Four types of mirrors; The bottom right shows visible scratch near the center and abundance of edge chips
Surface imperfections on optical surfaces such as lenses or mirrors, can be caused during the manufacturing of the part or handling. These imperfections are part of the surface and cannot be removed by cleaning. Surface quality is characterized either by the American military standard notation (eg "60-40") or by specifying RMS (root mean square) roughness (eg "0.3 nm RMS").[1] American notation focuses on how visible surface defects are, and is a "cosmetic" specification. RMS notation is an objective measurable property of the surface. Tighter specifications increase the costs of fabricating optical elements but looser ones affect performance.[2][3]
While surface imperfections can be labeled "cosmetic defects", they are not purely cosmetic. Optics for laser applications are more sensitive to surface quality as any imperfections can lead to laser-induced damage. In some cases, imperfections in optical elements will be directly imaged as defects in the image plane. Optical systems requiring high radiation intensity tend to be sensitive to any loss of power due to surface scattering caused by imperfections. Systems operating in the ultraviolet range require a more demanding standard as the shorter wavelength of the ultraviolet radiation is more sensitive to scattering.
There are many different standards used by optical element manufacturers, designers, and users which vary by geographic region and industry. For example, German manufacturers use ISO 10110, while the US military developed MIL-PRF-13830 and their long-standing use of it has made it the de facto global standard.[3] It is not always possible to translate the scratch grade by one standard to another and sometimes the translation ends up being statistical (sampling defects to ensure that statistically, the percentage rejected elements will be similar in both methods).[4]
Examining surface quality in terms of 'Scratch & Dig' is a specialized skill that takes time to develop. The practice is to compare the element to a standard master (reference).[3]Automated systems now replace the human technician, for flat optics, but recently also for convex and concave lenses.[3][5] In contrast, 'Roughness' characterization is done with more precise and easier-to-quantify methods.
Overview of types
The various standards separate two main categories for surface quality: scratch & dig and roughness.[1]
A scratch is defined as a long and narrow defect that tears the surface of the glass or coating.[6] There are standards that refer to the degree of visibility, which is the relative brightness of the scratch. In these cases, there is also a standard for the lighting conditions used for the test. Other standards classify scratches according to their dimensions. A dig is defined as a pit, a rough area, or a small crater on the surface of the glass (or any other optical material).[6] All standards measure the physical size of the dig. Some standards include small defects within the glass that are visible through the surface, such as bubbles and inclusions.
Roughness, texture or optical finish is a defect that originates from the element's manufacturing. Texture is a periodical phenomenon with a high spatial frequency (or in other words, in small dimensions), which affects the entire surface and causes the scattering of incident light.[7] A higher value of roughness means a rougher surface.[7] The texture is especially important in cases where the polishing is carried out using new processing methods such as diamond turning, which leaves a residual periodical signature on the surface, affecting the quality of the obtained image or the level of scattering from the surface. The amount of scattered light is proportional to the square of the RMS of the roughness.[1]
Scratch & Dig
Military standard MIL-PRF-13830B
This is the most common standard, stemming from a standard that was originally proposed by McLeod and Sherwood of Kodak back in 1945 and evolved in 1954 into the military standard MIL-O-13830A.[1][3] It defines the quality of the surface by a pair of numbers, the first is a measure of the visibility of the scratch and the second is the size of the dig.
Scratch visibility grades are described by a series of arbitrary numbers: 10, 20, 40, 60, and 80 where the brightest scratches, the easiest to see using the naked eye, are grade 80, while the most difficult to detect are grade 10. A scratch on a tested part is compared with an industrial or military standard (master) on which there are scratches of different degrees of visibility and the comparison is made using the naked eye, under controlled lighting conditions.[2] It is important to recognize that this is a subjective test and its results can vary between different people.[2] The scratches' visibility largely depends on their shape, and contrary to popular belief, there is little correlation between the scratch's visibility grade and its width.[3] One cannot measure the width of a scratch to determine its grade.[3]
On the other hand, a dig's grade is a precise and measurable value. It is the diameter of the largest dig that is found on the tested surface, in units of hundredths of a millimeter. It is customary to use discrete grades of 5, 10, 20, 40, or 50, where of course the larger numbers describe larger imperfections.
