Photographic method for monitoring visibility through measurement of transmittance and path radiance

A method for obtaining quantitative data useful to determine either the transmittance, path radiance, or both, of a sight path toward a target having both relatively light and dark areas using photographic film which comprises the steps of:

• (a) at each of at least two stations along a sight path, providing a camera with photographic film, a calibration scale in the camera's field of view, said scale having panels of known reflectances, a light meter for measuring the radiance of one panel of the calibration scale, and means for synchronizing the time of the photographic exposures;
• (b) at each station along the sight path, photographing a target having varying light contrast substantially synchronously with all other stations;
• (c) at each station along the sight path, at the time the target is photographed per step (b), measuring the radiance of one panel of the calibration scale using the light meter provided in step (a);
• (d) calibrating the film at each station using the light meter measurement of the radiance of one of the calibration panels at each station at the time of the target exposure to calculate the radiances of all panels of the calibration scale and to relate photographic film densities to the radiances of the calibration scale panels;
• and
• (e) obtaining radiance data for each station along the sight path by using the step (d) radiance calibration obtained by measuring the photographic film densities corresponding to the calibration scale panels and by measuring the film densities for each target photograph for contrasting portions of the target.