1) Fuel 1) Gas 
• (n.) That which illuminates or affords light; as, gas and petroleum are illuminants.
light radiation with a spectral power distribution defined over the wavelength range which influences the perceived colour of an object NOTE - In everyday English this term is also used more generally for any kind of light falling on an object or scene.radiation with a relative spectral power distribution defined over the wavelength range that influences object colour perception NOTE - In everyday English this term is not restricted to this sense, but is also used for any kind of light falling on a body or scene.
Il·lu'mi·nant noun [ Latin illuminans , -antis , present participle of illuminare .] That which illuminates or affords light; as, gas and petroleum are illuminants . Boyle. 
[n] - something that can serve as a source of light
illuminant Giving off light.
A table of spectral distribution as close as possible to that of the natural light source, usually daylight, to be duplicated.
an illuminating agent or material.
An illuminant is a mathematical representation of a theoretical real light source, used for calculating tristimulus values from a spectrophotometric measurement. The numbers represent relative power of the theoretical source at each point in the visible spectrum. The relative power distribution of a real source could be used for calculation, but re...