Grow Lights – Intro Part 1: All About Light For Growing

What Is the Best Light for Growing?

It was not so long ago now (5-10 years) that it was accepted wisdom that the most cost-effective grow lights were fluorescent for small vegging plants – Metal Halide (MH) for medium-sized vegging plants and High Pressure Sodium (HPS) grow lights for larger plants in flowering/fruiting.

Since then, new types of grow light have come along, and there have been some huge advancements in the arena of LED grow lights with some highly notable advancements. But Plasma lights, Ceramic Discharge Metal Halide (CDM) lights, and 400 volt HPS technology have also come onto the market, giving the grower even more choice.

Each type of grow light produces a different spectrum output, a different radiant intensity (which affects canopy penetration), and a different effective area of illumination (footprint). These factors can vary between brands, and even between different models from the same manufacturer.

So, with all this choice, how do we decide which grow light is best for us to use on our plants, and for which stage of growth?

The answer to that question does not just entail considering the output spectrum, radiant intensity and lighting footprint. We also need to consider environmental factors such as the dimensions of our grow space (the width and depth of the area, and also it’s height), and how much heat generation (from the light) can be dealt with by a given extraction or environmental control system.

In this article we will delve into light, and the aspects of it that we need to consider when scientifically assessing a grow light. The article following this one will go into the “whats” and “hows” of what we are going to measure. Subsequent articles will then test different types and various models of grow light. At the end there will a round-up, with conclusions, as to which grow light will be best for a given grower and their grow-space.

What Is Light?

What us humans call light is actually something called electromagnetic radiation. This electromagnetic radiation also includes things like radio waves, microwaves and x-rays. All electromagnetic radiation is just waves of energy that travel at the speed of light. The waves of electromagnetic radiation are classified as one type or another by measuring something called the wavelength. The wavelength is the distance between the peaks of the wave.

The electromagnetic radiation wavelength spectrum goes from thousands of metres (radio waves) right the way down to trillionths of a metre (billionths of a millimetre) on a continuous scale. The following chart shows us that for humans, the visible light part of the electromagnetic spectrum sits within a very narrow band of the electromagnetic wavelengths in the whole spectrum:

Visible Light and the Electromagnetic Spectrum

The visible part of the electromagnetic spectrum (for humans) lies roughly between 400 nanometres (billionths of a metre) up around 700 nanometres, although all people are different and some people can see wavelengths as far as 1000 nanometres.

The term “nanometres” is usually abbreviated to “nm”. We humans perceive the wavelengths just above 400nm as violet and wavelengths just below 700nm as red. All the wavelengths in-between 400nm and 700nm are attributable to all the colours of the rainbow:

Violet: 400nm – 450nm
Cyan: 490nm – 520nm
Green: 520nm – 560nm
Yellow: 560nm – 590nm
Orange: 590nm – 635nm
Red: 635nm – 700nm

The above numbers are not definitive. They are simply how most humans perceive visible light of different wavelengths (when seeing a light source) and what most of us would agree is the colour that corresponds to that range of wavelengths. This also applies to reflected light. When us humans see that an object is of a certain colour, it is because that object is reflecting the visible light of the corresponding wavelength of that colour. For example, we perceive blood to be red because it reflects mostly the red portion of the visible light spectrum (about 635nm to 700nm), while absorbing much of the other wavelengths of visible light.

Of course, if a light source produces (or an object reflects) more than one wavelength of light then we perceive the combined wavelengths as yet a different colour which is often similar to that of one of the single wavelengths in the spectrum. For example, if a light source contains red and blue (or if something reflects both red and blue) then we perceive the combined wavelengths as the colour purple. Purple looks similar to violet, but we can still perceive it as subtly different.

There are an infinite amount of combinations of the different wavelengths of visible light, and this infinite range of possible colours gives enormous richness to the way we perceive, and view, the world around us. The most important effect of this “combination” effect is that when we see all the wavelengths (in similar proportions) at the same time, then we perceive that light source or that light reflected from an object as white.

Wavelengths just outside the visible spectrum below 400nm are called ultraviolet (UV), and wavelengths just above 700nm are called infrared (IR). We humans cannot see these wavelengths but they still have affects on us. For example, UV causes our skin to tan and to become damaged. Infrared, meanwhile, is sometimes called thermal radiation and it is how we feel heat from the sun.

Plants do not have eyes like us humans but they do respond to the different wavelengths and radiant intensities of light in different ways, just like us. For plants, different colours (different wavelengths of light) stimulate different bio-chemical reactions within the plant’s cells.

How Does Light Travel, and How Does This Affect My Plants?

Electromagnetic energy travels in little packets called photons. Apart from the spectrum, the other very important measurement of visible light is how many photons per second arrive at our eyes. This dictates how bright something appears to us. Brightness can also alter our perception of a colour. For example, something that appears to be yellow-orange in normally lit conditions will begin to appear brown if the light level dims.

Now, plants need light – this we know. However, just the total brightness of a source of light is not the only thing that we need to consider when comparing grow lights. There is a measurement called radiant intensity which refers to a particular attribute of brightness. Radiant intensity refers not only to the brightness of a light, but also to the size of the source of the light.

For example, think of water coming out of the end of a garden hose pipe when you hold it  horizontally. If the hole in the nozzle on the end of it is quite large then the water will travel, maybe, only a foot or 2. However, if the size of the hole is reduced by either adjusting the nozzle or putting your finger over the end then the water will travel much, much further, about 6-10 feet or so!

So, going back to how this relates to grow lights, let’s take the example of a panel of fluorescent tubes about 2 feet by 2 feet which emits 6300 lumens. Those 6300 lumens of light is being emitted from quite a large area. This means that although quite bit of light is being produced, it is of low radiant intensity.

For comparison, if we consider a low power HPS lamp (70 watt or so) which produces a similar 6300 lumens, the light is coming from a small filament within a comparatively small lamp. A lot of light is coming from a small source and this means that it has high radiant intensity.

The large panel of fluorescent tubes will illuminate a large area, but the light does not have much depth penetration to get the light past the tops of the plants and to the lower leaves and branches further down. Meanwhile, the 70 Watt HPS, with it’s considerably higher radiant intensity (and therefore penetration), will drive the light further down to the lower portions of a plant.

Now we know a bit about what light is. We also know that the 3 main aspects of light that we need to consider when assessing and choosing  a grow light are: the output spectrum, the radiant intensity, and whether it will cover the area that we need it to. We need to take into account other characteristics too such as a grow light’s electrical efficiency and also it’s heat output, but those will be discussed in a later article.

In Part 2 of this series of articles we will take a plunge into how we can measure those 3 main aspects of a grow light’s light output. See you soon!