An optical sensor comprising: an optical emitter that is configured to direct light into a fluid sample, a first optical detector that is configured to detect light emitted by the optical emitter and transmitted through the fluid sample: a second optical detector that is configured to detect light emitted by the optical emitter and scattered by the fluid sample; a third optical detector that is configured to detect fluorescent emissions emitted by the fluid sample in response to the light emitted by the optical emitter; an optical emission filter positioned between the optical emitter and the fluid sample; a first optical detection filter positioned between the first optical detector and the fluid sample; a second optical detection filter positioned between the second optical detector and the fluid sample; and a third optical detection filter positioned between the third optical detector and the fluid sample, wherein the optical emission filter, the first optical detection filter, and the second optical detection filter are each configured to pass the same wavelengths of light while rejecting all other wavelengths of light so that substantially any light detected by the first optical detector and the second optical detector is light emitted from the optical emitter passing through the optical emission filter and passing through the fluid sample; the sensor further comprising a tube, the tube comprising an optical emitter window in the form of a ball lens (324, 326, 328,330) facing the optical emitter and the optical emitter filter, whereby the tube defines a geometric centre through which the fluid sample travels and the optical emitter window is offset relative to the geometric center of the tube so that light emitted from the optical emitter through the optical emission filter and subsequently through the geometric center of the ball lens optical emitter window does not pass through the geometric center of the tube. an optical emitter that is configured to direct light into a fluid sample, a first optical detector that is configured to detect light emitted by the optical emitter and transmitted through the fluid sample: a second optical detector that is configured to detect light emitted by the optical emitter and scattered by the fluid sample; a third optical detector that is configured to detect fluorescent emissions emitted by the fluid sample in response to the light emitted by the optical emitter; an optical emission filter positioned between the optical emitter and the fluid sample; a first optical detection filter positioned between the first optical detector and the fluid sample; a second optical detection filter positioned between the second optical detector and the fluid sample; and a third optical detection filter positioned between the third optical detector and the fluid sample, wherein the optical emission filter, the first optical detection filter, and the second optical detection filter are each configured to pass the same wavelengths of light while rejecting all other wavelengths of light so that substantially any light detected by the first optical detector and the second optical detector is light emitted from the optical emitter passing through the optical emission filter and passing through the fluid sample; the sensor further comprising a tube, the tube comprising an optical emitter window in the form of a ball lens (324, 326, 328,330) facing the optical emitter and the optical emitter filter, whereby the tube defines a geometric centre through which the fluid sample travels and the optical emitter window is offset relative to the geometric center of the tube so that light emitted from the optical emitter through the optical emission filter and subsequently through the geometric center of the ball lens optical emitter window does not pass through the geometric center of the tube.