The physical unit for describing the brightness of an object is the luminance. However, the eye does not behave like a linear receptor. That means that an object that has twice the luminance of another one does not necessarily appear twice as bright to our eyes. There is no linear relationship between the luminance and the perceived brightness.
How we perceive the brightness of an object depends on the luminance of the object and the state of the adaptation of the eyes. The same object can appear very bright when looked at in a dark environment or rather dark when put against a well lit background.
A typical example of how the perceived brightness depends on the environment is a TV screen. When we watch television during the day, the display is not much brighter then the surrounding. Since the eyes are adapted to the surrounding, the screen appears rather dim. On the contrary, given the same TV set put into a dark room, the picture will look much brighter.
Hit the button below to see how the appearent brightness of an object changes depending on the surrounding.