Solid-State Lighting and the Growth of LEDs for Horticultural Applications

The advent of light emitting diode (LED) lighting has reborn the lighting business. Due to their energy efficient and long-life characteristics, LED lights have become not only the de facto replacement lamp for traditional incandescent bulbs, but have also opened up many new use cases. Their size has been a key consideration, and they are physically more robust than glass bulbs. They have found applications in the home due to ever increasing electricity bills. Many national government organizations have implemented consumer campaigns to phase out bulbs that consume too much power in order to speed up adoption of more energy efficient lighting. As LEDs have gained acceptance for consumer and industrial applications, so has their development advanced. The ability to optimize and manufacture LEDs to emit particular wavelengths of light has been a fairly recent innovation that has generated a lot of interest from the horticultural community.

Prior to LED lighting, there was a lot of research into the use of other lighting technologies for indoor horticultural applications. Such work looked at the complex relationships between light wavelengths and plant growth, establishing the study of photobiology, biochemistry, and plant ecophysiology. Due to their energy efficient attributes, smaller dimensions and simplistic drive methods, LEDs have quickly become the ideal candidates for use in grow lamps. By comparison, traditional methods of grow lighting were not only much less energy efficient, but also generated a lot of excess heat that in turn increased the need for climate control within the greenhouse.

Much photobiology research has identified and focused on a plant’s response to different light wavelengths in what is termed the photosynthetic active radiation region (PAR). Typically, the wavelengths that produce the most growth, in terms of a photosynthetic response, are in the range from 400 nm to 700 nm. Specific wavelengths have been identified that promote the most amount of photosynthesis activity per proton, while other wavelengths are relevant for controlling germination and flowering. 650 nm, an orange-red color, for example, offers the highest amount of photosynthesis. Collaborative research between horticultural research centers, growers, and lighting manufacturers is now paving the way for standards to be established that correlate the different stages of plant growth and the wavelength for different plant crops. Additionally, given the environmental conditions in which high humidity grow lights operate, future standards will evolve to grow light specification, operation and selection. The research is also guiding LED manufacturers to produce devices that suit particular types of plant. Different plant varieties require a different spectrum of light and intensity in order to achieve optimal growth throughout the life cycle. For example, 450 nm (deep blue) light and 660 nm (hyper red) light yields the best photosynthetic results, 730 nm (far red) typically is used to control the plant from germination, while full spectrum white light encourages green growth.

Figure 1: Spectral power design of OSLON family of LED emitters from Osram.

An example of LED emitters specifically designed for use in horticultural grow lamp applications is the OSLON SSL family from Osram, offering several different horticultural lighting colors in 450 nm (deep blue), 660 nm (hyper red) and 730 nm (far red). Packaged in a robust 3 mm x 3 mm corrosion resistant ceramic package and with a number of radiation angle options (80, 120 or 150 degrees), the series suits tight clustering of LEDs within luminaires. The ceramic package offers a maximum operating temperature of 135°C, allowing for designs that do not require additional cooling for most greenhouse environments. Their maximum drive current is 1 A.

The series suits a wide range of growing installations such as overhead lighting, rack lighting, and multilayer growing. They can be used as the single source for light without any natural daylight, or as a supplement.

The availability of various wavelength LEDs constructed in the same package size gives manufacturers of solid-state growth lighting the opportunity to construct luminaires with different wavelength ratios without having to significantly change the PCB or the luminaire design. Also, the availability of LEDs with varying radiation angles can eliminate the need to design lenses to provide a given beam angle. Not only does this allow for the design of a much simpler luminaire, but also one that is a fraction of the weight of a luminaire using competing technologies.

In many industries, reducing the size of end products is a constant challenge. This is also true of horticultural lighting applications where the diversity of growing methods is increasing. There is more consumer interest in creating local, sustainable, yearlong indoor growing methods such as using vertical multiple racks to maximize crop yields in the minimum amount of space. The drive is on to further minimize the dimensions of grow luminaires for this market. LED manufacturer Cree offers its XLAMP XQ series of 1.6 mm x 1.6 mm emitters that facilitate the design of the smallest and highest density luminaires available. This series is available in white, royal blue and red, and as a high-intensity or high-density device. An example, shown in Figure 2, is the Cree XLAMP XQ-E red emitter. This high-density LED emits light at 625 nm wavelength and has a radiation angle of 130 degrees. Offering design flexibility without having to compromise on lumen output or reliability, the high-density devices allow manufacturers to reduce their luminaire dimensions while maintaining performance characteristics similar to 3.5 x 3.5 mm LEDs.

The high-intensity emitter versions utilize an integral optic that aids in delivering the maximum radiant flux from an extremely compact footprint.

Figure 2: Cree’s high-density and high-intensity XLAMP XQ series.

