Abstract: An in-line photoresist thickness measuring device wherein a plurality of projection optical fibers are disposed over a wafer processing track for illuminating portions of a wafer as the wafer proceeds along the track. The light scattered back from each illuminated portion is detected by a corresponding plurality of pickup optical fibers and communicated to an optical fiber multiplexer. The multiplexer sequentially selects the optical signal from each pickup optical fiber and communicates the light from the selected pickup optical fiber to a spectrometer. The spectrometer simultaneously diffuses the scattered light into a plurality of light bands, each light band having a different wavelength. The plurality of light bands are projected in parallel onto a pin diode array which acts as a photodetector. The electrical signals generated by each pin diode in response to the incident light bands are communicated to a processor which calculates resist thickness for each illuminated portion of the wafer.