These are retro-reflective optical devices. The sensors use bundled glass or quartz fibres to transmit light to and receive reflected light from target surfaces. The intensity of the reflected light is processed to provide the distance between the sensor tip and a target surface.
Reflectance Dependent sensors (D Models) provide an output signal that is proportional to the target gap and also proportional to changes in reflectivity of the target surface.
Reflectance Compensated sensors (RC Models) provide an output signal proportional to the target gap only. They are blind to (independent of) changes in reflectivity of the target surface.
D models (Reflectance Dependent) are used for single axis vibration and stroking motions, where the target reflectivity is constant.
RC models (Reflectance Compensated) are used for translating and rotating targets, where the target reflectivity is variable.