Calibration of PIV

To convert the velocity field from image coordinates to laboratory coordinates, length calibration is performed with a calibration sheet. Calibration sheet is a set of grid points on which the distance between two grid points is 10 mm. By putting the calibration sheet at the image plane, the image of the grid is captured and superimposed on the PIV image. Two types of calibration are followed depending upon the geometry of the flow field: linear and nonlinear calibration. In the case of plane geometry with a distortion free image, linear mapping is adequate. In this route, a minimum of four grid points is required. At the four grid points x and y coordinates of the image are specified and the corresponding pixel numbers identified from the image. By linear interpolation, all other x and y-coordinates are calculated for each pixel location. Once we get the real coordinates of each pixel of the image, the velocity vectors can be determined from the time interval between two images. Nonlinear calibration is recommended when image distortion is expected during recording by the camera.