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OPTICAL FILTER DESIGN APPLICATION NOTE THIN-FILM THEORY each other by producing a wave of zero amplitude. When two such lightwaves are is known to have both particle-like and exactly in phase with each other, they inter- wave-like characteristics. Optical thin-film fere constructively, producing a lightwave of theory is based principally upon the wave- higher amplitude. An optical thin-film coat- like characteristics of light. Key among these ing is designed so that the distances are , refraction, and interference. between the boundaries (often integral quar- All boundaries between media in which ter-) will control the phase differ- lightwaves travel create a division of reflect- ences of the multiple reflected and transmit- ed and transmitted portions of the energy. ted components. When this “stack of bound- Those lightwaves not reflected are transmit- aries” is placed in a light path, constructive ted across the boundary to a medium with interference is induced at some selected different electric and magnetic properties. wavelengths, while destructive interference These differences cause a refraction, or a is induced at others. change in the speed and angle of the light- waves. A material’s refractive index is With the aid of thin-film design software, we derived by comparing the velocity of a light- apply optical thin-film theory to optimize var- wave in that medium to the velocity in a vac- ious coating performance characteristics uum. such as: a) the degree of transmission and reflection; b) the size of the spectral range The amount of light reflected is related to the over which transmission, reflection, and the difference between the refractive indices of transition between them occur; and c) the the media on either side of the boundary; effects at off-normal angles of greater differences create greater reflectivity. incidence. These characteristics are influ- If there is an increase in refractive index enced by the number of boundaries, the dif- across the boundary, the reflected lightwave ference in refractive index across each undergoes a phase change of 180º; if there is boundary, and the various distances between a decrease, no phase change occurs. An opti- the boundaries within a coating. cal thin-film coating is a stack of such bound- aries, each producing reflected and transmit- The Coating Process ted components that are subsequently We select coating materials for their trans- reflected and transmitted at other bound- missive, refractive, and absorptive character- aries. If each of these boundaries is located istics at those wavelengths critical to at a precise distance from the other bound- the filter’s application. The coating process aries, the reflected and transmitted compo- requires that materials be selected for their nents are enhanced by interference. evaporation and condensation properties as well. Unlike “solid” particles, two or more light- waves can occupy the same space. When A coating is produced in a vacuum chamber lightwaves of equal occupy the at a pressure typically less than 10-5 torr. The same space, they interfere with each other in coating materials are vaporized by a resistive a manner determined by their difference in heating source or an electron beam. With phase and amplitude. When two such light- careful control of conditions such as vapor- waves are exactly out of phase with each ization rate, pressure, temperature, and other—by 180°—they interfere destructively, chamber geometry, the vapor cloud con- and if their amplitudes are equal, they cancel denses uniformly onto rotating substrates,

OPTICAL FILDER DESIGN APPLICATION NOTE 2 returning to the solid state. reflectors made of dielectric materials. The spacer, which is a single layer of dielectric As a layer of material is deposited, its material having an optical thickness corre- increasing thickness is monitored optically. sponding to an integral-half of the principal With exacting response to anticipated wavelength, induces transmission rather changes in the monitor signal, the operator than reflection at the principal wavelength. stops deposition by shielding the material Light with wavelengths longer or shorter and turning off the thermal source (or than the principal wavelength undergoes a electron beam), thereby precisely controlling phase condition that maximizes reflectivity the thickness of the layer. A multi-layer coat- and minimizes transmission. The result is a ing is produced by alternating this cycle (typ- passband filter. The size of the passband ically 20 to 70 times) with two or more mate- region, the degree of transmission in that rials. region, and the degree of reflection outside that region are determined by the number Quarter-Wave Stack Reflector and arrangement of layers. A narrow pass- The quarter-wave stack reflector is a basic band region is created by increasing the building block of optical thin-film products. reflection of the quarter-wave stacks as well Composed of alternating layers of two or as increasing the thickness of the thin-film more dielectric materials—each layer with an spacer. optical thickness corresponding to one-quar- ter of the principal wavelength—this coating In a metal-dielectric-metal (MDM) cavity, the has highest reflection at the principal wave- reflectors of the solid Fabry-Perot interferom- length and transmission both higher and eter are thin-films of metal and the spacer is lower than the principal wavelength. At the a layer of dielectric material with an integral principal wavelength, constructive interfer- half-wave thickness. These are commonly ence of the multiple reflected rays maximizes used to filter UV light that would be the overall reflection of the coating; destruc- absorbed by all-dielectric coatings. tive interference among the transmitted rays minimizes the overall transmission. 100

