Light is a form of radiant energy, and light waves are essentially electromagnetic waves. In the wavelength range of electromagnetic waves, sunlight is composed of wavelengths of 380~780nm. In the wavelength range of sunlight, the wavelengths in the range of 400~700nm are called visible light, which can be observed with the naked eye to distinguish various colors, as shown in Table 2-3 shown.
When a beam of light is projected onto the paper surface, optical phenomena such as reflection, transmission, scattering, absorption, and diffuse reflection occur.
① Reflection refers to the specular reflection of light from the paper surface, and the amount of reflection is evaluated by glossiness.
② Transmission refers to the light entering the paper layer to form scattered light, which is evaluated by the scattering coefficient.
③ Scattering refers to light passing through the paper layer to form refracted light, which is evaluated by opacity.
④ Absorption means that light is absorbed by paper and turned into heat energy, which is evaluated by absorption coefficient.
⑤ Diffuse reflection When a part of light enters the interior of the paper and reflects the same amount of light in all directions, a hemispherical diffuse reflection phenomenon is formed, which is evaluated by diffuse reflectance.
If the amount of light reflected by the specular surface is large, the paper will have a high gloss. When a certain color has a large amount of diffuse light, objects mainly appear in that color. If the light reflectance of all wavelengths is 100%, the object is perfectly white. When all light is absorbed by an object, the object appears absolutely black. If the amount of light transmitted through the object is large, the opacity is low; if 100% is transmitted, the object is absolutely transparent.
The optical properties of paper are evaluated based on the above conditions, such as whiteness, color, gloss and opacity.
