An optical sensor apparatus (100, 300), comprising: an optically transmissive light guiding structure (102, 302) having planar first, second, and third faces (F1, F2, F3); four or more light sources (106A, 106B, 106C, 106D, 306A, 306B) located outside the light guiding structure adjacent the first face, wherein the four or more light sources include two common wavelength light sources configured to emit light of a common vacuum wavelength and two different wavelength light sources configured to emit light at vacuum wavelengths different from each other and also different from the common vacuum wavelength of the common wavelength light sources; a photodetector array (108, 308) located outside the light guiding structure adjacent the first face, wherein the light guiding structure, light sources, and photodetector array are configured such that light from the four or more light sources that is totally internally reflected at an optical interface between the light guiding structure and a sample (111, 311) outside the light guiding structure proximate the second face is reflected at the third face and incident on a portion of the photodetector array that depends on a refractive index of the sample, wherein the two common wavelength light sources are positioned with respect to the light guiding structure and photodetector array such that light from each of the two common wavelength light sources that is totally internally reflected at the interface and reflected at the third face corresponds to a different range of refractive index of the sample and maps to a corresponding portion of the photodetector array. an optically transmissive light guiding structure (102, 302) having planar first, second, and third faces (F1, F2, F3); four or more light sources (106A, 106B, 106C, 106D, 306A, 306B) located outside the light guiding structure adjacent the first face, wherein the four or more light sources include two common wavelength light sources configured to emit light of a common vacuum wavelength and two different wavelength light sources configured to emit light at vacuum wavelengths different from each other and also different from the common vacuum wavelength of the common wavelength light sources; a photodetector array (108, 308) located outside the light guiding structure adjacent the first face, wherein the light guiding structure, light sources, and photodetector array are configured such that light from the four or more light sources that is totally internally reflected at an optical interface between the light guiding structure and a sample (111, 311) outside the light guiding structure proximate the second face is reflected at the third face and incident on a portion of the photodetector array that depends on a refractive index of the sample, wherein the two common wavelength light sources are positioned with respect to the light guiding structure and photodetector array such that light from each of the two common wavelength light sources that is totally internally reflected at the interface and reflected at the third face corresponds to a different range of refractive index of the sample and maps to a corresponding portion of the photodetector array.