Gas measurement apparatus

A gas measurement apparatus configured to measure a gas concentration by laser absorption spectroscopy, the gas measurement apparatus comprising:

• a first corner cube or first right angle prism;
• a second corner cube or second right angle prism disposed opposite the first corner cube or first right angle prism;
• a laser source configured to emit a laser beam to the first corner cube or first right angle prism;
• and
• a light receiving element configured to receive the laser beam that has been reflected multiple times through a target gas between the first corner cube or first right angle prism and the second corner cube or second right angle prism,
• wherein the second corner cube or second right angle prism is disposed so that a centerline that is parallel to incident and exit light centering at an incident position and an exit position of the first corner cube or first right angle prism is displaced from a centerline that is parallel to incident and exit light centering at an incident position and an exit position of the second corner cube or second right angle prism.