1. illuminant - Noun
2. illuminant - Adjective
That which illuminates or affords light; as, gas and petroleum are illuminants.
Source: Webster's dictionaryElectric lighting started displacing kerosene as an illuminant in the late 19th century, especially in urban areas. Source: Internet
When the illuminant is not a point source, the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1%. Source: Internet