Abstract: An external calibrating device (10) for a measurement device, which may be an acoustic camera. The calibrating device (10) includes a sound source (11) and a light source (12), which are preferably pointed to the same direction. This direction can be a horizontal direction, where the sources (11, 12) locate on the same vertical side surface of the calibrating device (10). An IR LED can be used as the light source (12). The measurement device may instruct a user how to place and align the measurement device during the calibration process. Guidance is given by instructions, volume range information, and focusing lines on the screen. When the instructions are fulfilled, the user may acknowledge the finished calibration process.