What Is a Reflectometer?
A reflectometer is a device used to check how much light returns to an object when it is illuminated.
The measurement of reflectance is based on the method for measuring reflectance.
Reflectance is very important in car mirrors. Because they become dazzling when the sun reflects off them or when the headlights of a following vehicle hit them, they are equipped with technology that automatically changes the reflectance.
It is also used for other purposes, such as measuring the roughness of plated surfaces.
Uses of Reflectometers
Reflectometers are available in direct and indirect measurement methods.
- Direct Reflectometry
This method measures the reflectance when a light source is shone directly on the object to be measured. True reflectance is obtained due to absolute reflection.
This method is used for parts with smooth surfaces, such as car mirrors and plastic products, where specular reflection is likely to occur.
- Indirect Reflection Method
This method measures reflectance by shining a light source indirectly on the object to be measured.
The light source is shined onto an integrating sphere. When light strikes the integrating sphere, diffuse reflection is repeated in the sphere, and light of uniform intensity, regardless of position, can be shone on the material.
This method is effective for materials that are uneven or prone to diffuse reflection.
Principles of Reflectometers
The principle of reflectometers is based on the Fresnel equation. Fresnel’s equation takes into account incident light, reflected light, transmitted light, and refractive index.
When light is shone on an object, incident light and reflected light are generated. The angle between the incident light and the center of the object is called the angle of incidence and the angle of reflection, respectively.
Not all incident light becomes reflected light, but some light is transmitted toward the object. If the refractive index of the object differs from that of the medium through which the light passes (air), the angle of transmission between the center of the object and the transmitted light differs from the angle of incidence.
The reflectance can be calculated by applying the difference between the angle of incidence (angle of reflection), the angle of transmission, and the refractive index to the equation.
A typical example of where Fresnel’s formula is used is in optical fiber. An optical fiber has a two-layer structure, with materials having different refractive indices in the inner and outer layers.
If there is no problem on the connecting surface of the layers, reflection does not occur, but if a gap is created due to cutting or other reasons, reflection will be repeated. The degree of reflection is used to determine the extent of the gap.