Optical specifications are utilized in the design and production of a component or system to enable the component or system to accurately meet specific performance requirements. Optical specifications are useful for two reasons: first, they can specify acceptable limits for key parameters that determine the performance of a system; and second, they can determine the amount of resources (i.e., time and cost) that should be spent on production.
Parameters of an optical system that are under- or over-specified can affect its performance, resulting in unnecessary waste of resources. Failure to set all necessary parameters correctly can lead to under-specification, which can degrade performance. Defining system parameters too tightly without taking into account any variations in optical or mechanical requirements can lead to over-specification, which can increase cost and production difficulties. In order to understand optical specifications, it is important to first understand what they mean, so knowing the most commonly used specifications will provide you with the strongest foundation for understanding the specifications of almost any optical product.
Diameter tolerances for circular optics provide a range of acceptable diameter values. This production specification will vary depending on the skill level and capabilities of certain optical processing companies that make the optics. While diameter tolerance does not have any effect on the optical performance of the optic itself, it is a very important mechanical tolerance that you must consider if you are mounting the optic on any kind of fixture. For example, if the diameter of a lens deviates from its nominal value, there is a risk that the mechanical axis in the mounted assembly will deviate from the optical axis, resulting in an eccentricity of light. Typically, production tolerances for diameter are: +0.00/-0.10 mm for general quality, +0.00/-0.050 mm for precision quality, and +0.000/-0.010 mm for high quality.
2. Center Thickness Tolerance The center thickness of an optical element (most typically a lens) measures the thickness of the material in the center portion of the optical element. The center thickness is measured by the mechanical axis of the lens, which is defined as the axis between the outer edges of the lens. Variations in the center thickness of a lens affect optical performance because the center thickness and its radius of curvature determine the length of the optical path of light through the lens. Typically, production tolerances for center thickness are: +/-0.20 mm for average quality, +/-0.050 mm for precision quality, and +/-0.010 mm for high quality.
The radius of curvature is the distance between the vertex of the optical element and the center of curvature. The radius can be positive, zero or negative, depending on whether the surface is convex, flat or concave. If the value of the radius of curvature is known, the length of the optical path of a ray of light through a lens or mirror can be determined and also plays an important role in determining the surface power. The production tolerance for the radius of curvature is typically +/-0.5, but can be as low as +/-0.1% for precise applications or +/-0.01% where very high quality is required. h3> The center of a center lens, also known as centripetal or off-center, is specified based on the beam deviation δ (Equation 1). Once the beam deviation is given, the wedge angle W can be calculated by a simple relationship (Equation 2). The centrifugal amount of the lens is the distance by which the mechanical axis is physically deviated from the optical axis. The mechanical axis of a lens is simply the geometric axis of the lens, defined by its external cylindrical surface. The optical axis of a lens is defined by the optical surfaces, which are lines connecting the centers of curvature of each surface. To perform a centripetal test, place the lens in a teacup and apply pressure to it. The pressure applied to the lens will automatically converge on the center of curvature of the first surface in the center of the teacup, and this center will also align with the axis of rotation. Parallel light coming in along this axis of rotation will pass through the lens and reach the focal point at the back focal plane. As the lens rotates with the rotation of the teacup, any eccentricity in the lens disperses the focused beam and creates a circular trajectory of radius Δ in the back focal plane.
The quality of an optical surface is used to measure the surface characteristics of an optical product and covers a number of imperfections such as scratches and pits. Most of these surface imperfections are purely cosmetic and do not greatly affect system performance, although, they may cause a small dip in system throughput and a finer scattering of scattered light. However, some surfaces will be more sensitive to these effects, such as (1) surfaces with image planes, as these imperfections can create focusing, and (2) surfaces with high power levels, as these imperfections can increase energy absorption and ruin the optical product. The most commonly used specification for surface quality is the scratch and pitting specification illustrated by MIL-PRF-13830B. Scratch names are determined by comparing scratches on a surface to a series of standard scratches provided under controlled lighting conditions. Thus, rather than describing the actual scratches thereof, the scratch name compares them to standard scratches based on the MIL specifications. Pit names, however, relate directly to dots or small pits on a surface. Pit names are calculated by dividing the diameter of the pits in microns by 10. Typically a scratch pit specification between 80 and 50 would be considered standard quality, between 60 and 40 would be accurate quality, and between 20 and 10 would be considered high precision quality.
p>Surface flatness is a type of specification for measuring surface accuracy, which is used to measure the deviation of flat surfaces such as mirrors, window pieces, prisms or flat mirrors. You can measure this deviation using an optical flat crystal, which is a high-quality, high-precision reference plane for comparing the smoothness of specimens.When the flat surface of the optical product under test is placed against the optical flat crystal, streaks appear, the shape of which indicates the smoothness of the surface of the optical product under test. If the streaks are equally spaced and are parallel straight lines, then the tested optical surface is at least as flat as the reference optical flat crystal.If the stripes are curved, the number of stripes between two dashed lines (one dashed line tangent to the midpoint of the stripe and the other dashed line passing through the endpoint of the same stripe) points to a smoothness error. Deviations in smoothness are usually measured in terms of ripple values (λ), which are composed of multiple wavelengths of the test source. One stripe corresponds to ½ of a wavelength.A smoothness of 1λ indicates a general quality level; a smoothness of λ/4 indicates a precise quality level; and a smoothness of λ/20 indicates a high precision quality level.
