 In this case, the spectrophotometer measures the intensity of light reflected from a sample (formula_3), and compares it to the intensity of light reflected from a reference material (formula_6) (such as a white tile).The ratio formula_7 is called the "reflectance", and is usually expressed as a percentage (%R).The basic parts of a spectrophotometer are a light source, a holder for the sample, a diffraction grating in a monochromator or a prism to separate the different wavelengths of light, and a detector.