![]() Since the area of a sphere one meter in radius is 4pi square meters, the total light output of a candle is 4pi (about 12.6) lumens. To light a surface of one square meter evenly at a 1 lux requires one lumen of total light. (In practice 10 lux = 1 footcandle is close enough.) If you hold the candle just a foot away the illumination is brighter, of course, and we call that lighting level one foot-candle, or about 10.76 lux. Here's a simpler explanation: 1 lux is the light level you get when you hold a lit candle one meter from the subject in a dark room. of lumens in a big church or a cathedral? anon16230 August 1, 2008 If I am sitting in a pew with my bible open in the dark, what light bulb would I need to choose to be directly 3 meters above my head to achieve enough light to read by? My guess would be Sylvania ULTRA RT 6 LED10W Downlight 700 lumens. But let me just take a stab in the dark for an answer. Anon2634 asks a complex question with little specificity. Luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry.What we in the west will have to do is just recalibrate our life's experiences with brightness to lumens instead of watts. Luminous intensity is the perceived power per unit solid angle. Luminous intensity should not be confused with another photometric unit, luminous flux, which is the total perceived power emitted in all directions. For instance, the measured responses of the eye to violet light varied by a factor of ten. This curve, denoted V(?) or, is based on an average of widely differing experimental data from scientists using different measurement techniques. The curve which measures the response of the human eye to light is a defined standard, known as the luminosity function. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. The human eye can only see light 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. ![]() The SI unit of luminous intensity is the candela (cd), an SI base unit. 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, a standardized model of the sensitivity of the human eye. ![]() A single candela approximates the now-obsolete unit of one candlepower, which was originally defined in England by as the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. It is defined as the power emitted by a light source in a particular direction per unit solid angle, weighted by the luminous efficiency function (a mapping of the human eye’s sensitivity to the different wavelengths). What is a candela? The candela is the base unit of measurement of luminous intensity.
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