Figure 1 illustrates the spectral performance of a quarter-wave stack reflector. Designed for maximum reflection of 550nm light 50

waves, each layer has an optical ransmission

thickness corresponding to one quarter % T of 550nm. This coating is useful for three types of filters: cut-on filters, rejection band 0 filters, and blockers. 400 500 600 700 800 900 Wavelength (nm) Fabry-Perot Interferometer Figure 1: Quarter-Wave Stack Reflector The solid Fabry-Perot interferometer, also Twenty-three layers alternating between zinc known as a single-cavity coating, is formed sulfide (n=2.35) and cryolite (n=1.35), at a princi- by separating two thin-film reflectors with a pal wavelength of 550nm. A slight design modifi- thin-film spacer. In an all-dielectric cavity, the cation could be used to eliminate the dips in the transmission regions, optimizing the element for thin-film reflectors are quarter-wave stack use as a longpass or a shortpass cut-on filter.

OPTICAL FILDER DESIGN APPLICATION NOTE 3 Multi-Cavity Passband Coatings trates the spectral performance of a 3-cavity The multi-cavity passband coating is made bandpass filter. Three features used to identi- by coupling two or more single-cavities with fy bandpass filters are center wavelength a matching layer. The transmission at any (CWL); full width at half maximum transmis- given wavelength in and near the band is sion (FWHM), which characterizes the width roughly the product of the transmission of of the passband; and peak transmission the individual cavities. Therefore, as the (%T). number of cavities increases, the cut-off edges become steeper and the degree of Anti-Reflective Coatings reflection becomes greater. The anti-reflective coating is a thin-film that does the opposite of a reflector. At the princi- When this type of coating is made of all- pal wavelength(s), it creates destructive dielectric materials, out-of-band reflection interference for the multiple reflected light- characteristically ranges from about waves and constructive interference for the (.8 x CWL) to (1.2 x CWL). If thin-films of multiple transmitted lightwaves. This type of metal, such as silver, are substituted for coating is commonly applied to the surfaces some of the dielectric layers, the metal’s of optical components such as lenses, mir- reflection and absorption properties extend rors, and windows. When deposited on the the range of far into the IR. surface of an interference filter, the anti- These properties cause loss in the transmis- reflective coating increases net transmission sion efficiency of the band. and reduces the intensity of ghost images. It should be noted that a properly designed The multi-cavity coating is used in many longpass or shortpass filter is anti-reflective variations on bandpass filters. Figure 2 illus- in its transmission.

100 CWL 90 T peak = 90%

λ1 λ2

50 45 ransmission

% T T peak = 45% 2

FWHM

0 571.3 576.1 580.9 550 560 570 580 590 600 Wavelength (nm) Figure 2: Multi-Cavity Passband Coating A bandpass filter that was made by depositing alternating layers of zinc sulfide and cryolite on a substrate according to a 3-cavity Fabry-Perot interferometric design. The filter’s CWL is located at 576.1nm; its FWHM is 9.6nm.

