Tuesday, July 5, 2011

Standard properties of a Lens

Refractive Index

The refractive index of a lens material indicates how much the material will refract or bend light as it enters the material from air, by comparing the speed of light in a given material to the speed of light in air. The higher the index number of a given material, the more the light will refract as it enters the material. If a material has a greater ability to refract light, less of a curve is required to obtain a specific power, resulting in a thinner lens. Plastic (CR-39) and Crown Glass are considered base index with indices of 1.498 and 1.52 respectively. Materials with an index between 1.53 and 1.57 are sometimes considered mid-index, while 1.58 and greater is considered high-index. More frequently, however, anything over 1.53 is called high-index.


Specific Gravity

Specific gravity describes the density of a lens material by comparing its density to the density of water. The higher the specific gravity of a lens material, the higher the density, and consequently, the heavier a lens of that material will be for a given power and size.


Abbe Value

White light is composed of the visible spectrum of wavelengths each corresponding to a different color. When light enters a prism it is bent toward the base of the prism. Shorter wavelengths (e.g., violet) are bent at a greater angle than longer wavelengths (e.g., red). Since a lens can be likened to two prisms (apex to apex for a minus lens and base to base for a plus lens), light passing through a lens has a tendency to separate into its respective colors as its varying wavelengths are focused at differing points. The tendency to of a material to separate light in this manner is called chromatic aberration and is measured by its Abbe value. The Abbe value of a material is inversely proportional to the chromatic aberration induced as light passes through it. In other words, the higher the abbe value, the lower the amount of chromatic aberration. Generally speaking, the higher the index of a lens material, the higher the chromatic aberration, and the lower the Abbe value.



The reflectance of the material describes the percentage of incident light reflected from a highly polished surface of that material and is calculated from the refractive index of a material. When light hits a lens surface in air normally, the percentage of light reflected at each surface is given by:

R = (n – 1)2/(n + 1)2 * 100%

Thus a material of refractive index 1.5, has a reflectance of

(0.5/2.5)2*100 = 4% per surface



The transmittance of a lens material describes the amount of light (usually specified for a given waveband) that will pass through that material.


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