Optical gas concentration measuring method by forming a differential signal using lights with different absorbabilities to a raw material in a gas flow path using a time-sharing method

An optical gas concentration measuring method when a chemical or physical process is to be implemented upon introduction of a raw material gas into a processing chamber from a gas supply line in which optical gas concentration measurement apparatus comprising a gas flow path is arranged in a predetermined position, the method comprising the steps of:

• irradiating light having a first wavelength that has an absorbability with respect to the raw material gas, and light having a second wavelength that has no or substantially no absorbability with respect to the raw material gas, or an absorbability that is relatively lower than that of the light having the first wavelength, along the same or substantially the same optical path and onto the raw material gas in the gas flow path using a time-sharing method;
• receiving in a photodetector a first exiting light on the basis of the light having the first wavelength and a second exiting light on the basis of the light having the second wavelength, the first exiting light and the second exiting light exiting from the optical path, in a photodetector disposed on an end edge of the optical path;
• inputting a first differential circuit input signal in accordance with a first signal based on the first exiting light, and a second differential circuit input signal in accordance with a second signal based on the second exiting light, the first signal and the second signal being output by the photodetector according to the received light, into a differential circuit;
• and
• comparing a measured value that is based on an output signal output from the differential circuit in accordance with the input, with data stored in advance in storage means to derive a concentration of the raw material gas, wherein
• the first signal and the second signal are formed by the photodetector in a first period (T13) that is arranged to be within a second period (T12),
• the photodetector is in an ON-state within second period (T12),
• the second period (T12) is arranged to be the same as or within a third period (T11),
• the light is irradiated onto the raw material gas within third period (T11), and
• said first, second and third periods satisfy the relationship T13<T12<T11.