In some examples, an apparatus can include a light transmitter, a light sensor aligned along a light transmittance axis of the light transmitter, an impeller positioned between the light transmitter and the light sensor. The impeller can in some examples include a blade to pass through the light transmittance axis during rotation of the impeller. The blade can in some examples be translucent to permit calibration of the light sensor based on a comparison of a first light sensor reading when the blade intersects the light transmittance axis and a second light sensor reading when the blade does not intersect the light transmittance axis.