brightness; luminance; cheerfulness
Refers to the degree of light (physical brightness) or to a cheerful, positive atmosphere or personality. The context usually makes the intended meaning clear.
Please adjust the brightness of the room.
彼女の明るさにみんなが元気づけられた。
Everyone was cheered up by her cheerfulness.
陰り describes a shadow of gloom or decline settling over something, while 明るさ describes brightness or a sense of improvement appearing instead.
Noun form of the adjective 明るい (bright, cheerful), formed by replacing the final い with さ, a common pattern for deriving abstract nouns from adjectives.