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The term candlepower was originally defined in the United Kingdom, by the Metropolitan Gas Act 1860, as the light produced by a pure spermaceti candle that weighs 1 ⁄ 6 pound (76 grams) and burns at a rate of 120 grains per hour (7.8 grams per hour).
As can be seen by referring to the above tables, if a photographer has a camera with an X-sync speed of 1 / 125 th of a second, is shopping for a flash device, and desires the capability to fill shadows from up to 2.5 meters (8 feet) away, a flash device with a guide number of at least 22 (m) / 72 (ft) will be required.
10 −6: μcd/m 2: 1 μcd/m 2: Absolute threshold of vision [4] 10 −5: 10 −4: 10 −3: mcd/m 2: 100 μcd/m 2: Cloudy night sky without moon [4] 400 μcd/m 2 "Darkest sky" [5] 1 mcd/m 2: Night sky [6] 1.4 mcd/m 2: Typical photographic scene lit by full moon [7] 10 −2: 5 mcd/m 2: Approximate scotopic/mesopic threshold [8] 10 −1: 10 0: cd ...
The candela [...] is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, [a] K cd, to be 683 when expressed in the unit lm W −1, which is equal to cd sr W −1, or cd sr kg −1 m −2 s 3, where the kilogram, metre and second are defined in terms of h, c and Δν Cs. [12]
Left to right: 3x AA to D parallel battery converter with rechargeable NiMH AA-size batteries inserted. MY DAY vintage flashlight. It uses 1.5 V D-size batteries. Sofirn SP36 flashlight. It features a 5 V 2 A USB-C charging port to load 3.7 V 18650 rechargeable lithium-ion batteries. The most common power source for flashlights is the battery.
If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous intensity of 1 candela. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela.
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