This page describes the construction of a simple photometer to be used with a microscope. The unit allows light intensity measurement of a small crystal positioned at the center of the microscope field in reflectance and transmission modes. Used in conjunction with a monochromator, the photometer becomes a part of a microscope spectrometer. Used in reflection modes with narrow band pass filters, the photometer can be used to measure the reflection coefficient of opaque minerals and thus help their identification.