The evidence for the non-uniform erosion of the MgO layer in a real a.c. plasma display panel was given by scanning electron microscopy and stylus surface profilometry. However, these techniques did not allow the quantitative study of the temporal and the spatial variation in MgO-layer thickness. Therefore, we constructed a microscopic spectrophotometer suitable for the transmission measurement from the area as small as 20 x 20 mum(2), which is necessary to probe MgO-layer thickness in a real micro-cell. The relative change in film thickness was determined either from the shift of the transmission extremum or from fitting the measured spectra using the parameterized optical functions.