Optical Filters 2013 - SCHOTT North America

203.5 Characterizationof edge filtersEdge filters are characterized by having a range of high transmittance (passband)followed by a range of low transmittance (blocking range) or vice versa. There aretwo types of edge filters:• Shortpass filters pass a shorter wavelength band i.e. have a range of hightransmittance of shorter wavelengths than the blocking range.• Longpass filters, on the other hand, pass a longer wavelength band (seeFig. 3.4), i.e. have a range of high transmittance of longer wavelengths thanthe blocking range.τ maxτ'' DSpectral transmittance τ(λ)τ' Dτ(λ C)τ''' SFig. 3.4A longpass filter as an example of an edge filtertype and characteristic values.τ' Sτ'' Sλ S1λ S2λ S3λ S4λ C(τ) λ D1λ D2λ D3Wavelength λThe main properties of edge filters are defined by (compare with Fig. 3.4):τ max :λ C :τ DM :λ D :τ' D , τ'' D etc.:τ SM :τ S :τ' S , τ'' S etc.:Maximum value of spectral transmittance within the passband(peak transmittance)Edge wavelength, whereby spectral transmittance reaches a certainspecific value, e.g. τ(λ C ) = 0.50Mean (average) value of spectral transmittance within the passbandMinimum value of wavelength within the passbandMinimum values of spectral transmittance within the passbandfrom λ D1 to λ D2 , from λ D2 to λ D3 , etc.Mean (average) value of spectral transmittance within the blockingrangeUpper limit for spectral transmittance within the blocking rangeUpper limit for spectral transmittance within blocking ranges λ S1 toλ S2 , from λ S2 to λ S3 , etc.S % : Slope of filter in percent, defined by: S % = λ 80 % of peak – λ 5 %λ 5 %· 100,where λ 80 % of peak is the wavelength at which the transmittance is80 % of τ max (correspondingly λ 5 % wavelength where transmittanceis 5 % of τ max ).