This master thesis deals with image processing of Petri dishes. The first part is about terms and procedures in laboratory practice. The following part is about devices and algorithms for image analysis of Petri dishes. The second part is about draft and implementation of own colony counting application, which was developed in cooperation with the microbiological laboratory. The application is developed for mobile devices running Android OS. The master thesis also includes the design and creation of two lighting units, which uses as an accessory for better results in the analysis of Petri dishes. In conclusion are presented results, compared lighting units and various types of mobile devices, on which the application was tested.