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For growers, light is an essential part of our tool-box. It is the fuel in our car, the plasma in our reactor, the only form of energy our plants can usefully take in. It’s central to everything we do, and photosynthesis is the process which determines how successful our plants are.

In the dark days of hydroponics, growers used standard fluorescent strip-bulbs to illuminate their plants, and received very mixed results. This is because fluorescent light is very different in a number of ways from the daylight provided by the sun, and cannot provide plants with the quality of light they need. To shine a light on this issue, we need to illuminate how plants use light and how we can use technology to maximise our grow room’s potential. Before we start, it might be a good idea to explain why a lot of the information on this subject looks so complicated.


Scientists are not really interested in the truth, but they are very interested in finding out how wrong they are, and where that error might be. This is a paraphrase of Sir Francis Bacon, who is the father of modern scientific thinking. Understanding this is the key to figuring out why scientists seem to make everything so over-complicated and hard to understand.

photosynthetic light spectrum, photosynthesis, measuring light, Hydromag, hydroponics

Each of the equations above describes the process of photosynthesis but as we move down the list the equations become a little less wrong, a little closer to the actual situation. Looking at ‘Water’ we see that it is not just one thing, but, as shown in the second equation, a molecule of two hydrogens and an oxygen atom. In the third equation, we can see that six of these molecules take part in the process. If we extend this process of becoming a little less wrong through several dozen stages we end up here:

photosynthetic light spectrum, photosynthesis, measuring light, Hydromag, hydroponics

Diagrams like this can give the impression that scientists are confusing people, but remember that they are just trying to become a little less wrong. Going back a few stages in complexity and working forward again will eventually switch on that light-bulb (OK, enough puns). Going back to our first diagram, both light energy and chlorophyll are not as simple as the word might appear. Just as ‘Water’ became ‘6H2O’, Chlorophyll can become ‘P700 Photosystem I’. Just identifying that the current level of understanding is certain to be wrong is a valuable insight.


Chlorophyll is the substance that allows light to be absorbed by plant leaves and then stored as carbohydrates.

Each photon or particle of light that is successfully absorbed enables the plant to manufacture more high-energy molecules such as glucose, which are used in a range of cellular processes. Sadly, chlorophyll is not just one molecule in the way that water (H20), or carbon dioxide (C02), or even glucose (C6H12O6) are. It is a family of different molecules that are known by a variety of different names and have different structures and functions. To begin with, we should look for things that they all have in common.

All the molecules in the chlorophyll family are attached to the surface of stacks of membranes called Thylakoids that are found in the Chloroplasts of leaf cells. They all take part in the process of photosynthesis. Other than that, they perform a wide variety of different functions within the process, like different machines on a production-line.

There are three main types of pigment, each of which has its own ability to absorb different colours or frequencies of photons of light.

The molecules in the family we are most interested in are the pigments or ‘Photosystems’. Pigments, such as those in dyes, work by absorbing certain colours of light photons from the spectrum but allowing other colours to pass through. These photosynthetic pigments work by giving the energy of the photon to a set of electrons that then move across the thylakoid membrane due to their suddenly increased energy levels. These high energy electrons then drive the rest of the photosynthesis system in the same way that water falling off a cliff can drive a mill-wheel.

There are three main types of pigment, each of which has its own ability to absorb different colours or frequencies of photons of light. To be even less wrong about things, each pigment is a complex array of different molecules arranged in a disclike structure known as a ‘Photosystem Complex’. These complexes are known as ‘Chlorophyll a’, ‘Chlorophyll b’ and ‘Carotenoids’. These last consist of a subfamily of different types of molecules that don’t actually contribute energy to photosynthesis but use the energy they absorb to prevent the photosystems from destroying themselves as they perform their function.

As you can see in the graph below (click to enlarge) that plants are not good at absorbing green and yellow light. They simply never evolved a pigment that could do the job of absorbing these frequencies of photons. Shining light of these colours at a plant is pointless, like firing bullets at a magazine; they just pass straight through the leaf.

photosynthetic light spectrum, photosynthesis, measuring light, Hydromag, hydroponics


Wavelengths of light that are absorbed by any of this chlorophyll family are called ‘Photosynthetically Active Radiation’ or PAR for short.

A lot of the light coming from light bulbs does NOT fall into this range. There are no types of pigment that can absorb it, and so it is called non-PAR. Non-PAR light is pretty much useless for our purposes. Plants can’t use it to grow faster, so it just wastes electricity and money. Common strip-lights or incandescent bulbs release a lot of non-PAR energy, usually in the form of excess heat or Infra-Red (IR) light. Just because light activates photosynthesis, does not mean that it is very good at activating it. Because plants are not very good at absorbing and using green light for photosynthesis, light in this range is also wasted. This means that it’s not enough just to buy a bulb with a high PAR rating, it has to have a high PUR, or Photosynthetically Useful Radiation as well. A bulb with a high PAR, but which releases a lot of its energy in the green wavelengths of the spectrum will have a lower PUR than one that shines out more red light.


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…

In part two we cover measuring light and the effects of different light levels on plants.


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

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