Problems with Foot-candles, Lux and Lumens By Erik Runkle and Bruce Bugbee

There are three ways to describe light intensity, but only one of them is appropriate for photosynthesis. Increasing use of light-emitting diodes (LEDs) means that the appropriate unit for light measurement is more important than ever. Although light quality and timing are powerful tools for altering plant shape, light intensity provides the horsepower to drive photosynthesis.

Photometric: Foot-candles and lux. These units of measurement are based on the perceived brightness to the human eye. Our eyes perceive green and yellow light much better than blue or red light, so this measurement system is completely biased toward people and is not appropriate for plants. Since most lighting applications are for people, this system is used by lighting professionals and utility companies, who usually optimize lighting for people. Therefore, the efficiency of lamps is almost always reported as the luminous efficacy, which is the number of lumens per watt of energy. (One lumen per square meter equals one lux, and there are 10.8 lux per foot-candle.)

Radiometric: Watts. This unit is used to determine the energy input and output of lighting applications. It describes the power of light that a light source emits or consumes. The light energy increases as wavelength decreases, so it takes more energy to make blue light than red light. Thus, this measurement system is based on power and is used to predict the heating value of light. It is not specific to plants or people.

Quantum: Moles of photons. The Stark-Einstein Law tells us that one photon excites one electron in photosynthesis, regardless of color, so counting photons is the best way to predict photosynthesis. The number of photons in the photosynthetic waveband (400 to 700 nm) is typically reported as micromoles per square meter and second, or µmolám-2ás-1. This system considers all colors of light equally and is the most appropriate way to measure light intensity for photosynthesis and plant growth. Measuring moles of photons is not biased toward human vision; in fact, an intensity of deep blue and deep red light that appears dim to us can be bright light for plants.

Measuring light intensity in foot-candles or lux is especially unsuitable when using LEDs for plant growth. Blue and red LEDs are most often used for plant lighting because:
1. They are the most energy-efficient colors of LEDs
2. They are considered to be more photosynthetically efficient than green or white light
3. The addition of 10 to 20 percent blue light to red light produces a more "normal" plant shape; most plants grown under only red light are elongated.

However, since we don't perceive either blue or red light as well as green and yellow light, the luminous output of blue and red LEDs is low and they look dim (Table 1). Don't let your eyes mislead you; lamps that emit these wavelengths can deliver bright light for plants.

There is an even more dramatic difference in measuring horticultural lighting in lumens/foot-candles/lux versus µmolám-2ás-1 when delivering far-red light. To us, far-red light (around 730 nm) is barely perceptible yet it is necessary for photoperiod extension lighting of some crops. This difference is apparent with both incandescent lamps and far-red LEDs; their luminous output is low compared with their output in photons.

For these reasons, the horticultural industry should discontinue use of photometric units such as lumens, lux and foot-candles. These units are based on people's perception of light and are a highly misleading indicator of plant growth and flowering applications. We need to work with lighting professionals and utility companies to develop and adapt plant lighting standards based on photons and not lumens.



Erik Runkle and Bruce Bugbee

Erik Runkle is associate professor and floriculture extension specialist in Michigan State University's department of horticulture. Bruce Bugbee is a professor of crop physiology at Utah State University. Erik can be reached at runkleer@msu.edu or Bruce can be reached at bruce.bugbee@usu.edu.



Latest Photos see all »

Lupine Staircase series (Green Fuse Botanicals)
IMG_0518 copy
IMG_0469 copy
IMG_1488 copy
IMG_1587 copy
IMG_1517 copy
40under40
GPN recognizes 40 industry professionals under the age of 40 who are helping to determine the future of the horticulture industry. These individuals are today’s movers and shakers who are already setting the pace for tomorrow.
Hancock 1 2
MattNagle_0
McGinty 1 My favorite xoxo
Screen-Shot-2012-05-21-at-10-07-12-AM.png
PJs
0112katedav
vbn-dvb-df-001
Ryan 2
Benoit 2 11295675_1596556550596534_623036435284185172_n

75 Applewood Drive, Suite A
P.O. Box 128
Sparta, MI 49345

616.887.9008
GPN September_Cover
Interested in reading the print edition of Greenhouse Product News? Preview our digital edition »

Get one year of Greenhouse Product News in both print and digital editions for free.

Subscribe Today »


Be sure to check
out our sister site.
website development by deyo designs