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Lighting Basics

by Bob Taylor

Although artificial lighting produces an imperfect spectrum compared to the sun, it does allow control of variables such as duration and intensity.

Equipment Overview

Artificial lighting requires the use of a ballast (used to ignite then regulate current to the lamp), a shade or reflector (directs light towards the plants and usually incorporates the lamp holder/socket), a timer (controls when the lights turn on and off) and the lamp itself.

Lamp Selection (Optimizing Color Spectrum)

Photosynthesis occurs mainly within the visible light spectrum (wavelength range 400 to 700 nanometers). Within this range, 445 nanometers (blue) and 650 nanometers (red) are needed most.

HID: High Intensity Discharge lamps (HID) are generally used for the vegetative and flowering/fruiting phases due to their high lumens per watt rating. (HIDs produce five times as many lumens per watt than incandescent lamps). The following types of HID are commonly used:

  • Metal Halide (MH): Produces light predominantly blue in color. This is ideal for strong vegetative growth (e.g. larger leaves, thicker stems) and a shorter, dense plant (i.e. shorter internodal spacing), which will utilize light more efficiently.
  • High Pressure Sodium (HPS): Produces more red light. This promotes flower onset and production and is, therefore, more suited to the flowering/fruiting phase.

Unfortunately, MH and HPS lamps each require a different ballast and lamp holder. Therefore, it is common practice to use a HPS lamp for both vegetative and flowering - especially if plants have a very short vegetative phase. However, using HPS throughout can promote tall and spindly growth in many species.

Blended light lamps: To save needing a separate lamp kit (lamp, ballast and lamp holder) for both vegetative (MH) and flowering (HPS), "blended light" lamps are available. These produce a more balanced spectrum of red and blue light.

Fluorescent: Fluorescent lamps produce less lumens per watt compared to HID lamps, hence their use is limited to plants needing low to medium light intensities, such as seedlings, clones, herbs, orchids and lettuce. They are simpler to use than HID lamps because they generate less heat.

These are broadly categorized as either 'strip' fluorescents (SL) or 'compact' fluorescents (CFL) and are available in different wattages and color temperatures. Those of high color temperature (known as "cool white") are bluer in color and more suited to vegetative growth and development of seedlings/clones. Lower color temperatures ("warm white"), are redder in color and, therefore, better for flowering. CFLs are generally available in higher wattages than SLs and thus, are suitable for species needing higher light intensities.

Optimizing Light Intensity

Insufficient light will produce sparse foliage, spindly branches and poor flowering. This can occur because lamp size (and type) is inadequate; lighting duration is inadequate; foliage is too far away from the lamp; or foliage is being shaded by other plants.

Lamp size: A lamp's size is measured in watts (W). The size of the growing area will partly determine the required wattage.

Plant height: A plant's height must be considered when determining lamp size because light intensity diminishes rapidly as distance from the lamp increases. For example, imagine your plants covered an area of three by three feet. You may be tempted to use a 400 watt lamp instead of a 600 watt HPS (i.e. power costs are 50 per cent less, and it generates less heat). However, if a significant proportion of the foliage is three feet from the lamp, the intensity is only 5,555 lumens per square foot. This intensity may be insufficient for the species being grown. By choosing a 600 watt HPS, the intensity would be roughly double (assuming equal gap between lamp and foliage).

HINT: It is also preferable to keep plants as short (and therefore wide) as possible. This can be achieved by removing the 'growing tip' and also restraining upwards growth (and training sideways growth) by erecting netting at an appropriate height.

Plant density/shading: Shading becomes an issue when plants are positioned too close to one another. Hence, it is generally more productive to plant less than more.

Lamp height: To best utilize lamp output, position the lamp as close as possible to the top of plants without causing photo-respiration or burning of foliage. Air cooled shades should be used because they enable lamps to be positioned much closer to foliage - particularly beneficial for 1,000 watt lamps.

