This page will teach you how to understand the terminology used to describe the light color that comes from LED lamps.
Unlike the other pages on this site, some of this content might be little difficult. Fortunately for you, gardener, the manufacturer has probably put a friendly label on lamp or box, saying something like full-spectrum, or flowering, or fruiting. These are products, and they want to sell them to you. So, usually, all this information is unnecessary for you. For the vast majority of plants, you can purchase full spectrum grow lights, keep them 15 cm (6 inches) above your plants, and it will usually work. If you are satisfied with the easy version, you could go on to the next page, about controlling the moisture in your garden room.
While the history of all indoor gardening reaches back to ancient times, the history of growing vegetables indoors under lights is really my focus. The natural light levels most people have in their homes, unless they have an area built for gardening, under large skylights, perhaps, are not usually sufficient to grow vegetables without adding artificial light.
Indoor gardening under lights can be categorized into three different historical periods based on the predominant lighting technology. The Incandescent Period, The Fluorescent Period, and the current LED Period.
The Incandescent Period
Conventional outdoor growing has always had its limits, most obviously growing seasons, in temperate or cold areas.
Farmers who both had electricity and lived in colder places with short seasons were pretty motivated to move ahead with the idea of artificial lighting and plants. Farmers have used glass enclosures of various sorts for centuries in order to extend the growing season. Even with these, however, you are risking a night so cold that it simply is too much for your cold frame or cloche.
You might only need to be starting the plants for a couple of months before transplanting them outdoors. The young plants aren’t taking up nearly as much space as they will be in the field later, and so farmers could put lighting in heated basements and barns, in order to start their plants early without the risk of freezing. In this situation, it’s cold outside and the heat from the lights was more of a help than a hindrance.
There were also specialized lighting applications for plants that required very controlled growing conditions, or in order to force or suppress flowering, and other things of that nature.
These uses were mostly limited to people with financial interests beyond growing their own food. The farmers I mentioned extending seasons are an easy example, but there were many commercial applications of growing plants under incandescent lighting.
Incandescent lamps work by heating a wire, known, as a filament, to very high temperatures. There is a direct relationship between the temperature of that wire and the color of the light that is coming from the lamp. These are very high temperatures, and so they are measured in degrees Kelvin. The bulb around the filament is filled with an inert gas, usually nitrogen, so the filament does not burn.
The color of the light emitted from electric lights is still generally expressed in terms of an imaginary black body, and what temperature it would have to be heated to, in order to produce light of that color. It’s not the actual temperature of the real filament.
The range of colors that we are concerned with, at the moment, and these have been in common use since the incandescent days, are 2700 degrees Kelvin to 6500 degrees Kelvin. Where it falls on this range would be printed, typically both on the bulb and its packaging as something like 5000K, for a common example, that might be used for a reading lamp.
It takes a lot of power to get a wire to, and keep it at, those temperatures, and most of the energy you put into it is lost as heat, instead of being emitted as light. To put it another way, incandescent bulbs make more efficient heaters than they do light sources. Before 1938, there was simply no better available choice.
The Fluorescent Period
Though fluorescent bulbs do not have filaments, they inherited the same terminology for light colors.
This standard terminology is fortunate for us because it means that a lot of the horticultural research that had gone into the use of incandescent lighting was still applicable in the fluorescent world. Unfortunately, it is not completely applicable.
One thing that incandescent bulbs do better than fluorescent lamps is dial in a specific light color. You get broader minimum spectrum ranges with fluorescent lamps. Also, the light is not as penetrating as that of high-powered incandescent lamps. That is to say, leaves shade lower leaves more using fluorescent lamps than they do use incandescent lamps. It will still work, but it affects the plant’s growth habits.
Fluorescent lamps are glass tubes coated inside with a material called a phosphor, and that material lights up when an electrical charge is passed through mercury vapor within the tube. It’s as obsolete now as the incandescent lamps because of the LED lighting technology we will use. The word ‘lamp’ in this context means what most people would call the ‘bulb’. It’s lighting industry jargon.
For our purposes, it’s more important to understand that the switch to fluorescent lighting caused indoor gardening to become much more widespread because it produced a lot less heat and used less energy. This made them much more cost-effective for many applications.
The Fluorescent Period of indoor home vegetable growing under lights began in 1938 when General Electric began manufacturing them in large numbers. With fluorescent lighting, the farmers I wrote about in the Incandescent Era section no longer had to spend as much on electricity to provide adequate light for the seedlings or deal with the risks of excess heat.
Across time, advancements in florescent lighting further reduced the power usage, encouraging many home and hobby growers, even those who were not farmers and did not live in places with particularly short seasons. The concept works on your little home cactus garden or whatever just fine. Some people wanted to grow something tropical, or provide some sort of produce for themselves year-round in a temperate or cold area.
This lighting technology was universally available in areas with electric power, and so people all around the world were doing this. The writings of these people are the source of much of the lore that has come to us about growing vegetables under lights. As with the shift from incandescent lamps to fluorescent lamps, not all of that lore is entirely applicable to the current technology.
The LED Period
Today, we have LED lighting, it’s much more efficient than fluorescent lighting, using less power for light emitted and making a lot less heat. It is so much better in most ways that it has quickly replaced both incandescent and fluorescent lighting in general use.
LED stands for Light Emitting Diode. If you have no idea what the next paragraph says, let me assure you now that your not understanding it, will not affect your ability to garden with these lights or to understand the rest of these pages. LED lighting works in a non-obvious way that requires significant education to understand.
According to Wikipedia:
A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the bandgap of the semiconductor. White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.
That’s right, electron holes in semiconductors release energy in the form of photons. No, I don’t understand it either, but I do grow vegetables with them. That’s much harder to understand than a glowing hot wire. I only really had to explain that to talk about light color, and why it is expressed in some number followed by the letter K.
