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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.

Cut-off filters
Cut-off 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.


Dielectric mirrors

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.

Cut-off filters
Cut-off filters

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.