By Vaughn Hopkins
Candle: Candles were the first reproducible means of generating a specific amount of light. So, a candle was once used as a measure of how much light was being emitted, and that amount of light was one candlepower.
Candela: A modern version of one candlepower, but defined independent of actual candles.
Foot Candle: While a candela is a measure of the total light emitted by a source, a foot candle is the intensity of the light at a surface being illuminated, which is one foot from a one candela source. It is the intensity of the light cast on the inner surface of a one foot radius sphere surrounding a one candela source. It is measure of radiant power striking a unit area.
Lumen: A foot candle is also the intensity of the light on a one square foot surface which is evenly illuminated by a flux of light from a one lumen light. Lumens are a measure of the total light emitted by a source.
Lux: One lux is the intensity of the light on the inner surface of a 1 meter radius sphere surrounding a one lumen source of light. It, like a foot candle, is a measure of radiant power striking a unit area.
Note: All of the above units are based on light as perceived by typical human beings. Because our eyes are more sensitive to green light than red light, for example, a meter that reads in any of these units has its reading adjusted to more accurately indicate light as seen by people.
PAR, photosynthetically active radiation: PAR is analogous to Lux, except without the adjustments for human eye sensitivity, and limited to light in the 400 to 700 nanometer band of wave lengths of light. The unit of PAR is micromols of photons per second per square meter. This is a measure of intensity of light on a surface as “seen” by plants, the light that plants can use for photosynthesis.
Note: There is no single conversion constant for converting PAR to Lux. The conversion depends on the spectrum of the light which is providing the PAR. For example, the conversion constant for cool white fluorescent light to PAR is 74 Lux per micromol per second per square meter. For sunlight it is 54 Lux per micromol per second per square meter.
Celsius: Temperature is measured in degrees Celsius, or degrees Fahrenheit. The two scales differ in that the Celsius scale is based on the freezing temperature of water being zero, and the boiling temperature of water being 100 degrees, while the Fahrenheit scale is based on the freezing temperature of water being 32 degrees and the boiling temperature being 212 degrees.
Kelvin: Kelvin degrees are the same as Celsius degrees, except that the zero point for the Kelvin temperature scale is zero at the temperature where molecular motion ceases (-273.13 Celsius). It is a measure of absolute temperature.
Color Temperature: A perfect radiator, a black body having 100% emissivity, emits radiant energy over a range of wave lengths. If you make a chart for black body radiation at a given temperature, showing the emitted radiant energy versus the wave length of the emission, it is a smooth curve gently sloped to a maximum, with a shorter curve sloped back to zero. If you make a chart of the emitted light versus the wave length of the light, for a light bulb, you can, theoretically, match that chart to a chart for the radiant emission of a black body at a specific absolute temperature. That temperature is the Color Temperature, or Kelvin Rating of the bulb. Fluorescent light bulbs emit light with several sharp peaks in the emitted light versus wave length curve, which prevents them from matching any black body radiation curve. The Color Temperature marked on a fluorescent tube is largely an advertising gimmick, although some standardization among manufacturers is used.
Wave Length: Light can be viewed as electromagnetic radiation of a specific wave length, measured in nanometers (billionths of a meter). Every color of light has a specific small range of wave lengths. The wave length for red is 620 to 750 nanometers, for green it is 495 to 570 nanometers, for blue it is 450-475 nanometers.
CRI: CRI stands for Color Rendering Index. It is a measure of how accurately a light makes different colors appear, compared to how an incandescent light makes them appear. (An incandescent light has a CRI of 100, while a light emitting only one color of light, like a sodium vapor light, has a CRI of zero.) Unfortunately, for typical planted tank lights, with a color temperature of 6500K or above, the CRI ratings mean nothing and can be ignored.
Watts per Gallon: This is an obsolete method for determining a good lighting level for a planted tank. It is measured by determining how much electric power the lights you are considering will use, in watts, and dividing that by the tank size in gallons. It was once thought that 2 watts per gallon was an adequate amount of light, and 3+ watts per gallon was “high light”. This would be a workable measure of light only if light acts like a fertilizer, where you pour an amount of it into the tank, to get a concentration of X parts per million of light. Obviously, you can’t pour light into water. If you have a single 40 watt fluorescent tube sitting on a 40 gallon tank, you have 1 watt per gallon, and if you have two 20 watt fluorescent tubes sitting on that same tank, you also have 1 watt per gallon. But the latter lighting would be twice as intense as the former, at least right under the bulbs. And, if the 40 watt bulb is in a T5HO light, and the 20 watt bulbs are in a T12 light, you would have far more light intensity with the 40 watt bulb than with the two 20 watt bulbs.
Fluorescent Light: Fluorescent light bulbs are hollow tubes of glass, with only tiny amounts of argon, neon, xenon, and other actinide gases, in otherwise vacuum filled interiors. The inner walls of the tubes are coated with various phosphors. When a voltage is applied across the length of the tubes, an electric gas discharge occurs, which emits ultraviolet radiation. The ultraviolet photons are captured by the phosphor coatings which emit visible radiation through the glass tube walls. Fluorescent lights of any one “generation”, like T12, or T8, use power almost directly proportional to their length.
T5, T8, T12: Fluorescent tube light bulbs are labeled as T5, etc. depending on the diameter of the tube they are made of, in eighths of an inch. T5 = 5/8 inch diameter tubes, etc.
T5NO and T5HO: These represent two generations of technology. T5NO bulbs (NO means normal output) are higher in efficiency than T8 bulbs. T5HO bulbs (HO means high output) are designed to operate at about twice the power of T5NO bulbs, but at a bit lower efficiency.
PC: PC is an abbreviation for Power Compact bulbs. They are T5 size bulbs manufactured as two closely spaced tubes, each with two electric connections, making the bulbs about half as long per power used as T5 bulbs. They are driven by their ballast at HO power.
CFL: CFL is an abbreviation for Compact Fluorescent Light bulbs. They are made of fluorescent tubes smaller in diameter than T5 bulbs, and twisted into spirals or other shapes to make their size comparable to incandescent bulbs. Unlike other fluorescent bulbs, they have their ballast built into the bulb, so they simply screw into a standard incandescent bulb socket. These are commonly called “Power Saver” bulbs.
LED: LED is an abbreviation of Light Emitting Diode. A diode is a semiconductor material with only two electric connections. One version of a diode has the property of emitting light when current flows in one direction through it, and that is the LED. LEDs produce light in proportion to the electric current flowing through them. The junction of the electric connectors to the semiconductor is very delicate, so LEDs also include a transparent plastic protective cover over the junction, which is usually shaped to act as a lens, concentrating the light into a cone of light going in one direction. LEDs are very efficient at converting electric power to light, but still generate enough heat that some form of heat sink is necessary for all but the lowest power LEDs to keep them cool when operating.