Lamps for Growing Plants

(LED or otherwise)

Now, for wavelengths and how much light is needed:

Chlorophyll has two spectral ranges of utilization - red and blue.

Plants also have two kinds of chlorophyll - A and B.

Basic photosynthesis will proceed if even only one chlorophyll is utilizing light. Chlorophyll A is the basic one for making chemical energy from light energy. Chlorophyll B is the main "accessory pigment" in green algae and "higher plants", and serves to utilize light at wavelengths not absorbed well by Chlorophyll A and transfer the energy from such wavelengths to the process that uses Chlorophyll A. Other accessory pigments are mainly carotenoids, which mainly utilize blue and violet light.

One source with utilization spectra of various plant photopigments and an overall "action spectrum" for a specific plant frequently studied for photosynthesis:

A Photosynthesis Page at the Maricopa Comunity Colleges.

The same action spectra are also available here.

The specific plant species chosen here appears to me to have spectral response peaks corresponding to those of Chlorophyll A, but has significant utilization of red wavelengths 625 to 680 nm and violet, blue and green-blue wavelengths 400 to 495 nm.

Another "action spectrum" is available at:

this bean plant photosynthesis action spectrum paper", specifically Fig. 7 on Page 4.

This bean plant action spectrum has a red peak and a violet-blue peak at wavelengths similar to those of chlorophyll A. However, the red utilization wavelength range has action at least 85% of peak from about 585 nm in the slightly orangish yellow to 680 nm in the very deep red. Wavelengths in the orange-red range at which action is 90% of peak or more are from roughly 595 nm (yellowish orange) to 675 nm (very deep red).

Unlike many other published action spectra, the blue peak is not as great as the red peak. The blue utilization region has action at least 80% of that at the blue peak at wavelengths from about 410 nm (violet) to about 463 nm (mid-blue).

There is a minor peak in the blue-green at close to 500 nm.

One more datapoint is that high pressure sodium lamps have been used for growing plants. The spectral content of those is mostly in the range from 580 to 620 nm, mid-yellow to orangish red. High pressure sodium lamps have a significant very yellowish green spectral emission feature around 568-569 nm, but that one is not as well utilized by plants as red and blue wavelengths are - that wavelength is mainly useful for usefulness to human photopic vision.

Although stimulating only one photopigment is sufficient for photosynthesis, many plants have some requirement of stimulating more than one photopigment or at least one other than the red utilization of Chlorophyll A for proper growth regulation, flowering and fruiting. Proper growth regulation, flowering and fruiting often requires significant blue light.

Most published spectral action curves do not show well actual utilization, and a few show well ratio of absorption to transmission. Fewer still show ratio of light utilized to light not utilized. If these curves are redrawn to show ratio of light utilized (or absorbed) to incident light, the peaks often become wider and flattened compared to most published "action spectrum" curves. Many of these curves are also inaccurate, showing the peaks as more symmetric than they actually are. The peaks are asymmetric, with wavelengths shorter than peak being used better than wavelengths longer than peak.

The red region has a utilization peak around 660-670 nm for Chlorophyll A and around 635-645 nm for Chlorophyll B, depending on the source of information. Plants generally make good use of all red wavelengths except for ones much longer than 670 nm. 700 nm and longer is close to useless for plants. Most plants actually make good use of orange and even yellow-orange wavelengths, to such an extent that high pressure sodium lamps have been used for growing plants. Keep in mind the above-cited bean plant "action spectrum".

The blue utilization peaks of chlorophyll are mostly reported to be around 430-440 nm for Chlorophyll A and around 453-470 nm for Chlorophyll B. Unlike the red peaks, the blue peaks are usually shown with substantial asymmetry indicating that wavelengths shorter than peak are used well while wavelengths longer than peak are not used well. Beta carotene is a major blue-absorbing accessory pigment, with a double peak at about 450 and 480 nm.

As for how much light is needed:

This page at Wisconsin Center for Space Automation and Robotics describes a plant growth unit illuminated by 670 nm red and 470 nm blue LEDs. Illumination intensity is adjustable, with maximum values (my translation to watts of light per square meter) of about 98 watts per square meter for red and about 18 watts per square meter for blue. If you need to get by on much less than this, I would make these figures more equal in case your plants have specific blue light requirements that need to be satisfied. I suspect plants that do not have especially high needs can fare reasonably well with 25 watts per square meter of red and 9 watts per square meter of blue.

Most Cost-Efficient Lamps for Growing Plants

The fluorescent "grow", "gro", "Gro-Lux" and "Plant and Aquarium" fluorescents by GE and Sylvania are fairly optimized for this. The "similar" Philips lamps appear to me to be different but only slightly worse (from having some significant though minority phosphor content at less-utilized green wavelengths).

Keep in mind that plants and humans are different in lighting needs, so brightness of a lamp as it appears to humans easily fails to correlate well with usefulness to plants.

More ideal I consider to be a hypothetical metal halide lamp using lithium (to get a close match to the red peak of Chlorophyll A) and indium (for blue with some violet spectral content), and a bit of sodium (to provide a bit of stimulus for Chlorophyll B and other photopigments).

However, I find such a metal halide solution to be something that I have yet to be aware of so I consider it "vaporware".

Some Notes on Using Specifically LEDs

As of early 2008, the most suitable LEDs for growing plants are only about as efficient at producing the desired light as better fluorescent lamps of types made for this. LEDs also cost a lot more than fluorescent lamps of the same output.

GaAlAsP red LEDs (peak wavelength typically 660 nm) and Agilent's similar "TS AlGaAs" (peak wavelength near 655 nm) match the Chlorophyll A red peak well. However, Lumileds InGaAlP red LEDs with peak wavelength in the upper 630's nm and dominant wavelength (color specification) in the 620's of nm are better because they are so much more efficient. The shorter wavelength is utilized not much less by most plants than more optimum red wavelengths are.

Keep in mind that red Lumileds Luxeons will achieve a much higher visual level of illumination than fluorescent lamps made for growing plants will at the same amount of watts of light per square meter. This is because red Luxeons produce shorter wavelengths of red, selected because the human eye is more sensitive to wavelengths closer to 555 nm.

The blue response bands of both chlorophyll A and chlorophyll B and also beta carotene are served well by Lumileds royal blue LEDs, which typically have peak wavelength near 450 nm.

Photosynthesis works most fundamentally from red light and secondarily from blue light. However, many plants have some need for blue light for proper growth regulation and/or flowering and/or fruiting.

As for how much light is needed:

Keep in mind that red Luxeons are typically about 26% efficient when the junction temperature is 25 degrees C, and typically about 16% efficient in the more reasonable situation of heatsink temperature of 35 degrees C.

The most efficient royal blue Lumileds Luxeons currently available are LXML-PR01-0225 (of the Rebel series), which are typically 20% efficient at 350 mA with a 25 degree C junction temperature. Thankfully, blue LEDs have a much lower temperature sensitivity than red ones have.

It is likely that a Luxeon K2 TFFC royal blue version and a more efficient royal blue Rebel will become available in the near future, with efficiency around 30%.

Cree already has royal blue XRE series LEDs that are around 22-30% efficient at 350 mA, with a typical peak wavelength around 455 nm - still good.

Keep in mind that plant-growing fluorescents are about 25% efficient at producing desirable wavelengths of light for F40T12, and 20% efficient for the F20T12.

Keep in mind that blue LEDs operate less efficiently with more power (current more than 350 mA, or power greater than roughly 1 watt per LED), even if the amount of power and current is well within their ratings.

Written by Don Klipstein.