def air_index(lam, pressure=600, temperature=7,
                  f=8):
    """ Returns index of refraction of air-1 at
        lam in micron at vacuum
        p is pressure in mm Hg
        t is temperature in deg C
        f is water vapor pressure in mm Hg
    """

    k1 = (1/lam)**2
    nm1e6 = 64.328 + 29498.1/(146-k1) + 255.4/(41-k1)

    nm1e6 *= pressure * (1 + (1.049-0.0157 * temperature)*1e-6*pressure) / (720.883 * (1 + 0.003661 * temperature))

    nm1e6 -= 0.0624 - 0.000680 * k1 / (1 + 0.003661 * temperature) * f

    return nm1e6/1e6