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BACKGROUND & TECHNOLOGY
Interference bandpass filters are relatively inexpensive wavelength selectors that allow transmission of a predetermined wavelength while rejecting or blocking other wavelengths. Interference filters are widely used in instrumentation for clinical chemistry, environmental testing, colorimetry, elemental and laser line separation, flame photometry, fluorescence, immunoassays, etc.
Optometrics Corporation produces thousands of filters specified by and designed for other instrument manufacturers. If our standard filters do not conform to your requirements, Optometrics can design and manufacture filters to your performance and dimensional specifications.
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FILTERS AS WAVELENGTH SELECTORS
Interference filters should be considered when the application requires a limited number of known wavelengths, when energy throughput is more critical than wavelength resolution or when cost is more important than flexibility.
The relative low cost and simple installation of interference filters makes them the preferred wavelength selector for applications such as clinical chemistry, environmental testing, laser line separation and flame photometry, where the required wavelengths are well known.
The energy throughput of an interference filter is usually much greater than can be achieved with a monochromator, where throughput is dependent on slit size, grating efficiency, input optics, etc. The entire clear aperture of an interference filter can be illuminated, resulting in high throughput and an excellent signal-to-noise ratio.
An application that requires numerous analytical wavelengths, variable resolution, far ultraviolet analysis, spectral analysis, etc., will require a prism or grating monochromator rather than interference filters.
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An interference filter is fabricated in three sections, one of which determines the central wavelength (CWL), halfbandwidth (HBW), and shape of the transmittance curve while the other two control the degree and range of blocking outside the passband. After thin film deposition is complete, the three sections are scribed, laminated, cut and mounted.
The bandpass section of an interference filter is made by repetitive vacuum deposition of thin layers of partially reflecting dielectric compounds on a glass substrate. A typical interference filter can have over fifty such layers, each one precisely controlled and evenly deposited over the preceding layer. The thickness of each layer is equal to a quarter wave (λ/4) of the filter central wavelength (λ). Alternating layers of dielectric materials with high and low refractive indices make up a stack. A half wave (λ/2) layer, or a multiple thereof, deposited between symmetric stacks, forms a spacer layer. The halfbandwidth of an interference filter is determined by the ratio of the indices of the high and low dielectric
materials, the number of layers in a stack and the number of half waves in a spacer. A spacer layer and adjacent stacks form a “cavity," the basic element of an interference filter. The number of cavities in the bandpass section determines the overall shape of the transmittance curve. Most Optometrics filters are made with three cavities, resulting in filters with steep slopes, improved blocking close to the passband and relatively flat tops.
Rejection of wavelengths resulting from destructive interference is limited to within 15% of the central wavelength. Therefore, additional glass or metallic blockers must be added to reduce out-of-bandpass transmittance. Metallic blockers, which consist of layers of silver deposited on the dielectric spacer layer, reflect and absorb radiation outside the filter passband and negate higher order passbands from X-ray to the far IR. The blocking capabilities of metallic blockers are augmented in high-performance filters by the addition of color glass and custom dyes that absorb UV or long wavelength radiation.
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The central wavelength of an interference filter can shift with increasing or decreasing temperatures. This effect, which is due primarily to the expansion or contraction of the spacer layers and the concomitant change in their refractive indices, is extremely small over normal operating ranges (≈ 0.01 nm/°C). Prolonged operation at high temperatures (>75° C) will irreversibly set the central wavelength lower. Temperatures above 125° C should be avoided.
Though interference filters will function at -50° C or lower, the cooling rate should not be allowed to exceed 5° C per minute. An excessive cooling rate can cause the glass substrate to crack or the filter to delaminate due to differential thermal contraction.
ANGLE OF INCIDENCE
An interference filter should be illuminated with collimated radiation normal (perpendicular) to the surface of the filter. The central wavelength will shift slightly to a lower wavelength if the illuminating radiation is not normal to the filter. A deviation of less than three degrees results in a negligible wavelength shift. At large deviations, the wavelength shift is significant, transmittance decreases and the shape of the passband changes.
When noncollimated radiation impinges on the filter, the result is similar to that stated above. In this case, the effect is dependent on the cone angle of the illuminating radiation. Varying the angle of incidence from normal can be used to “tune” an interference filter within a limited wavelength range.
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Blocking refers to the degree to which transmitted radiation outside the filter passband is restricted. A blocking specification should state the wavelength range over which it is measured. Both the degree and range of blocking required are application dependent. Too little blocking will result in unacceptable stray light (high noise); too much will decrease throughput (low signal) and increase costs. Blocking is one of the most important specifications to be considered when selecting an interference filter.
