The Collaborative International Dictionary of English v.0.48:

Photogen \Pho"to*gen\, n. [Photo- + -gen.] (Chem.)
   A light hydrocarbon oil resembling kerosene. It is obtained
   by distilling coal, paraffin, etc., and is used as a
   lubricant, illuminant, etc. [Written also photogene.]
   [1913 Webster]