There are many default definitions in the MIL standard. For example, the grade that must be required outside the clear aperture (the part of the lens to which the standard applies, also called "effective diameter" or CA) is, in the absence of another definition, 80-50.[8] This is a very basic surface characterization and is easy to achieve. It describes a scratch whose brightness is less than that of a scratch at visibility grade 80 and a dig with a diameter of up to 0.5 mm (50 hundredths = 50/100=0.5). 60-40 is considered "commercial" quality, while for demanding laser applications 20-10 or even 10-5 are used.[6] The scratches on a 10-5 or 20-10 surface can be hard to see, making the visibility standard more subjective.[1] Other standards may work better when precision surfaces are required. Optical coating can change scratch visibility, so for example an element that passes 40-20 before coating can be worse than 60-40 after coating.[1]
Accumulation and concentration rules regulate common situations in which there are multiple defects on the surface of an optical element, and clarify how they should be added up.[2][8] For example, if one or more scratches are found with the maximum visibility allowed, to pass the test, the sum of the length of these scratches is limited to a quarter of the diameter of the element.[2][8] The number of digs at the maximum permitted level is determined by dividing the measured clear aperture diameter (in millimeters) by 20, and rounding up. For example, for a clear aperture of 81 mm, 5 digs are allowed at the maximum level.[4]
Since the comparison master is only in possession of the US Army, several commercial masters have been developed that are intended to be compatible, but due to the complexity of the factors that make a scratch visible, these masters are not always compatible with the original and there is no way to match one set to another.[3] For example, a visibility grade 10 scratch on one master can appear brighter than a visibility grade 60 scratch on another master.[1] For this reason, it is recommended to also indicate on the drawing the type of master set to which it must be compared during the test.[1]
Examples of such commercial comparison sets made of plastic or glass are Davidson Optronics, Brysen Optical, and Jenoptik Paddle – sold by ThorLabs and Edmund Optics.[1][3]
ISO 10110-7
This standard is used in the USA, China, Japan, Russia, and all of Europe.
The notation as of 2007 is: 5/ N x A; C N' x A'; L N" x A"; E A''', where N and A represent the number of defects and the maximum size of the defect, N' and A' represent the number of imperfections on the coating and their maximum size, N'' and A'' represent the number of scratches allowed and their maximum size and A''' represent the maximum size of an edge chip (a defect on the rim of the optical element).
A scratch in this case is defined as a defect longer than 2 mm. Only the first part of the characterization, N x A, is mandatory. The rest of the details can be omitted. A and A' are given as the square root of the area of the defect and are indicated by discrete values from the series: 4,2.5,1.6,1,0.63,0.4,0.25.[4]
In addition to the limits on the number of defects and their size, the total area of all imperfections must not exceed A*N2.[4] Long defects (scratches) are summed up by their width, independent of length. There is no limit on the number of edge chips, and the concentration of imperfections is limited by the rule that at most 20% of the defects, allowance can be concentrated in an area of 5% of the clear aperture.[4]
A fundamental advantage of ISO is a relatively simple translation between the percentage of light scattered from a surface and the characterization of its surface, according to the formula:[4]
Scatter% = 4 x [(N x A2)+(N' x A'2)+ N" x A" x Φ]/(π x Φ2)
Unlike MIL-PRF-13830B which is cheap and fast to use, but suffers from inaccuracies, the use of the dimensional standard of ISO 10110-7 is more accurate but takes a longer time to test and is therefore expensive.[2] The relatively long test time is derived from the fact that testing according to this standard is carried out using a microscope, comparing sizes of each defect to defects on a master, and because of the large magnification needed the field of view is small, requiring several measurements to map each optical element.[2]
David Aikens, director of Optics and Electro-Optics Standards Council,[9] presented a recommended conversion chart that preserves the level of quality control, or percent fall, in ISO scratch & dig testing versus the military standard. For example 5/2x0.40; L 3 x0.010 is a statistically-equivalent standard to 60-40 of the strict military standard, over a 20 mm opening.[4]
The logical flaw of this dimensional standard is in defining a scratch according only to its width. For example, if a lens with a diameter of 100 mm has a requirement of L 1 x 0.025, a single scratch with a thickness of up to 25 microns is allowed, even if it covers the entire 100 mm diameter. However, if the manufacturer polishes the surface and removes the scratch from the central 95 millimeters of the lens, there will be two scratches each 2.5 mm in length and now the lens will fail the acceptance tests because the characterization allows only one scratch. The illogicality here is obvious: it is not acceptable to reject a component due to a process that improves its quality.[4]
As of 2017, to support quick measurements intended for less sensitive surfaces, ISO 10110-7 also allows the definition of scratches according to their visibility, and the definition of digs according to their diameter, just like MIL-PRF-13830B, using the same grades, for example 60-40.[2][4]
It is possible to expand and also mark coating imperfections as well as edge chips, similarly the definition in the dimensional standard: 5 / S - D; C S' - D'; E A''' where S and D are the definitions for scratches and digs, S' and D' for these defects on the coating and A''' characterizes edge chip as defined above. As explained about the military standard, it is important to explicitly specify which master set the scratches brightness are to be compared to.[4]
MIL-C-48497A ו- MIF-F-48616
These standards are almost as popular as MIL-PRF-13830B but they have become less popular with time.