Lamp shades: These help maximize the amount of light directed towards the plants. Shades need to be hung so that their height can be easily adjusted as plants grow. For safety, ensure mounts are securely fastened to the ceiling.

Light cycles: Plants generally require 18 hours of light per day during the seedling and vegetative phase. During flowering/fruiting however, the duration can be reduced to 12 hours. Growth can suffer if lighting intervals are irregular. Therefore, employ a timer to help ensure consistency. Note also, vital processes occur during the night (lights off) period, therefore, avoid interrupting it by turning lights on.

Reflective material: The use of reflective material on walls (etc.) helps ensure that light is not wasted through absorption.

Hints for Set-up and Maintenance

Light meter: Useful for determining whether light levels are adequate throughout the garden, and for routinely checking if lights are operating to specification.

Compatibility: Ensure the ballast and lamp suit one another. For example, lamps of different type (e.g. MH or HPS) or different sizes (wattages) may require a specific ballast. Also, if using an electronic ballast, ensure the lamp is compatible, otherwise premature lamp failure and blackening may occur.

Handling HID lamps: Lamps can explode if they are cold when lit or have fingerprints on glass. Always wrap in clean paper or towel before handling. To clean the lamp glass use window cleaner (or rubbing alcohol) and allow to dry thoroughly before use. Lamps should not be restarted immediately after being switched off (MH lamps should generally be left off for at least 20 minutes; HPS for at least three minutes).

Effective working life: A lamp's intensity will diminish with use, and may also use more power. Note that MH lamps have a shorter effective working life than HPS. Also note that the capacitor in magnetic ballasts will deteriorate with use and consequently the light intensity will diminish. Ask the manufacturer for the useful working life of these items.

Lamp orientation: Lamp directions will specify whether a lamp can be orientated vertically, horizontally, etc. Failure to comply can cause poor lumen or color output, and shorten the lamp's life span.


Consider safety at the design stage:

  • Keep power devices and junctions away and/or above any potential water spillages/ floods.
  • Determine whether the electrical lines can handle the current draw.
  • Where multiple lamps are being lit ensure to stagger start up times. HID lamps draw large amounts of electricity, therefore, safety must be a priority.
  • As an overall safety measure, install a miniature circuit breaker (MCD) and residual current device (RCD). If there is a faulty circuit these will cut the power supply.

Calculating Electricity Costs

Cost of power per day = Cost of power per kilowatt* x Total kilowatts** x daily duration e.g. If the cost of power is $0.10 per kW and there are two 600 watt lamps (1.2kW) alight for 12 hours per day, the daily cost of electricity is $1.44 (i.e. $0.10 x 1.2kW x 12hrs)

*Refer to your last power bill. **1,000 watt = one kilowatt (kW)

*The preceding article was contributed by Maximum Yield Magazine

In Addition...

Digital Ballast- Advances in the reliability of digital ballast are rapidly establishing them as the new first choice for most serious gardeners. Their benefits over magnetic ballast include; total silence (no annoying humming from your garden), greatly reduced heat output, and the option to run either an MH or an HPS bulb, making them the most cost effective for people wishing to use both. Another recent advance in digitals are the dimmable versions which enable you to use a higher wattage bulb with reduced output when your plants are smaller and increase as needed, saving both energy and money.

T5 Fluorescents- T5's are quickly replacing MH bulbs as the number one choice for vegetative growth. These high output fluorescent bulbs run MUCH cooler allowing them to hang just inches over the canopy of the plants. This leads to much denser and more even growth during veg.

LED Lights- Don't believe the hype. At this point in time, they simply don't live up to the claims. For more information on LED's click here. LED Lighting

Below is some specific information about both MH and HPS lighting.

Metal halide bulbs are very efficient and produce between 65 and 115 lumens of light output per watt of electricity used. MH bulbs produce a light that is very close to full summer sun, with a spectrum rich in the blue end. This promotes fast vegetative growth and compact, stocky plants with short internodal leaf spacing.

