Broad, Narrow, and Dual Bandpass Filters
Bandpass coatings transmit only a limited range of wavelengths; while a broad range of wavelengths are effectively blocked (i.e., reflected and/or absorbed). They are widely used wherever there is a need to separate signals from ambient background. Narrow bandpass filters are generally defined as those in which the bandwidth (FWHM) is 10% or less of the center wavelength (e.g. 100 nm FWHM for a 1000 nm center wavelength filter). Wide bandpass filters are just the opposite. Dual bandpass filters feature two passbands, rather than a single one.
THE DSI DIFFERENCE
DSI has extensive experience designing and manufacturing bandpass coatings for use at wavelengths ranging from the visible through the mid-wave infrared. These include broad bandpass, narrow bandpass and even dual (or multi-wavelength) bandpass designs. DSI bandpass filters are characterized by high in-band transmission, strong out of band blocking and immunity to shifts due to changing humidity.
DSI BANDPASS FILTER ADVANTAGES
- High in-band transmission
- Sharp cut-on cut-off slopes
- Excellent out-of-band blocking
- No wet/dry wavelength shift
- Extremely small thermal shift
DSI BANDPASS FILTER APPLICATIONS
- Laser systems
- Spectroscopy and fluorescence imaging
- Military imaging, target designation and countermeasures
- Digital cinema
- Medical imaging
- Hyperspectral and multispectral sensor systems
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DSI utilizes both evaporative and MicroDyn® sputtering methods for the fabrication of bandpass filters. That enables us to tailor coating performance, cost and durability to meet your particular performance specifications and budget.
MicroDyn®: DSI employs this technology to deliver environmentally stable and exceptionally durable coatings from the UV to the midwave IR, making it ideal for many aerospace applications. Because MicroDyn® deposition can be performed at relatively low temperatures, these bandpass coatings can be applied to a wide range of substrate types, including temperature-sensitive active device.
Evaporation: DSI produces evaporated bandpass coatings from the midwave infrared through the longwave infrared. Ion assisted deposition (IAD) technology is utilized to improve coating adhesion to the substrate, and can also be used to lower film stress for greater environmental durability.
SPECIFYING COST EFFECTIVE BANDPASS COATINGS
For the buyer of bandpass coatings, it’s useful to understand some of the most common design and production tradeoffs in order to avoid writing specifications in a way that drive up cost unnecessarily. The graph here shows the key performance parameters commonly used to specify a bandpass coating.
Definitions of bandpass filter performance parameters. The Out of Band blocking may be specified to start at the first 1% or 0.1% point on either side of the bandpass, or sometimes at a given wavelength away from the half-power point, or even at a particular wavelength. It may be specified as a maximum value, average value, or total integrated power within the blocking range
WAVELENGTH ACCURACY VS. COST
Tight tolerances (below ±0.5%) on center wavelength or edge placement can lower yield and thus adversely impact price. While extremely tight tolerances for these values can be achieved by measuring and selecting filters after fabrication, this can become expensive as part size increases, and may not be entirely practical in higher volumes.
OUT-OF-BAND BLOCKING VS. COSTHigh absolute out-of-band blocking specifications (greater than OD3, or <0.1%) can increase cost because these require “over designing” the blocking region to ensure there are no possibilities of a leak. In contrast, an equivalent tolerance on average blocking is much easier, and therefore less expensive to achieve.
USEFUL COST TIPAchieving very high out-of-band blocking can compromise the in-band transmittance because the very thick blocker coating layers can cause absorption and scatter, and the larger number of layers causes the design to be more sensitive to manufacturing variations.. Thus, try to avoid simultaneously specifying both high peak transmittance and strong out-of-band blocking.
Wide bandpass filter for use in the visible spectrum.