Candles per square inch or candlelight per square inch were units of measure for light illumination, as determined by the amount of light focused per square inch from a candle.
In 2267, Spock calculated that 1 million candles per square inch was required to kill the flying parasite encountered on Deneva. When he expressed his intention to be exposed to that amount of light as a test case, Doctor Leonard McCoy protested, asking rhetorically, "Do you know what one million candlelight per square inch can do to your optic nerves?" When James T. Kirk agreed with the proposed treatment, McCoy said he'd get some goggles for the Vulcan. Spock declined, saying that nobody on the planet's surface would be wearing any, with which Kirk also agreed. (TOS: "Operation -- Annihilate!")