Abstract: A system for quantifying the presence of at least one chemical and/or a physical condition in ocular tissues is provided. The system uses a membrane having at least one photo-reactive chemical- and/or physical condition-sensing indicator dye and a photo-reactive reference dye wherein both the photo-reactive chemical- and/or physical condition-sensing indicator dye and the photo-reactive reference dye are responsive to the same wavelength of an excitation light source but emit different wavelengths of light in a light spectrum. An optical data detector receives the emitted light and a processor splits the data into two or more colours of the light spectrum and measures the intensity of the two or more colours.