A switch on the side of the Personal Series LCD Clock sets the backlight to ‘High’, ‘Low’, or ‘Off’. ‘High’ and ‘Low’ select the intensity of the backlight. When set to ‘High’ or ‘Low’, an optical sensor on the top of the clock samples available light to determine whether to turn on the backlight.
Backlight use significantly affects battery life. To extend battery life, turn the backlight to ‘Off’ or operate the backlight on ‘Low’