How to measure, with timestamps, changes in the received luminance?
If I were designing it I'd probably use a little PIC or AVR with a FT2232 (serial-to-USB) adapter and a light sensor from Sparkfun. The PIC would just use its ADC and measure the light level and then write to its UART a timestamp + light level reading. Some PICs (PIC18) have built-in USB interfaces so you could forego the FT2232, but the FT2232 is cheap and is a zero-programming-time cost adder, since all the PIC sees is a regular serial port (and all the PC sees is also a normal USB serial port).

Beyond the basics, I'd probably add in a way to zero the timestamp and to calibrate the return value on the light sensor... Maybe a software PLL along the lines of what the Network Time Protocol uses so you could synchronize multiple sensors (if your application needed it).