These standards define scratches and digs according to their physical size and mark their grade with the letters: A, B, C, D, E, F, G (and H which is used only for digs).[3] The letter A represents the narrowest scratch, which is 0.005 mm wide, and the smallest dig, which is 0.05 mm in diameter. On the other hand, the letter G represents a scratch that is 0.12 mm wide and a dig that is 0.7 mm in diameter. A microscope or magnifying glass is used for testing, or sometimes even just using the naked eye to compare to a master.
ANSI OP1.002
This American standard was first published in 2006.[3] Just like in the MIL-PRF-13830B standard, ANSI OP1.002 defines digs according to their diameter.
ANSI OP1.002 also supports two separate methods for scratches: visibility and size.
The visibility method defines scratches according to their visibility and is identical in design and terminology to the MIL-PRF-13830B standard. Just like the military standard, it uses two numbers, the first for scratches and the second for digs, maintaining their meaning as in the military standard. Examples: 80-50, 60-40. This method takes advantage of the speed and low cost of the visual inspection and is used for elements with looser tolerances.
The dimensionality method for scratches is based on the MIL-C-48497A standard, which is considered easy to use and functional.[1] The dimensional method uses two letters, the first for scratches and the second for digs. For example: A-A or E-E. This standard is intended for parts with tight surface quality tolerances, such as CCD cover glasses or demanding laser applications.[1]
The OP1.002 standard allows using a microscope to compare with the master.[1]
This standard allows a relatively easy translation between the desired scattering level and the surface quality, as mentioned above.[1]
Roughness
US military standard MIL-STD-10A
This original standard was common in nature, not intended for the characterization of polished surfaces per se. It used parameters that are not typically used for the characterization of optical elements such as average roughness.[1]
ASME B46.1-2002
An example of measuring surface roughness. Rq is RMS.
This standard replaced MIL-STD-10A and defines more than forty different parameters including RMS (root mean square), slope, skew, PSD (Power Spectral Density, which is the most comprehensive characteristic), and more. There is a significant improvement in this standard because it allows the characterization of machined surfaces, at different spatial frequencies, which is especially important in cases where the optics were produced using techniques that leave periodic marks, such as caused by diamond turning. For most uses it is sufficient to use RMS.[1] In all cases, it is important to specify the range in which the calculation is performed because without defining the spatial frequency range in which the measurement is performed, this standard is meaningless.
ISO 10110-8 (2010)
This popular standard, similar to ASME B46.1, also defines the RMS of the surface over a specific length scale, PSD and more. It differs from the ASME specification by using symbols instead of words.[1][7]
A mirror, also known as a looking glass, is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the image in an equal yet opposite angle from which the light shines upon it. This allows the viewer to see themselves or objects behind them, or even objects that are at an angle from them but out of their field of view, such as around a corner. Natural mirrors have existed since prehistoric times, such as the surface of water, but people have been manufacturing mirrors out of a variety of materials for thousands of years, like stone, metals, and glass. In modern mirrors, metals like silver or aluminium are often used due to their high reflectivity, applied as a thin coating on glass because of its naturally smooth and very hard surface.
A corrective lens is a transmissive optical device that is worn on the eye to improve visual perception. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal but can be used for purely refractive purposes.
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes when viewing distant objects. Most binoculars are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal-mounted military models.
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.
An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.
Cheesecloth is a loose-woven gauze-like carded cotton cloth used primarily in cheesemaking and cooking.
A telescopic sight, commonly called a scope informally, is an optical sighting device based on a refracting telescope. It is equipped with some form of a referencing pattern – known as a reticle – mounted in a focally appropriate position in its optical system to provide an accurate point of aim. Telescopic sights are used with all types of systems that require magnification in addition to reliable visual aiming, as opposed to non-magnifying iron sights, reflector (reflex) sights, holographic sights or laser sights, and are most commonly found on long-barrel firearms, particularly rifles, usually via a scope mount. Similar devices are also found on other platforms such as artillery, tanks and even aircraft. The optical components may be combined with optoelectronics to add night vision or smart device features.
