There are two ways two generate Radiance calibration files for Field Spec Pro. One method uses a Panel, and the other method uses an Integrating Sphere.In the ‘Panel’ method, the illumination from a NIST traceable ‘Irradiance Source’ falls onto a nearly 100% reflectance, diffusepanel (White Reference Panel). Per NIST the panel is located 50cm away from the source. This is far enough away that the Flux density, or Irradiances at points a and c differ insignificantly from the Irradiance at point b. This also means that the distances from the Source to points a and c are insignificantly different from the distance, D from the Source to point b. The NIST traceable certificate provides the Source Irradiance values at point b for distance, D. The spectrometer input views an area safely within a and c. The source enclosure contains black baffles and apertures to prevent light from bouncing off surfaces and the calibration room (such bouncing light could contribute sources of error).