This standard describes a method for measuring the blue light directional diffuse reflection factor Rrs7 (also known as paper and Cardboard whiteness) of paper and Cardboard using a Das illuminator and 45/0 illumination observation conditions. Suitable for paper and Cardboard specimens that are nearly white, contain fluorescent whitening agents, or do not contain fluorescent whitening agents. In the definition, the reflection factor R is the ratio of the radiation flux reflected by an object to the radiation flux reflected by a complete reflection diffuser under the same conditions, expressed as a percentage. The blue light directional diffuse reflection factor R4s7 refers to the internal reflection factor measured under blue light conditions with an effective wavelength of 457 +/- 2nm and a half-wave width of 44nm. The standard also specifies the requirements for the whiteness meter instrument, including the angle of the incident ray axis to the normal of the specimen plane, the diameter of the test hole, and the ability to eliminate mirror reflections.