Reliable and realistic methods for assessment of thermal infrared signature properties for military purposes are important. With a basis in ongoing developments of imaging technologies, especially towards mass markets, including small handheld cameras or automotive sensors, thermal infrared sensors are expected to pose an increasing detection threat in the future. In this paper, we present a field-based approach that evaluates thermal contrast of camouflage nets, as well as mobile camouflage systems. In the proposed method, relative differences in thermal behavior between target and background are evaluated in a controlled manner in an outdoor environment over extended periods of ten days or more. The camouflage materials under test are mounted identically, in operationally realistic environments, and recorded with a thermal sensor at an image rate of 6 images per hour. Hence, thermal contrast values between each target and selected parts of the scene background are obtained during a full 24 hour period of time. Weather data are collected along with the thermal image data. In the subsequent analysis, average thermal contrasts between targets and selected backgrounds are calculated for certain well-defined time slots, such as night, day and transition between day and night. Only time slots that satisfy weather conditions requirements are analyzed, as changing weather is expected to affect the thermal response to camouflage systems. We believe the proposed method is a good compromise between controlled lab-tests, which are hampered by their lack of transfer value to thermal behavior in theatre, and field measurements during operations, where reproducibility of data can be low.