Blocking is sometimes defined in “absolute” terms, which refers to the ratio of the largest peak outside the passband to the peak within the passband. Absolute blocking does not measure the total radiation (energy) outside the passband and has little meaning in spectroscopy, where all radiation outside the passband is considered stray light.
Integrated blocking is a more useful way to define blocking. It is the ratio of the total radiation (energy) outside the passband to the total radiation within the passband. For an integrated blocking value to be meaningful, the conditions under which the filter is to be used must be known. For example, the integrated blocking value of a 340 nm filter in an optical system with a UV source and photomultiplier will be considerably better than the same filter used with a tungsten lamp and silicon photodiode. The spectral response of a UV source and PMT detector system may overlap from about 200 nm to 400 nm, with considerable energy and detector sensitivity at 340 nm (high signal). Under these conditions, radiation detected through the filter outside the passband (stray light) is limited by both source and detector and can be easily controlled by standard blocking. If, however, the same 340 nm filter is used with another source and detector, stray light could be a problem and additional blocking may be required. The spectral response of a tungsten source and a silicon photodiode detector system may overlap from about 320 nm to 1100 nm, but with very little source energy or detector sensitivity at 340 nm (low signal). These conditions require that the filter have additional blocking to compensate for the source radiation and detector sensitivity from 400 nm to 1000 nm (ultra low noise).
Several equivalent notations are used by various manufacturers to specify blocking including absorbance, optical density, transmittance, scientific notation, rejection ratio and signal-to-noise ratio. To establish a blocking specification, Optometrics utilizes an optical system with a tungsten halogen lamp with a color temperature of 2800° K and a UV-enhanced silicon photodiode. A transmittance notation is used since it is universally understood.
For spectroscopic applications, the degree of blocking should be consistent with the sample being used. Integrated blocking to 0.1%T (standard performance filter) will not cause an appreciable error with a low absorbing sample. For a highly absorbing sample (Abs ≥ 2.0), the 0.1% stray light would be 10% of the total transmitted signal, grossly affecting the accuracy of an assay. Therefore, a high-performance filter is required, where integrated blocking is 0.01%T.
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Interference filters in the UV-VIS-NIR range are subject to environmental deterioration due to moisture penetration of the hygroscopic dielectric layers. Though the bandpass and blocking sections of interference filters are laminated with epoxy, a high humidity environment can cause delamination.
A process known as scribing results in excellent moisture protection. Scribing removes all dielectric material from the periphery of a filter, allowing a glass-to-glass epoxy seal that minimizes moisture penetration. Optometrics filters are also sealed in a metal ring, but the primary purpose of the ring is to protect the filter from physical damage, particularly the relatively soft color glass. Optometrics randomly tests its filters in a humidity chamber and Optometrics’ filters routinely pass MIL standard 810E aggravated test protocols.
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An interference filter will function with either side facing the source. It is recommended, however, that the side with the “mirror–like” reflective coating be oriented toward the source. This will minimize any thermal effect that could result from the absorption of heat by the color glass or blockers on the other side.
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CHOOSING A FILTER
Select from our extensive range of standard filters or contact Optometrics with your requirements for an OEM quotation.
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TYPES OF FILTERS
This type of interference filter finds wide application in spectral analysis, particularly those in clinical chemistry, spectral radiometry, environmental testing, laser line separation, flame photometry and color separation where the required wavelengths are well known. (See pages 11-12 for more information)
Used in applicationrs where input energy levels are low and where a wide viewing field is needed, the line of filters covers the region from 450 to 700 nm, stepped every 50 nm and each has a halfbandwidth of 80 nm. (See page 14 for more information)
Short and Long-Pass Filters
Often called Edge filters, the transition between the 50% cut-off or cut-on rejection of these filters is quite sharp, making it much easier to separate excitation form emitted wavelengths without interfering with wavelengths of interest. They can be used as emission filters in fluorescence applications, to eliminate any unwanted radiation, in Raman spectroscopy and as order sorting filters. They are also used in laser-induced fleorescence to isolate source radiation. (See pages 17-18 for more information)
Laser Line Filters
While lasers are generally assumed to emit monochromatic light, there is often unwanted radiation that the user wishes to eliminate and Optometrics' Laser Line filters are designed for this purpose. (See pages 14-15 for more information)
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INTERFERENCE FILTER TERMINOLOGY
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