General Parameters of Optical Lens
Optical specifications are utilized throughout the design and manufacturing of a component or system to characterize how well it meets certain performance requirements. They are useful for two reasons:
1) First, they specify the acceptable limits of key parameters that govern system performance;
2) Second, they specify the amount of resources (i.e. time and cost) that should be spent on manufacturing.
In order to understand optical specifications, it is important to first review what they mean. To simplify the ever-growing number, consider the most common manufacturing, surface, and material specifications for lenses, mirrors, and windows. Filters, polarizers, prisms, beamsplitters, gratings, and fiber optics also share many of these optical specifications, so understanding the most common specifications will provide a great baseline for understanding nearly all optical lenses.
The diameter tolerance of round optics affords an applicable vary of diameter values. Although the diameter tolerance has no impact on the optical overall performance of the optical component itself, if the optical aspect is to be mounted in any kind of holder, a very vital mechanical tolerance should be considered. For example, if the diameter of the optical lens deviates from its nominal value, the mechanical axis can also deviate from the optical axis in the mounted component, ensuing in eccentricity (Figure 1). Typical manufacturing tolerances for diameters are +0.00/-0.10 mm (typical quality), +0.00/-0.050 mm (precision quality), and +0.000/-0.010 mm (high quality).
Center Thickness Tolerance
The thickness at the middle of an optical element, mainly a lens, is the thickness of the component cloth measured at the center. The middle thickness is measured on the mechanical axis of the lens, described as the axis that lies simply between its outer edges. The exchange in the thickness of the lens middle will have an effect on the optical performance, due to the fact the middle thickness and the radius of curvature decide the optical course size of the mild passing via the lens. The regular manufacturing tolerances for core thickness are +/-0.20 mm for common mass, +/-0.050 mm for precision mass, and +/-0.010 mm for excessive quality.
Radius of Curvature
The radius of curvature is described as the distance between the apex of the optical thing and the core of curvature. It has many manifestations, which can be positive, zero, or negative, relying on whether or not the floor is convex, flat, or concave. Knowing the price of the radius of curvature can decide the optical course size of the mild passing via the lens or mirror, however, it additionally performs a giant position in figuring out the energy of the surface. The manufacturing tolerance of the radius of curvature is generally +/-0.5, however, it can be as low as +/-0.1% in precision applications, and as low as +/-0.01% for extraordinarily excessive nice requirements.
The centering of the lens is also called center or eccentricity. The amount of eccentricity in the lens is the body displacement between the mechanical axis and the optical axis. The mechanical axis of the lens is just the geometric axis of the lens, which is described by its outer cylinder. The optical axis of the lens is described by the optical bottom surface, which is a line connecting the surface curvature facilities. To check the alignment, place the lens in the cup and strain it.
Parallelism describes how parallel two surfaces are with respect to each other. It is useful in specifying components such as windows and polarizers where parallel surfaces are ideal for system performance because they minimize distortion that can otherwise degrade image or light quality. Typical tolerances range from 5 arcminutes down to a few arcseconds.
Glass corners can be very fragile, therefore, it is important to protect them when handling or mounting a component. The most common way of protecting these corners is to bevel the edges. Bevels serve as protective chamfers and prevent edge chips. They are defined by their face width and angle.
Bevels are most commonly cut at 45° and the face width is determined by the diameter of the optic. Optics with diameters less than 3.00mm, such as micro-lenses or micro-prisms, are typically not beveled due to the likelihood of creating edge chips in the process. It is important to note that for small radii of curvature, for example, lenses where the diameter is ≥ 0.85 x radius of curvature, no bevel is needed due to the large angle between the surface and edge of the lens. For all other diameters, the maximum face widths are provided in table below.
