Provided are turbidometers and fluorometers having a unique form-factor to accommodate a number of optical components in a confined geometry. This provides the ability to compensate for change in light intensity from an optical source even in a closed-loop manner. The ability to package reference and signal detectors, along with a relatively large diameter LED light source in a confined geometry is particularly suited for applications requiring small-diameter sensors, such as multi-parameter sonde devices having a total diameter that is in the sub-two inch range.