Spheric and aspheric lenses

IMOS precision plastic injection molding technology

makes it possible to supply aspherical lenses, which are very sophisticated in terms of technical production, at the same price as spherical lenses. The advantage accruing to the customer is that they can focus purely on technical aspects during the selection process.

Aspherical variants generally facilitate much more precisely directed illumination. This is because even in lenses with a short focal length, the light from the peripheral areas of the lens can also be directed to the focal point.

Previously, it was tremendously difficult to make such lenses with the necessary precision, such as by means of manual polishing. Today, however, the procedure of ultra-precision turning or also DTO (as is available as standard at IMOS) enables the manufacture of high-precision aspherical variants for optical use as prototypes or as a negative component in metal for the production of injection molded parts. No subsequent manual polishing is then necessary. The elements are ready to be used after the ultra-precision machining. Prototypes can thus be shipped to the customer straight from the spindle, making a very short lead time possible.

Aspherical lenses

As part of a spherical surface with radius R, spherical lenses have the same curvature throughout. Aberrations thus increasingly appear, especially if their focal length is comparable to or even less than the lens diameter. A mere arbitrary parameter (the radius R) is simply not sufficient to then form the lens surface in such a manner that, for instance, parallel incident light rays are bundled in the focal point. Thus, lenses are then used that deviate from the spherical form – namely aspherical variants (from the Ancient Greek in which “A” is used to negate the attribute). Aside from a radius R, to describe rotation-symmetrical aspherical variants, one then also needs the so-called conical constant k, as well as, if required, the coefficients of polynomial correction terms. With this, a higher level of freedom in terms of design is clearly achieved. Thus, the lens surface can be significantly better adjusted to the required curvature progression so as to appropriately deflect the light rays.

Spherical lenses

If an optically effective surface of a lens can be described solely by citing a radius, what is involved is part of a spherical surface, and so this surface is termed according to the Greek word for a globe, sphere or spherical. Spherical surfaces are relatively easier to manufacture than are other more complicated surfaces. The reason for this is that the surface described by only one parameter evinces significant symmetry. This is why the very first machine-manufactured lenses were predominantly spherical lenses. This high degree of symmetry however explains why a spherical lens, especially in the case of a small focal length, shows clear aberrations. With specificity to small focal lengths, outer light rays, i.e. those that are further removed from the optical axis, have to be deflected more sharply from their direction of incidence than those rays further inside for purposes of focusing. As one can clearly see from the image below, a spherical lens, which has a uniform curvature, cannot do this optimally. In such cases, the use of aspherical lenses is recommended.

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