Illustration of a measurement system for transmission spectrum. Here, 1: deuterium lamp, 2: tungsten lamp, 3: optical filter, 4: chopper, 5: monochromator with double grating, 6: collimating lens, 7: flat mirror, 8: sample holder, 9: voltage adjustable DC power supply, 10: silicon detector, 11: InGaAs detector, 12: integrating sphere, 13: lock-in amplifier, and 14: data acquisition system.