OPTICAL FILDER DESIGN APPLICATION NOTE 4 Partial Reflectors material to increase their durability. When made from all dielectric materials, the partial reflector is similar to the quarter-wave Refractory oxide surface coatings, while stack reflector except that fewer layers trans- eliminating additional substrates and their mit only a given portion of the incident light. related problems, are inherently unstable as The portion not transmitted is reflected, a result of coating densification and inconsis- since virtually none of the light is absorbed. tent reduction. The reactive coating process Partial beamsplitters used at off-normal for oxides is critically dependent on deposi- angles of incidence are common to all dielec- tion parameters at the physical chemistry tric partial reflectors. A 50/50 beamsplitter level. Methods such as ion beam, sputtering, will reflect 50% and transmit 50% of the inci- and plasma coating have been developed to dent light over a given spectral range. A improve coating stability through energetic 60/40 will reflect 60% and transmit 40%. A bombardment to make the coating more partial reflector, made with thin-films of dense. A minor process variable, however, metal or a combination of metal and dielec- can result in long-term inconsistency, such tric material, absorbs some portion of the as wavelength drift, which is unacceptable in incident light. A neutral density filter, coated some instrument applications. Surface coat- with the metal alloy inconel, is a common ings can be more expensive because of long metal partial reflector. manufacturing cycles, but provide extreme durability, very good Transmitted Wavefront Front Surface Coatings specifications, and can survive high tempera- Front surface coatings are used when the ture applications. light must interact with the coating before passing through the substrate. Reflective sur- Protected Coatings face coatings eliminate multiple reflections in Dielectric coatings are protected by a cover products such as and dichroic beam- glass laminated with optical grade cement. splitters. They also reduce the amount of This allows use of materials which have wide energy absorbed by the substrate in prod- ranging indices of refraction that result in ucts such as hot mirrors and cold mirrors. increasing spectral control at a reasonable Anti-reflective coatings, which reduce the cost. A glass-to-glass lamination around the degree of difference in refractive index at the perimeter of the assembly, created by boundary of a filter and its medium, are removing the coating at the edge, adds effective on both the front and back surfaces moisture protection and shear strength. of a filter. Dielectric materials suitable for optical thin- Refractory oxides, fluorides, and films yield the highest spectral performance metals are surface coating materials chosen of any materials. Research has shown that for their durability. Indeed, many optical although more fragile than refractory oxides, components are protected by durable sur- a single pair of dielectric materials permit the face coatings. Common surface coatings most complicated and highest performing have undergone testing that simulates many interference designs. Coating durability has years of environmental stress with historically been achieved by bonding a pro- no observable signs of cosmetic deteriora- tective glass coverplate to the coated sub- tion and only minimal shift in spectral strate. This laminated design limits perform- performance. Metal coatings are often ance to some degree, however, through the overcoated with a layer of oxide or fluoride introduction of adhesives and a second layer

OPTICAL FILDER DESIGN APPLICATION NOTE 5 of glass. The potential problems, which man- components are often combined with the ufacturing processes attempt to control, principal coating in a single assembly. include auto- and transmitted wavefront distortion. On the positive side, Adding attenuating components always protected dielectric coatings provide the end- results in some loss in transmission at the user with deep out-of-band blocking, very desired wavelengths. Therefore, blocking high phase thickness coatings with low strategies are devised for an optimum bal- residual stress, minimal crazing and sub- ance of transmission and attenuation. For strate deformation, consistent and stable example, if a detector has no sensitivity spectral performance, and simplicity of depo- beyond 1,000nm, the filter’s blocking is sition which results in affordable cost. designed only to that limit, conserving a criti- cal portion of the throughput. Absorption Color absorption glasses attenuate some Extended attenuation sometimes is achieved portion of the spectral range through absorp- by selecting materials that absorb the tion—either simple absorption by ions in unwanted wavelengths but transmit true solution or strong scattering by micro- the desired wavelengths. Absorptive color crystals. glasses are commonly used as coating sub- strates or laminated to filter assemblies. Extended Attenuation Thin-film coating materials sometimes pro- When used with a light source or a detector vide attenuation by absorption. Also, dyes that performs over a broad spectral range, it can be added to optical cement to provide is often necessary to extend the range of absorption. The choices of absorbing media attenuation provided by a single-coated sur- are many, yet limited. Absorbing media are face. Additionally, an increased level of ideal for some blocking requirements such attenuation might be necessary if a high- as the “short wavelength side” of a visible intensity source or a highly sensitive detec- bandpass filter. However, these materials tor is used. While some optical systems pro- don’t provide the best levels of transmission, vide space for separate reflectors or levels of absorption, or transition slopes in absorbers, these attenuating, or blocking, all situations. Furthermore, the temperature

