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Cheshire OpticalOptical Filters for Research & Technology Manufacturer and Supplier of Optical Interference Filters, Optical Bandpass Filters, Narrowband Filters, Hot Mirrors, OEM, Color Process Filters and Neutral Density Filters for Research and Technology |
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Technical Information
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Filters used at an angle or F#
Typical Filter Construction of bandpass filters Our Standard Filters are scribed, a process where a ring of coating is removed around the OD of the filter. This does two things for the product, it provides a hermetic seal to keep out moisture and makes a better epoxy bonding surface for the other components in the assembly. Both of these added benefits make a more stable and longer lasting filter when compared to an un-scribed filter assembly. In an un-scribed filter the edges of the coating are exposed to cutting oils and solvents during the configuring process that can start the failure of the component before it is even sealed in an aluminum ring or edge painted. (drawings not to scale) Ring Mounted Filter Construction
Painted Edge Filter Construction
Blocking Considerations
Out of band blocking is an important aspect to consider when specifying a custom optical filter. the blocking range and depth has a big effect on the performance of a filter. in general the broader the blocking range is the more it impacts the final transmission of the pass-band. the blocking depth to a lesser degree also has a negative effect on transmission. the main effect of greater blocking is that the filter requires more layers in the coatings and that in turn adds to the cost. One needs to consider the light source, detector, wavelengths needing to be blocked and what needs to be transmitted. differences in blocking arrangements can have big effects on the total filter performance and cost. A normal band-pass filter will have near band blocking on its own from ~ 0.8 X CWL on the low side to ~1.2 X CWL on the high side, outside of that other components are needed to complete the blocking of unwanted wavelengths. Typical band-pass filters in the visible are blocked "complete" meaning they use a metal dielectric blocker that blocks to the far IR on the high side, near band blocking is done by the band-pass component and a long-pass absorbing component takes care of the low side blocking. in this arrangement the transmission is limited by the metal-dielectric blocker and they typically pass %45 to %60 T. Filters blocked to 1150nm or 1200nm are intended to be used with a Silicon detector where high throughput is needed, most wide-band filters are blocked this way. In this arrangement one or more short-pass dielectric components are used to block from the high side of the filter to the desired wavelength. this method of blocking offers much higher transmission usually anywhere from %65 to %80 T since the dielectric coatings have much higher transmission than the metal-dielectric coatings. the only drawback to the all dielectric blockers is that they have limited range of blocking. The depth of blocking can be increased in almost any filter and sometimes has a small impact on overall transmission, but the biggest impact is on price as greater blocking requires more layers witch typically make the filter more difficult to make. The cost of a custom filter is mostly effected by the quantity needed, requested blocking range and depth, and overall transmission. in most cases a metal-dielectric blocked band-pass filter is the least expensive solution in the visible but in many cases higher transmission is needed. when this is the case the best solution is to block only the out of band energy that might be picked up by the detector in use. there is a limit to what can be done with dielectric blocking on the high side of a filter, if for instance one needs a 650nm filter and a PbS detector is in use it would not be practical to use dielectric blocking since so many short-pass blockers would be needed that the transmission would be about the same as a metal-dielectric blocked filter. and the metal-dielectric blocked filter would be much more cost effective and perform just as well. Typical blocking range offerings by detector PMT /GaAs 200nm to 900nm Si 200nm to 1050nm or 1200nm (depending on type) Ge 200nm to 1800nm InGaAs 200nm to 1700nm PbS 200nm to 3000nm
Filters used at an angle or F# Interference filters are sometimes used with a camera or detector that needs to "see" a wide field of view for its application. an interference filter is a good choice for imaging or detecting a dedicated spectral band because they are compact and cost a lot less than an imaging spectrograph that is bulky, uses gratings and still needs order sorting filters. The only problem with interference filters in these applications is wavelength shift of the pass-band at an angle, this limits the effective field of view if simply placed in front of a camera or detector. the best solution to this problem is the use of an optical system that collects and collimates the light before it is filtered , then after light passes through the filter it can be refocused onto the camera CCD or detector. most all interference filters will work as intended up to f8, and still function well up to f4 in some cases, but if collection and collimation optics are not possible then the following will explain some solutions and limitations. ----------------------------------------------------------------------------------------------------------------------------------------------------- When a filter is used at an angle, a range of angles or a fast f# special designs need to be used for the filter to function as needed or at all in some cases. Standard band-pass filters are made to be used at angles of up to +/- 5 deg and light beams of f8 (~3.5 deg half cone angle) or slower. anything outside of that would be considered a special application although some function can still be had using standard filters outside of the normal angles. Applications that need a filter to function at a high f# can be a challenge. often a wide-band filter of special design is needed in order to pass the desired band at the edge of the detectors field of view. narrower band filters simply will not work as well at f#'s below 6, there will be energy at the normal CWL of the filter passing through the very center of the filter but the outer areas of the filter will shift to lower wavelengths as the light entering there is at a steeper angle. this has the overall effect of greatly cutting the energy throughput, allowing a range of wavelengths to reach the detector and limiting the effective field of view at the desired band . Small angles up to 10 deg tilt with a standard filter will just shift the pass-band towards the blue a little and will only change the transmittance and band shape slightly, it is angles of 15 to 45 deg or more and optical systems of f4 to f1 that have a an impact on the filters ability to function properly. In these faster systems the blue shift of the filter is radical as well as complete band shape change and loss of transmittance. blocking can also be compromised in these situations unless the filter is designed for use at angle or low f#. Polarization is a major factor in the change in performance of thin film optics at angles. for the most part we will only talk about the "mean" or average polarization ( also referred to as random at times), but briefly what happens is that in the P plane the band will widen and lose out of band reflectance with increased angle and in the S plane the band will narrow and gain out of band reflectance, the mean will be the average of the two. One of the main factors in the amount of blue shift in the pass band is the overall refractive index of the filter, low index filters can shift up to 50nm at a 25 deg angle where a high index filter will shift only 25nm or so. the throughput of some filters can almost completely collapse at angles of greater than 20 deg while others with proper design can work in some fashion up to 45 deg. Instrument systems that use optical interference filters to scan a spectral region by tilting through a range of angles is not new, some very good non-destructive inline systems use sets of filters that are tilted to "scan" a sample spectrum eliminating the need for gratings and collimating optics. But these filters need to be specially designed for the purpose to work as intended. There are different solutions to filters that need to see light coming in at off angles, being used at an angle or used at a high f#, for a custom application call us to discuss your needs, we have provided filters for use as described here for many customers with great success.
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