def a(interval=1.0e-9):
    """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.
    """
    # 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