The apparent magnitude scale we still use for comparing the brightness of stars builds on an ancient Greek system refined and codified in 1856. It remained essentially our only source even in 1875, when the International Bureau of Weights and Measures (BIPM) was created to help nations agree on standard measurement units.Ī definition of luminance eluded us at that point-today we describe it as candelas per square meter-but the idea of relative brightness reaches back to early astronomers. Flame was our first source of artificial light.
PastĪmong the definitive early achievements in humanity’s development is our mastery of fire. Manufacturers need to take these ideas into account as they design virtually every lighting product on the market, a task that relates back to the candela-the oddity of the SI, the sole unit that remains tied to human perception. We are more sensitive to some colors than to others, and this sensitivity changes as day falls into night-and as youth gives way to age. And what we have learned about the human eye continues to inform NIST’s efforts to measure light’s effect on it. What we have learned about visible light has led scientists to create specific terms, both verbal and mathematical, to describe ideas of brightness. The creation of incandescent lamps less than 150 years ago up through our contemporary LEDs and smartphone screens has demanded a means to describe how intense these artificial light sources would appear to the humans who use them. But old though the candle is, our need for the candela has been driven largely by modern commerce. Its name is the Latin word for “candle,” and from our earliest attempts to create a measurement unit, we have always reached back to this familiar and ancient light source as an easily grasped reference for brightness-which modern scientists refer to as intensity.
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