In this math activity related to light, learners assemble a photometer and use it to estimate the power output of the Sun. Learners construct the photometer by making a grease spot on white paper and compare the brightness of the Sun to the brightness of a lamp. Learners calculate the power output by identifying the position at which the Sun is as bright as the lamp. This resource guide includes a calculation example and explains three complications that arise in this experiment. Instructions for constructing a plastic photometer are also provided.