put a PPFD of 490 on every point in the growing area or even on the area under the light, which in this case is about 12 inches square. But we know what conditions will be at the center. This light claims to cover a 2’ x 2’ growing area. That is, of course, arbitrary, but it’s a reasonable area for the use of this light and it will be approximately within the beam angle. So does it work for our lettuce.
If we look at the DLI chart under 500 PPFD, a value close to the rating of this light at 18 inches, we find that if we run it for 12 hours, we will actually hit a DLI of about 22. Far more than enough. And not so much that it will damage the plants. Enough, in fact that we could use it for our tomatoes, except that large plants like tomatoes present other lighting problems.
If we wanted to mount it lower, say to fit a shelf, we would see that we would probably be wise to reduce the hours to something more like 6 hours with the light at 12 inches where the PPFD is 809. (You can also buy grow lights capable of being dimmed, either by a control on the light or an external control. Dimmable lights are, of course, more versatile, but most growers won’t need them.)
We could, if we wished, increase the height above 18 inches. But what PPFD to use? That maker didn’t provide data for higher mounting. We can get a reasonable feel from how the data does change for the other heights, concluding perhaps that at 24 inches, PPFD will be somewhere around 350 to 400. And that would give us a comfortable DLI of 15 to 17 at 12 hours. Perfectly acceptable.
But if our light is not quite so lavishly endowed as this one is for our lettuce job that we could barely meet the needs of our lettuce at the center, what would it provide enough at the edge? Without a meter, we aren’t so sure here because the maker has not provided those numbers.
But we can us a similar light from a more informative maker to get an idea of how much the light might fall off at the edges and corners. Here’s a very similar light from a top maker, Mars Hydro. And Mars provide more information in the form of a PPFD map of the projected 2’ x 2’ growing area.
Quite similar to the other numbers in the center. Because the designs are so similar, when we see how the intensity of this light falls off to the edges and corners, we can be pretty confident our first light will behave similarly. And here’s the interesting part.
Notice that at 18 inches, the light falls off very little. But at 14 inches, although more intense than at 18 inches, it falls off to about 2/3 at the corners. And at 12 inches, although the center is even more intense, the corners are less than half the center intensity. The higher the mounting, the more evenly it lights, at the expense of intensity.
We should be aware of such things when we are making decisions about intensity and mounting height. If we have a somewhat weaker light that is barely adequate at 12 inches at the center, the half intensity at the corners may not be what we would like to have. We might compensate for that my increasing the mounting height where we know the fall-off is less severe. We would, of course, have to compensate by nearly doubling the hours of light operation. The DLI chart will tell us if we can do it and still reach or DLI target.
The fall-off is because while all the LED’s contribute to the center intensity, when we move to one corner, we are losing the light from the far corner. Look at a map for a long rectangular light we might consider to light a shelf or a channel system.
The light falls off much more abruptly to the sides than at the ends. The rectangular shape changes everything by allowing greater or lesser numbers of LED to contribute to different areas. There are far more LED’s along the long axis than the short axis. Maps like this can tell us a lot we would like to know when we start matching the shape of a light to a growing area.
Note, too, that large commercial growers who use many powerful lights mounted very high don’t have to worry about such things. In their case, the entire ceiling acts like a vast grow light and the individual lights are more like the individual LED’s in our smaller lights. Nor do commercial growers using very long channels have this problem. Their lights are light bars mounted end to end, one row to one channel. The whole thing is “center” so to speak, and if they continue the light bars a bit beyond the end of the channel, every point gets the same light.
But what if you have no map and no other product’s map that you believe is similar enough. You can use any decent light meter to compare relative intensity across the growing area. Of no real value in metering actual intensity believably, but since the mix of light is the same across the field, it doesn’t matter than it’s no good for getting a PPFD number. You already know the center PPFD. The losses to the sides will be in the same proportion to what the meter tells you.
I have a simple LUX meter for just finding shadow spots and such in the greenhouse where I used lights as supplement to filtered sunlight. You can also use a free LUX app on your phone. Just don’t believe any of those clickbait, like-hunting YouTubes and such that claim to show you how to use a LUX meter as a PAR meter. It’s low grade baloney. I strongly suspect even fairly expensive “PAR meters” are faking it. The YouTubers and such who have monetized their sites or channels want you to like them because they tell you what you’d like to hear. If it worked, Apogee would be out of business and Dr. Bugbee would have to go back to hustling research grants. And I would have saved $500.
For those who do want to reliably meter PAR light, the minimum device is the Apogee MQ-500. The new one with the blue sensor has such small correction factors for different light sources that you can really use it for routine things with doing any conversions. But it’s a somewhat expensive toy and not really necessary if you can follow what we’ve talked about here. I depend on it, though, because in my greenhouse with mixed filtered natural light changing throughout the day, supplemented by grow lights, none of the methods here can tell me much.
One might well ask if the PPFD numbers from grow light makers and seller are accurate. My feeling is that they are reasonably accurate. It would be foolish to lie about something that could so easily be detected. I’ve only ever actually tested one light, the GVG LED 600W Grow Light Full Spectrum LED, which is the one I used to get the first numbers to demonstrate. According to the Apogee PAR meter, the maker numbers checked out well.
