A system for measuring the gap in a proximity exposure tool of a plasma display panel (PDP) is developed that uses single or dual position-sensitive detectors (PSD's). The resolution of the gap distance is 0.5 mum, with a capture range of 500 mum. Signal processing is simple and fast and easy because analogous PSD's are used as the position sensors. One PSD is used to detect the position of the reference beam, which is reflected from the upper surface of the gap; the other PSD is used to detect the position of the signal beam, which is reflected from the lower surface of the gap. A charge-coupled-device sensor is also employed to monitor the reflected beams and the region of measurement. In the gap-measurement system that uses a single PSD, first the reference beam is incident upon the PSD and then the signal beam is incident upon the same PSD. Then the separation between the two beams is calculated from the position of the reference beam and from the average beam position.

A noncontact optical system that can measure both the vibration characteristics of the shadow mask and the relative distance (Q value) between the shadow mask and the phosphor surface of the glass panel of a cathode ray tube (CRT) is developed. Two position-sensitive detectors (PSDs) were employed to detect the positions of two beams that were reflected by the shadow mask surface and the inner surface of the glass panel. The relative distance between them is critical for the performance of the CRT. The vibration signals obtained from the PSDs were displayed in both temporal domain and spectral domain by using LabView software. The resonance frequencies of 20, 75, 400, and 490 Hz were identified and the low 20 Hz component was responsible for generating discomfort to the human eye.