Remove the photocell from the heater and connect an ohmmeter on the 1000 OHM(RX1k)scale.
Hold the lens up to a light source like an 80 watt light bulb or direct sunlight, with the lens pointed toward the light. The resistnace of the meter should be low(less than 500ohms).
Block off the light source by putting a thumb over the opening of the photocell. The resistance on the meter should be high. Replace if a change in resistance is not observed during the test.
