The optical contact method was proposed by SM Chapman, and the test principle is shown in Figure 2-10. Place the sample on the base of the platform, which can be adjusted up and down to adjust the pressure between the sample and the prism. The optical glass prism is formed by merging three prisms, and a whole prism can also be used. When the base is adjusted upward, optical contact is made between the peaks of the uneven surface of the paper and the prisms. The so-called optical contact refers to the contact where the gap between the convex part of the paper and the prism is less than or equal to the wavelength of the incident light. If the gap is larger than the wavelength of the incident light, it is considered that there is air between the prism and the paper, which is called non-optical contact.
Figure 2-10 Schematic of the optical contact method Figure 2-11 Reflected light in the contact area
1.4-photosensitive element; 2-human light source; 3-prism; 5-sample; 6-base
When the incident light along the normal direction hits the contact part, diffuse reflection is formed from the glass-paper interface to the space inside the prism, as shown in Figure 2-11(a). Therefore, there is reflected light in the range of ±90° from the incident direction. When the incident light hits the non-contact part, it passes through the glass-air interface to the surface of the convex and concave parts of the paper, and then the reflected light from the convex and concave parts of the paper is refracted into the prism body through the air-glass interface. When the wavelength of visible light is 380~780nm, the refractive index of glass is n=1.5~1.53. If n=1.52, it can be seen from the definition of refractive index that the light refracted into the prism body is concentrated in the space of ±41°, as shown in Figure 2- As shown in 11(b), there is no reflected light in the space greater than 41° from the incident direction. Therefore, at "A" at a direction of 30° to the normal incident light, the reflected light on the contact area and non-contact area is reflected, and at "B" at 60° to the normal incident light, But it can only reflect the reflected light of the contact part. The SM Chapman optical contact method is to use the above phenomenon to set two photosensitive elements at the two positions of "A" and "B" to measure the reflected light intensity of the two parts of "A" and "B", and compare them to determine The percentage of contact area.
To sum up, it can be seen that the optical contact method can measure the size of the contact area of the paper between the pressurized objects, but the data obtained from the test are related to the applied pressure and the compression characteristics of the paper, so it can only be used under the specified pressure. To a certain extent, it reflects the relative flatness of the paper surface. In addition, the contact area percentage obtained by the optical contact method cannot reflect the depth of the concave part of the paper.
