A blood chamber for optically monitoring blood flowing through an extracorporeal tube, the blood chamber defining a flow path through an internal blood flow cavity which provides a flat viewing area for optical monitoring of the blood, the blood chamber comprising: a chamber body having a flat internal wall and a peripheral wall extending around the flat internal wall, the flat internal wall and peripheral wall forming part of the internal blood flow cavity, a first port and channel in fluid communication through a first opening in the peripheral wall with the internal blood flow cavity, a second port and channel in fluid communication through a second opening in the peripheral wall with the internal blood flow cavity, and a viewing lens made of a clear material commensurate with at least a portion of the flat internal wall on the chamber body to provide viewing access of blood flowing through the internal blood flow cavity, wherein at least a portion of the chamber body is made of a material that is opaque to red light having a wavelength defined as a third wavelength (λ3); and a lens body having a flat internal wall that is attached to the chamber body to form the internal blood flow cavity along with the flat internal wall and peripheral wall of the chamber body, wherein the lens body is attached to the chamber body with the flat internal wall on the lens being substantially parallel to the flat internal wall on the chamber body and also being separated from the flat internal wall on the chamber body by a predetermined fixed distance, and further wherein the lens body comprises a viewing lens made of a clear material commensurate with at least a portion of the flat internal wall on the lens body to provide viewing access of blood flowing through the internal blood flow cavity; wherein the opaque portion of the blood chamber is configured to attenuate the red light at the third wavelength (λ 3) ducting through the chamber body and is configured to isolate the lenses from ducted light at the third wavelength (λ 3), characterized in that the opaque portion of the blood chamber is molded from a blue-tinted polycarbonate material that is opaque to red light at the third wavelength (λ 3) and otherwise transparent to visible light so as to attenuate red light ducting through the chamber body. a chamber body having a flat internal wall and a peripheral wall extending around the flat internal wall, the flat internal wall and peripheral wall forming part of the internal blood flow cavity, a first port and channel in fluid communication through a first opening in the peripheral wall with the internal blood flow cavity, a second port and channel in fluid communication through a second opening in the peripheral wall with the internal blood flow cavity, and a viewing lens made of a clear material commensurate with at least a portion of the flat internal wall on the chamber body to provide viewing access of blood flowing through the internal blood flow cavity, wherein at least a portion of the chamber body is made of a material that is opaque to red light having a wavelength defined as a third wavelength (λ3); and a lens body having a flat internal wall that is attached to the chamber body to form the internal blood flow cavity along with the flat internal wall and peripheral wall of the chamber body, wherein the lens body is attached to the chamber body with the flat internal wall on the lens being substantially parallel to the flat internal wall on the chamber body and also being separated from the flat internal wall on the chamber body by a predetermined fixed distance, and further wherein the lens body comprises a viewing lens made of a clear material commensurate with at least a portion of the flat internal wall on the lens body to provide viewing access of blood flowing through the internal blood flow cavity; wherein the opaque portion of the blood chamber is configured to attenuate the red light at the third wavelength (λ 3) ducting through the chamber body and is configured to isolate the lenses from ducted light at the third wavelength (λ 3), characterized in that the opaque portion of the blood chamber is molded from a blue-tinted polycarbonate material that is opaque to red light at the third wavelength (λ 3) and otherwise transparent to visible light so as to attenuate red light ducting through the chamber body.