Large plants take more thought about lighting. Not only are they larger and taller, but their lighting needs are more critical. Remember in the charts with the lights how the all the numbers dropped as the light was raised. With a tall plant, like a tomato, the top of the foliage may be two or three feet above the lower foliage. We have to avoid harming the top foliage while providing useful light to the lower leaves.
We probably won’t be able to do it all with any precision. Grow lights are often mounted so that they can be raised as a plant grows larger. If a plant is allowed to grow close to the light, it may suffer light burn. Light burn is simply when a plant gets excess light and is stressed. Leaves close to the light may turn brown and die. Different plants have different tolerances. My basil plants often grow up, into and beyond the light fixtures without suffering harm. Lettuce often shows it is light burned when the young leaves at the center of the forming heads begin turning brown. With lettuce, this usually happens because someone was unable to calculate the correct DLI and made a mistake in guessing that the lights needed to be very close.
There is no clear rule about how much light is too much. You have to let the plants tell you. But you can avoid the burn by knowing the DLI at the highest part of the plant and keeping that from being several times what is normally required. Of course, that may mean shorting the lower foliage. But most plants can work with a considerable range of light intensity. Remember that they evolved in nature where conditions vary and can change.
You can also mount a light in such a way that the distances from the light to all parts of the plant are more similar. The extreme end of the situation is lighting the tower. They are usually lighted using bars hung or mounted vertically to the sides of the tower on all sides used for growing. Something similar can be done for a tall plant. If two lights are offset to the sides of the growing area the distances to all parts of the plant will vary less. You can use the methods learned here to figure the DLI’s in such situations.This is not an exact science. Think your way through to reasonable solutions, and consider such issues when selecting lights.
What are the consequence of getting it wrong and providing inadequate light? Plants will seek to find good light. They assume that the best light is up, but they can also sense even weak light from the side and lean into it. Low light makes plants “leggy,” meaning they put everything into trying to get to better light. They grow taller, but they grow thinner because they care only about height. Their branches are fewer and more widely spaced up the stem. They don’t have the light to produce chlorophyl, so foliage become pale.
These are leggy tomato plants, likely from an attempt to grow by window light.
Which brings me to another issue. Can you grow vegetables by window light? Generally, no. The exception is a south-facing window where the plants can get direct sunlight for several hours a day. It must be actual direct sunlight falling on the plants. Otherwise the only light that comes through a window is “skylight,” almost entirely blue light and far less intense than sunlight. Even when there is direct sunlight, it will be directional, and you may want to keep rotating the plats every few days so they don’t lean, and all sides get light.
A note on shopping for grow lights.
This is mainly for new growers who have not shopped much for lights. Note that the GVG light looks like they’re claiming it’s a 600 watt light. It’s no such thing. I had a talk with them, and whether it was me or not, they now on their web site call it just the GVG LED Grow Light Full Spectrum LED Grow Light.
Using a model number to mislead about intensity is very common. Sadly, the best, like Mars, do it, too. The one I used for the second demonstration is their MARS HYDRO TS 600W LED Grow Light. They would, of course, say TS 600W is just their model number. Right. The Mars light actually consumes 100 watts. The GVG is 60 watts. At least in both those cases, the correct wattage is fairly easy to find on the product pages. It’s not always so. Many shifty sellers flatly state much higher than actual wattage and never reveal the truth. Most of them also fail to provide PPFD data.
What’s up? It became the almost universal practice in the amateur grow light business to use the equivalent wattage required in a different lighting technology, like incandescent, for the same effect. That’s their excuse, anyway. Deplorable since LED lights have become the standard. But now you’ve been warned to search descriptions until you find true consumption. Sometimes, it doesn’t appear at all or appears only in the electrical data like “120 v. 0.5 amps”, meaning 60 watts.
Regular offenders are clip-on gooseneck lights. This is an honest one.
“LED Grow Light,6000K Full Spectrum Clip Plant Growing Lamp with White Red LEDs for Indoor Plants”
“As for LED power draw this mini succulent lights just consume about 10watt and is equivalent to a 50W Halogen Bulb. …It is a great gift if you growing [sic] some small house plants”
They show it only being used for small house plants with low light needs. They clearly reveal the true wattage. And they say it is for “small house plants.”
Many don’t do that. Beware. All lights that look like that will be too weak for real growing, although they can be used in germination.
There have been many attempts to establish standard for describing grow lights. None have found much success at being adopted. But progress is being made. The DesignLights Consortium® (DLC) publishes technical requirements required for them to list products. If you are interested, here is the horticultural section. The requirement are a good list of issues you might want to consider.
https://www.designlights.org/our-work/horticultural-lighting/technical-requirements/hort-v2-1
We have one more rough and ready tool. It works like this. 35 watts of actual LED wattage, mounted at 18 inches, to light one square foot of growing area. So, to light a line of 12” buckets, four on a five-foot shelf, about 140 watts. This works pretty well in practice and can serve as a rough guide to whether we made a gross mistake somewhere. The DLI’s produced depend on hours of light.
That’s it. Here are links to the most important tool I used:
DLI Chart and Plant Needs
