COMPOSITE FILTER AND THE USE OF SUCH A FILTER

Abstract: A composite filter for intercepting light of a radiation spectrum in the region of 600-900 nm comprising at least one organic film filter and at least one glass filter, in which said organic film filter absorbs light of a radiation spectrum in the region of 600-700 nm and said glass filter absorbs light of a radiation spectrum in the region of 700-900 nm. An observa­ tion system for dark field utilizing a multi-channeled photo­ multiplier tube, which is provided with said composite filter is disclosed. Further disclosed is a filamentary display device containing plural bar-type coiling filaments encased in a cas­ ing having a transparent window portion in which said trans­ parent window portion is provided with said composite filter.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a composite filter, and the use of such a filter. Description of Prior Arts A technology of detecting an image in dark field such as at dark night with no moonlight has been recently developed. This technology utilizes starlight as the light source and its detection system contains a multi-channeled plate and a photoelectric conversion element matching with the starlight source. It is known that the earth goes through a space in which approx. a hundred thousand stars are dispersed, for one day. Although most of the stars emit light, the ultraviolet (UV) rays emitted by new-born stars (age: 100,000 - 1,000,000 years old) are scattered or absorbed in an atmospheric gas, etc. so that the UV rays hardly reach the surface of the earth. On the contrary, the near infrared (IR) rays emitted by stars of approx. 5 x 10⁶ years old reach the surface of the earth. The spectrum of light emitted by the stars and received by the earth is shown in FIG. 1 as the curve (i). Accordingly, the photoelectric conversion element employed in the system has, for instance, such photoelectric conversion characteristics as shown in Fig. 1 as the curve (ii). The observation system utilizing the starlight has been developed and now employed generally in the military and police works, because such observation system needs no employment of artificial light. Since the observation system now utilizes a multi-channeled secondary electron multiplier, the gain is at very high level. The detection device employed for the observation system contains a number of artificial light sources such as a lighting device, various display devices, etc., most of which emit light having a radiation spectrum overlapping in part with the radiation spectrum of starlight. Generally, the luminance of light emitted by these artificial light sources is several millions times to several ten millions times as high as that of the starlight. Therefore, if any light of the artificial light source is incidentally received in the visual field directed to eyes of the observator, not only such light causes noise but also the highly amplified artificial light causes burning and further destroys retinas of the observator's eyes. GB-A-1388684 discloses organic filters. Melles Griot catalogue from 1975, page 123 discloses a glass filter absorbing light in the 700 nm to 900 nm wavelength region. SUMMARY OF THE INVENTION The present invention has an object to provide a composite filter which is capable of rendering the dark field observation system substantially free from the above-described drawbacks and dangerous phenomena. Another object of the invention is the use of such a composite filter. The present invention provides a composite filter according to claim 1 and the use of such a filter according to claim 4. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 are spectra in which (i) shows a radiation spectrum of light emitted by the stars and received by the earth, and (ii) shows representative photoelectric conversion characteristics. FIG. 2 shows representative absorption spectra of the organic film filter. FIG. 3 shows a representative absorption spectrum of the glass filter. FIG. 4 shows a representative absorption spectrum of the composite filter according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The composite filter provided by the present invention is designed for intercepting light of a radiation spectrum in the region of 600 - 900 nm, and which comprises at least one organic film filter and at least one glass filter. The organic film filter absorbs light of a radiation spectrum in the region of 600 - 700 nm and can be prepared, for instance, from an organic polymer such as cellulose derivative (e.g., cellulose diacetate, cellulose triacetate, etc.) dyed with an appropriate dye having an absorption spectrum in the region of 600 - 700 nm. Examples of the appropriate dye