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The quality of optics in medical devices can mean the difference between correct or incorrect diagnoses. Hence, a decision as to what material to make the optics from can affect system performance. In addition to material, the optics in medical devices often require selecting high quality precision coatings.

A wide variety of specifications govern lens quality. A good grasp of these specs helps weed out unsuitable optics and focuses on a few of the remaining alternatives. To better select lenses, consider the specifications for materials and manufacturing, and those that define coatings and filters.

Material specs

Understanding how material and manufacturing specifications relate to the quality of optics is a first step to choosing the right ones for an application. The material, for instance, is important because it has unique properties, and different applications require different characteristics.

Along with conventional parameters such as cost, availability, strength, and durability, key factors for optical materials include index of refraction, dispersion, and transmission characteristics. Each helps describe how the material affects light passing through it.

Index of refraction is the ratio of the speed of light in a vacuum to the speed of light through the material at a given wavelength. The higher the index, the slower light passes through it. The index and the curvature of each surface, determines a lens' focal length.

Dispersion measures how the index of refraction varies with wavelength. In a material with dispersion, light of different wavelengths may follow different paths. Materials with a high dispersion are used for prisms which separate white light into its constituent colors. Low dispersion materials are better suited for imaging where it is important that all colors focus at the same point. A material's Abbe number identifies the amount of dispersion. A high Abbe number means less dispersion.

A material's transmission characteristics describe how readily it transmits light as a function of wavelength. For example, BK7 glass (a borosilicate) is commonly used in visible light systems but is a poor choice for working with ultraviolet (UV) light because it poorly transmits UV wavelengths. Fused silica is a better choice for UV applications.

Wavelength

Manufacturing specs

The manufacturing parameters key to optics include surface quality, flatness, parallelism, and centration tolerance. Surface quality is especially critical in applications involving lasers. Polishing a lens creates small defects such as scratches and digs. (A scratch is any mark or tear along the surface and a dig is a pit or divot.) These small imperfections scatter light. In lasers, scattering leads to undesired effects that are potentially dangerous. The optics for lasik eye surgery, for instance, must have high surface quality to precisely administer the appropriate amount of energy without damaging the patient's eye. Scattered laser energy due to a len's poor surface quality decreases the equipment's reliability in energy delivery and could injure the patient.

The scratch-dig specification defines a standard surface quality as an amount of acceptable variation on the optic surface. The specification combines two numbers: a scratch number followed by a dig number, such as 20-10. The figures come from a visual comparison to a set of standard surfaces in accordance to MIL-O-13830A, a U.S. military specification for inspecting optical components. Lower numbers indicate higher surface quality. It is important to note that these figures do not directly correspond to the number of defects on the surface. The scratch specification includes the total length and number of allowable scratches. As a common reference, the scratch number relates to the “apparent” width of an acceptable scratch. Dig numbers, though, do relate to a specific value. For example, a dig number of 10 relates to 0.10 mm or 100-µm diameter pit.

Flat optical surfaces are typically specified as the greatest deviation from peak to valley across the clear aperture (usable area) of the optic. Flatness is measured in terms of fractions of a reference wavelength, for instance, ¼ wave or ¼ λ. Thus, if an optic is to have 1/10λ flatness at a reference wavelength of 633 nm, the maximum deviation from flatness will be less than 63.3 nm.

Windows can also have a defined parallelism. This specification indicates how parallel one surface is to the second. Parallelism is typically given as an angular measurement such as arcsec or arcmin. A window with loosely controlled parallelism or one that is not flat can distort images.

A lens has at least one curved surface and typically a centration specification. Centration is a measure of how well the physical center of the lens (mechanical axis) aligns with the optical center of the lens (optical axis). The optical axis is the line connecting centers of curvatures of both lens surfaces.