Interference Filters and Special Filters Catalogue
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There is a solution behind every filter problem Interference Filters and Special Filters

2 Contents Page 1. Foreword 3 2. Optical radiation filters 4 2.1 Spectral transmittance ô(ë) 4 2.2 Graphic illustration of ô(ë) 4 2.3 Spectral optical density D(ë) 5 2.4 Characterization of bandpass filters 6 2.5 Characterization of edge filters 7 3. Terms and definitions Design and properties of interference filters 8 4. General comments on filter descriptions 16 5. Standard program of bandpass interference filters 16 6. Linear variable interference filters VERIL 29 7. i-line filters 31 8. Interference filters with hard coatings 32 8.1 Edge filters (hard coatings by reactive ion plating or...

3 1. Foreword Technological development in the field of optical measuring and analyzing techniques has led to greater demands being placed on the properties of interference filters and special filters used as optical components. SCHOTT fulfils these requirements by constantly improving its established filter types and by developing filters with properties suitable for new fields of application. This catalog contains descriptions of the filter program and includes information and hints necessary for the selection and us of optical filters. We will of course be glad to advise you on the...

4 2. Optical radiation filters Optical radiation filters are characterized by the fact that they more or less selectively alter the spectral composition of a radiation beam, by absorption or reflection, within the optical spectral range. Optical radiation filters are usually characterized quantitatively by indication of spectral transmittance ô(ë). Spectral transmittance ô(ë) is the ratio of the transmitted radiant flux (Öeë)ô to the incident radiant flux Öeë. 2.1 Spectral transmittance ô (ë) 2.2 Graphic illustration of ô (ë) ô(ë) = (Öeë)ô Öeë Spectral transmittance ô(ë) Ordinate: ô(ë) in...

5 Figs 1 and 2 show two forms of illustrating spectral transmittance as a function of wavelength. The diabatic form has an advantage over the linear form: Both the passband and the blocking range can be clearly seen (cf. figs 1 and 2). In both figures the course of spectral transmittance of the same bandpass filter is used to ensure proper comparison. In some cases the quantitative characterization of a filter is given in terms of spectral optical density D(ë). The relationship of spectral optical density to spectral transmittance is governed by the formula: 2.3 Spectral optical density...

6 Bandpass filters are characterized by having a range of high transmission (passband) bounded both towards the shorter and longer wavelengths by ranges of low transmission (blocking ranges). 2.4 Characterization of bandpass filters Spectral transmittance ô(ë) Wavelength ë Fig. 3: Characterization of bandpass filters The most important properties of bandpass filters can be defined by the following values (see also fig. 3): ômax: Maximum spectral transmittance within the passband (peak transmittance). ëm: Center wavelength: If ë´1/2 and ë´´1/2 are the wavelengths at which spectral...

7 Tenth width Äë1/10 ZW Q value: Q = = = Half width Äë1/2 HW Thousandth width Äë1/1000 TW q value: q = = = Half width Äë1/2 HW ôSM: Mean (average) value of spectral transmittance within the blocking range. In the case of bandpass interference filters that are specified as having an “unlimited” blocking range (see also section 3.), the end of sensitivity range of a commonly used detector is taken as the long-wave limit, when ôSM is evaluated. ôS: Upper limit for spectral transmittance within the blocking range. ô’S, ô’’S etc.: Upper limits for spectral transmittance within blocking ranges...

8 3. Terms and definitions Design and properties of interference filters Interference is a characteristic of the wave nature of electromagnetic radiation: Two or more coherent wave trains of the same wavelength and polarization state that are superimposed enhance or compensate each other, depending on the phase relationship and amplitudes of the electric field strength. These filters utilize the interference effect to transmit or reflect certain spectral ranges of the electromagnetic radiation. Hereto numerous thin layers with differing refractive indices are brought up to a substrate. The...

9 All Dielectric Interference filters consist of alternating thin layers of materials with differing refractive indices that are practically absorption-free within the spectral range in question. ADI filters Spectral transmittance ô(ë) Wavelength ë [nm] Fig. 5: Spectral transmittance curve of a non-blocked ADI shortpass filter (general curve) Depending on the multilayer system selected, the following filter types mainly can be achieved: mirrors, edge filters (shortpass or langpass filters and dichroic beam splitters), band stop filters and bandpass filters (line, band und broadband...

10 Metal-Dielectric Interference filters are mainly produced as bandpass filters. Their multilayer system mainly consists of thin, partially-transmitting metal layers separated by essentially absorption-free dielectric spacer layers. The thickness of the spacer layers in the main determines the spectral position ë1 of the passband with the longest wavelength. Further ranges with maximum transmission are approximately obtained for ëK = ë1/K (K = 2, 3, 4...). Wavelength ë1 is also referred to as the first-order wavelength, ë2 as the second-order wavelength, and so forth. As the refractive...

11 A mirror is essentially formed by a thin layer of metal or a series of alternating dielectric layers with differing refractive indices whose optical thickness is a quarter of a given design wavelength ë0. This multilayer system functions as a selective mirror with a high degree of reflection at and around wavelength ë0. By an appropriate matching of certain layers, the distorted areas of high transmission outside the reflection range can be smoothed. Hence, depending on the matching employed, interference filters with shortpass, longpass or band stop character can be fromed. Mirrors in...