Geometrical Optics

Geometrical optics refers to the physical laws giving an explanation for the path of a light beam in materials. Basic concepts in geometrical optics are reflection, refraction, transmission, absorption and diffusion.
Reflection occurs when a beam of light meets a surface. Light is reflected from the surface following the law of reflection: the angle between an incoming ray and a line normal to the surface is equal to the angle of the reflected ray and the normal one.

Depending on the quality of the surface, reflection can be specular (polished surface), spread (rough surface) or diffuse (matte surface).

When a beam of light coming from one material enters another material, it refracts, i.e., changes angle and velocity. Refraction depends on two factors: incoming angle of the light beam and refractive index of material. Refractive index is the ratio of the speed of light in a vacuum to the speed of the light in that material. E.g., the magnificent glittering of a diamond is caused by the high refractive index of diamonds. Diamond is the most refractive optical material of all.

When a beam of light comes from a material having a greater index of refraction to one with a lower index of refraction, it bends away from normal. When this angle increases, it reaches finally a point, from which all light is no longer refracted but reflected. This phenomenon is used, e.g., in fiber optics and light guides where light has to be transported long ways. When a beam of light goes through an object, it is called transmission. The phenomena that affect light transmission are reflection, refraction, absorption and diffusion. How much they affect is depending on the object’s material properties and surface quality. Many materials selectively absorb more of certain areas of radiation. Normally absorbed radiation is converted into heat. Degree of absorption is defined by material thickness and concentration of the absorbing component. When a beam of light hits a rough surface, it is reflected or transmitted in many directions. This is called diffusion or scattering. The amount of diffusion depends on the difference in refractive index between the materials and the size and the geometry of the diffusing particles compared to the wavelength of the light.

The choice of the optical material normally depends on the optical, mechanical and environmental requirements set to the end-product, total costs and manufacturing possibilities. We use optical polymer, coated or uncoated, in most of our products. The quality of a lens material is characterized by the transmission and dispersion factor (so called Abbe factor). Glass is the best of lens materials, solely when it comes to these figures. Plastic materials can be used, when developing a system with fewer requirements for optical precision and with optical polymer and there is a need for good integration of the optical system to the rest of the mechanical structure. Suitable standard optical polymer materials for lenses are, e.g., PMMA, PC, PS, SAN and PA12. When using optical polymers, special attention must be paid to material specific properties, to material behavior in the molding process and to manufacturing & design of molds.