Rationale

The Stefan-Boltzmann constant given in the Standard Practice for Calculating Solar Reflectance Index of Horizontal and Low-Sloped Opaque Surfaces is currently 5.6691 x 10-8. Since the development of the Standard, the Stefan-Boltzmann constant has been revised to be more precise. As such, using the full length of the constant 5.670374419 x 10-8, as defined by NIST would avoid discrepancies.