More than you wanted to know about lights

SUMMARY: Plants require a certain quantity of light; mostly they do not care so much about which lights you choose to use to give them this light. A 4 ft bulb should be rated at 3000 lumens or more for best efficiency.

Types of lights

Incandescent- Normal household bulbs with a filament --- inefficient
Halogen Usually tubes- burn brightly because of special gas in the small tube-- inefficient 
High Pressure Sodium-very efficient - ugly -used for street lighting and intensive growing hydroponics setups
Mercury- very efficient - ugly -used for street lighting and intensive growing hydroponics setups 
Fluorescent-efficient- best choice for smaller hobby growers 

Sizes of Fluorescent
Available in tubes of various sizes. The 4 foot tubes are the least expensive since they are produced in huge quantities for use in offices, etc. A 4 ft bulb should be rated at 3000 lumens or more for best efficiency.

Light output and labeling
Every bulb should be labeled for "lumen" output (either on the bulb, the packaging, or the store shelf). Lumen output tells you how much light the bulb will put out. Bulbs will also specify a "watt" number. This is the amount of power/electricity the bulb will use. You can compare lumen output of bulbs that have the same "watt" rating (eg a 4 ft. 40 watt tube)  to see which bulb will be the most efficient for you.

Bulbs are also labeled as to light color:  cool white, warm white, full spectrum. You may have seen how a prism can split sun light into various colors of the rainbow. All artificial lights contain only some parts of natural sunlight. 

Full spectrum lights have a light that is much more balanced with the various parts of the full spectrum of light. Full spectrum make your blossoms look best, but the also tend  to give much less light output for a given amount of electricity and the bulbs are very expensive to buy.

Cool white bulbs are commonly the cheapest bulbs and give a high light output. They have less red and more blue, so they look harsh (skin tones look flat-blossoms look washed out). Stores and offices use these because they are cheap and efficient. You can use these but may not like the light if you use them in a living area.

Warm white bulbs  were developed for home use for people who objected to the ugly colors of cool white bulbs. Warm white bulbs  add more red tones to show better skin tones. These used to be more expensive, but are becoming more common and no longer much more expensive than cool white. They often give a little less light output than cool white, but with careful shopping you can find warm white bulbs that give nearly the same lumen light output. Some people suggest using one cool white and one warm white in your fixture so that you have better balance of all the components in the light spectrum. 

Any of the above bulbs will work fine for the plants. Selection is mostly based upon cost considerations along with which the light you find most pleasant. Note: our local Home Depot store has quite a good display of lighted fluorescent lights so that you can directly compare differences in light output and color tone.  

Fixtures and ballasts
Fluorescent lights were first developed with something called a "magnetic ballast." Fluorescent tubes cannot use regular house old current directly, so they need this "ballast"  to change the electricity to higher voltage. Older ballasts used  heavier transformers, newer ballasts use solid state electronics. There are several key improvements in the newer electronic ballasts.

Older style ballasts give off extra heat, are less efficient, can produce a humming sound, often show a flickering light that can give people headaches and the bulbs wear out more quickly. Newer style electronic ballasts are a little more expensive, are more efficient, produce less heat, do not hum, lights don't flicker, and bulbs last longer. Over all you are much better to buy a fixture with the newer ballast. Boxes are often not labeled about type of ballast so look for a fixture that advertises that it works well in cold since this is another feature of electronic ballasts- you could also check to see if the fixture supports "T-8" bulbs. Avoid the very cheapest fixtures since they are most likely to have the older style ballast. I have some some very good prices on 4 ft fixtures at Wal-Mart, but they do not always have them in stock.

Technical detail: The old ballasts produce a light the turns off and on 60 times per second which is why the lights flicker for some people. The new electronic ballasts use a much higher frequency which avoids the flicker and other problems. 

