The application note contains things related to the input frequency, interface, and output. Inside the application note you can learn description like frequency, input frequency, and sensor.
Lots of things such as computer, sensor interface, and frequency converter are described in this application note. These are selected from this application note:
To keep the circuit simple and small the internal 4MHz clock is used with satisfying accuracy. In case of very high accuracy measurements an external oscillator is recommended. The frequency is converted to the intensity of light and transmitted to the computer which displays the irradiance of light with suitable software on the computer. This design displays irradiance of the light source (10 µW/cm2 to 1000 µW/cm2) on the computer with a serial port (with very little modiﬁcation a serial LCD display module can also be used). The circuit is designed keeping in view to make it the smallest light sensor in the world (using SOIC packages).
In addition, the application note gives the reader more around sensor, and light meter.