I keep coming back to this because light colors are very important in indoor gardening under lights. The difference between many vegetables maturing or bolting young, that is preparing to go to flower, is the color of the light that you shine on them, and for how long you shine it. Many of the plants that people eat for salads become more bitter at that point, and they stop putting their energy into the leaves, as they flower.
Applied to the light color scale, the values we are concerned with now are between 2700K and 6500K. Those lower numbers are toward the red end of the light spectrum, and generally, promote flowering and fruiting. The higher numbers are toward the blue end of the spectrum and generally promote proper leaf and stem development. The industry refers to light colors near the red end as warm, and those near the blue end as cool. Modern grow lights go beyond those numbers, because they include light that is used by plants but is invisible to humans. I get to that in the LED section.
Outdoors, sunlight contains all these colors. You don’t have any control over that, the way you will in your garden room. In the garden room, you might have widespread 6500K range lighting, and more focused lighting in the 2700K – 3000K range on your fruiting plants. This is because your fruiting plants, of course, also have stems and foliage, and so benefit from all these colors.
I use specialized LED grow lights, exclusively, now, and I get to those later, but I did have some experience using lights designed for human lighting use, and have better results than I did with regular lighting. However, in this section, I am talking about interpreting light colors in K. Even LED lights meant for human lighting use are expressed in this way, and these cheap lamps will grow plants.
Some plants, like peppers and tomatoes, will both grow and fruit under the 5000K lighting. This color is usually called Cool or Daylight in household lighting. Check the box for the number followed by the letter K.
Much of my ambient lighting is currently 6500K lighting, commonly available as shop lights, and I have had great success with plants reaching maturity without bolting.
There are specialized LED lamps for growing plants that have multiple lighting peaks to promote both foliage and fruiting. These are the most efficient in terms of plant growth for the amount of electricity used. Some of the more common ones use only 9W and are suitable for growing a single plant with low light requirements, like most leafy greens.
Lumens
I didn’t mention it before, but there is another aspect to light besides color, and that is its intensity. This is measured in units known as lumens. One lumen is equivalent to a theoretical specific candle flame’s amount of light.
Normal human eyes are very good at adjusting for light, which means that we are terrible judges of how much light we are actually seeing.
If you have a cell phone, you can probably install a free light meter application. Once you have done so, turn on some lamp and, holding your phone very near it, observe the number displayed while you move your phone away from it. Be aware that the cell phone is very directional so you may have to move it a little to get the best reading. They aren’t the best light meters, but they are good enough for our purposes, so long as you take that into account.
They typically work in units called lux, or other units called foot-candles (FC).
A unit of lux is equivalent to 1 lumen, that is 1 candlepower, lighting an area of 1 square meter.
A unit of FC is equivalent to 1 lumen, that is 1 candlepower, lighting an area of 1 square foot.
We are expressing the intensity of lamps in lumens, which measures how much the lamp makes, but ultimately what matters is the amount of light reaching your leaves, which will be expressed in lux. We have chosen to use lux over foot candles for international use, and even in the USA, they are more commonly used these days. Your light meter app almost certainly supports both of these units.
As you will see if you do that exercise, the numbers drop quickly with distance from the lamp. Sometimes, when it is a sunny day, try the same thing with the sun, and you will see what the maximum reading is on the light meter. On mine, it reads 65,535 lux. I and other computer programmers recognize this value as the maximum positive number a certain sort of variable can hold. The full sun, at least where I live, in New Mexico, is off the scale of my light meter.
The good news is that your vegetables only need a fraction of the light the sun provides. Sunlight is so intense it is penetrating, that is it will shine through leaves and onto those below them. This is much less the case with the LED lighting that we will be using and you will find that the plants look somewhat different when grown indoors or outdoors, and may not respond very well to being moved in and out, if like me, you live in an area of extreme sunlight.
You can certainly start plants indoors, and move them outside, and you can, in many cases, move them indoors for a few months. However, they probably will not do very well during that period. Plants adapt to their conditions, and you have changed them radically.
Now, I personally never bring in plants from the outside, and always start from seeds. This, however, has to do with not introducing pests into the garden room. I’ve had some hard lessons.
The PAR Range
A certain range of light colors are usable by plants. This is known as the range of Photosynthethic Active Radiation (PAR). The PAR range has portions, which I briefly described above, in the section under light color. These are the same colors, but extended, beyond visible light, to the full range used by plants.
It’s hard to make a detailed description, that applies to all lamps. This is because some are made for flowering and fruiting, while others are made to promote stem and leaf growth. This will be described on the box that the lamp comes in. Usually, there will also be a graph showing the PAR range and how much of what parts that lamp produces.
To make matters more confusing, light color in this case is not expressed in terms of degrees kelvin, but rather in nanometers (nm). There’s this formula, known as Wren’s Law, that enables this.
I’m writing to help you garden indoors, and don’t want to get too far into the weeds on all this. I’m trying to get you to the level that you can interpret what you see on the box, or the website, that is telling you about the lamp. I also hope to give you the language you would need to look up for information on the internet, without boring you too much.
The PAR range exceeds what is visible to the human eye. Before, I spoke of a blue and a red high and low lighting temperatures. In PAR lighting, the blue end extends all the way to what people call ultraviolet, and the red end also extends into the invisible infrared. 380 nm light is the ultraviolet end, while 750 nm is the infrared end of the PAR spectrum.
PPF (Photosynthetic Photon Flux)
PPF is the measurement of how much PAR light is produced by a lamp, while PPFD is the amount of PAR light produced in a given area of the lamp. Usually, this is the whole plant facing surface area or the lamp.