Light and Lighting Systems
Here’s some background information on light, what it is; how it’s measured and what light(and lamp) is best for your plants at different stages of growth –
Light is essential for all living organisms and plants use light in the photosynthesis process to generate energy for growth. Plants also use light in other processes too, so its vital that a plant receives the right light at the right stage of growth.
Light is a type of radiation in the form of electromagnetic waves. These waves can be described by three physical properties – intensity (or amplitude), frequency (or wavelength) and direction of vibration (polarisation). See the diagram(Fig 1) below for Here’s where photons come in! Photons are the smallest measurement of an electromagnetic wave.
When we describe the electromagnetic or light spectrum, it’s better to talk about wavelength than about colour. That’s because visible light for humans comprises only a small portion of the light spectrum as a whole – namely the range of wavelengths between 400 and 700 nanometres (or nm, which is one billionth of a metre or 1/1000,000,000m!)).
Light can be measured in a few different ways, including lumens, lux, photons and PAR. The best light measurement for plants is in PAR.
Fasten your seat belts! – Here’s some technical stuff on PAR and why it’s used to measure light, especially for plants.
Photosynthetically Active Radiation (PAR) is the range of light that can be used by plants to photosynthesise(400-700nm). Photosynthetic Photon Flux Density(PPFD) is a count of all PAR over a set area(m2) over a set time (s1). However, there is no one-to-one relationship between PPFD and spectral distribution. It also means that when we compare light sources, we need to consider spectral distribution data as well as PPFD. So, when you look at the information on a lamp you want to buy, a key bit of data is the spectral distribution diagram. Most light units are supplied with one.
Because photon numbers emitted from a lamp are huge, the photon number is divided by a million to give umol. PPFD light is expressed as μmol/m2/s and tells us how many light photons will reach a predetermined surface area (m2) in a specified length of time (a second). As a general rule, most plants need a minimum of 30 – 50 μmol/m2/s PPFD to stay alive.
Have a look at the diagram(Fig 1) below to get an idea of just how narrow the visible band of light is (in fact, it is less than 1 percent of the total spectrum.) –
Fig 1 – Radiation/Light Spectrum Diagram
What makes a good light spectrum for growing?
To answer this question, we need to think about the plant’s response to different light spectra. Because these fall mainly under visible light, we can speak about ‘colours’, starting with the most important ones for plant development.
Blue light (400 – 500 nm)
A larger proportion of blue light has an inhibitory effect on cell elongation, which leads to shorter stems and thicker leaves. Conversely, a decrease in the amount of blue light will cause a larger leaf surface area and longer stems. Too little blue light will negatively affect the development of plants. Many plants need a minimum amount of blue light, which ranges from 5 to 30 μmol/m2/s for lettuce and peppers to 30 μmol/m2/s for soybean.
Interaction between red (600 – 700 nm) and far-red (700 – 800 nm) light
Because red and far-red light have a higher wavelength, they are less energetic than blue light. Combined with the profound influence of the red-induced phytochromes(plant light receptors) on plant morphogenesis(shape and form development), relatively more red and far red light is needed for plants to develop. The two forms of phytochrome, Pfr and Pr, play an important part in this process. Because red and far-red light are both present in sunlight, plants in nature will almost always contain both Pfr and Pr phytochromes. A plant senses its environment by the ratio between those two forms; this is called the phytochrome photostationary state (PSS). The Pr phytochrome has a light absorption peak at a wavelength of 670 nm. When the Pr absorbs red light, it is converted to the Pfr form. The Pfr form acts the other way around – when it absorbs far red light at a peak of 730 nm, it converts into a Pr form. However, because Pfr molecules can also absorb red light, some of the Pfr molecules are converted back to Pr. Because of this phenomenon, there is not a linear relationship between PSS and the ratio of red to far red. For example, when the ratio of red to far red light exceeds two, there is barely any response in the PSS and thus plant development is not affected. It is therefore better to speak about PSS than the red to far red ratio of the light.
The amount of Pr and Pfr tells a plant which light it is receiving. When there is a lot of Pr present, this means that the plant is receiving more far red light than red light. When there is less red light, the opposite conversion (from Pr to Pfr) is hampered, meaning that there is relatively more Pr. In environments in which many plants grow close together, all the red light from the sun is used for the photosynthesis process (between 400 and 700 nm) and much of the far red light is reflected by the plants (>700 nm). Most of the plants, especially those in the shade, will receive more far red than red light in this situation. As a consequence, Pr increases, and when this happens, the plant senses that it needs more light for photosynthesis and stem elongation is triggered. The result is taller plants with a bigger distance between the internodes and a thinner stem. This is a clear example of a shade avoidance response, where plants seek to capture more light in order to survive. Taller plants can absorb more red light which increases the quantity of Pfr forms. This will trigger greater branching, shorter distances between the internodes and less vertical growth in order to maximise the light absorption for the photosynthesis. As a result, plants expend less energy on growing as tall as possible and allocate more resources to producing seeds and expanding their root systems.