100

50 ransmission % T

0 400 450 500 550 600 650 700 750 800 850 900 Wavelength (nm) Figure 3: Bandpass Without Extended Attenuation The limited range of attenuation of a single bandpass coated element with a CWL of 576.1nm and a FWHM of 9.3nm. The filter leaks unwanted light below 500nm and above 700nm.

OPTICAL FILDER DESIGN APPLICATION NOTE 6 100

Longpass absorption glass

50 Interference

ransmission coated blocking element #1 % T Interference Assembled coated blocking Bandpass element #2 filter 0 400 450 500 550 600 650 700 750 800 850 900 Wavelength (nm) Figure 4: Bandpass With Extended Attenuation The bandpass filter’s region of attenuation is extended by the addition of blocking elements. The red curve represents the assembly of the bandpass element in Figure 3 and three blocking elements (#1, #2, and Longpass absorption glass). While light “leaks” are eliminated, notice how transmission is reduced.

increase caused by absorption can be great ing the attenuation of a single-coated surface enough to cause significant wavelength shift are referred to as “blocking optimized,” for or material damage. filters used with detectors sensitive only in a limited region, and “blocking complete,” for Dielectric thin-film coatings—either longpass, filters used with detectors sensitive to all shortpass, or very wide bandpass—are com- wavelengths. An optimized blocked filter monly used to extend attenuation through- combines a color absorption glass for the out the required spectral region. Deposited short wavelength side of the passband with onto substrates and laminated to the assem- a dielectric reflector for the long wavelength bly, they are highly transmissive in the side of the passband. A completely blocked desired spectral region and highly reflective filter includes a metal thin-film bandpass where the principal coating “leaks” unwant- coating, which is often combined with a ed wavelengths. Figures 3 and 4 illustrate color absorption glass to boost short-wave- how several blocking components increase length attenuation. the attenuation of a principal filter compo- nent. Signal-To-Noise (S/N) Ratio Signal-to-noise (S/N) ratio is often the most Metal thin-film bandpass coatings extend important consideration in designing an opti- attenuation to the far IR (>100 microns). This cal system. It is determined by: approach is simpler than the all-dielectric method in that a single component attenu- S/N = S + (N1 + N2 + N3) where: ates a greater range. Metal layers are S = desired energy transmitted by absorptive, however, and can reduce trans- the filter mission at desired wavelengths to levels N1 = unwanted energy transmitted by between 10% and 60%. A comparable all the filter dielectric filter, blocked to the desired wave- N2 = other light energy reaching the length, would allow transmission of 45% to detector 85%. N3 = other undesired energy affecting the output (e.g., detector and Our two most common strategies for extend- amplifier noise)

OPTICAL FILDER DESIGN APPLICATION NOTE 7 The optimum filter is one that reduces complicated and a coating’s specific damage unwanted transmitted energy (N1) to a level threshold is dependent on a number of fac- below the external noise level (N2 and N3), tors including coating type, wavelength of while maintaining a signal level (S) well the incident energy, angle of incidence, and above the external noise. pulse length.

