Light.  It’s easy to take it for granted but what is it and how does it work?  UGM tries to shed some lumens on the subject and discovers that there’s a lot more to light than meets the eye…

The existence of nearly all life on earth is fuelled by light from the sun. Plants are no exception.  They use the energy of sunlight to turn air into simple sugars – a process known as photosynthesis. These sugars are the building blocks which allow plants to grow.

Light can be thought of as the “motor” of photosynthesis. Plants need light to grow and bloom – they absorb light primarily using the pigment chlorophyll, which is the reason that most plants have a green colour.  Photosynthesis occurs when green leaves catch light and transform it, using water as a fuel, into chemical energy. This energy is preserved as matter (sugar composed from carbon dioxide).

When it gets dark, photosynthesis stops. The plant closes its pores as much as possible and the stream of water inside the stem slows down and almost comes to a stop. When light returns, the first pores open to admit carbon dioxide and pass a sort of “perspiration water”. Carbon dioxide is the raw material for sugar – the final product of photosynthesis.  Through this process of evaporation the juice stream, beginning at the roots, commences once again.

In nature, light comes from the sun. The sun radiates the whole spectrum of light, some of which is invisible to the human eye.  We can however feel and detect the influences of the non-visible radiation. We feel infrared as heat and experience the effects of sunburn and pigments of UV-light. Plants, on the other hand, are completely adapted to sunlight. A plant mainly uses the light elements of the visible spectrum from blue to red.

Over the last thirty years a lot of research has been undertaken into the light requirements of plants.  We know that plants need light outside the visible spectrum – the light we cannot see. Whilst some reactions to light have been accurately described and certain photo-active matters have been discovered, the ‘secret of light’ remains a mystery. Assimilation bulbs (such as the Philips Son-T Plus) used for growing plants try to emulate the sun as accurately as possible.

Light influences plants in the following ways:

• The direction of incoming light determines the direction of the plant’s growth.
• The composition of light (the spectrum) influences the metabolism.
• The intensity of light determines the height of the plant.
• The duration and order of the light periods can trigger a new life-cycle (e.g. grow / bloom).

The quality of light

Light moves in waves and the distance between the two highest peaks is known as the wavelength. Each colour we see has a specific wavelength. When the waves change, such as when they are reflected, the colour of the light also changes. The wavelength is measured in nanometres (nm).  One metre is the same as 1.000.000.000 (one billion) nanometres, i.e. one nanometre is a billionth of a metre!

The visible range of light is from blue to red. Visible blue light has a wavelength of 400 nanometres. Visible red light has a wavelength of more than 700 nanometres. Blue light is comprised of more waves than red light over the same distance so it moves more often and with greater speed. Consequently, blue light has more energy than red light.

Beyond blue light is ultraviolet light (UV) – this has even more energy! Further along the spectrum are roentgen and gamma radiation. And, at the other end of the spectrum, red is followed by infrared (IR) and long-wave radio radiation.
In spite of photosynthesis only taking place with light from the visible spectrum, scientists have discovered that plants which receive only light of 400-700 nanometres succumb to more diseases.  They also found that these plants did not grow and bloom as successfully, even when the amount of Watts was increased. We know that plants need both the blue and red parts of the light spectrum. When a plant is growing in the vegetative stage (especially a mother plant) it requires extra blue light. Extra red light stimulates blooming plants insofar as they bloom more often and have a greater number of larger flowers.

Blue parts of light will cause plants to become thick and stout. Light with a lot of blue parts is ideal for mother plants because the plants remain squat and produce a lot of side branches. These form ideal stock for healthy, quick-growing cuttings. Metal halide bulbs produce light with more blue parts.
Light streams, lumens and lux

Just as important as the structure of light is the light flux or luminosity. The luminosity (measured in lumens) of a bulb depends on its electric power (measured in Watts). The specification of assimilation bulbs often refers to lumens (=lm). A bulb of 400 Watts can produce between 30,000 and 60,000 lumens, and 600 Watt bulbs about 90,000 or more.
The luminosity that shines on one square meter is measured in Lux.

Examples of values in Lux include:

Cloudless day: 100,000 Lux
Closed grey layer of clouds: 10,000 – 18,000 Lux
Well-illuminated living room: 500 Lux
Candle at a distance of one metre: 1 Lux
Night full moon: 0.25 Lux

A bulb’s luminosity not only depends on its capacity (i.e. the number of Watts) but also on the distance of the measurement point from the light source. The luminosity decreases if this distance is increased. A young plant that receives too little light will become long and skinny and the distances between the side branches (also known as the internodal length) will increase. In nature a plant only knows the light of the sun. If there is not enough light the plant’s survival mechanism kicks in and it tries to outgrow whatever is shading it. The plant concentrates on becoming taller in order to access more light.  Of course, this vertical growth spurt requires a great deal of energy and if the light conditions don’t improve the plant will begin to die. Leggier plants also require more energy to transport water. A plant that has grown under good light conditions has much more energy, disease resistance, and greater health.

Read the second part of this article…