Step Tablet, which consists of 21 steps covering a density range from 0.05 to 3.05 OD. The mean gray value of each step was translated into its corresponding OD value. The accuracy of this calibra�on was verified by construc�ng a calibra�on curve with an R 2 value of 0.99. All POM images were cropped to the region of interest and converted into 8-bit grayscale for analysis.