This research was part of the collaborative study "Improvements in spraying techniques against potato late blight". The study was a joint project of MTT Agrifood Reseach Finland and the Potato Research Station (PETLA). Different application techniques were studied to determine the efficacy of chemical blight control in potato canopies. An important aim of the study was to measure the canopy penetration of the spraying liquid. The penetration was measured with a multitude of water-sensitive papers placed among the potato stems. Consequently, an efficient measurement system was needed for the paper analysis. The main aim of this research was to develop a usable method for photographing and analysing water-sensitive paper. This required use of digital photography and digital image processing. The chosen measurement method was tested and calibrated in experiments in which water-sensitive papers and potato leaves were sprayed with a fluorescent spraying liquid at rates of 40, 100, 200, 300, and 400 l/ha. The degrees of coverage were analysed after spraying. Papers and potato leaves were photographed with an Olympus C-1400 TM digital camera, and the images were analysed with Global Lab Image TM software. According to the results, water-sensitive paper has a higher degree of coverage than potato leaves. Two linear models were developed to depict the relationship between the degree of coverage of water-sensitive paper and that of potato leaves. Model A relates the degree of coverage of the paper (x) and that of the leaf's upper side (y): y = 0,6246x + 0,2593. Model B relates the degree of coverage of the paper (x) and that of the leaf's lower side (y): y = 0,7747x - 9,0085. The models are valid for paper coverage degrees between 15 and 75 % when the spray rates are 100 - 400 l/ha.The digital photography and analysis system developed in the study is efficient. During normal working hours, it is possible to take 300 - 400 pictures or to analyse about 300 image files. The method is suitable for different spraying applications, and it is easy to improve by replacing the digital camera with a newer and more precise one. Further studies should concentrate on the basics: coverage requirements, droplet size, and dosages of different protective agents. Without question, it is possible to develop more efficient and more environmentally friendly application techniques, but only after we actually know the exact specifications needed.