Wavelength Selection, Band Pass, Minus, And Notch Filters

Wavelength Selection Filters:  have many names and operate differently.  But, the intent is to select or remove one or more wavelengths, for a particular interest.    

Most of the Band-Pass filters are designed either by combining Long-Wave and Short-Wave Pass filters (Broad-Band) or by utilizing Fabry-Perot Multi-Cavity Designs approach (Narrow- Band).  The inverse of this is to stack quarter waves with alternating extreme high and low indices of refraction to create a Minus Filter (Broad-Band) or stack quarter waves with slightly different high and low indices creating a Notch Filter (Narrow-Band)

Transmission Band-Pass Filters are usually divided in two groups, Broad-Band or Narrow-Band Pass Filters. Although there are no Bandwidth values that define Narrow or Broad Filters in the UV, Visible, or IR, it is common to consider filters that have a Bandwidth of 2% or less of the central peak wavelength as a Narrow Band Pass. Thus, according to this convention, the Band-Pass filter centered at 500nm is considered as a Narrow-Band if the Pass Band has a Bandwidth of 10nm or less. Band-Pass Filters are defined by the Bandwidth or Full Width at Half Maximum Transmission(FWHM). Where the 50% Peak-Transmission points are defined for each rising and falling slope.  It is also necessary to specify the rejection zones(Band Width about either side of the Pass-Band).  In addition, the slope of the Transmission curve on both sides of the Pass-Band can be defined together with so-called Cut-On and Cut Off points. The Cut-On and Off point are both 5% of Tmax(from left to right) 

Band-Pass Filters are traditionally thought only to be deposited by means of Sputtering Technologies. But, with tighter controls, than ever, we have been able to pull our 50nm FWHM in to 25nm FWHM.  But, for example, for the common Telecom Dense Wavelength Division Multiplexing(DWDM) of 10nm FWHM we still defer to Sputtering.  Given the extraordinary costs of that technology, many customers find what we have to offer an inviting alternative and suitable for their requirements.

Reflecting Band-Filers are another option.  Specifically, there are Multiple Reflection systems, Color correcting, Laser line blocking, Minus, and Notch Filters to help enhance or reject wavelengths.  For example, in vacuum ultraviolet, below 200nm or above 9µ where very few materials transmit, a Transmission Band Pass is virtually impossible.  One solution is to utilize a multiple reflector filtering system where incident radiation is reflected from each surface in the system, thus, resulting in exponential filtering of the out-of-band spectrum.