N(A): the Avogadro constant

• This last constant is arguably one of the most artificial and human- centered. • To explain it, let’s decode the entire second half of the definition first. “Monochromatic radiation of frequency 540 × 1012hertz” is simply green light, specifically the shade of green light that the human eye can pick up most sensitively in a well-lit room. • And “luminous efficacy” is basically the total amount of visible light that a source produces using a certain amount of power. • Kcd is equal to 683 lumens...