A measurement system has a photosensitive element, an analog-to-digital converter (ADC), a reference voltage source, and a transmission interface. The photosensitive element senses variations of brightness of a display panel of a liquid crystal display (LCD) to generate a voltage signal. The ADC converts the voltage signal into a digital signal. The reference voltage source provides a reference voltage to the ADC to drive the ADC to dynamically adjust a convertible voltage range of the ADC for any voltage inputted to the ADC according to the reference voltage. The transmission interface transmits the digital signal to a computer to trigger the computer to calculate a response time of the LCD according to the digital signal.