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Most light sources will have an intrinsic brightness, expressed in candelas, which varies with direction. Note that the candela is defined as intrinsic brightness as seen from one particular direction. However, the candela has now been re-defined by the lumen: 1 cd = 1 lm sr −1. The candela was formerly defined as 1/60 of the luminous intensity of a projected 1 cm 2 black body at the temperature of melting platinum (2044 K). If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured. A common candle emits light with a luminous intensity of roughly 1 cd. The candela (symbol: cd) is the SI base unit of luminous intensity that is, power emitted by a light source in a particular direction, weighted by the luminosity function. The international standard, at present, is a theoretical point source that has a ‘luminous intensity’ of 1 ‘candela’ (the Latin word for ‘candle’). These were replaced in 1909 by an international standard based on a group of carbon filament vacuum lamps, and again in 1948 by a crucible containing liquid platinum at its freezing point. However, the standard wax candle is variable and inconvenient.
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Wax candles were used as national light source standards during the eighteenth and nineteenth centuries in England. The photometric principles in terms of the existing light source, a wax candle, were first discussed in 1729 by Pierre Bouguer in his book, L’Essai d’Optique. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. The human eye can see light only in the visible spectrum, and has different sensitivities to light of different wavelengths within the spectrum. Photometry deals with the measurement of visible light as perceived by human eyes. In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function. Asim Kumar Roy Choudhury, in Principles of Colour and Appearance Measurement, 2014 1.5.2 Luminous intensity