Maximum Face Width of Bevel
3.00mm – 5.00mm
5.01mm – 25.4mm
25.41mm – 50.00mm
50.01mm – 75.00mm
Clear aperture is defined as the diameter or size of an optical component that must meet specifications. Outside of it, manufacturers do not guarantee the optic will adhere to the stated specifications. Due to manufacturing constraints, it is virtually impossible to produce a clear aperture exactly equal to the diameter, or the length by width, of an optic. Typical clear apertures for lenses are show in following table.
3.00mm – 10.00mm
90% of Diameter
10.01mm - 50.00mm
Diameter – 1mm
Diameter – 1.5mm
The surface quality of an optical surface describes its cosmetic appearance and includes such defects as scratches and pits, or digs. In most cases, these surface defects are purely cosmetic and do not significantly affect system performance, though, they can cause a small loss in system throughput and a small increase in scattered light. However, certain surfaces, however, are more sensitive to these effects such as: (1) surfaces at image planes because these defects are in focus and (2) surfaces that see high power levels because these defects can cause increased absorption of energy and damage the optic. The dig designation, however, does directly relate to the dig, or small pit in the surface. The dig designation is calculated at the diameter of the dig in microns divided by 10. Scratch-dig specifications of 80-50 are typically considered standard quality, 60-40 precision quality, and 20-10 high precision quality.
Surface flatness is a type of surface accuracy specification that measures the deviation of a flat surface such as that of a mirror, window, prism, or plano-lens. This deviation can be measured using an optical flat, which is a high quality, highly precise flat reference surface used to compare the flatness of a test piece. When the flat surface of the test optic is placed against the optical flat, fringes appear whose shape dictates the surface flatness of the optic under inspection. If the fringes are evenly spaced, straight, and parallel, then the optical surface under test is at least as flat as the reference optical flat. If the fringes are curved, the number of fringes between two imaginary lines, one tangent to the center of a fringe and one through the ends of that same fringe, indicate the flatness error. The deviations in flatness are often measured in values of waves (λ), which are multiples of the wavelength of the testing source. One fringe corresponds to ½ of a wave. 1λ flatness is considered typical grade, λ/4 flatness is considered precision grade, and λ/20 is considered high precision grade.
Power, a type of surface accuracy specification, applies to curved optical surfaces, or surfaces with power. It is tested in a fashion similar to flatness, in that a curved surface is compared against a reference surface with a highly calibrated radius of curvature. Using the same principle of interference caused by the air gaps between the two surfaces, the interference’s pattern of fringes is used to describe the deviation of the test surface from the reference surface (Figure 6). A deviation from the reference piece will create a series of rings, known as Newton's Rings. The more rings present, the larger the deviation. The number of dark or light rings, not the sum of both light and dark, corresponds to twice the number of waves of error.
The picture above show power error tested by comparing to a reference surface or using an interferometer.
Power error is related to the error in the radius of curvature by the following equation where ∆R is the radius error, D is the lens diameter, R is the surface radius, and λ is the wavelength (typically 632.8nm):
(Power Error[waves or λ] = ΔRD2/8R2λ
Irregularity, a type of surface accuracy specification, describes how the shape of a surface deviates from the shape of a reference surface. It is obtained from the same measurement as power. Regularity refers to the sphericity of the circular fringes that are formed from the comparison of the test surface to the reference surface. When the power of a surface is more than 5 fringes, it is difficult to detect small irregularities of less than 1 fringe. Therefore it is common practice to specify surfaces with a ratio of power to irregularity of approximately 5:1. For more detailed information on optical flats and interpreting fringe patterns to test surface flatness, power and irregularity, view Optical Flats.
Surface finish, also known as surface roughness, measures small scale irregularities on a surface. They are usually an unfortunate by-product of the polishing process. Rough surfaces tend to wear faster than smooth surfaces and may not be suitable for some applications, especially those with lasers or intense heat, due to possible nucleation sites that can appear in small cracks or imperfections. Manufacturing tolerances for surface finish range from 50Å RMS for typical quality, 20Å RMS for precision quality, and 5Å RMS for high quality.
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