In astronomy, the illuminance stars cast on the Earth's atmosphere is used as a measure of their brightness. The usual units are apparent magnitudes in the visible band. V-magnitudes can be converted to lux using the formula E v = 10 ( − − 14.18 − − m v ) / 2.5 , where E v is the illuminance in lux, and m v is the apparent magnitude. The reverse conversion is m v = − − 14.18 − − 2.5 log ⁡ ⁡ ( E v ) .