The curvature of the surface of a drop, which correlates with the contact angle, is determined from the distance of reflected light spots in a video image. The light spots originate from small light sources (LEDs) which are arranged in a plane above the drop. For this purpose, only variables which are determined by the apparatus must be known. These include the working distance, the distance of the LEDs from one another, the enlargement and the dosed drop volume.
This method has a very good resolution in the lower measuring range as, for small contact angles, the spacing of the reflected spots is particularly strongly dependent on the change of contact angle. The method is also frequently used for making measurements on curved solid surfaces and in depressions which are not easily accessible to the classical shadow image measurement in drop shape analysis.