An optical sensor can include first and second optical emitters configured to emit light into a fluid sample via an optical pathway. Light from the emitters can cause fluorescence from the sample and/or scatter off of the sample. Scattered and fluoresced light can be received by an optical detector in the sensor via the optical pathway, and used to determine at least one characteristic of the fluid sample. A second optical detector can provide reference measurements of the amount of light emitted to the sample. The second optical emitter and second optical detector can be included in an optical emitter assembly removably disposed in the optical pathway of the optical sensor such that the second optical emitter emits light into the optical pathway toward a fluid sample.