An optical sensor may include multiple 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. In one example, the optical detector can detect scattered and fluoresced light simultaneously. In another example, light is emitted and detected alternatingly. The sensor can be part of a system that includes one or more controllers configured to control the emitting and detecting of light to and from the fluid sample. The controller can use detected light to determine at least one characteristic of the fluid sample.