Visibility Measurements Using a Commercial Digital Camera

Abstract

The measurement of visibility has many different applications from meteorological uses to air quality measurements. In fact, the amount of particles and molecules in the atmosphere can be estimated from visibility measurements, through the extinction coefficient, ext b . Also, visibility is an important parameter, for example, in airborne traffic control. It is well known that the extinction coefficient can be obtained from the contrast C between two black objects positioned at different but known distances, 1 x and 2 x from the observer (Horvath, 1996). From the extinction coefficient, visibility can be inferred using either the Koschmieder formula Visibility = 3.9 bext (Koschmieder, 1925) or its modified version 3.0 ext Visibility = b (Middleton, 1968). Our approach to obtain visibility is based on digital photography made at three different channels (red, green and blue) of an adequate landscape as shown in the picture. In this case, the objects chosen were a tree group located at 1 x = 6km from the observation point and a mountain background which contains the same type of trees sited at a distance of 2 x = 24km. The objects are not black, but are sufficiently dark for our purposes, in a similar fashion as the one used by Horvath et al. (1996). The contrast between the two objects and the background sky is measured by the camera for the three colors, and the extinction coefficient is obtained using the Lambert Beer Law, e.g. ( ) ( ) 0 exp ext C x = C −b x , where 0 C is a calibrating constant. By using two different objects relative to the same (identical) background, the calibrating constant can be eliminated. Hence this method can be used with any background. For the case shown in the picture the extinction coefficient and the visibility are shown in Table 1. A human observer could estimate the visibility of 25 km which corresponds to the value of the green channel. At the conference, further first results as a daytime diurnal cycle and other cases under a variety of different visibility conditions will be presented. It will be shown that the extinction coefficient and atmospheric visibility can be retrieved by means of a digital camera. Hence this method has the advantage of being very low-cost, robust, easy to set-up and it works for both clear and cloudy skies. Using this method, measurements up to 60km visibility were obtained, which enlarge the capabilities of the weather services in Portugal, where these high visibility values are currently not reported.