Metal Halide bulbs create light by passing electricity through an clear inner arc tube that is enclosed in the vacuum of an outer clear glass tube. This inner arc tube contains mercury and other metals in iodide form. When electricity is applied to these metal iodides they give off very intense light and heat. The outer casing can be either clear or phosphorus coated. Most gardeners prefer the clear bulb as it produces the brightest white light available.

MH bulbs come in sizes from 70 to 1500 watts with the 250 w, 400 w and the 1000 w being the most popular sizes for gardening. All the bulbs need to run with a ballast (basically a transformer to step up the voltage to the proper amount), that are designed to run a specific size bulb.

The bulbs themselves need to be burned in a specific position. They come in three types: Vertical (marked BU or BD), Horizontal (marked HOR) and Universal (marked U). The universal bulbs can be burned in any position, but they are more efficient when burned vertically.

The bulbs also come in a variety of styles, among them are regular MH bulbs and Super Bulbs (which produce 10% - 12% more light but use the same amount of electricity). The super bulbs cost slightly more, but they are more efficient, making them a good choice in the long run.

Medal Halide bulbs (except the 1000 w Super Bulb) should be replaced about every 9500-10000 hours or aprox.18 months (assuming an 18 hour / day on cycle). The 100 w super bulbs do not last as long as any of the others and should be replaced every 6500 hours, which is about 12 months if you run the bulb 18 hours a day.

HPS bulbs are the most efficient bulbs that are commonly used by the hobby gardener. They are high in the red and yellow parts of the spectrum and low in the blue, this imitates the fall sun. Because of this spectrum some plants that are grown with HPS lights only can grow elongated and rather leggy, while many other plants are not effected by the light spectrum. For plants that normally bud and flower in the fall this is usually the light of choice because the light spectrum promotes flower production.

There are now available color corrected HPS bulbs such as the Son Agro and the Planta bulbs. These are designed specifically for indoor horticulture, and have a more balanced color spectrum.

HPS bulbs are made out of a translucent ceramic arc tube containing a mixture of sodium, mercury and xenon gas. This arc tube is suspended in a outer glass shield (bulb). HPS bulbs range in power from 35w to 1000w, with the 250w, 400w, 600w and 1000w being the most popular for indoor horticulture.

HPS bulbs should be changed out after about 24 months. The bulbs slowly lose their brightness over time and to maintain good light quality the bulbs must be changed before the end of their rated life.

Sulfur lighting is the new kid on the block. I have had no dealings with this type of lighting. At this point in time sulfur lighting is extremely expensive when compared to H.I.D. lighting. There have been a lot of fantastic claims about the efficiency and brightness of these lights, however from what I have heard lately the claims were greatly exaggerated. There are companies working on a marketable sulfur lighting system for the hobby gardener and I will keep you posted when I learn more.

Using indoor grow lights is a less than ideal substitute for the sun. Grow lights are obviously less intense and don't cover a very big area. Grow lights lose intensity very rapidly as you get further away from the bulb, which means that plants need to kept as close to the bulb as possible for the best possible growth. There are two major problems with putting the light very close to the plants, the coverage area of the light is diminished and the plants can burn if kept too close to the light. Keeping plants under a stationary light can also cause another problem. Plants that are directly under the light will grow better than those that are just slightly off to one side, resulting in a pyramid effect. To combat this problem you can move the plants around or you can move the light around. Moving the plants can be difficult if not impossible, so moving the light is the only option in most cases.

There are basically two kinds of light movers, linear and circular. Linear movers move the light back and forth in a straight line (most models have a pause momentarily at each end further reducing the pyramid effect), these work well in a rectangular shaped garden. The circular light movers move 2 or 3 lights around in a circle and are designed for use in a square or round garden.

The use of light movers allows you to bring the lights closer to the plants without fear of burning them and they allow for more light coverage (you can increase coverage by 25%-35%). Light movers also give your garden better light distribution, and sends light to the plants from many angles instead of only one as with a stationary light.

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