Light is the form of radiant energy that powers plant growth and development. However the light we see is only a small part of the whole electromagnetic spectrum and plants need certain wavelengths of lumens that are not within the human visible spectrum. We can see best in light of 500 - 600 nm (the green, yellow and orange range which is optimized in the yellow-green range of 500nm). Plants on the other hand prefer the 400-700nm waveband, but different plant processes require different wavelengths. Photosynthesis occurs within this 400 -700 waveband and this is termed 'photo synthetically active lumens, but maximum photosynthesis actually occurs in two peak wavelengths 435(blue) and 675 (red). So while we know exactly which wavelengths provide the correct spectrum for maximum photosynthesis in plants, the next step is to calculate just how much light (termed ‘quanta') of these wavelengths is required for maximum growth.

Knowing how much light or 'quanta' is delivered by lighting equipment is important, but often confusing since there are a number of ways of expressing just how much light is falling on a plant surface. Light energy is delivered via photons - this is why the amount of light falling on a surface is often measured as photosynthetic photon flux density (PPFD),which is the number of photons falling per square meter of leaf per second. These photons are measured in milli moles - but not all photons are created equal. Photons emitted in the 400nm wavelength have less energy than those emitted at700nm, so blue light photons have greater energy than red ones. Modern light meters can use a range of units to measure the light energy present on a surface - watts per square meter is a common unit, as is lux or foot-candles.

With horticultural lamps what we are interested in is the light emitted in the photo synthetically active radiation range of 400 - 700nm. For this reason the light output of the lamp compared with the energy input(wattage) is termed luminous efficacy and expressed in lumens per watt(lm.W-1). What we are looking for are bulbs or lamps with a high luminous efficacy - this means the most light in the correct waveband for plant growth per watt of energy required to power the lamp/bulb.

Which Lamp/Bulb

High intensity discharge lighting (HID) consists of two different bulb types: High pressure sodium (HPS) and metal halide bulbs (MH). MH bulbs come in a range of watt sizes from 175 - 1500 watts, HPS bulbs come in150 - 1000 watt sizes. The luminous efficacy of metal halide lamps is high at between 40 - 100 lm.W-1,  HPS lamps also have a high luminous efficacy of 50 - 140 lm.W-1.  Briefly, the difference between these two light bulb types is that the high pressure sodium bulbs provides more of the red spectrum while the metal halide provides more light in the blue spectrum. These differences are important since various stages of the plant's life cycle are more responsive to different light spectrums and we need to provide the correct spectrum for plant growth. Flowering and stem growth are influenced by the red to far red light, while vegetative growth is more influenced by the blue light spectrum.

There are also lamps which combine both the red and blue light spectrums to provide a more balanced light range for mixed crops which have vegetative and flowering stages present at the same time. These lamps sometimes contain an incandescent filament that emits the red light (in the 540 - 700nm)range and a HP mercury discharge tube which as the blue range(400-40nm).

How Many Lamps/Bulbs are Required What needs to be taken into account is firstly the wattage of your bulb, but also how many bulbs of each wattage are used within a given space and the proximity to the plants. The reason for this is not only the amount of light that will be reaching the plants surface, but also the heat output of the bulb. The higher the bulb wattage, the further away the plant must be from the light source to prevent heat stress that can cause excessive water loss through transpiration and even burning of the foliage. These problems can be reduced with the use of 'water cooled’ lamps. Water is more efficient than air at absorbing the excess heat and removing it from the areas surrounding the plant.

Reusing Light
Reflector use in a lighting system. Redirects light into the plants - the shape and design of this reflector will determine just where the light is reflected . Installing reflective material on all surfaces surrounding the plants will reflect light back into the plant canopy where it can be 're used’ for photosynthesis. Reflection of light however is not as simple as it may seem - light can bounce in all directions and it can also be concentrated into spots which can burn foliage and flowers. There are excellent materials available for light reflection. Flashgro and black and white poly . Never use materials such as tin foil - this can burn plants as the reflected light is never evenly redirected back into the plants. FHD