Generally described, one or more embodiments of the present disclosure are directed to pulse oximeter test instruments for testing pulse oximeters. The pulse oximeter test instruments are configured to linearize a relationship between an input signal and an output signal of a light emitting diode (LED). In some embodiments, the linearized relationship may be obtained by minimizing an amount of ambient light detected by a photosensor in a feedback loop. The photosensor may be located in a housing that limits the amount of ambient light that may be detected.
