An electrically modulated light source is provided. The electrically modulated light source comprises a carbon nanotube film structure. The electrically modulated light source heats up to a highest temperature and emits thermal radiation in less than 10 milliseconds after a voltage is applied, and the electrically modulated light source cools down to an initial temperature of the electrically modulated light source in less than 10 milliseconds after the voltage is removed. An modulation frequency of the electrically modulated light source is greater than or equal to 150 KHz. A non-dispersive infrared spectrum detection system used the electrically modulated light source, and a method for detecting gas used the electrically modulated light source are also provided.