Aperture number is a type of specification for measuring surface accuracy, which is applicable to curved optical surfaces or surfaces with power.The aperture number test is similar to a flatness test in that the surface is compared to a reference surface with a collegiate radius of curvature. Using the same principle of interference generated by the gap between the two surfaces, a striped interference pattern is used to indicate the deviation of the test surface from the reference surface. The deviation from the reference will produce a series of rings called Newton's rings.The more rings present, the greater the deviation.The number of dark or bright rings, rather than the total number of both dark and bright rings, is equal to twice the wavelength error.
Irregularity is a type of specification that measures the accuracy of a surface and describes the deviation of the surface shape from a reference surface shape.Irregularity is measured in the same way as aperture number. Irregularity is the spherical circular streak formed by comparing the test surface to a reference surface. When the surface has an aperture number of more than 5 stripes, it will be difficult to detect small irregular shapes smaller than 1 stripe. Therefore, it is common practice to specify the ratio of the number of apertures to the irregularity of the surface so that it is approximately 5:1. surface finish, also known as surface roughness, is used to measure some of the small irregularities of a surface. They are usually the result of poor polishing processes. Rough surfaces tend to be more abrasion-resistant than smooth surfaces and may not be suitable for some applications, especially in applications using lasers or in overheated environments, due to the possibility of small breaks or imperfections at the nucleation site. A production tolerance of 50 Å RMS for surface finishing indicates average quality, at 20 Å RMS indicates precise quality, and at 5 Å RMS indicates high quality.
The refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in the medium. The refractive index of glass generally ranges from 1.4 to 4.0, and the refractive index range of vision glass is somewhat smaller than that of glass optimized for infrared light. For example, N-BK7 (a general purpose visible glass) has a refractive index of 1.517, however germanium (a general purpose infrared glass) has a refractive index of 4.003.The refractive index of optical glass is an important property because the power of an optical surface is derived from the difference between the radius of curvature of the surface and the refractive index of the medium on either side of the surface.The glass manufacturer specifies inhomogeneity as the variation in the refractive index of the glass. Inhomogeneity is specified according to different grades, where grade and inhomogeneity are inversely related to each other, with inhomogeneity decreasing as the grade increases.
Another material property of glass is the dispersion coefficient, which is used to quantify the amount of dispersion presented by the glass. It is the refractive index of the material at the wavelengths f (486.1nm), d (587.6nm) and c (656.3nm) (Equation 3). (4) The range of dispersion coefficient values for vd = nd-1nf-ncvd = nd-1nf-nc is usually between 25 and 65.When the dispersion coefficient of a glass is greater than 55 (less dispersion), the glass is considered to be a corona glass, while those with dispersion coefficients less than 50 (more dispersion) are considered to be flint glass. Due to dispersion, the refractive index of a glass will vary depending on the wavelength.The most significant result of dispersion is that the focal length of the system will be slightly different for different wavelengths of light. h3>Laser Damage ThresholdThe laser damage threshold is the maximum amount of laser power that can be tolerated by the surface of each area prior to laser damage. Both pulsed lasers and continuous wave (CW) lasers have corresponding laser damage thresholds. Laser damage thresholds are a very important material specification for mirrors due to the fact that they are used in conjunction with laser products rather than any other optics, however, any laser-grade optics will provide thresholds. For example, consider a Ti: Sapphire laser reflector with a damage rating threshold of 0.5 J/cm2 @ 150 femtosecond pulses and 100kW/cm2 CW. This would indicate that the reflector can tolerate an incoming high repetition femtosecond pulsed laser with an energy density of 0.5J per square centimeter, or a high power CW laser with an energy density of 100kW per square centimeter. If the laser beam is concentrated over a smaller area, then measures must be considered to ensure that the overall threshold is not exceeded by the specified value.
Although there are a range of other production, surface and material specifications, confusion can be significantly avoided if the most commonly used optical specifications are understood. Lenses, mirrors, windows, filters, polarizers, prisms, beamsplitters, gratings, and optical fibers have a variety of attributes, so understanding how they relate to each other and how they will affect overall system performance will help you select the best components to integrate into your optical, imaging, or optoelectronic applications.
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