Types of Optical 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.
Used in applications 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 half bandwidth of 80 nm.
Short Wavepass and Long Wavepass 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 from 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 fluorescence to isolate source radiation.
Laser Line Filters
While lasers are generally assumed to emit monochromatic light, there is often unwanted radiation that the user wishes to eliminate and our Laser Line filters are designed for this purpose.
The central wavelength of a bandpass 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/oC). Prolonged operation at high temperatures (>75o C) will irreversibly set the central wavelength lower. Temperatures above 125o C should be avoided. Though bandpass filters will function at -50o C or lower, the cooling rate should not be allowed to exceed 5o C per minute. An excessive cooling rate can cause the glass substrate to crack or the filter to delaminate due to differential thermal contraction.
A bandpass 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.
Angle of Incidence
A bandpass 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 3 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” a bandpass filter within a limited wavelength range.
Application and Filter Lifetime
Bandpass 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 bandpass 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. Our 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. Dynasil’s optical filters routinely pass the aggravated test protocols of the MIL 810E standard.
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 a bandpass 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, we utilize an optical system with a tungsten halogen lamp with a color temperature of 2800o 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.
Absolute Blocking: The ratio of the largest peak outside the passband to the peak within the passband. Expressed as an area or %T.
Absorbance: The logarithmic function of transmittance. Sometimes used to express the degree of blocking. A = log(Iø/I).
Angle of Incidence: The angle formed by radiation arriving (incident) at the filter surface and the perpendicular to the surface at the point of arrival.
Angstrom (A): Unit of length used to measure wavelengths of light. One tenth of a nanometer (nm). One Angstrom is equal to 1 x 10-10 meters.
Bandpass Filter: A filter that, operating on the principals of constructive and destructive interference, transmits radiation in a discrete, narrow wavelength range while rejecting other radiation. Also known as an interference filter.
Bandwidth: Specified wavelength interval of transmitted radiation.
Blocking: The degree to which detectable radiation outside the passband is rejected. Expressed as transmittance, absorbance, optical density, scientific notation, signal-to-noise or rejection ratio. Blocking requirements are specified over a useful wavelength range.
Cavity: Basic component of a bandpass filter consisting of two layers of reflective stacks separated by a spacer layer. Also known as a period.
Clear Aperture (CA): The central, usable area of a filter through which radiation can be transmitted.
Central Wavelength (CWL): The mean of the two wavelengths corresponding to the half power points.
Half Power Points: Points on both sides of the passband curve of a filter, with a value 50% of the peak transmittance. Used to calculate HBW and CWL.
Half Bandwidth (HBW): The wavelength interval of the passband measured at the half power points (50% of peak transmittance). Expressed as halfbandwidth (HBW), full width half maximum (FWHM) or half power bandwidth (HPBW).
Incident Radiation (Iø): The radiation, usually polychromatic, that impinges on a filter.
Interference Filter: See bandpass filter.
Integrated Blocking: The ratio of the total transmitted radiation (energy) outside the passband to the total transmitted radiation within the passband. Integrated blocking is influenced by the source output and detector response as functions of wavelength.
Micron (μ): Unit of length used to measure wavelengths of light. One micron is equal to 1,000 nm or 10,000 angstroms.
Nanometer (nm): Unit of length used to measure wavelengths of light. One nanometer is equal to 1 x 10-9 meters.
Near Infrared (NIR): Light from the region of the electromagnetic spectrum with wavelengths between (approximately) 750 nm and 3.0 μ.
Optical Density (OD): Used to express the degree of blocking. Also known as Absorbance.
Passband: A wavelength interval through which incident radiation is transmitted. The first order passband is at the filter design wavelength.
Peak Transmittance: The highest transmittance value of a filter.
Peak Wavelength: The wavelength at which a filter has its peak (highest) transmittance.
Period: See cavity.
Rejection Ratio: The ratio of the maximum transmittance outside the passband to the total transmittance within the passband.
Signal to Noise Ratio (S/N): The ratio of detected energy transmitted through the passband to the detected energy transmitted outside the passband. It is source and detector dependent.
Stray Light: Unwanted energy transmitted through the filter.
Transmittance (Tx): The ratio of the transmitted radiation to the incident radiation, expressed as a percent. %T = I/Iø x 100.
Transmitted Radiation (I): Radiation passing through a filter, either inside or outside the passband.
Ultra-Violet (UV): Light from the region of the electromagnetic spectrum with wavelengths between 150 nm and 400 nm.
Visible (VIS): Light from the region of the electromagnetic spectrum with wavelengths between 400 nm and 750 nm.