Flowering is also influenced by the Pr and Pfr forms. The length of time for which Pfr is the predominant phytochrome is what causes the plant to flower. Basically, the levels of Pfr tell the plant how long the night is (photoperiodism). As the sun sets, the amount of far red light exceeds the amount of red light. During the darkness of the night, the Pfr forms are slowly converted back to Pr. A long night means that there is more time for this conversion to happen. Consequently at the end of the night period, the concentration of Pfr is low and this will trigger short-day plants to flower.
Green light (500 – 600 nm)
It’s often assumed that only blue and red light help plants to grow and develop but that’s not completely true. Although much of the green light is reflected back off the plant’s surface (that’s why we humans see plants green), green light itself can also be beneficial for a plant. The combination of different light colours can lead to higher photosynthesis than the sum of its parts. Research conducted on lettuce also shows that plant growth and biomass increased when 24% green light was added to a red-blue LED, while maintaining equal PAR levels (150 μmol/m2/s) between the two objects. This indicates that even green light can have a positive influence on plant growth.
Ultraviolet light (300 – 400 nm)
Ultraviolet (UV) light has an effect on plants, too, causing compact growth with short internodes and small, thick leaves. However, too much UV light is harmful for plants, since it negatively affects the DNA and membranes of the plant. Photosynthesis can be hampered by too much UV light. Research shows that this happens at UV-values higher than 4 kJ/m2/day.
This brings us back to the general question of ‘what makes a good light spectrum for growing?’ It’s quite hard to give a general answer to this question, since it depends heavily on the type of plant and the requirements of cultivation. For a ‘normal’ plant development these specs are recommended:
• Most plants needs a minimal amount of 30 – 50 μmol/ m2/s photosynthetic light to stay alive
• A minimum amount of blue light is required, which varies between 5 and 30 μmol/m2/s
• A somewhat larger portion of red and far-red light is required, compared to the blue light
• Balance between red and far-red light: preferably a red to far red light ratio of less than 2
• A limited amount of UV light, less than 4 kJ/m2/day
Bear in mind the following points –
• More blue light will lead to shorter stems and thicker leaves
• Too much far-red light or an unequal balance with the red light will result in elongated plants
• A low red to far-red ratio and consequently a limited amount of red light at the beginning of the night is important for the flowering of short-day plants
• Far red light alone does not regulate flowering
• Green light is beneficial for the photosynthesis, although it does not affect the flowering or plant development
Have a look at Fig 2 for the light spectra ranges used by a plant’s chlorophyll to photosynthesise.
Fig 2 – Chlorophyll Absorption Diagram
The next step is to provide the best light spectrum for your growing needs. If there isn’t enough sunlight, or you prefer indoors, then a good grow lamp is needed. There is a wide range of grow lamps to cover all your plants growing needs, from propagation lighting; CFL lighting; HID and HPS lighting; plasma lighting and the ‘new kid’ on the block LED lighting.
The emergence of effective light-emitting diodes (LEDs) in plant production(through the recent developments in new technological advancements), makes it easier than ever before for growers to optimise the light spectrum.
Have a look at our summary of the options available –
This type of lighting is ideally suited for the early stages of plant growth (germination, seedling and cloning) and give a blue spectrum of light at lower emission levels to restrict the amount of heat and energy a plant receves at these sensitive stages of a early growth.
Typically, T5 flourescent and CFL lamps are ideal during this stage of a plant’s growth. Avoid High Intensity Lights (HID) at this stage as the give off too much heat and light intensity.
Here are a few examples of propagation lighting –
See our Propagation Lighting section for a product range.
High Intensity Discharge(HID) Lighting
HID lamps have made indoor gardening practical by providing plants with high levels of light emission.
Essentially, there are two types of HID lamps, Metal Halide(MH) and High-Pressure Sodium (HPS), each provides different light spectral ranges.
HID lamps help plants thrive during their vigorous vegetative growth stage(after seedling stage) by supplying high levels of light towards the blue end of the light spectrum and HPS lamps aid a plant’s flowering stage by supplying high levels of light towards the red end of the light spectrum.
There are also, dual spectrum HID lamps that provide both blue and red spectral light.
