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Artificial Lighting

Introduction: First lets start off saying, there is no substitute for the sun. These plants, along with all others, have adapted over millions of years to grow under the big star. Fortunately we now have decent technology that we can grow these plants without the sun's direct shine. There are some basic things we need to keep in mind, as mentioned below.

Photosynthesis and light spectrum: Almost all plants turn light energy into sugar, via photosynthesis. The main things in the plant that do this are chlorophyll. Other pigments, such as carotene, also turn light energy into sugar, but chlorophyll is the main one. Chlorophyll is also what gives many plants their green colour. We can use this and compare it to the light spectrum.

Plants mainly absorb the visible spectrum, this is the light spectrum that is visible with the eye as "white light". Most people are familiar with the blue/violet end and the red end (yellow and green are in the middle) of the spectrum. If you go past the violet end of the spectrum you get Ultra-violet light, UVA, UVB and UVC (this is what gives you a sunburn) and goes on to X rays, Gamma rays etc. On the other end of the spectrum, the red end, you go into Infra-red (IR) and radio waves. What we want to look at is the visible spectrum, violet to red.

Light is measured in by the wavelength with the unit "Nanometer" (Nm). With the visible spectrum the measurements are from 400 NM (violet) to 700 Nm (red), see photo below of the light spectrum. In the middle (generally speaking) there are green and yellow, which are around 500-600 Nm.

The light spectrum divided up with focus on the visible light spectrum. Click photo for enlarged view.

Most cacti are green (with the exception of variegated plants). Because they are green, actually the chlorophyll is green, they are not very efficient at absorbing green light. The reason is the various pigments are more efficient at absorbing violet-blue and yellow-red, so these colours do not reflect off the plant well. None of the plants pigments are especially good at absorbing green light, this is why green light is reflected away from the leaves, allowing us to see green. When we go light shopping this is a very good thing to keep in mind.

Most studies indicate that the rate of photosynthesis is highest at around 450 Nm (violet-blue) and 650 Nm (red) for the main pigments in plants, Chlorophyll A and B. Fortunately, common household lights generally come in these general wavelengths (although they also have a wider spectrum that often includes a little of all the visible spectrum).

Light Efficiency (Lumens)

Without getting into too much detail, lumens are the measure of brightness. In very general terms, the more lumens the more luminous flux! Many lights will show a L/W rating or at least an L rating. L=Lumens (how bright), L/W= Lumens per watt (how bright for every watt used). The more lumens per watt, the better. You get more light for less energy/money. In another VERY general way, the hotter the bulb the more inefficient it is because much of the power is being "wasted" in creating heat energy.

Types of Lights

HID: High IntensityDischarge. These lights are often used by nurseries and greenhouses because they are very efficient and give off the right spectrum for plants to absorb light. There are 2 main types of HID lights; Metal Halide (MH) and High Pressure Sodium (HPS). HPS bulbs are on the red end of the spectrum and MH are on the blue end.

HID lighting is probably the only well suited lighting for growing cacti year round without ill effects, other lights just don¡¦t have the power to make to stop stretching.

HID lights are often 100 or more Lumens per Watt

Fluorescent: Fluorescent lighting is a great form of lighting. It comes in the right colours to grow plants. Often these lights, and most other lights, are given a colour temperature rating using something called Kalvins. In general terms, the lower the K rating, the more red (2500K), the higher the more blue (6000K+). One problem with these lights is that they loose efficiency incredibly fast over distance. If placed 2 meters away from the plant, for example, not as much usable light is going to go the plant. If you put the same light 5cm away from the light, the plant is going to react in a much more positive manner as it is provided much much more light. Reflectors help a lot, but they are still loosing light through absorbing it.

In short, if using these lights, place them close to the plants or they will be pretty much useless. No further away than say 60cm, 2 feet.

Fluorescent lights are often 60-95 Lumens per Watt. In most cases, the longer tubes (4 feet) are more efficient, and smaller tubes are less efficient. Compact Fluorescents (CFL) are generally 55-70 L/W.

Incandescent: These lights are basically dinosaurs. They are very inefficient and provide the wrong colours usually and waste most of their energy in heat.

Incandescent lights are often 15 Lumens per Watt.

Halogen: Halogen lights are also about as useless for plant growth as incandescent lights. They spend much of their energy producing heat, not usable light for photosynthesis. If your looking for heat, these are good lights for heating an area.

Halogen lights are often 15-20 Lumens per Watt.

LED: LED lights are a new lighting option that show more promise than any other. Although they are still fairly new to the scene they already show HUGE promise. They have the advantage of being very specific to a certain wavelengths. For example, you can buy 640 Nm LED's where as a fluorescent may be 400-700 Nm with a spike in the 650 Nm area. These bulbs also happen to be VERY energy efficient. They have already surpassed most other commercially available lighting as far as L/W go, and are very cool in temperature making them much safer to use. Right now their draw backs are: expensive and not able to penetrate through things well, which is not a big concern with cacti.

LED lights are often 50-100+ Lumens per Watt, and are seemingly always going up (there is 130+ L/W being made now).

When choosing your lights look for a few things. The right spectrum, often can be seen in K ratings. Choose the 2700K bulbs for the red end and 6500K for the violet end. Also the amount of light, look for higher lumens per watt.

Also read about Plant Pigments that absorb light.