Spherical Lenses

Spherical lenses, also known as singlets, are optical lenses with curved surfaces that cause light rays to converge or diverge. They are widely used in imaging applications. Conjugate ratio, lens shape, radius of curvature, lens material, transmission, wavefront distortion, scattered light, and types of coating are all features that must be considered when choosing a lens.

Shanghai Optics produces high precision spherical lenses in plano-convex, double convex, and plano-concave shapes. Both plano convex and double convex lenses are what we call positive lenses; they have positive focal lengths. Plano convex lenses are spherical on one surface and flat on the other. They are often selected when one is working with infinite or near-infinite conjugate, for light collimation applications and monochromatic illumination. Their special shape minimizes spherical aberration up to an approximately 5:1 conjugate ratio, and they are often the most practical choice for demanding applications. In an ideal setup the curved surface would face the infinite conjugate or largest object distance.

To learn more about spherical lenses, read our blog post.

Double convex lenses feature two convex surfaces with equal radii and both horizontal and vertical symmetry. They are ideal to form an image at close conjugates as well as for image relay. At 1:1 magnification, when object and image distance are equal, the symmetry of this lens mean that coma, distortion and chromatic aberration are cancelled out and spherical aberration is minimized. We recommend double convex lenses for applications where conjugate ratios are between 5:1 and 1:5.

Magnifying glasses typically use double convex lenses. When the object distance is close, the rays will be bent toward the focal point at the center of the lens. This causes the virtual image to appear much larger than the real image.

Plano concave lenses have negative focal lengths, and are appropriate for situations where the light rays should diverge after passing through the lens. For instance, in light projection, in beam expansion, or when one wishes to expand an optical system’ focal length. When used with other lenses, the negative spherical aberration can be used to cancel out aberrations created by convex lenses. These lenses feature one flat surface and one that is concave, and in most situations, the curved surface should face the largest object distance. The one exception is in high energy lasers, where the flat surface should be directed toward the infinite conjugate or largest object distance in order to avoid the possibility of a virtual focus.

Surface Quality and Material Selection

We offer our lenses in a wide range of substrates, including BK7, Fused Silica, N-SF11, Zinc Selenide, Calcium Fluoride, and Silicon. The material of construction largely determines the wavelength, and custom spherical lens are available for infrared, visible, and ultraviolet applications. For infrared applications in defence and security, environmental, medical or agricultural, ZnSe, ZnS, BaF2, CaF2, MgF2 or germanium substrates are usually selected. For visible wavelengths, we recommend Schott, Ohara, CDGM, or Corning Fused Silica.

Surface quality ratings are an evaluation of the smoothness of a lens, and enable our spherical lenses to be selected to meet any wavefront and low-scatter requirements. This is especially important for laser applications, as surface imperfection in lenses have the potential to initiate laser-induced damage. The MIL-PRF-13830B rating system most commonly used in spherical lens manufacturing provides both a scratch and a dig number, given in an x-y format. Scratches are assigned a number from 10 to 80, with brightness of allowable scratches increasing as the number increases, and dig numbers represent the diameter of the largest allowable dig in 1/100th of millimeters. The total number of digs may not exceed the diameter divided by 20, and the sum of all scratch lengths with the specified scratch number may not exceed ¼ of the diameter of the optic.The standard quality for most optical applications is 40-20. Precision laser requires 20-10 surface quality.

Optical coatings provide fine tuning of the reflection/transmission ratios of our custom spherical lenses, and can also extend the life of optical components. Custom BBAR coatings are available upon request, in single or multi-layer applications.