1. Bandpass filters
1. Bandpass filters. Categories
Bandpass Filters (BPFs) select the bandwidth in the ultraviolet, visible or near-infrared regions.
Photooptic Ltd. produces BPFs with a half-width from 6 to 100 nm and more. As a rule, BPFs involve a set of colored glasses and optical coatings, their thickness is from 2 to 6 mm.
Our multilayer interference optical coatings made of alternating layers of refractory oxides show high mechanical and climatic resistance.
Fig. 1. Examples of typical band-pass filter transmittance.
Categories of band-pass filters.
We produce band-pass filters of four main categories.
They are presented in Table 1 in order of increasing the number of layers, slope factor and stop-zone density.
Table 1. Categories of band-pass filters
By clicking on the links, you will be taken to the technical information on the spectral properties of this category of light filters.
Forming coatings of filters with the t-index (tilted) are assembled at small angles (about 10 deg.) to each other, as described in the monograph Philip W. Baumeister "Optical Coating Technology", SPIE PRESS, 2004.
Figure: 2 Assembly diagram of light filters with the -t index.
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Specification of band-pass filters. Terms and definitions.
Ω - controlled spectral interval and its end points λ¹, λ². (Outside the interval Ω, the T values are not significant when using a light filter in a device of this particular design).
T - spectral transmission of the BPF measured in the interval Ω.
Tmax – maximum of T in the passband.
λm - wavelength at the maximum of T(λm) = Tmax.
λ¹₀․₅, λ²₀․₅ half-pass wavelengths T (λ₀․₅) = 0.5 * Tmax, determine the position of the bandwidth of the band-pass filters.
Central Wave Length (CWL): CWL = 0.5*(λ¹₀․₅ + λ²₀․₅).
FWHM (Full Width at Half Maximum) – width of the bandwidth equal to λ²₀․₅ - λ¹₀․₅.
Decilambda λ¹₀․₁ (λ²₀․₁) - wavelength corresponding to 10% transmission from Tmax. The filter width (BW) at this level is Δ₀․₁ = λ²₀․₁ -λ¹₀․₁.
Santillambda λ¹₀․₀₁ (λ²₀․₀₁) - wavelength corresponding to 1% transmission from Tmax. The width of the filter (BW) at this level, Δ ₀․₀₁ = λ²₀․₀₁ - λ¹₀․₀₁.
Millilambda λ¹₀․₀₀₁ (λ²₀․₀₀₁) - wavelength corresponding to 0.1% transmission from Tmax. The filter width (BW) at this level is Δ₀․₀₀₁ = λ²₀․₀₀₁ - λ¹₀․₀₀₁.
Stop band - spectral interval in which T is small and the signal-to-noise ratio decreases as T increases.
Pass band - spectral interval in which T is relatively large; as T increases, the signal-to-noise ratio increases.
Fig. 3. Transmittance spectrum of the BPF. The decimal points and bandwidth for optical densities
OD=1, 2, 3, 4 are shown.
The shape of the bandwidth is described by the Shape factors S1, S2, S3, S4:
Where Δλ0. 5 is the BPF’s width at the transmission level T = 50% of the maximum,
Δλ0, 1 – BPF’s width at the transmission level T = 10%,
Δλ0, 01 – BPF’s width at the transmission level T = 1%,
Δλ0, 001 - BPF’s width at the transmission level T =0.1%,
Δλ0, 0001 – BPF’s width at the transmission level t = 0.01%.
Form factors describe in detail the transition from the pass zone to the stop zone. For an ideal filter with a rectangular bandwidth shape, S1= S2 = S3 = S4 = 1.0.
Terms and definitions are borrowed from Philip W. Baumeister monograph "Optical coating technology", SPIE Press, 2004.
Filters manufactured by "Photooptic" work in space.
The photo shows the results of assembling the instrument of the Federal State Unitary Enterprise “Institute of Applied Geophysics named after academician E.K. Fedorov" at the ISS station. All cameras have our filters installed.
Dawn shadows from cumulus clouds over the Indian Ocean.