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Lighting Education & Terminology FAQ's

Where can I find lighting terms and technical information?

Our lighting glossary can answer some of your questions about technical lighting terms (like "What's an amp?" and "How do fluorescent lamps produce light?")

 

What are lamp BTUs?

British Thermal Units (BTUs) are the measurement of heat output. One BTU is the heat that will raise the temperature of one pound of water by one degree Fahrenheit. It is approximately the amount of heat generated by burning one blue-tip kitchen match. There are approximately 3.4 BTUs per watt per hour. High bulb temperature usually does not affect the life of the lamp unless the temperature is extreme. The filament of an incandescent lamp operates at a very high temperature (nearly 4750 degrees Fahrenheit for a 200-watt standard lamp). An increase of a few hundred degrees in bulb temperature does not noticeably increase the filament temperature, and there is practically no effect on the lamp life or light output. Glass in most general service bulbs withstands 700 degrees Fahrenheit safely.

 

What is color temperature, and how does it indicate to the color of the light output from a lamp?

Color temperature (or correlated color temperature, CCT) is a number indicating the degree of yellowness or blueness of a white light source. Measured in kelvins, CCT represents the temperature an incandescent object (like a filament) must reach to mimic the color of the lamp. Yellowish-white (warm) sources, like incandescent lamps, have lower color temperatures in the 2700K-3000K range; white and bluish-white (cool) sources, such as cool white (4100K) and natural daylight (6000K), have higher color temperatures. The higher the color temperature the whiter, or bluer, the light will be.

 

What is efficacy?

Efficacy is a term used to describe the efficiency of a lighting product. This is measured in LPW, or lumens per watt. To determine the efficacy of a lamp, divide the lumen output of the lamp by the watts consumed. For example, a 60-watt soft white has 840 lumens and has an efficacy of 14 LPW.

 

How is quality of light determined?

Quality of light is determined by the relationship between the color temperature of the light, and the color rendering index.
The color temperature indicates the appearance created by the light source. The higher the color temperature, the "cooler" the color.
Typically, color temperatures of 2000K -3000K create a warm (yellow) appearance; above 4000K are cool (blue) in appearance. Between 3000K and 4000K are considered intermediate and tend to be preferred.
Color rendering index (CRI) rates a light source's ability to render colors in a natural way, based on a scale from 0 to 100. In general, light sources with high CRI (80-100) will make people and things look better than those with lower CRIs.

 

What is the mean lumen output of a fluorescent lamp, and how is it different from the initial lumens?

Since fluorescent lamps have such a long life compared to incandescent lamps, eventually the brightness (lumen output) of the lamp will begin to decrease.
Because of this, fluorescent lamps have an initial lumen rating and a mean lumen rating. The mean lumen output is measured at 40% of the lamp life.
NOTE: Decorative or colored bulbs do not have a lumen rating since they are not considered a primary source of lights, only decorative.

 

How is lamp diameter measured? How can I determine the diameter of the lamp from the lamp name or description?

The shape of a bulb is followed by its maximum diameter, expressed in eighths of an inch. For example, a R40 has a maximum diameter of 40 eighths of an inch which equals 5".
The chart below will help you to quickly make these conversions.
Eighths of an inch - Inches

1 - 0.125 2 - 0.25 3 - 0.375 4 - 0.5 5 - 0.625 6 - 0.75 7 - 0.875 8 - 1 9 - 1.125 10 - 1.25
11 - 1.375 12 - 1.5 13 - 1.625 14 - 1.75 15 - 1.875 16 - 2 17 - 2.125 18 - 2.25 19 - 2.375 20 - 2.5
21 - 2.625 22 - 2.75 23 - 2.875 24 - 3 25 - 3.125 26 - 3.25 27 - 3.375 28 - 3.5 29 - 3.625 30 - 3.75
31 - 3.875 32 - 4 33 - 4.125 34 - 4.25 35 - 4.375 36 - 4.5 37 - 4.625 38 - 4.75 39 - 4.875 40 - 5
 

What is a foot-candle, and how can I calculate it for lighting a room?

A foot-candle is unit of illuminance or light falling onto a surface. It stands for the light level on a surface one foot from a standard candle. One foot-candle is equal to one lumen per square foot.
To calculate the foot-candles of a lamp, divide the lamp's lumens square footage of the room. The lumen rating is usually listed on the lamp package. It's a lot easier (and probably more precise) to just take a reading with light meter.