| Light Up My Life: Page 6 | |||
| FOR YOU TO READ How Intense Is a Light Source? |
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| What unit of measurement can be used to discuss and compare the intensity of light sources? One candlepower is the intensity of one candle. Since there are many different sizes of candles, this method of measuring light is not very precise. "One candlepower (cp) is the luminous intensity of a standard candle made of whale wax, weighing 1/6 lb., 7/8 in. in diameter, and burning 120 grains per hour." That is better but it is an old definition. Today there is a modern name and definition of luminous intensity: One candela (cd) is the same as one candlepower. One candela is defined in terms of the power that travels outward from a source of a specific green light.* |
* Formal SI (Systeme international d"unites, or International System of Units) definition: The candela (cd) is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 3 1012 Hz (Hertz) and that has a radiant intensity in that direction of (1/683) W/sr (watts per steradian). | ||
 To relate the candela (cd) to a familiar light source, the standard 100-W incandescent light bulb oriented as shown in the diagram has a luminous intensity of about 180 cd in the horizontal direction and about 65 cd in the direction opposite the bulb's base. |
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