Glass microspheres are microscopic spheres of glass manufactured for a wide variety of uses in research, medicine, consumer goods and various industries. Glass microspheres are usually between 1 and 1000 micrometers in diameter, although the sizes can range from 100 nanometers to 5 millimeters in diameter. Hollow glass microspheres, sometimes termed microballoons or glass bubbles, have diameters ranging from 10 to 300 micrometers.
Metallography is the study of the physical structure and components of metals, by using microscopy.
Gloss is an optical property which indicates how well a surface reflects light in a specular (mirror-like) direction. It is one of the important parameters that are used to describe the visual appearance of an object. Other categories of visual appearance related to the perception of regular or diffuse reflection and transmission of light have been organized under the concept of cesia in an order system with three variables, including gloss among the involved aspects. The factors that affect gloss are the refractive index of the material, the angle of incident light and the surface topography.
An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths than electrical cables. Fibers are used instead of metal wires because signals travel along them with less loss and are immune to electromagnetic interference. Fibers are also used for illumination and imaging, and are often wrapped in bundles so they may be used to carry light into, or images out of confined spaces, as in the case of a fiberscope. Specially designed fibers are also used for a variety of other applications, such as fiber optic sensors and fiber lasers.
Optical manufacturing and testing is the process of manufacturing and testing optical components. It spans a wide range of manufacturing procedures and optical test configurations.
A laser beam profiler captures, displays, and records the spatial intensity profile of a laser beam at a particular plane transverse to the beam propagation path. Since there are many types of lasers—ultraviolet, visible, infrared, continuous wave, pulsed, high-power, low-power—there is an assortment of instrumentation for measuring laser beam profiles. No single laser beam profiler can handle every power level, pulse duration, repetition rate, wavelength, and beam size.
Anti-scratch coating is a type of protective coating or film applied to an object's surface for mitigation against scratches. Scratches are small surface-level cuts left on a surface following interaction with a sharper object. Anti-scratch coatings provide scratch resistances by containing tiny microscopic materials with scratch-resistant properties. Scratch resistance materials come in the form of additives, filters, and binders. Besides materials, scratch resistances is impacted by coating formation techniques. Scratch resistance is measured using the Scratch-hardness test. Commercially, anti-scratch coatings are used in the automotive, optical, photographic, and electronics industries, where resale and/or functionality is impaired by scratches. Anti-scratch coatings are of growing importance as traditional scratch resistance materials like metals and glass are replaced with low-scratch resistant plastics.
A subwavelength-diameter optical fibre is an optical fibre whose diameter is less than the wavelength of the light being propagated through it. An SDF usually consists of long thick parts at both ends, transition regions (tapers) where the fibre diameter gradually decreases down to the subwavelength value, and a subwavelength-diameter waist, which is the main acting part. Due to such a strong geometrical confinement, the guided electromagnetic field in an SDF is restricted to a single mode called fundamental. In usual optical fibres, light both excites and feels shear and longitudinal bulk elastic waves, giving rise to forward-guided acoustic wave Brillouin scattering and backward-stimulated Brillouin scattering. In a subwavelength-diameter optical fibre, the situation changes dramatically.
Precision glass moulding is a replicative process that allows the production of high precision optical components from glass without grinding and polishing. The process is also known as ultra-precision glass pressing. It is used to manufacture precision glass lenses for consumer products such as digital cameras, and high-end products like medical systems. The main advantage over mechanical lens production is that complex lens geometries such as aspheres can be produced cost-efficiently.
Ballistic eyewear is a form of glasses or goggles that protect from small projectiles and fragments. For the U.S. military, choices are listed on the Authorized Protective Eyewear List (APEL). Ballistic eyewear including examples that meet APEL requirements are commercially available for anyone who wishes to buy it. The history of protective eyewear goes back to 1880 and extends through to World War I and the present. There are three standards that are currently used to test the effectiveness of ballistic eyewear. These include a U.S. civilian standard, a U.S. military standard (MIL-PRF-31013), and a European standard.
There are two different types of haze that can occur in materials:
Veiling glare is an imperfection of performance in optical instruments arising from incoming light that strays from the normal image-forming paths, and reaches the focal plane. The effect superimposes a form of noise onto the normal image sensed by the detector, resulting in a final image degraded by loss of contrast and reduced definition.
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