It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation).
ɪɾ ɪz mˈɛʒɚd ˌɔn ɐ skˈeɪl fɹʌm zˈiəɹoʊ kˌɔːɹɪspˈɑːndɪŋ tʊ ɐ blˈæk bˈɑːdi ðæt ɐbsˈoːɹbz ˈɔːl ˈɪnsɪdənt ɹˌeɪdɪˈeɪʃən tə wˈʌn kˌɔːɹɪspˈɑːndɪŋ tʊ ɐ bˈɑːdi ðæt ɹᵻflˈɛkts ˈɔːl ˈɪnsɪdənt ɹˌeɪdɪˈeɪʃən.