Abstract: A method is composed of providing a mask and placing a substrate to face the mask. The mask includes an array of patterns, and a window disposed between two of the patterns. Each of the patterns corresponds to a display device. The method includes projecting an incident laser beam onto the substrate through the window of the mask and determining a gap between the mask and the substrate in a middle region of the substrate in response to first and second reflected beams. The first reflected beam is generated by the incident laser beam reflected by the mask, and the second reflected beam is generated by the incident laser beam being reflected by the substrate. Determining the gap between the mask and the substrate in the middle region advantageously provides a step for removing or releasing undesirable deflection of the mask.