QUOTE(boscarol @ Feb 27 2010, 04:38 PM)
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Hai una visione molto romantica della colorimetria. Ufficialmente una candela (nel senso di unità di misura fotometrica) è definita così (dalla Conferenza dei pesi e delle misure nel 1979), pensa un po':
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian.
Aggiungo questa citazione:
"Like other SI base units, the candela has an operational definition—it is defined by the description of a physical process that will produce one candela of luminous intensity. By definition, if one constructs a light source that emits monochromatic green light with a frequency of 540 THz, and that has a radiant intensity of 1/683 watts per steradian in a given direction, that light source will emit one candela in the specified direction.[1]
The frequency of light used in the definition corresponds to a wavelength of 555 nm, which is near the peak of the eye's response to light. If the source emitted uniformly in all directions, the total radiant flux would be about 18.40 mW, since there are 4π steradians in a sphere. A typical candle produces very roughly one candela of luminous intensity.
Prior to the definition of the candela, variety of units for luminous intensity were used in various countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition, or the brightness of an incandescent filament of specific design. One of the best-known of these standards was the English standard: candlepower. One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. Germany, Austria, and Scandinavia used the hefnerkerze, a unit based on the output of a Hefner lamp.[2] In 1881, Jules Violle proposed the Violle as a unit of luminous intensity, and it was notable as the first unit of light intensity that did not depend on the properties of a particular lamp. All of these units were superseded by the definition of the candela."
Come vedi la candela di cera c'entrava eccome...
QUOTE(boscarol @ Feb 27 2010, 04:38 PM)
Figurati quanto c'entrano le candele di cera.
Purtroppo sbagli. Intanto kelvin si scrive in minuscolo (come scrivi metro e non Metro), e poi non sono gradi, ma semplicemente kelvin, per esempio "6000 kelvin".
Cito inoltre questo retalivo ai kelvin..
"By international agreement, the unit kelvin and its scale are defined by two points: absolute zero, and the triple point of Vienna Standard Mean Ocean Water (water with a specified blend of hydrogen and oxygen isotopes). Absolute zero, the lowest possible temperature, is defined as being precisely 0 K and −273.15 °C. The triple point of water is defined as being precisely 273.16 K and 0.01 °C. This definition does three things:
1. It fixes the magnitude of the kelvin unit as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water;
2. It establishes that one kelvin has precisely the same magnitude as a one-degree increment on the Celsius scale; and
3. It establishes the difference between the two scales' null points as being precisely 273.15 kelvins (0 K = −273.15 °C and 273.16 K = 0.01 °C).
Temperatures expressed in kelvins are converted to degrees Rankine simply by multiplying by 1.8 as follows: T°R = 1.8TK, where TK and T°R are temperatures in kelvin and degrees Rankine respectively. Temperatures expressed in degrees Rankine are converted to kelvins by dividing by 1.8 as follows: TK = T°R⁄1.8."
Se ricordo bene è prassi comune in colorimetritra parlare di gradi kelvin per indicare una "temperatura colore" e le temperature si esprimono in gradi!
Direi che è ora di chiudere la discussione, visto che indastria chiedeva suggerimenti sui suoi riscontri e penso che non possa interessargli tutto il resto!
Federico