Fig. 1: Example of how the full width at half max (FWHM) can be determined. This simulated symmetrical curve represents the spectral output of a narrow waveband LED and peaks at x=6. Once the height of the peak is determined (0.4, as shown along the vertical axis), the FWHM can be determined by measuring the width of the curve at half the maximum (peak). The FWHM value for this example is 2.4 as can be determined from the length of accompanying line segment drawn along the horizontal axis.
Photos courtesy of Rutgers University/A.J. Both

Recent advances in LED technology have reignited an interest in plant lighting research because of four key advantages: reduced energy consumption, increased lamp life, options to manipulate the spectral output, and dimming capabilities. Most of these advantages are of special interest for assimilation lighting strategies that typically require high(er) light intensities.

Some LED lamps are capable of producing radiation outside the photosynthetically active radiation (PAR; with wavelengths between 400 and 700 nm) waveband, including ultraviolet (UV-B and UV-A; 280-314 and 315-399 nm, respectively) and far-red (FR; 700-800 nm) radiation. One nanometer (nm) equals a billionth of a meter. Note that visible light covers the 380-780 nm waveband and includes wavelengths associated with the colors of the rainbow (ROYGBIV).

Of late, research conducted using LEDs that combine PAR with UV and/or FR radiation has revealed new insights into plant growth and development and, in some cases, has opened up new market opportunities for crops that are sold for their enhanced appearance (e.g., leaf color).

All about PAR

Plant scientists commonly divide the PAR waveband into three distinct ranges: blue (400-500 nm), green (500-600 nm), and red (600-700 nm). While convenient, this approach causes a couple challenges: the wavebands overlap (e.g., 500 nm is considered both blue and green light), and it ignores some color wavebands used to describe visible light (e.g., indigo, yellow and orange).

McCree (1972) reported that single plant leaves that were acclimated to relatively low light intensities exhibited the highest rate of photosynthesis when exposed to blue (with peak sensitivity at 430 nm) and red (with peak sensitivity at 610 nm) light.

While these findings do not necessarily apply to the sensitivity of whole plants grown under a variety of light conditions, McCree’s results have long been used as the standard description of a plant’s sensitivity to light. This explains why the 3-way (blue, green and red) division of PAR has worked well and why many LED lamps used for assimilation lighting are designed with some ratio of blue and red LEDs corresponding to the peak plant sensitivities (resulting in an output consisting of variations of the color magenta/purple).

Overcoming waveband overlap

To prevent the issue of waveband overlap, we used the following waveband intervals to define the various (color) ranges: UV = 280-399 nm (including both UV-B and UV-A), Blue = 400-499 nm, Green = 500-599 nm, Red = 600-699, Far-red = 700-800 nm. Using this approach, the PAR waveband (400-700) is slightly different from the combined B-G-R waveband in that the PAR waveband also includes the 700 nm wavelength.

While some scientists only report the peak wavelength when describing the specific light color(s) they used, we believe that this is insufficient to fully characterize the light output from lamps. Some LEDs emit specific colors over a relatively narrow waveband, while others that deliver a spectrum with peaks at the same wavelength can emit light over a much wider waveband. When evaluating color (waveband) ratios, it is important to know how wide each waveband is so as to avoid any overlap from impacting the calculations.

Therefore, we suggest that in addition to the identification of the peak wavelength, the so-called full width at half max or peak (FWHM) of each narrow-band LED type be determined and reported (Fig. 1). Using both the peak wavelength and the FWHM of the spectral peaks more accurately describes the light output and its potential impact on plant growth and development.

Occasionally, scientists report lamp output ratios by counting the number of LEDs that emit across a specific waveband. For example, based on such assessment, a lamp composed of 80 red LEDs and 20 blue LEDs would emit radiation with a R:B ratio of 4 (80/20). This determination is incorrect since it does not include an accounting of the number of red and blue photons emitted by such a lamp. Thus, we strongly encourage the use of a spectroradiometer for all light ratio calculations.

Study methods detailed

We mounted a variety of lamps including incandescent (INC), compact fluorescent (CFL), ceramic metal halide (CMH), high-pressure sodium (HPS; both mogul base and double ended), and several LED fixtures (with different color output and wattage) in a 3-by3 meter darkroom. All lamps were mounted 2 feet above the sensor head of a calibrated spectroradiometer.

The sensor head was placed directly below the center of each lamp and several spectral scans were collected after each lamp had stabilized (i.e., warmed up sufficiently). The scans were averaged and the data were used to determine the spectral output across the various wavebands of interest, as well as the ratios of various waveband combinations (Tables 1 and 2).

The results of the lamp scans were compared to an outdoor scan of sunlight that was collected at solar noon on a clear day that occurred within three days of the summer solstice (June 21) in New Brunswick, NJ, USA.

Take home results

Table 1 shows the total light output (photon flux) across the 280-800 nm waveband (expressed in the units µmol m-2 s-1) for each of the light sources evaluated and for sunlight. The 280-800 nm waveband is sometimes referred to as the biologically active radiation. The INC and CFL lamp were included for comparison only since their low output will make it unlikely that they will be used for assimilation lighting.

