The application note gives the reader things around gain, light levels, and intensity. Lots of things such as sensor, ambient light, and light intensity are presented inside this application note.
Inside this application note you can get information like light, input voltage, and ambient. Here are taken from the application note:
Ambient light is increasingly considered as a source for harvesting energy to power heartbeat monitors, bathroom fixtures, remote weather sensors, and other low-power devices. At the heart of an energy-harvesting system is the ability to measure ambient light accurately. This design idea describes a simple, cost-effective circuit that provides a voltage proportional to the intensity of ambient light. The sensor is a light-dependent resistor (LDR)—photoresistor Model 276-1657 from RadioShack—which has a resistance that varies with ambient light intensity,
Even more, this application note contains discussion about amplifier, circuit, and ldr.