Filter Orientation For a given set of conditions, a surface oxide In most applications, an interference filter coating will be the most damage resistant. A should be placed with the most reflective, protected dielectric coating will be the most metallic looking surface toward the light susceptible to damage. Surface fluoride, sur- source. The other surface usually can be dis- face metal, and protected metal coatings will tinguished by its more colored or opaque fall between these two extremes. Extensive appearance. When oriented in this way, experience with laser applications guides the the thermal stress on the filter assembly selection of substrate materials and coating is minimized. Spectral performance is design best suited to meet specific spec- unaffected by filter orientation. Typically, trophotometric and energy handling require- our filters are labeled with an arrow on the ments. edge, indicating the direction of the light path. Special markings are made for those Angle of Incidence and Polarization customers who require consistency with Most interference coatings are designed to instrument design. filter collimated light—parallel rays of light— at a normal angle of incidence where the Incident Power coated surface is perpendicular to the light Excessive light energy can destroy a filter by path. However, interference coatings have degrading the coating or by fracturing the certain unique properties that can be used glass. Heat induced glass damage can be effectively at off-normal angles of incidence. avoided by proper substrate selection and by Dichroic beamsplitters and tunable bandpass ensuring that the filter is mounted in a heat filters are two common products which take conducting sink. Coating damage is more advantage of these properties.

100

P-plane at 45º

50 0º incidence S-plane at 45º ransmission

% T 45º incidence unpolarized light

0 450 500 550 600 650 700 750 Wavelength (nm)

Figure 5: Angle of Incidence Polarization Effects—Longpass Filter Longpass filter has cut-on at normal incidence of 665nm and at 45º incidence of 605nm. Graph illustrates the separation of the P- and S-planes of polarization at 45º angle of incidence.

OPTICAL FILDER DESIGN APPLICATION NOTE 8 The primary effect of an increase in the inci- coating, both of which are variables in the dent angle on an interference coating is a design process. For filters with common shift in spectral performance toward shorter coating materials such as zinc sulfide and wavelengths. In other words, the principal cryolite, effective refractive index values are wavelength of all types of interference filters typically 1.45 or 2.0, depending upon which decreases as the angle of incidence increas- material is used for the spacer layer. This es. For example, in Figure 5 the 665LP long- relationship is plotted in Figure 6. The actual pass filter (50% T at 665nm) becomes a shifts will vary slightly from calculations 605LP filter at a 45° angle of incidence. based solely on the above equation. To a near approximation, the relationship A secondary effect of angle of incidence is between this shift and angle of incidence is polarization. At angles greater than 0º, the described as: component of lightwaves vibrating parallel to the plane of incidence and reflection (P- lf = √N2-Sin2 f plane) will be filtered differently than the l0 N component vibrating perpendicular to the plane of incidence (S-plane). The plane of Where: f = angle of incidence incidence is geometrically defined by a line lf = principal wavelength at angle of incidence f along the direction of lightwave propagation l0 = principal wavelength at 0º angle and an intersecting line perpendicular to the of incidence coating surface. Polarization effects increase N = effective refractive index of the coating as the angle of incidence increases. Figures 5 and 7 illustrate the effects of polarization on The effective refractive index of a coating is a longpass and a bandpass filter. Coating determined by the coating materials used designs can minimize polarization effects and the sequence of thin-film layers in the when necessary. 1.00 System Speed 0.98 As noted above, the filtering of collimated N = 2.0 light rays at a greater angle of incidence pro- 0.96 duces a spectrum at a shorter wavelength. 0.94 When filtering a converging rather than colli- CWLθ CWL0 N = 1.45 mated beam of light, the spectrum results 0.92 from the integration of the rays at all of the 0.90 angles within the cone. At system speeds of f/2.5 and slower (cone angle of 23° or less), 0.88 the shift in peak wavelength can be approxi-

0.86 mately predicted from the filter’s perform- ance in collimated light (i.e., the peak wave- 0 5 10 15 20 25 30 35 40 45 50 55 60 length shifts about one-half the value that it Angle of Incidence θ (º) would shift in collimated light at the cone’s Figure 6: Angle of Incidence Effects most off-axis angle). Decrease in CWL as a function of angle of incidence for two bandpass filters with the In addition to the shift in peak wavelength, same coating materials (zinc sulfide and cryolite) but different effective refractive indices (N). system speed can also have significant effect N = 2.0 for the filter with a zinc sulfide spacer. on both transmission and bandwidth. Faster N = 1.45 for the filter with a cryolite spacer. system speeds result in a loss in peak trans-

