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LEDs and Photosynthesis

Posted on  biology light indoor

Let’s take a look at how plants convert light into energy today. Knowing a little bit about how plants work can help when you need to provide supplemental light indoors, so I’ll also go over some different types of light sources and how good or bad they might be.

Photosynthesis

At its most basic, a plant in its natural habitat requires water from the soil, carbon dioxide from the air, and light from the sun, along with a dozen or so other elements in increasingly small amounts. I’ll save those lesser requirements for another post on fertilization, and will focus on light in this post.

Photosynthesis is the process by which plants use these compounds to create energy for growth and fruit production. Several compounds within the plant are involved, but the main one is called chlorophyll and comes in two flavors: a and b. You can think of them as little green bubbles inside of every plant cell.

Chlorophyll inside a plant cell

These react to specific wavelengths (we see them as colors) of light. Below you can see the exact wavelengths that chlorophyll and various other compounds present in some plants can absorb and use to create energy.

Pigments involved in photosynthesis and the light frequencies they absorb

As you can see above, chlorophyll a & b absorb mostly blue (400-500nm) and red (600-700nm) light, which explains why we see plants as green. Green light is not absorbed, and is instead reflected back off the leaf and into our eyes.

The other compounds in the chart are produced in some or all plants and help the plant to use more of the available light. For example lycopene, which is present in autumn olives, allows the plant to make better use of green light (500 - 550nm).

For the curious, here is a more in-depth explanation of photosynthesis. For now, though, just remember that chlorophyll love red and particularly blue light frequencies.

While blue light is very important for plants, don't assume that they can do without red light after the seedling stage! Red light is essential for plants to tell time using a red-sensitive pigment called phytochrome. This means that red light is important for initiating new growth, flowering, fruit production and hardening off for the winter.

Light Types and Spectrums

Society has come a long way since the days of candlelight. We now have dozens of options when choosing lights for our homes. There are light temperatures, energy use, heat production, and of course cost to think about when choosing light sources.

Daylight

First, I want to look at the one true reference light for life on Earth: our sun. It produces a full range of colors with lots of blue and green light.

Daylight spectrum

As you read about other sources of man-made light, keep the daylight spectrum and the chlorophyll ranges in mind.

Incandescent

Incandescent lights have gotten a bad reputation for being inefficient. These days, a 60 watt incandescent can easily be replaced with a 9-14 watt alternative, saving you a lot of electricity. They also produce most of their light in the infra-red range, which is invisible to the human eye but can be felt as heat on your skin. Here is the light’s spectrum:

Incandescent light spectrum

As you can see, it produces almost exclusively red light, which is okay but not great for photosynthesis. This combined with the excessive heat output and high energy use make incandescent lights a poor choice for plants.

Incandescent Bulb Value
Electricity use (lower is better) high
Heat output (lower is better) high
Cost (lower is better) low
Produces blue light poorly
Produces red light well

Fluorescent

Fluorescent lights come in both long tube worklight bulbs and in more compact traditional sizes. Both produce about the same spectrum and use significantly less electricity than incandescent lights.

Fluorescent light spectrum

A 60 watt equivalent fluorescent bulb will use about 13 watts. It will generate some heat, but not anywhere near as much as the incandescent. There is a good amount of blue spectrum, so overall fluorescent bulbs are a good choice for plants so long as you use a cool color temperature (e.g. 5000K).

Fluorescent Bulb Value
Electricity use (lower is better) medium
Heat output (lower is better) low
Cost (lower is better) medium
Produces blue light well
Produces red light okay

Metal Halide (HID)

Metal halide is a type of high intensity discharge light source with inexpensive bulbs available.

Metal halide spectrum

A 60 watt equivalent metal halide bulb will use about 12 watts. It will generate quite a bit of heat and not much light in the blue spectrum. Most of the light it generates is yellow and is not readily used by most plants, making this a poor choice.

Metal Halide Bulb Value
Electricity use (lower is better) medium
Heat output (lower is better) medium
Cost (lower is better) low
Produces blue light poorly
Produces red light okay

High Pressure Sodium (HPS)

HPS is a popular type of grow light sold in many places. They are available in very high wattages which allow you to light a large area with few lights. The bulbs themselves aren’t too expensive, but require special digital ballasts which can be expensive and require cooling.

HPS spectrum

A 60 watt equivalent HPS bulb (if it existed) would use about 13 watts. It will generate a lot of heat and not much light in the blue or red spectrum. Most of the light generated is wasted or poorly absorbed. Given enough power to generate a good amount of blue and red light these can be a decent choice, but because there are such better alternatives I think HPS lamps are a poor choice for plants today.

High Pressure Sodium (HPS) Bulb Value
Electricity use (lower is better) medium
Heat output (lower is better) high
Cost (lower is better) medium
Produces blue light poorly
Produces red light poorly

Light Emitting Diodes (LEDs)

LEDs are the newest technology on the market and are rapidly both dropping in price and generating more light with fewer watts used every year. LEDs are special in that they can be built to generate specific wavelengths of light, so I’ll first focus on the general household bulbs available and then take a look at specialized LEDs which focus on the plant’s absorption spectrum.

Daylight Household Bulb

This is the type of bulb that fits a regular E26 socket or into a worklight fixture that would normally fit a tube fluorescent. You can buy these at home improvement stores and probably even your local supermarket.

