An optical sensor is configured to determine an amount of at least one material in a sample without interference from any of the material of interest present in an ambient environment in which the optical sensing is effected. The optical sensor may include a compensation detector positioned a different distance apart from a source of monitoring radiation than the distance that a primary, measurement detector is positioned from the source. Alternatively, the optical sensor may include an optically transparent material that consumes space within the sensor and, thus, eliminates ambient amounts of a material of interest from at least a portion of an optical pathway through the sensor. A calibration element transmits calibration radiation to one or more detectors of an optical sensor to facilitate correction of any changes in the manner in which the one or more detectors sense radiation. Optical sensing methods are also disclosed, as are calibration methods.