How do we define a standard candle? Is the brightness of all the candles the same, or does it depend on the chemical properties of candles? If the brightness of different candles differs, how did Bouguer determine which candle to use as the "standard" candle?
@SainanChen: The definition was originally quite arbitrary, "One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour." Now it is a precise value in terms of fundamental constants. Wikipedia has a nice history of the different definitions over time - https://en.wikipedia.org/wiki/Candela
This is regarding the "equally bright" point--I remember we briefly mentioned in class that human eyes/ears don't perceive the intensity of light/sound linearly to their physical intensity (i.e. energy).
In the case of light, I learned in one episode of MKBHD that when phone companies say there is a X% increase in brightness, they usually mean increase in nits. For example, 400 nits to 500 nits is a 25% increase. However, this doesn't mean the screen will appear 25% brighter to the human eyes, because we don't perceive brightness linearly w.r.t. nits.