A measurement system, comprising: (a) a light source; (b) a detection system; (c) a set of at least first, second, and third light ports, which set transmits light from the light source to a sample and receives and directs light reflected from the sample to the detection system, wherein a distance between the first port and the third port comprises a first detection distance and a distance between the second port and the third port comprises a second detection distance, wherein the first detection distance is larger than the second detection distance; wherein the first and second ports are transmitting ports and the third port is a receiving port; and wherein the detection system generates a first set of data corresponding to the first detection distance and comprising information corresponding to both an internal target within the sample and features overlying the internal target, and a second set of data corresponding to the second detection distance and comprising information corresponding to features overlying the internal target; and (d) a processor configured to remove information characteristic of the overlying features from the first set of data using the first and second sets of data to produce corrected information representing the internal target, wherein the detection system is a spectral detection system, the first and second sets of data comprise first and second sets of spectra, and wherein the processor removes spectral information characteristic of the overlying features from the first set of spectra using the first and second sets of spectra to produce corrected spectral information representing the internal target, wherein the processor is further configured to remove from the corrected spectral information representing the internal target spectral information that is characteristic of variations in optical scattering properties of the internal target that are unrelated to an analyte of interest therein by orthogonalizing the corrected spectral information by means of principal component analysis to remove analyte-irrelevant contributions to spectral reflectance variations (a) a light source; (b) a detection system; (c) a set of at least first, second, and third light ports, which set transmits light from the light source to a sample and receives and directs light reflected from the sample to the detection system, wherein a distance between the first port and the third port comprises a first detection distance and a distance between the second port and the third port comprises a second detection distance, wherein the first detection distance is larger than the second detection distance; wherein the first and second ports are transmitting ports and the third port is a receiving port; and wherein the detection system generates a first set of data corresponding to the first detection distance and comprising information corresponding to both an internal target within the sample and features overlying the internal target, and a second set of data corresponding to the second detection distance and comprising information corresponding to features overlying the internal target; and (d) a processor configured to remove information characteristic of the overlying features from the first set of data using the first and second sets of data to produce corrected information representing the internal target,