Using LEDs as Light-Level Sensors and Emitters
This paper discuses the Altera MAX IIZ CPLD and how it measures the analog light level of its environment and then drive an LED at a proportional analog intensity level. In portable electronic products, a common use for LEDs is a "heartbeat" indicator that shows power status, battery condition, or BlueTooth connection activity.
The LED can be a major factor in determining battery life, as its intensity is directly proportional to power drain. LEDs are designed to be easy to see in bright ambient light, yet it can be assumed that much of the time a portable device is in a dark purse or pocket. A low-intensity LED indicator extends battery life but is useless in a bright environment. Modulating LED power based on ambient light level would increase battery life, a particularly helpful feature in a device where battery life is measured in days.