Light Emitting Diodes and other light sources

Light Emitting Diodes

Light emitting diodes provide a more precise lighting system. Narrow wavelengths are available, including the three major wavelengths of interest in plant science research- red, far red and blue light. Each of the three wavelengths affects plant growth in different ways. LEDs offer the following advantages:

• monochromatic light source with narrow band width
• very little heat output, low voltage
• instant start up, rapid switching options
• very high relative intensities are possible as the LEDs can be placed very close to the plants.

LEDs are available in specific narrow band wavelengths and unlike our fluorescent monochromatic light sources there are no other peaks to filter out. Thus when used in our light proof cabinets, the only source of light is in the wavelength of interest. The spectral distribution charts below illustrate the narrow monochromatic nature of our LED light sources.

Figure 1: Spectral distribution of red fluorescent and red LEDs.

Figure 2: Spectral distribution of blue fluorescent and blue LEDs.

Figure 3 Spectral distribution of far red fluorescent and far red LEDs.

When designing our LEDs we calculated the best spacing for us to achieve a compromise between intensity and coverage. The LEDs were placed 10mm apart and encased in 60cm lengths of polycarbonate tube for protection. Perspex tends to craze and longer tubes build up heat in the middle. We constructed frames to mount the tubes in that can be placed in any of our growth cabinets. Intensity is adjusted by height, spacing or variable output on the transformers.

Figure 4: LED light mountings.

The instant start capability of LEDs allows us to examine the effect of rapid pulses of light. We can also examine R/FR switching and B/R/FR ratios by placing different coloured tubes in the frames and switching them separately.

Other light sources

Figure 5. The commonly used light sources for plant science research are cool white fluorescent, incandescent and high intensity discharge lamps. Incandescent globes (Figure 5) have a high proportion of far red light, so are good for photoperiod studies however, have low photosynthetic efficiency.

Figure 6. Cool white fluorescent have good photosynthetic efficiency but low levels of far red light.

Figure 7. In growth cabinets the two light sources are often mixed to provide a broader spectral range

Figure 8. For higher light intensities, high intensity discharge lamps are often used in growth cabinets. However, the heat output must be managed via separately ventilated light lofts. Thus a glass or plexiglass barrier separates the lights from the plants and for very high light intensities (and thus high heat loads), a water barrier filter is often also included. This reduces the thermal load without altering the photosynthetically active wavelength transmission significantly. High intensity discharge lamps for plant growth are usually metal halide, high pressure sodium, or a combination of both. Metal halide (Figure 8) have higher blue proportions than sunlight.

Figure 9. While high pressure sodium (HPS) have lower blue and more red

Figure 10. Mixing the two sources together provides a more even spectral distribution

At the School of Plant Science, all of these lighting options are available to provide a range of light intensities and spectral options for specific projects. With accurate control over temperature and photoperiod, our growth cabinet options can meet most requirements. Our controlled environment glasshouse offers the option of accurate temperature control under natural light conditions