2. Cut-off filters
2. Cut-off filters. Categories
Cut-off filters divide the radiation flux into transmitted, reflected fluxes of different intensities and different spectral composition.
Two types of cut-off filters are distinguished: Short Pass filters or SP filters and Long Pass filters or LP filters.
Cut-off interference filters produced by "Photooptic" Ltd. are characterized by high (85-95%) transmission in the useful spectral region - pass band and high (more than 99.95%) reflection in the blockage zone - stop band ").
Multilayer optical interference coatings made of alternating layers of refractory oxides provide high mechanical and climatic resistance of our light filters.
Figure: 1 Examples of typical transmission spectra of LP cut-off filters
Figure: 2 Examples of typical transmission spectra of SP cut-off filters
Categories SP and LP filters.
Photooptic Ltd. produces three categories of SP filters and three categories of LP filters.
All of them are presented in Table 1.
Short-wave transmission filters. SP
Table 1. Categories of cut-off filters
By clicking on the links, you will be taken to technical information about the spectral properties of this category of light filters.
Long-wavelength transmitting filters. LP
The shaping coatings of the -t (tilted) filters are set at small angles (about 10°) to each other, as described in Philip W. Baumeister's "Optical Coating Technology", SPIE PRESS, 2004.
Figure: 3 Assembly diagram of light filters with the -t index.
To speed up the execution of your order, we recommend that you fill out a special order form .
Specification of cut-off filters. Terms, concepts, definitions.
For the specification for cut-off filters it is recommended to use the concepts and definitions presented below and clearly illustrated in Fig. 4 and 5.
1. Ω - controlled spectral interval and its extreme points λ1, λ2. (Outside the Ω interval, the T values are not essential when using a light filter in a device of this particular design).
2. T is the spectral transmission of the filter, measured in the range Ω.
3. Tav - the measured average spectral transmission of the filter in the pass zone (dashed line in Figures 4, 5).
4. λ₀․₅ - half transmission wavelength T (λ₀․₅) = 0.5 * Tav.
5. Decilambda λ₀․₁ is the wavelength corresponding to 10% transmission of Tav.
7. Centilambda λ₀․₀₁ is the wavelength corresponding to 1% transmission of Tav.
8. Milliambda λ₀․₀₀₁ - wavelength corresponding to 0.1% transmission of Tav.
9. Stop band is a spectral interval in which the signal-to-noise ratio decreases with increasing T.
10. Pass band - the spectral interval (in Figures 4, 5 it is indicated by the darkened area), in which the signal-to-noise ratio increases with increasing T.
Figure: 4 Transmission spectrum SP of the filter.
The decimal points λ₀․₁, λ₀․₀₁, λ₀․₀₀₁ are shown
for optical densities OD = 1, 2, 3.
Figure: 5 Transmission spectrum of the LP filter.
The decimal points λ₀․₀₁, λ₀․₀₁, λ₀․₀₀₁ are shown
for optical densities OD = 1, 2,
Table 2. Example of spec and its interpretation.