Substrate processing apparatus, substrate processing method, and computer-readable recording medium

A substrate processing apparatus comprising:

• a first laser and light receiving element including a first lens configured to irradiate a first light to a processing liquid flowing through a flow path supplied from a supply source, receive light irradiated to a portion of the flow path, and output a first signal;
• a second laser and light receiving element including a second lens configured to irradiate a second light to an upstream portion than the portion of the flow path, receive light irradiated to the upstream portion, and output a second signal;
• and
• a controller configured to control the first laser and light receiving element and the second laser and light receiving element,
• wherein the controller is programmed to:
• determine whether color information of the processing liquid flowing through the upstream portion is identical to predetermined color information that readily absorbs the first light, based on the second signal,
• determine whether the first light is irradiated to the processing liquid by the first laser and light receiving element, based on the result of the determining, and
• when determined that the color information of the processing liquid is not identical to the predetermined color information, irradiate the first light to the processing liquid by the first laser and light receiving element and obtain status of the processing liquid based on the first signal obtained by the irradiating.