OPTICAL FILDER DESIGN APPLICATION NOTE 9

100

0º incidence

50 P-plane at 45º

ransmission S-plane at 45º

% T 45º incidence

0 500 550 600 650 Wavelength (nm)

Figure 7: Angle of Incidence Polarization Effects—Bandpass Filters A bandpass filter at normal incidence with a CWL at 590nm and a FWHM of 40nm. At 45° angle of incidence, in random polarization, the CWL is 547nm and the FWHM is 42nm. At 45° angle of incidence, there is a separation of the P- and S-planes of polarization.

mission, an increase in bandwidth, and a ating a glass to glass seal. In addition, sever- blue-shift in peak wavelength. These effects al layers of proprietary moisture-rejecting can be drastic when narrow-band filters are sealants are then applied to the edge. To fur- used in fast systems, and need to be taken ther prolong filter life, filters should be into consideration during system design. stored in a low to moderate (less than 70%) relative humidity environment whenever Temperature Effects possible. The performance of an interference filter shifts with temperature changes due to the Filter Life expansion and contraction of the coating Laminated interference filters, particularly materials. Unless otherwise specified, filters those with narrow-bands, are subject to are designed for an operating temperature of gradual blue shift with age. This tendency is 20°C. Filters will withstand repeated thermal somewhat stabilized through a process of cycling, provided temperature transitions are repeated heat cycling, or curing, at moder- less than 5°C per minute. An operating tem- ately high temperatures for short durations perature range between -60°C and +60°C is during the manufacturing process. recommended. Filters must be specifically designed for use at temperatures above 120°C or below -100°C. Although the shift is Wavelength range Thermal coefficient (nm) (nm of shift per 1°C change) dependent upon the design of the coating, coefficients in Figure 8 provide a good 300 – 400 0.016 approximation. 400 – 500 0.017 500 – 600 0.018 Humidity Effects 600 – 700 0.019 Perhaps the greatest cause of filter deteriora- 700 – 800 0.020 tion is humidity, which can be absorbed by 800 – 900 0.023 the coating. To protect “soft dielectric” coat- 900 – 1000 0.026 ings, a narrow 2mm “scribe” in the coating

is removed around the filter’s perimeter, cre- Figure 8: Wavelength and Thermal Coefficients

OPTICAL FILDER DESIGN APPLICATION NOTE 10

Image Quality Filters Prolonged exposure to light, particularly A filter’s effect on the quality of an image short UV wavelengths, results in solarization results from the degree it distorts transmit- and reduced transmission. Whenever possi- ted lightwaves. ble, we use substrates which are less prone to solarization, especially at the outer sur- In high-resolution imaging systems, faces of filter assemblies. To further filters require multiple layering of various control solarization, protection from intense materials (i.e., glasses, coating materials, light is recommended when a filter is not optical cements, etc.) for high spectral in use. performance. These materials if used indis- criminately can degrade a filter’s optical Transmission and Optical Density performance. This effect can be significantly Accurate spectral measurements of sub-com- diminished through material selection, ponents and assemblies are critical to the design, process, and testing. production of high-precision filters. Transmission (T) and Optical Density (OD) To enhance image quality we select optical are two common ways to express the grade materials with the highest degree of throughput of a filter or filter assembly. homogeneity, the lowest number of inclu- Throughput is the portion of the total energy sions, and the best match in refractive index at a given wavelength that passes through at contacted boundaries. Special coating the filter. A throughput value is always a por- designs minimize the required number of tion of unity (less than 1 and greater than 0). contacted surfaces that cause internal reflec- When describing the transmitting perform- tion and fringe patterns. Before coating and ance of a filter (usually when throughput is assembly, all glasses are polished to requi- 1%–99%), the preferred expression site flatness and wedge values. Our coating is “transmission.” When describing the and assembly techniques assure uniformity attenuating performance of a filter (usually in material as well as spectral properties. when throughput is less than 1%), the After the filter is assembled, transmitted preferred expression is “optical density.” wavefront distortion is diminished further Transmission can be expressed either as a through a cycle of polishing, evaluating, and percentage (e.g., 90%) or decimally (e.g., repolishing both outer surfaces. Durable anti- .90). Optical density is always expressed as reflective coatings are then deposited onto the negative of transmission. Unit the outer surfaces, reducing the intensity of conversions are: ghost images while boosting transmission. OD = -log10 T or T = 10-OD (See Figure 9.) The attainable level of opti- mal performance depends in part upon size, thickness, spectral region, and spectral demands of each particular filter.

