A method and apparatus are provided for a spectroscopic measurement for determining a lateral recess depth in the sidewall of a microstructure. The structure is formed on a larger substrate with the sidewall in an upright position relative to the substrate, and the recess extends essentially parallel to the substrate. The recess may be an etch depth obtained by etching a first layer relative to two adjacent layers, the layers oriented parallel to the substrate, the etch process progressing inward from the sidewall. An incident energy beam falling on the structure generates a spectroscopic response captured and processed respectively by a detector and a processing unit. The response comprises one or more peaks related to the material or materials of the substrate and the structure. According to the method, a parameter is derived from said one or more peaks, that is representative of the lateral recess depth.