Light sensor and source are in a small, rectangular block at the end of a long flexible cable. Circuitry, controls, speaker and batteries are in a small box at the other end of the cable. When the light is detected, the unit produces a tone whose pitch is proportional to the amount of light falling on the sensor; the higher the tone, the more light is being detected. Unit can also operate as an active light probe. It has its own light source and can, therefore, measure the amount of light from t