Grow Lights? Intro Part 2: The Light Spectrum

Welcome back, growers!

In this next instalment, we will be delving into what colours of light we want a grow light to produce. This will explain the 'whats' and 'whys' of the colour spectrum part of our grow light testing. First of all, let's examine what we are going to look for in the output spectrum of a grow light.

So, what effect do different colours in the spectrum have on plants?

As we have already mentioned in part one of this blog series: plants use different parts of the light spectrum for doing different jobs, and their exact requirements change somewhat as plants go through their different growth stages. It is accepted wisdom that blue in the spectrum is good for vegetative growth and helps keep internodes short, while red in the spectrum is great for powering the flowering/fruiting stages. 

It is worth mentioning that far red in the spectrum triggers something called "shade avoidance", causing lengthening internodes and petioles (leaf stalks) in the vegetative stage and it is not regarded as a good thing during that time. However, once flowering/fruiting is under way, far red light does a good job of encouraging more flowers/fruits. Used correctly, far red can also shorten the flowering/fruiting season and many say it also helps to stimulate terpene and essential oil production a bit like uvb does.

Most growers are aware that what gives plant's leaves their green colour is a substance called chlorophyll. Chlorophyll is a compound that has the unusual ability to use the energy in light (of certain colours) to combine water (H2O) and carbon dioxide (CO2) together to create sugar (and also oxygen as a by-product). This process is called photosynthesis:

What Are the Different Types of Chlorophyll?

There are 2 main different types of chlorophyll. Each of those 2 types can use a part of the blue region of the colour spectrum of light and a part of the red region of light to power the photosynthesis process. Chlorophyll type A is powered by slightly different shades (wavelengths) of blue and red  than chlorophyll type B. Here is a graph of the how the 2 types of chlorophyll can use light in the visible spectrum:

By Chlorophyll_ab_spectra2.PNG: Daniele Pugliesiderivative work: M0tty - This file was derived from:  Chlorophyll ab spectra2.PNG:, CC BY-SA 3.0,

..and here is the summed total absorption for both types of chlorophyll:

It might seem from the graph that plants absorb blue light (around 450nm) better than red (around 650nm) and would therefore drive more photosynthesis. However, absorption of light is not the whole story because chlorophyll  photosynthesise with red light more efficiently than blue.

Creating sugar to power its own growth is most certainly the most important thing that a plant needs to do with the light that hits its leaves. Without that sugar made by photosynthesis, a plant will stop growing, become weak (etiolation), and eventually die. Many growers still believe that the bulk of the light that a plant needs is just for fuelling photosynthesis in chlorophyll A & B.  However, providing those wavelengths of light so that a plant can make sugar is not the whole story.

Apart from sugar, plants need to make other substances (such as carotenoids) for general health, vigour and disease resistance. Here is a graph which shows the spectrum of the absorption of the light used by plant leaves to create the other important substances:

Plants don't need very much light in the non-red and non-blue regions to produce enough of these other substances, but they do like to have at least some in order to have good structure, to be healthy and to grow vigorously.

There is a common misconception that plants do not respond to green light. However, green light does actually play a big part in optimising photosynthesis. Although green light does not power photosynthesis in the chlorophyll directly, it indirectly causes the plant to photosynthesise more effectively. The science behind this fact is somewhat out of the scope of this article, but it might be an interesting topic for another day. However, this effect of green light is real, particularly at higher light intensities.

Some time ago, a researcher called McCree plotted the photosynthetic efficiency curve which shows how effectively similar levels of light across the spectrum  produce different amounts of photosynthesis. It shows that red is more effective than blue, but green actually isn't all that far behind! Here is the graph showing his results:

Giving plants at least some of the whole of the visible spectrum of light (including the green region) really does make a difference. It not only improves the health and the structure of the plant, but it also increases growth and yields. This is why there is an increasing number of growers who are fans of full-spectrum grow lights.

How Does the Power of Electricity Factor into This?

As indoor growers, we have to pay for the electricity to power our grow lights. Also, every watt that we use will generate some heat that we have to deal with in one way or another. Therefore, it makes sense that we want our grow lights to use this electricity as efficiently as possible for us. This means we don't just want our grow light to be as bright as possible for the amount of power that it uses - we also want it to produce the light spectrum that most closely matches what our plants can use to best effect and cause as much photosynthesis as possible.

There are several ways to represent the ideal (most efficient) spectrum to give to our plants.  The McCree curve shows us where best on the light spectrum our plants use light. We know that for healthy plants it is best to provide some of all the wavelengths, but, they don't need a lot of green.

A given amount of red light drives more photosynthesis than the same amount of blue (or green). In flowering we are best using our electricity producing mostly red light (around 600-650nm), However,  some blue (around 450nm) is good for plant health. About a third to a fifth of the amount of red light is ideal.

In veg, blue light keeps the plants short with tight internodes and stimulates vegetative growth more. Giving our plants that  blue light is how we are best spending our electricity money.

Because the output spectrum has a lot of importance, it is useful to be able to take it into account when assessing and comparing grow lights. We would then be able to see how well a grow light uses electricity to produce the light to power plant growth.

For this reason, we will be using a Sekonik C-7000 Spectromaster spectral analyser to demonstrate the light spectrum that grow lights put out:

The graph that this device produces shows the relative amounts of all the different wavelengths in the visible light spectrum that are being produced. The graphs can then be uploaded to a pc, along with a csv spread sheet which gives the relative values of every single wavelength from 380nm to 780nm. Uploaded graphs look like this one of a cool white T5 fluorescent grow light:

The unit boasts a whole host of other facilities as well, such as lux/lumen/ppfd measurements to mention just one. Taking a reading is as easy as pointing the unit at a light source and pressing the "measure" button:

So now we've covered the light spectrum, its importance, and how we are going to measure it.

In our next article we will be looking into light intensity, depth penetration and spread (light footprint). We'll take a look at what we need to know, and how we are going to measure it.

Until then, happy growing! :)

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