For each of the wavebands shown, the table includes the amount of radiation as a percentage of the total radiation across the 280-800 nm waveband. In addition to the efficacy (number of photons emitted per unit of electric power consumed) of the light sources evaluated, Table 2 shows several ratios of the light (photons) emitted in each of the wavebands: R:B, R:G, G:B, R:FR and UV:B.

Obviously, other ratios can be calculated and, most importantly, it should be clear that the outcome of the ratio calculations is dependent on how the various wavebands are defined (i.e., what specific interval each waveband covers). Using different ranges for the wavebands (e.g., using the ROYGBIV wavebands as is common for visible light) will result in different ratios, and may therefore result in different interpretations about how the various lamps compare. This issue highlights the importance of standardization of the waveband ranges so that all researchers interpret their data based on widely accepted definitions.

As Table 2 clearly shows, the calculated ratios for each of the lamps varies substantially. Using sunlight as the benchmark, HPS lamps (and one of the LED lamps) have substantially higher R:B ratios. Similarly, the HPS lamps have much higher G:B ratios compared to sunlight. Not surprisingly, the magenta-colored output of two of the LED lamps has a much higher R:G ratio because their spectrum contains almost no green. Except for the INC lamp, the R:FR ratio for all other lamps is generally (much) higher compared to sunlight. While it is not clear at what point a much-increased R:FR ratio starts to substantially impact typical plant growth and development, these higher ratios resulted from the (very) small amount of FR light produced by most of the lamps.

The recent increased interest in the impact of UV radiation on plant growth and development prompted us to include data on the UV:B ratio. The data show that these ratios can be substantially smaller compared to sunlight. Especially the large amount of UV-A radiation in sunlight contributed to a higher UV:B ratio for sunlight compared to the lamps. Crops that can benefit from (some) UV radiation may need to be radiated with lamps that produce more UV radiation than the ones we tested. Obviously, working with UV radiation can be hazardous to biological tissue, so proper precautions are needed for both humans and plants exposed to such conditions.

While published literature on LED lighting applications is expanding, few reports include accurate information about light ratios across the biologically active radiation spectrum. And when ratios are described, they typically are the R:FR and the R:B ratios. Our results indicate that there can be substantial differences among the calculated ratios for sunlight and electric lamps, and not only for the R:FR and R:B ratios.

We hypothesize that other ratios (e.g., R:G, G:B and UV:B) are also important in some plant species for typical plant growth and development, but more plant research is needed. Fortunately, LEDs offer myriad opportunities to investigate the impact of spectral composition on plant growth and development. We anticipate future research will discover that a balanced light spectrum can be more effective than a spectrum solely based on the peak sensitivity of plants (red and blue) as defined by the McCree curve.

Of all the wavebands important to plants, outside the PAR waveband, the FR waveband has probably been studied the most. FR radiation has been reported to increase the rate of photosynthesis in some plant species. The most common peak wavelength associated with this effect is around 735 nm. Therefore, defining the 700-800 nm waveband as representing FR radiation, as was done here, includes perhaps too wide of a waveband. Researchers have suggested that it may be sufficient to use a narrower range centered around the peak wavelength. Ranges of 735 ± 25 nm, or even 735 ± 10 nm may adequately represent the impact of FR radiation on photosynthesis. Obviously, using narrower ranges compared to the 700-800 nm waveband we used will result in different ratio values depending on the specific spectral output of the light source.

While rather coarse, the division of the PAR waveband into blue (400-499 nm), green (500-599 nm), and red (600-699 nm) seems to work for plant lighting applications. However, this division seems arbitrary from the plant’s perspective and may need to be refined if, and when, research shows that more narrowly defined wavebands have specific and important impacts on plant growth and development. We encourage the scientific community to develop standardized definitions for important wavebands and adjust these definitions as warranted by new research findings.

Research has shown that LEDs designed to produce different colors can depreciate at different rates. That is, their output characteristics (intensity and spectral composition) change at different rates while the LEDs are operated for extended time periods. Therefore, the radiation ratios produced by lamps that contain such LEDs can also change over time. Users of LED lamps should be aware of this, and periodic testing should be performed to ensure that the desired radiation ratios are still provided by the lamps.

The bottom line

We strongly recommend using spectroradiometer data for all radiation ratio calculations.

We report substantial differences in radiation ratios among sunlight and a variety of common lamps used for plant lighting.

Standardization of the definitions of waveband ranges seems appropriate and desirable for plant growth applications.

Lamp efficacy values are but one of several performance characteristics that should be considered when deciding on the best lighting system for a particular application.

A.J. Both is a professor and extension specialist in controlled environment engineering in the Department of Environmental Sciences in the School of Environmental and Biological Sciences at Rutgers University–New Brunswick. Timothy Shelford is a part-time research specialist at Rutgers who also works at Cornell University. Claude Wallace is a Rutgers graduate and part-time employee.