Illuminants refer to the type of light used to calculate the best match during a scan. Each illuminant stands for the following: D50 (Sunrise or Sunset), D65 (Noon Daylight), F2 (Fluorescent Office Light), F7 (Fluorescent Daylight), A (Incandescent Light). You should select the illuminant closest to the light the surface you are painting will be in. The app has defaulted to D50, the most common illuminant used.