Extinction reduction factor f_ext for zodiacal light as a function of the line-of-sight top-of-atmosphere intensity I₀ of the zodiacal light in dex. The intensities correpond to those given in Table 17 of Leinert et al. (1998), i.e. the unit is 10^-8 W m^-2 μm^-1 sr^-1 (=1.690 photons   s^-1 m^-2 μm^-1 arcsec^-2 at 0.5 μm). The factors were calculated for different ecliptic longitudes of the Sun, sidereal times, and line-of-sight zenith distances (up to 50° in general and up to 70° for log I₀ > 2.4) and azimuths. Only night-time data points with solar zenith distances greater than 108° are considered. For Rayleigh scattering (black “ × ” symbols), a zenithal optical depth of 0.27 is taken, corresponding to a wavelength of 0.4 μm for the Cerro Paranal extinction curve (see Sect. 2.1). For Mie scattering (green “+” symbols), τ₀ is fixed to 0.01, a value reached at about 1.2 μm. The figure also shows average f_ext in 0.1 dex I₀ bins for both scattering modes (Rayleigh: blue circles; Mie: red triangles). The resulting fits based on these average values are displayed by solid (Rayleigh) and dashed lines (Mie).