T-8 vs. T-12 Bulbs (the thick and thin of it)
There are now two thicknesses of fluorescent tubes available: T-8 and T-12. The number refers to the thickness of the bulb- measured in 1/8 inch I think- so a T-12 would be an inch and a half in diameter. 

T-12 (thick bulbs) are the older style and by far the most commonly available. The older style magnetic ballast will only work with T-12 bulbs. 4 foot T12 bulbs are usually 40 watt bulbs (though sometimes 25 watt is available.) You can also use T-12 bulbs in the new fixtures. I have only seen full spectrum bulbs as T-12 size.

Gradually, you will start to see the new thinner T-8 bulbs. These bulbs run at 32 watts of electricity and give off nearly the same light as a 40 watt T-12. If you are running a lot of lights, this can be a substantial saving. You MUST have a new electronic ballast to use the new T-8 bulbs. 

Some people have posted that they have poor growing results with the new T-8 bulbs. Technically, I can see no reason why this would be the case. And we have been growing with both types of bulbs and we see no problems with the new bulbs.

Distance of plants from bulbs: One further consideration is the distance of the plant from the bulb. This can be a little complicated. Different plants like different environments. Typically, variegated plants and minis tend to want/need more light. If your plant leaves are reaching for the sky, they want more light. Most people have standards 10-16 inches from the lights. Minis can be very close; we have some small plants 6-8 inches from the light. We leave lights on for 10-12 hours per day. (To minimize mildew, keep a fan running in the area but not directly blowing on the plants.) 

Over all, the amount of light that a plant gets is based upon a combination of the following factors: lumen output of the bulbs, distance from the lights and  how long the lights are on. You need to balance all factors to get the correct amount of light. For example, you could increase the light a plant gets either by moving it closer to the light, or by increasing how long the lights are on. 

Many people are surprised to learn that light intensity varies by the square of the distance.  So a plant two feet from a light source will receive 1/4th the light of a plant that is one foot from the bulbs. In the case of sunlight, of course, distance is no factor since the distance of the earth from the sun is constant no matter where you have your plant.

Bulb burn out: Florescent bulbs do not burn out like normal incandescent light bulbs do. Instead they gradually grow dimmer and dimmer over the months and years. Because it is such a gradual process, you may not even notice that your bulbs are dimmer. When used many hours per day, the older style fluorescent bulbs running on older style magnetic  ballasts should be replaced every year- or year and a half. The newer style bulbs with electronic ballasts last much better. You might be able to use the same bulb for 2-3 years.  If you are not sure whether to replace your bulbs, simply buy one new bulb and install it in a fixture along side an older bulb. The comparison should show you how much the old bulb has faded.

You might stretch the life of an older light tubes by leaving  your lights on for an extra hour or two. But remember that your plants do need some period of rest/darkness. Also, remember there is the added electricity cost for running your lights longer.

Cost: Here is how to figure out how much your lights are costing you. 

Bulbs: 
if you replace tubes every year, and if a fixture uses two tubes, and if your tubes cost $5 each... then you are paying $0.80 per month per fixture for bulb replacement cost . If your tubes last longer or you buy cheaper tubes, the cost is accordingly less. 

Electricity. 
If you use newer electronic ballast with two 32 watt tubes and pay $0.12 per kilowatt hour, and leave your lights on for 12 hours per day-- you will pay about 10 cents per day per fixture... or $3 per month  (70 watts  per fixture x 12 hours x .12 per hour divide by 1000 kilowatts per kilowatt-hour x 30 days per month) 

If you use older style bulbs and fixtures, you will have 40 watt bulbs with a ballast wasting excess heat--- so the calculation would be   (95 watts  per fixture x 12 hours x .12 per hour divide by 1000 kilowatts per kilowatt-hour x 30 days per month) equals about $4.10 per month

Of course your number may vary somewhat depending upon how long you leave your lights on and how much your electricity costs... but this will give you a general idea of how much your lights are costing you... and why it is better to pay a little extra to get the newer electronic ballast fixtures and lights.