For designs that utilize the popular 3.45 mm footprint, the Cree XP and XT series provide variants offering high lumen and ultra-high reliability. The XT-E L04 is an example of a royal blue (453 nm) device that has a radiant angle of 140 degrees. Figure 3 illustrates its spatial distribution curve with radiant intensity plotted against viewing angle.

Figure 3: Spatial distribution of Cree XT-E royal blue.

An example of a white high-efficacy emitter is the Cree XLAMP XT-E 4500 K neutral white emitter. Figure 4 illustrates the typical spectral power distribution across the XT-E white family of devices. The 3.45 mm x 3.45 mm footprint of this series lends itself to the established ecosystem of luminaire parts and PCBs, which aids manufacturers in simplifying the design process and speeding time to market.

Figure 4: Spectral power distribution of XT-E series of white LEDs.

To help design engineers faced with developing a horticultural lighting application, Cree provides an introductory guide to the portfolio of products mentioned above. This includes two luminaire reference designs that showcase the capabilities possible using Cree’s LED product line-up. The first one is a small lightweight linear luminaire that provides up to 311 umol/s at its highest power. The second design highlights a higher power luminaire that is designed to provide a similar output compared to an industrial 1 kW high pressure sodium (HPS) grow light. By comparison, the LED design yields comparable levels of photosynthetic photon flux density with half the power (553 watts). Also, the lifetime of an HPS light is typically 10,000 hours, while the Cree LED reference design lifetime is in excess of 90,000 hours. Further, the concept behind solid-state LED grow lights is that since they comprise individual LED emitters, it is possible to design a luminaire that meets the optimal spectral performance that a particular plant requires. This is not the case with an HPS lamp since there is no possibility of tuning the lamp’s spectral output.

Figure 5 illustrates the spectral density output of both a 1 kW HPS lamp and the Cree reference design against the McCree horticulture research curve of the optimal photosynthesis response.

Figure 5: Spectral output comparing reference design, HPS lamp and McCree curve.

Another supplier of LEDs specifically designed for horticultural applications is LED Engin. Its LZ1 series includes products such as the LZ1-00R202, a 660 nm deep red emitter that delivers 5.7 umol/s at 2.6 watt power dissipation, and is packaged in a surface mount ceramic package measuring 4.4 mm x 4.4 mm. This robust device suits applications with high ambient temperatures and high humidity. A maximum drive current of up to 1.2 A can be accommodated.

Providing the power to solid-state LED luminaires requires a constant current driver, such as the Cree LMD800. Accepting an input voltage range from 120 V_AC to 277 V_AC, it provides a constant current output of up to 2 A with an output voltage range of 28 V_DC to 54 V_DC. A 0 to 10 V_DC analog dimming function is included. Anticipated operational life is 50,000 hours at an ambient room temperature of 25°C. Mounting studs provide a means of attaching the 104 watt driver directly to a luminaire, or to its supporting structure. The unit is IP20 rated.

Another LED driver example is the VLED15-120-1250 from CUI. This constant current driver provides a maximum output current of 1.25 amps and an output voltage in the range of 8 to 12 V_DC. For those applications requiring significantly more power, Delta Electronics has recently launched a 320 watt external driver unit, the LNE-12V320WDAA. This supply accommodates the universal wide range of input voltages from 90 V_AC to 305 V_AC, and provides a 12 V_DC output capable of delivering a maximum output of 22.5 amps.

There are many vendors of LED driver modules which helps manufacturers select second source products. Examples include CUI, TDK-Lambda and XP Power. When selecting a suitable driver for use in horticultural applications, the design engineer should review the environmental characteristics that the supply needs to operate in as much as the electrical parameters required. This includes moisture and fluid ingress, conditions highly likely to be encountered in many growing applications. Also, a decision is required on whether the supply will be packaged within the luminaire either separately or on a PCB, or perhaps externally mounted. Further, with many grow lighting systems designed for unattended operation, the capability to control driver units from a host computer would be a beneficial feature making system integration a more straightforward task. All of these factors need to be carefully considered.

Conclusion

As we search for more creative ways to feed the planet’s growing population, horticulture finds itself at the leading edge of plant and crop research. Whether used for research purposes or commercial crop production, the use of solid-state LED lighting techniques is becoming crucial. With such an energy efficient light source, growers can save money in addition to increasing crop yield.

There has been much research into the use of artificial light sources and the impact on plant growth over the past thirty years, but it is only with the recent innovations in LED technologies that cost effective growth lighting has been possible. In particular, achieving high lumen outputs from compact emitters as well as being able to tune spectral output, has been key.

With such innovations, horticulture is now advancing to embrace micro-growing projects for homes and communities in addition to traditional, large-scale greenhouse crop production.