Kcd : the luminous efficacy of monochromatic radiation of frequency 540 × 1012hertz

• 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 per watt and helps to define the candela.