Technologies are described for monitoring characteristics of layers of integrated computational elements (ICEs) during fabrication using an in-situ spectrometer operated in step-scan mode in combination with lock-in or time-gated detection. As part of the step-scan mode, a wavelength selecting element of the spectrometer is discretely scanned to provide spectrally different instances of probe-light, such that each of the spectrally different instances of the probe-light is provided for a finite time interval. Additionally, an instance of the probe-light interacted during the finite time interval with the ICE layers includes a modulation that is being detected by the lock-in or time-gated detection over the finite time interval.