The Illuminant of the instrument is long -life LED with 457nm, conforming to the light source. The measurement of the instrument adopts the principle of photoelectric effect. The diffuse reflectance Radiant Brightness Energy Value of the sample surface is receive by the optical cell. Then, the value is amplified by high precision auto zeroing operation amplifier, inputted to the calibration circuit for energy calibration. Then, the whiteness value of the sample is directly shown on the digital display.