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MEASURING LIGHT

As well as understanding chlorophyll and the absorption spectrum, we need to know how to measure the different quantities and qualities that light can have. One of the obstacles to doing this effectively is that light is one of the central fundamentals of reality; it does not change, but our perceptions of it do as we try to measure it in different ways.

Wavelength, Frequency & Colour

photosynthetic light spectrum,

Measured one way, light is a wave of electro-magnetic energy moving through space at a constant speed. As with all waves, there are several numbers that describe the qualities of the wave being measured. These are the speed of the wave (which for light is constant), the length of the wave (usually measured in micrometers (μm) or nanometres (nm)), and the frequency of the wave (measured in Hertz (Hz) which is waves per second).

Because the frequency is the speed divided by the wavelength, and speed for light always stays the same, as the wavelength of light increases it’s frequency falls in direct proportion. These measurements are used to determine the colour of the light that is emitted from an object.

Particle Flux & Illuminance

Measured another way, light is a series of particles called photons that are emitted from a light-source at a certain measurable rate, and fall upon a surface at another measurable rate. Particles, unlike waves, can be counted, and large numbers of particles are usually counted in moles. 1 mole is about the same as 6×1023 particles, which is the number 6 with 23 zeroes behind it.

When measuring photons, the number is usually measured in micromoles (μmol) or one millionth of a mole. One millionth is 1×10-6, which when combined with moles gives us about 6×1017 particles in a micromole. We are more interested in light falling upon a surface (incident light) than that given out by the bulb (emitted light), because we want to know how much light is falling upon the leaves of our plants.

When photons fall upon a surface, we can now measure how many photons (μmol) fall on one metre squared of surface (m2) per second (s-1) or (/s). Combined together this gives the Photon Flux, which is measured in μmol m2 s-1, and is also called the Illuminance, and can be measured in Lumens (lm). One lumen (lm) is one μmol m2 s-1. If we only count PAR photons, that fall within the photosynthetic spectrum we end up with a measure called the Photosynthetic Photon Flux (PPF).

The total amount of usable light a plant receives over the 12 hour cycle will be equal to x μmol m2 s-1 x (12h x 3600sec/h), where x is the YPF of your illumination.

Even though a photon is of the correct wavelength, and falls on a plant’s leaf, it does not necessarily mean that it will be absorbed by a chlorophyll molecule and contribute to photosynthesis. There is a fairly good chance it will just pass straight through the leaf, or hit and be absorbed by some non-photosynthetic molecule like starch or cellulose.

A more useful measurement is generated by using the absorbtion spectrum for leaves above and combining it with the PPF to give the Photon Flux that the plant actually uses, which is called the Yield Photon Flux (YPF). As the PPF increases within the PAR range of light, the YPF will increase in direct proportion, but the YPF will always be smaller than the PPF because of the various inefficiencies of the leaf. Assuming that your grow room has a twelve hour light cycle, and since there are 3600 seconds in an hour, the total amount of usable light a plant receives over the 12 hour cycle will be equal to x μmol m2 s-1 x (12h x 3600sec/h), where x is the YPF of your illumination.

Energy, Luminosity & Irradiance

A more familiar way to measure light is as a type of energy, in Watts (W), which is Joules per second (Js-1). Watts are a measurement of Power, which is just energy per second. Here, we need to know how much energy the bulb is using (input), as that will affect our electricity bill. We are also interested in how much energy the bulb is emitting (output), especially output within the PAR and PUR range. This is where the ideas of PARwatts and PURwatts come from.

The amount of energy per second given off by a bulb is known as the Luminosity of the bulb. The original measurement of luminosity was the foot-candle (fc or ft-c) which is how much light in lumens is falling on an area of one square foot that is one foot away from a candle. The metric equivalent is called the Lux (lx) or metre-candle, which is the same but with feet replaced by metres to give the energy of light falling upon one square metre of area which is one square metre from a candle.

The total amount of energy falling on one m2 of surface per second is called the Irradiance. This is similar to Photon Flux, but instead of measuring the number of light particles, we are measuring their energy. This is important because high-frequency low-wavelength light has higher energy than low-frequency high-wavelength light. We measure this in W m2 s-1.

Although PUR light is the only kind that can be utilised for photosynthesis, plants rely on many other wavelengths of light to gather information about their surroundings.

LIGHT EFFECTS

Now that we can measure the light and understand how plants use it, we can turn our attention to what varying the quality and intensity of our lighting can have on our plants.

Although PUR light is the only kind that can be utilised for photosynthesis, plants rely on many other wavelengths of light to gather information about their surroundings. Due to their relative immobility in comparison to animals, plants have become extremely effective at gathering data about their external environment and using that information to alter their interior processes.

Plants that do not receive enough light grow quickly upward and become etiolated, or long and thin. This is an evolutionary adaptation, as growing quickly upward is usually a good strategy when light levels are insufficient. The mechanism that allows this to happen relies on the destructive effects of light, in particular Ultra-Violet or UV radiation. The light falling on the plant can break molecules apart, and causes the number of intact growth hormone molecules to fall. This fall in growth hormone stops the plant growing upward so quickly, and so prevents the etiolation of the plant.

Too much light can cause the plant to deliberately alter the interior cellular and molecular structure of it’s leaves.

Higher light intensities can also cause problems for plants. The chlorophyll family is in constant balance between creation and destruction, and the chemicals and energies that the plant uses to harvest energy from the sun have the potential to be intensely destructive to the delicate molecules of the photosystem.

A great deal of the photosynthetic system is concerned with harvesting energy which is then used to prevent the destruction of the photosystems and their accompanying proteins. Too much light can cause the plant to deliberately alter the interior cellular and molecular structure of it’s leaves, tilting the receptors away from the incoming radiation so as to reduce it’s harmful effects.

The plant can also grow defences against prolonged exposure to excessive light, thickening its cell walls. These processes take time however, so quickly varying the light intensity to high levels can damage the delicate photosystems by overloading the leaf with the more destructive products of photosynthesis.

More modern research has indicated that Infra-Red or IR light is used by plants as a way of synchronising their flowering and reproductive patterns. As the quantity of infrared light emitted by the sun varies over the year, this allows plants of the same species to grow and develop their flowers at the same time. This is again an evolutionary adaptation, as it maximises the chances that pollen from another plant of the same species will be transported to the flower and result in fruit. Therefore, the amount of IR light can be increased artificially by the grower in order to induce flowering at the desired time.

In conclusion

This is a vast subject that people spend their entire lives working in, so this article can’t do anything more than give a quick look over the main topics relevant to hydroponic growers. Future articles will look more closely at how bulbs emit particular qualities of radiation, how to maximise the efficiency of your lighting, and what kinds of plant do well under different qualities of lighting.

(Read Part One of this article here.)

This article was originally published in Issue 002 of HYDROMAG (November – December 2012).

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