The problem of calibration of linear scan camera

Question

I need to calibrate a line-scan camera. Currently, I am using a method similar to Zhang Zhengyou's calibration technique, but with fewer parameters. My calibration tool is a swinging light source with holes. Using this method, when I measure the width of the calibration tool within its swinging range, the accuracy is very high, with an error of about 0.1 mm. However, after calibration, when I raise the calibration tool and measure its width, the error increases. The higher I raise it, the larger the error becomes. What could be the reason for this?