Digital Indicators rely on three major components including an LED, a glass scale, and a reticle.  The components all need to be precisely aligned with each other to give accurate measurements. First, the LED, has one function to shine an infrared light through the glass scale.  Secondly, the heart of the indicator is the glass scale which is plated with a chrome pattern allowing light to pass through some areas while blocking light in other areas. This combination of light and dark areas will change as the glass is moved. The light pattern is detected by the third component, the reticle, using a series of infrared light sensors.  Since this light pattern in any position on the tool is unique, the processor is able to convert it to an absolute measurement which is then displayed on the LCD. This is why users can turn the indicator off, move the glass scale along its travel while it is off and still have an accurate measurement as soon as the tool is turned back on.