include Zapon BG First (trade name, available from BASF A.G). The dye can be employed singly or in combination, the latter being adoptable for adjusting the absorption spectrum to a more appropriate region. The organic film filter generally has a thickness in the range of 0.01 - 2 mm, more preferably in the range of 0.1 - 0.5 mm. Such organic film filter can be prepared by coating an solution containing the organic polymer, a plasticizer (if desired), and dye on a tentative support such as glass plate or plastic plate. Otherwise, the organic film filter can be prepared directly on the glass filter. The glass filter absorbs light of a radiation spectrum in the region of 700 - 900 nm, and can be a glass plate colored with a cobalt-containing salt or other coloring material having an absorption spectrum in the region of 700 - 900 nm. The glass plate generally is phosphate glass. The coloring material can be employed singly or in combination, the latter being adoptable for adjusting the absorption spectrum to a more appropriate region. The glass filter generally has a thickness in the range of 0.1 - 10 mm, more preferably in the range of 0.5 - 5 mm. Such glass filter can be prepared in a conventional manner. The composite filter of the present invention can be prepared by directly forming the organic film filter on the glass filter. Otherwise, the independently prepared organic film filter and the glass filter can be combined in layers, preferably, by means of an adhesive. Examples of the adhesive include epoxy resin adhesives and other known adhesives employable for combining a glass plate with a plastic film. The composite filter can be provided in the observation system for preventing the visual field from receiving the aforementioned artificial lights such as those emitted by an illuminating device, a lighting device, a display device, a television-type monitor, etc. For instance, the composite filter can be provided on transparent faces of such devices for effectively intercepting (or shielding) the light having the radiation spectrum in the region of 600 - 900 nm. EXAMPLE A glass filter (thickness: 3 mm) was prepared in a conventional manner from a phosphate glass and a cobalt salt. The absorption spectrum of the prepared glass filter is given in FIG. 3. Independently, an organic film filter was prepared by coating on a glass plate a polymer solution of 2 kg. of cellulose diacetate, 20 g. of dibutyl phthalate and 1 g. of Zapon BG First in 1.5 ℓ of butyl acetate. The absorption spectrum of the prepared organic film filter (thickness: 0.3 mm) is given in FIG. 2 as P₁. P₂ and P₃ in FIG. 2 are absorption spectra of organic film filters prepared in the same manner except that the amount of the dye was slightly decreased. The glass filter and the organic film filter (having the spectrum P₁) were combined by an epoxy resin adhesive to prepare a composite filter. The aborption spectrum of the obtained composite filter is given in FIG. 4. As is apparent from comparison between the radiation spectrum of the light emitted by stars and received by the earth (i) in FIG. 1 and the absorption spectrum of the composite filter in FIG. 4, the obtained composite filter is appropriate for intercepting the starlight at the radiation spectrum in the region of 600 - 900 nm.

Claims: A composite filter for protecting an observation system utilising starlight as the ambient light source from artificial light in the 600nm to 900 nm wavelength region of the radiation spectrum, the composite filter transmitting visible light of wavelength shorter than 600nm and comprising an organic film filter and a glass filter, the organic film filter absorbing light in the 600nm to 700nm wavelength region of the radiation spectrum and the glass filter absorbing light in the 700nm to 900nm wavelength region of the radiation spectrum. A composite filter as claimed in claim 1, wherein said organic film filter comprises a dyed cellulose derivative. A composite filter as claimed in claim 2, wherein said dyed cellulose derivative is a dyed cellulose diacetate or a dyed cellulose triacetate. Use of a filter according to any of claims 1-3 for preventing light in the wavelength region 600nm-900nm from artificial light sources, such as an illuminating, lighting or display device, from reaching an observation system including a photomultiplier tube. Use as claimed in claim 4, wherein said composite filter is provided on a transparent face of said artificial light source. Use as claimed in any of claims 4 to 5 wherein said observation system utilizes a multi-channel secondary electron multiplier.

Inventors: NAGAI, KIYOSHI
Applicants: NAGAI, KIYOSHI
Application year: 1984
Main classification: G02B5
IPC codes: G02B5