"This brings us to the discussion of light flux and brightness or light intensity. Light flux is a flow rate of light energy. Light flux can be compared to the flow rate of water; it describes how much light is flowing per unit of time. Brightness or light intensity is like the velocity of water flow. If there are two pipes that discharge equal amounts of water every second and the diameter of one pipe is half that of the other pipe, then the velocity of water in the smaller pipe must be twice as great as the water flowing through the larger pipe"