The Collaborative International Dictionary of English v.0.48:

irradiance \ir*ra"di*ance\ ([i^]r*r[=a]"d[i^]*ans), irradiancy
\ir*ra"di*an*cy\ ([i^]r*r[=a]"d[i^]*an*s[y^]), n. [From
   irradiant.]
   [1913 Webster]
   1. The act of irradiating; emission of rays of light.
      [1913 Webster]

   2. That which irradiates or is irradiated; luster; splendor;
      irradiation; brilliancy. --Milton.
      [1913 Webster]