Figure 9: Transmitted Wavefront Interferogram A transmitted wavefront interferogram of a narrow-band filter used for tele- . Transmitted wave front distortion is measured at the filter’s principal wavelength on a Broadband Achromatic Twyman-Green Interferometer. Although many interferometers can measure transmitted wavefront distortion, most are fixed at a single wavelength (often 633nm). For filters that don’t transmit this wavelength, these instruments must produce reflected, rather than transmitted, interferograms. Although reflected interferograms are often used to represent the quality of a transmitted image, there are no reliable means for such interpretation.

OPTICAL FILDER DESIGN APPLICATION NOTE 11 Optimizing Custom Filter Sets microscope cubes, instrument mounts, and Omega Optical specializes in the design and filter wheels. production of integrated filter sets for use in new instruments or unique research. Sets A conventional filter assembly, with all filter that perform serially (e.g., separating a components laminated into a single unit, is source light and sending it along to further sometimes limited by inadequate transmis- separations and channels) are optimized for sion of the desired signal, over-transmission combined performance. Over-attenuation, or of undesired signals, or overheating from the redundant blocking, is avoided while signals absorption of radiation. In many cases, these and images are retained. Filter sets that func- limitations can be overcome by arranging tion in parallel (e.g., when each filter isolates the filter components into a “baffle” configu- a particular spectral band from the same ration and using the interference coating source) are optimized for uniform property of selective reflectance. performance characteristics such as optical thickness, bandshape, throughput, sensitivity Coatings and Special Substrates to system speed, sensitivity to temperature, In addition to our standard capabilities, we and image quality. In addition, we can build deposit many types of coatings on a variety these sets to “correct” for unbalanced sys- of optical components such as fiber , tem output by adjusting the throughput of pellicles, prisms, lenses, wedges, etalons, each filter to the system’s relative sensitivity flats, spheres, and parabolas. We can pro- at the principal wavelength. duce or procure these components with state-of-the-art surface accuracy and in vari- Physical Configurations ous materials such as , alloys, opti- Omega Optical offers a wide range of cal glasses, crystals, and . We also standard filter sizes depending on the coat customer-supplied substrates. In addi- product line, and has the capability to pro- tion to any of our standard product coatings, duce custom filters to nearly any dimension. we offer a range of anti-reflective, conduc- Any shape—including rectangles, ellipses, fil- tive, metal, UV wedge, and spectral profile ter-wheel segments, and free-form—is avail- coating services. able in sizes ranging from 1mm on the short- est axis to 250mm on the longest axis. Thickness can be as minimal as 0.2mm for an exposed coated filter. It should be noted that an unusual size and/or thickness require- ment is sometimes limiting to the perform- ance of a finished filter.

The edges of our filters are treated with a proprietary organic polymer that chemically bonds to the various glasses and provides an opaque hermetic seal. We assemble our standard round filters into anodized aluminum rings. Other shapes and sizes are chamfered, sealed, and painted at their edge. For special mounting require- ments, we fabricate a wide variety of rings,

OPTICAL FILDER DESIGN APPLICATION NOTE 12