Light: Intensity vs Brightness
As far as I know, brightness is defined in optics as a component of color.
Therefore it depends not only on the intensity of the light, but on our perception of it. It is more closely related to luminous intensity than to radiant intensity. For a beam with a given spectrum, however, the luminous and radiant intensities will be related by fixed proportionality.
For example, we could observe Malus's law using infrared light, but this light would have no perceived brightness because we can't see infrared light.
Intensity is the power per unit area; it is a physical quantity. Brightness involves how the human visual system perceives light, and it is not a physical quantity.