Making High Quality
Led Grow Lights since 2010

Photosynthesis & Plants

1. Photosynthesis

Absorption spectrum graph of chlorophyll

Photosynthesis is the process whereby plants convert carbon dioxide and water into organic compounds using energy from light. To do this plants use two types of a chemical called chlorophylls, specifically chlorophyll a and chlorophyll b, which are contained in proteins called photosynthetic action centres. Each of these chlorophylls absorbs light in a fairly narrow band of both the red and blue ends of the visible spectrum. The graph on the left shows the absorption spectrum of the two chlorophylls.

Absorption spectrum graph of chlorophyll

As well as chlorophylls, plants also contain other substances such as beta carotene which are known as ‘antenna pigments’. These absorb light across a wider range of wavelengths and channel the energy into the photosynthetic action centres, however they do so at a much lower efficiency than when the light is directly absorbed by the chlorophylls. The following graph on the right shows the chlorophylls, beta carotene and a couple of other photosynthetic chemicals.

You may well have seen this same graph on different led grow light sites, often with the explanation that they have included leds in their lights at around 550nm and 610nm for the phycoerythrin and phycocyanin in your plants. The joke is that phycoerythrin and phycocyanin do not occur in higher plants, just in some primitive algae and bacteria! They are just included in the graph as they ARE photosynthetic chemicals. Well I find it amusing anyway!

There is a second type of graph you will often see called a ‘Photosynthetic Action Curve’. This looks (unsurprisingly) very similar to the absorption graphs above and shows how much photosynthesis occurs in the plant at each wavelength. One thing to be aware of is that the photosynthetic action curve is not the same for different plants, so to be of any use you need the specific one for the plant you are interested in. Many websites show a curve with no reference to what plant it is for.

A complete description of photosyntheseis can be found here:

This next part is a bit complicated. When I first started my research I thought it would be easy – read a few textbooks and scientific papers and design the perfect light. I was astonished to find that science still has a lot to learn about plants, in fact the exact mechanisms behind flowering are not fully understood! Plants are enormously complex and many different factors appear to affect growth and flowering. Plants don’t just use light to provide energy for growth, they can also sense their environment through light. The reason certain types of light work best for different stages of growth is because they trigger a certain response in the plant. Below is a simplified explanation of the main factors for the different growth stages, more detailed information can be found by following the links.

2. Vegetative Growth Stage

When you have a plant in its vegetative growth stage, you normally want a stocky bushy plant with a short inter-nodal distance (the vertical distance between branches or leaves growing from the stem). A light source rich in blue light will produce this type of growth, whereas one with little or no blue will produce a tall thin plant with a large inter-nodal distance. The reason for this is that blue light is blocked/absorbed more by plants than red light, so if a plant is over shadowed by other plants the light it gets is mainly in the red part of the spectrum. If it detects that it is getting mainly red light it switches on a type of growth called etiolation which is characterised by rapid growth with a thin stem with few leaves. This allows the plant to use all its energy to growing higher than its competitors and reaching the light. When it reaches the light the increased level of blue switches off the etiolation response and the plant develops normally. More information here:

3. Light and the Seasons

Why do plants flower better under lights with a lot of red such as HPS? The reason has to do with why we have seasons and the way light changes over the year. Most people will tell you the reason we have seasons like summer and winter has to do with the earth being closer to the sun in summer and further away in winter. This sounds reasonable until you consider the fact that when it is summer in the northern hemisphere it winter in the south and visa versa! The real reason is that the earth is tilted on its axis. As the earth travels around the sun, first one pole and then the other points (slightly) toward the sun. When the north pole is pointing in the direction of the sun, the northern hemisphere gets summer (and the south gets winter), when its pointing away the north gets winter (and the south gets summer). The reason is that when your hemisphere is pointing toward the sun the light from the sun is shining more directly at the ground (it appears more directly overhead at noon) and its light has a shorter path through the atmosphere, resulting in more light and heat reaching you. In winter the light has to pass through more atmosphere and you get a weaker light and less heat. So what has this got to do with flowering? Well travelling through more of the atmosphere not only reduces the amount of light and heat reaching the ground it also changes the spectral balance. The air actually bends the light, affecting different colours to different degrees like a prism. This effect is know as ‘Rayleigh Scattering’.

Blue light is bent and scattered more than red, which results in sunlight being redder in the winter than in the summer. So basically as the growing season goes from summer (vegetative growth period) to autumn (flowering period) the light progressively contains a higher red/blue ratio. This as well as the shorter days triggers flowering in most plants. See the section below for information on the mechanisms involved. By the way, the reason you don’t notice this seasonal light change is because your eyes and brain have a sort of ‘auto white balance’ ability like that in a digital camera!

More information here:

4. Flowering and Light

Well whilst most people know that plants react to shorter days (the kind you get in the autumn) by flowering, they also respond to the balance of red and blue light which is detected by the photoreceptor proteins Cryptochrome and Phytochrome. Phytochrome is especially important as it appears to be a trigger for flowering. Phytochrome has to forms Pr and Pfr. Pr is the biologically active form and absorbs light at 660nm which causes it to change to Pfr which absorbs at 740nm. When Pfr absorbs light it converts back to Pr. Pfr also converts back to Pr during darkness.

Blue light triggers Cryptochrome which is responsible for phototropism (when plants grow in the direction of light) but it also seems to be involved in flowering. Plants which have been modified to have more than normal levels of cryptochrome have lots of large leaves but do not produce flowers or have very delayed flowering.

Plant Photonics lights have the highest levels of 660nm light of any light on the market to give optimal flowering!

More information here:

5. Lumens/Lux and why they don’t mean anything

One thing that confuses a lot of people is the way many grow light manufacturers give a rating for their lights in lumens. Unfortunately this is both meaningless and misleading! First of all the lumen reading varies a huge amount depending how far away from the light you place the meter as lumens is just a measure of perceived (a very important word which I will explain later) brightness with no measure of distance from the light or brightness over an area. It is very easy to ‘fake’ a comparison between 2 lights with a lumen meter, all you have to do is hold the meter level, close to and in the centre of one light and then at a slight angle and a bit further away from the second light and the first light will give a limen reading much higher than the second! A slightly more meaningful measure is Lux which is lumens per square meter, but even this is no indication of how good a grow light really is. Why? Because Lumens and Lux are a measure of PERCIEVED brightness. Perceived brightness means how bright a light appears to the human eye, not how bright it really is! The sensitivity of the eye varies a huge amount depending on the wavelength (colour) of the light. The eye is most sensitive to green light. As the colour of the light moves towards either red or blue, the sensitivity drops off a lot. Below is a diagram showing the sensitivity of the human eye VS colour. The black line is normal daylight vision, the green line is night vision. Lumen meters use the daylight response of the eye.

Graph Lumex/Lux

As you can see from the graph, at 650nM (fairly deep red) and 470nM the eye would see the light as only being about 1/10 as bright as a light at 560nM (green) light!

So why does this matter? Well the problem is that plants response to light is completely the opposite to the response of the human eye!

Plants use (or see!) red and blue light the best and green light little or not at all. So a light that is bright to the eye is dim to a plant and a light that is bright to a plant is dim to the eye – and a lumen/lux meter.

A more comprehensive (and complicated!) explanation can be found here: