[aiaiaiai]

you can add "さ（sa）" to the stem of adjective and use it as a noun,or changed into a noun.
When adjective change into noun, the rest except for the stem is delited.
by the way,When whose tense change is so.

明る（akaru）;the stem of adjective

The examples are as follows:
明るい（akarui）;adjective→明るさ（akarusa）；noun→明るか った（akarukatta）;the past
暗い（kurai）→暗さ（kurasa）;noun→暗かった(kurakatta);the past