Lighting Basics, by Bob Taylor, Maximum Yield Publication 08/01/2009

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Although artificial lighting produces an imperfect spectrum compared to the sun, it does allow control of variables such as duration and intensity.
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.
 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.
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 more blue 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. 
Even though bulbs can burn much longer than the recommended usage, it is wise to replace them frequently for many reasons. Our eyes might not notice the gradual decline of lumen intensity but the plants surely will. The plants will also notice the gradual shift in spectrum. Plants grown under older bulbs that have been used longer than 12 months will stretch in search of light and become spindly when they were once squat and bushy. Flowering plants without the light intensity they require will increasingly provide smaller yields with poorer quality flowers and fruits.
Once you have ensured that your bulbs are of the highest quality, it is time to fire them up in your ballast and make sure they work. Even though the light and ballasts are being tested prior to the indoor growing season, it is highly recommended to have backup ballasts and bulbs just in case of failure. A burnt out capacitor or a burnt out light can spell disaster for a crop if it cannot be replaced quickly enough. The best test for the ballast is to screw in a bulb and turn it on for a couple hours. If the ballast gets hot enough to burn or start a fire, chances are it is too old and dangerous and needs replacing.
Also check the bulbs for defects. If the bulb is flickering, dim or not staying in the proper spectrum, (ie. changing from red to blue), then there is a good chance that either the bulbs or the ballast may need replacing.  Inspections should be repeated with every piece of equipment in the grow room. Whether it is fans, pumps or lights, it doesn’t hurt to test it to be sure of its performance before plants are introduced to the room.
The same should be done for nutrients.  Take a look at your current supply and take note of anything that you have run out of as well as anything that is running low. Also take a close look at the nutrient. Is there a lot of sediment at the bottom? Has the color or consistency changed? If you purchase quality nutrients this should not be a problem but if you see these symptoms, your best bet is to replace the nutrient. If an organic nutrient is being used, keep in mind that they tend to smell. However, bad smell is not an indicator of an organic nutrient going bad. A good indicator that the organic nutrient needs replacing is mold growing on the surface. To prevent spoiling, place your favorite organic nutrient in the fridge after it has been opened.
Once you have gone through this simple, easy to follow checklist, the guesswork will be taken out of garden. Following this advice will consistently lead to healthy, happier gardens that produce superior crops.

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