Light level sensor. The sensor is at the end of the metal rod, and has been pointed at an incandescent light bulb (top). The light bulb symbol on the sensor indicates that it is a light sensor. The cable at bottom is a data logging cable, allowing regular or continuous readings of the light levels to be logged on a computer for later analysis. Light levels are measured in a unit called a lux. When working in artificial light, it is important that light levels are not too dim or bright, as this can aggravate eye problems. This sensor is from Phillip Harris.