Visibility in turbid media with colored illumination

Abstract

Reduced visibilities were simulated in a hydrosol, which was illuminated with light from iodine quartz lamps. Visibility observations were performed through different color niters, thus having the same effect as if the hydrosol were illuminated by the colored light obtained by passing the white light through the filter. Two types of hydrosols were used : One had a wavelength-dependent extinction coefficient similar to the atmosphere with higher extinction in the blue than in the red while the other consisted of monodisperse latex spheres of 2.02 μm diameter having a higher extinction in the red. For observations in white light, the visual range using the extinction coefficient at 550 nm underestimates the visibility for the hydrosol similar to the atmosphere and overestimates the visibility for the latex hydrosol. In colored lights, the visual range has no meaning; calculation of the visibility from the extinction coefficient at either the dominant wavelength of the filters or at their wavelength of maximum transmission gives wrong results, too. However, if the wavelength of maximum perception is used instead, the experimentally determined visibilities agree in all cases with the visibilities calculated from the extinction coefficient. Since the wavelength of maximum perception is calculated from the wavelength dependent contrast of the object seen through the turbid medium, the energy distribution of the illumination and the spectral sensitivity of the eye, it depends both on the aerosol and the illumination. For daylight conditions and the normal atmospheric aerosol the wavelength is 580 nm and it is suggested that the extinction coefficient at this wavelength be used, rather than that at 550 nm for the calculation of the visual range. When color filters are used for visibility observations, the atmospheric extinction coefficient can easily be obtained by means of the visibility formula at different wavelengths. For several conditions likely to occur in the atmosphere, the wavelength of maximum perception has been calculated and is listed in a table.