Daylight household LED spectrum

As you can see, other than the cyan valley it pretty closely matches daylight sun, with lots of usable light in the blue range and a good bit of red as well. Unfortunately a lot of the green and yellow is not really usable. A 60 watt equivalent daylight (5000K) LED bulb uses about 9 watts and produces very little heat. I can pick my bulb up in my bare hand after it has been on for 14 hours. These make for great general purpose lights for plants, but can be a bit expensive.

Household LED Bulb Value
Electricity use (lower is better) low
Heat output (lower is better) low
Cost (lower is better) medium to high
Produces blue light well
Produces red light okay

Specialized LED Fixtures

Specialized fixtures are available in various places online (Amazon, eBay, AliExpress) and locally at hydroponic grow shops. While the grow shops probably deal with a lot of marijuana growers, there are plenty of other reasons to get specialized lighting for plants: vegetable starts, berry propagation by cuttings, indoor overwintering in harsh climates, etc.

Specialized LED for plant growing spectrum

As you can see, the LEDs are specially designed to output just the right frequencies for chlorophyll to do its magic, wasting very little light and heat in the process. The blue and red peaks will depend on just how many blue and red LEDs are in the light fixture (and recently manufacturers have been adding UV and IR LEDs as well). The household daylight LED included some wasted green and yellow light, so you can expect to get up to 30% more usable light for the same wattage with these specialized LEDs, which are extremely efficient.

The lights look purple and dimmer than a normal light, which is something to consider if you will be around them. Personally, I’ve built a box around mine to keep the lights from bothering me during the day and making my house glow purple at night. This is an example of how they look, to give you an idea:

Specialized LED for plant growing looks dull purple to the human eye

They make an excellent source of light for plants, but they cost a bit more than household lights and can’t be repurposed during the summer for other tasks.

Specialized LED Bulb Value
Electricity use (lower is better) low
Heat output (lower is better) low
Cost (lower is better) high
Produces blue light well
Produces red light well

Light Intensity

Another incredibly important aspect of growing plants indoors is the light intensity, which you typically see measured in lumens. Many bulbs today list the lumen output, and in fact that is how manufacturers determine what a 60 watt equivalent bulb is: their lumen values will be similar to a 60 watt incandescent light.

Lux, which is another important metric to know, is the measure of intensity over area. Specifically one lux is defined as one lumen per square meter.

Illuminance Illuminated by
1 lux Full moon on a cloudless night
50 lux Family living room
400 lux Sunrise / sunset
1,000 lux Overcast day
10,000+ lux Daylight (not direct sun)
32,000+ lux Full sunlight (up to 100,000 lux!)

What does this mean? If you have an area of one meter by one meter and a 100 watt equivalent daylight LED bulb (14.5 watts, 1500 lumens) you would have the same light intensity for your plants as an overcast day. An LED tube light that uses 22 watts would give off around 2,200 lumens. With four of those you can get into the 10,000+ lux range and your plants will be much happier, but it will use more electricity.

If you want to cover an area that is two meters by one meter then you will now have a value of 4,400 lux (8,800 lumens spread over two square meters). Remember, the larger the area the more light you will need!

The Problem with Lumens

The problem with lumens, however, is that it is a value weighted to the human eye, which sees mostly green/yellow (that’s why purple lights look dull). Remember, plants respond much better to blue and red light. Here is what the human eye sees best:

Human eye spectral efficiency

For comparison, here is the total plant response, within which is the active region for chlorophyll. Notice that it’s almost the opposite of what the human eye sees:

Photosynthetic Active Radiation spectrum

Unfortunately most manufacturers only list lumens. If you do purchase specialized LED grow lights then some manufacturers may list information about the photosynthetic active radiation (PAR) intensity in watts or photosynthetic photon flux density (PPFD). That is a much better measure of usable intensity, but feels a lot like comparing apples to oranges when looking at household bulbs that only list lumens.

Keep in mind that a specialized LED that produces 1500 lumens of mostly red and blue is significantly better for plants than a household daylight LED that produces 1500 lumens with a lot of green light. The specialized light will be much brighter (to the plants) and produces almost no or very few wasted frequencies. You’re better off comparing the wattages and assuming the specialized LED will be 30% better at the same watt usage.

You may see the household daylight LED has an efficiency of 103 lumens per watt while the specialized bulb has only 30-50 lumens per watt - just remember that's comparing it to the human eye and not to what plants need. Use what you know about the light's spectrum along with the wattage to determine what is best for your use case!

Good luck!

My general rules for picking lights:

  1. Plants love blue and red light, so choose a high color temperature (5000K) to produce lots of blue. These are mainly marketed as ‘Daylight’ bulbs or special LEDs which produce blue light.
  2. Try to keep your heat production low and you won’t need to worry about cooling.
  3. Try to keep your energy use low because the lights will run long hours every day. Expect to run them 14+ hours a day.
  4. Produce enough lux to approach daylight. If you can get PPFD values, shoot for around 400 to 500 µmolm2·s photons (around 26 molm2·day).

Hopefully that has given you some insight into the different types of lights and why LEDs can be a great choice for providing supplemental or full light indoors for plants. Happy growing!