code: def a(interval=1.0e-9):
      
      # https://en.wikipedia.org/wiki/Standard_illuminant#Illuminant_A
      lmbda = numpy.arange(300e-9, 831e-9, interval)
      c2 = 1.435e-2
      color_temp = 2848
      numpy.exp(c2 / (color_temp * 560e-9))
      vals = (
          100
          * (560e-9 / lmbda) ** 5
          * (
              (numpy.exp(c2 / (color_temp * 560e-9)) - 1)
              / (numpy.exp(c2 / (color_temp * lmbda)) - 1)
          )
      )
      return lmbda, vals
docs: CIE Standard Illuminants for Colorimetry, 1999:
      CIE standard illuminant A is intended to represent typical, domestic,
      tungsten-filament lighting. Its relative spectral power distribution is
      that of a Planckian radiator at a temperature of approximately 2856 K. CIE
      standard illuminant A should be used in all applications of colorimetry
      involving the use of incandescent lighting, unless there are specific
      reasons for using a different illuminant.