Factors of wear for HID lamps come mostly from on/off cycles versus the total on time. The highest wear occurs when the HID burner is ignited while still hot and before the metallic salts have recrystallized.
At the end of life, many types of high-intensity discharge lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage. As they heat up during operation, however, the internal gas pressure within the arc tube rises and a higher voltage is required to maintain the arc discharge. As a lamp gets older, the voltage necessary to maintain the arc eventually rises to exceed the voltage provided by an electronic ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly.
Another phenomenon associated with an HID lamp’s wear and ageing is discoloration of the emitted light beam (fading). Commonly, a shift towards blue and/or violet can be observed. This shift is slight at first and is more generally a sign of the lamps being “broken in” whilst still being in good overall working order, but towards the end of its life, the HID lamp is often perceived as only producing blue and violet light. This is a direct result of the increased voltage and higher temperature necessary to maintain the arc.
More sophisticated ballast designs(see below) detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts.
See our HID Lamps section for a product range.
A typical HID lamp can be seen in the picture below –
A ballast is an essential part of running an HID lamp. They provide the lamp with the correct electrical current and ignite your lamp.
Magnetic vs Digital
Historically, magnetic ballasts were the standard choice, however, with the advent of electronic or ‘digital’ ballasts that offer a range of advantages, magnetic model sales have diminished. Digital ballasts use solid state circuitry to transform and regulate the voltage a lamp uses, and are much smaller and lighter than magnetic ballasts. They also offer significant improvements in consistency of light; better electrical efficiency; lower heat output and reduced noise. Dimmable digital ballasts offer greater adjustability of the current supplied to a lamp and give your lamps better light output and longer life by exciting the gases in your lamp more efficiently(by running much higher frequencies than magnetic ballasts).
Here are the major benefits of choosing a digital ballast over a magnetic ballast –
- Fully compatible with Metal Halide and High Pressure Sodium lamps
- No more blown contacts! By using ‘Soft Start’ technology there is no ‘spiking’ of current so there is a lower startup and running current making the digital ballasts much more power efficient
- Full lamp brightness reached in a much shorter time (upto 10 times quicker)
- Control of output (dimmable digital ballasts can reduce wattage down to 50% of a lamp’s maximum)
- Cooler running with a selaed bearing cooling fan.
- Higher light output, typically 20-30% per watt than a magnetic ballast
- A standard minimum manufacturers warranty of 2 years
Have a look at an example digital ballast below –
For more information see our Ballast section.
Plasma or LEP (Light Emitting Plasma) lighting solutions combine the energy efficiency, reliability and controls of solid state lighting with the high output of HID (metal halide and high-pressure sodium) lamps. LEP differs from traditional HID light sources, such as Metal Halide or High Pressure Sodium lamps, in a number of ways. Firstly, LEP is powered using solid-state electronics that allow it to achieve very high reliability and have precise control over its operation. Also, unlike a traditional HID light source, the LEP bulb does not need metal electrodes to drive power into the source and therefore has a more robust quartz vessel eliminating early failure and light degradation. In addition, the light output from a LEP source is directional, easily dimmable, turns on and re-strikes rapidly, and can operate in any orientation. All of these advantages translate into lower installation, maintenance and operating costs, higher energy efficiency, and an overall better lighting experience to the end user. The lamp (a small sealed capsule) contains an inert gas which gets turned into a plasma via electromagnetism. This plasma gives off an intense white light(full spectrum) that is ideal for plant growth.
See our Gavita LEP products
LED lights have historically had a bad reputation in the growing industry as the LED technology just wasn’t capable of offering HID lamps any serious competition… but no more!
In the last 5 years LED technology advancements have been rapid, bringing LED lights to the fore. By offering much lower electricity consumption; significant heat output reduction and huge improvements in light intensity, LED lights have become serious players in the growing market.
We have reached a point where many growers are now seriously deciding whether to invest in LED lights(that have a relatively high initial cost compared to HID lights) but now offer significantly higher light emission in full spectral light.
CREE LED manufacturing has moved the goalposts both in terms of safety development an light efficiency so much so that choosing it as a light source has become the standard for serious growers. Just looking at the facts and figures comparing LED CREE technology with HID lighting has shown exceptionally higher lux readings, providing much higher crop yields.
Is it time to switch to a new, proven light source that significantly ‘outshines’ others in the growroom light market?
Many who have tried these latest LED advancements say ‘YES’, especially when you offset the initial, relatively high product cost, with the electricity savings; much lower heat output; massively increased light intensity and higher crop yields.
Have a look at our ranges of LED lights –