The present invention relates to an illuminating light source apparatus for a photographic printer. More particularly, the present invention relates to an illuminating light source apparatus which enables an improvement in the light source efficiency of a photographic printer, and consequently allows an increase in the quantity of light usable for the printing process.
In a photographic printer (hereinafter referred to as simply "printer"), as shown in FIG. 4, light that is emitted from a lamp S is converted into a parallel or convergent beam of light by a reflector R and the light is then diffused through a first diffuser D1, a mirror box M, and a second diffuser D2, thereby producing diffused light that is required as a printer light source. Then, the diffused light is applied to a film F to print an enlarged image of the film F on photographic paper P through a lens L, whereby blemish or dust on the image recorded on the film F is made inconspicuous.
A conventional light source for a printer has a structure such as that shown, for example, in the sectional view of FIG. 5. The reflector R has a reflecting multilayer film ML deposited on the inner surface of a glass substrate G. The reflecting multilayer film ML has such spectral transmittance characteristics that, among light that is emitted from the lamp S, only a light component in and near the visible region, which is useful for printing, is reflected, as shown by the solid lines in the figure, whereas infrared radiation, which makes up most of the light emitted from the lamp S, is transmitted and thus removed. FIG. 6 shows an example of such spectral transmittance characteristics of the reflecting multilayer film ML.
A reflecting mirror that separates and removes the infrared component from light emitted from a light source lamp as described above is called "cold mirror". Arrangements in which such a cold mirror is disposed behind or in front of a light source lamp have been proposed in Japanese Utility Model Application Post-Examination Publication Nos. 49-26665 and 50-2016, and Japanese Patent Application Unexamined Publication (KOKAI) No. 59-79236.
The reason why infrared rays among light rays emitted from the lamp S are not used for photographic printing is that, if infrared rays are absorbed by the film F and consequently the temperature of the film F rises, the density of dyestuffs constituting the image of the film F undesirably reduces. This phenomenon is sometimes called "thermotropy". Owing to the phenomenon, when a multiplicity of prints are produced from the same film image, the hue of the finished prints gradually changes. This is unfavorable for color prints, in which importance is attached to the stability of color tone. It is necessary in order to suppress such phenomenon to cut off infrared rays so that no heat will be generated from illuminating light that is applied to the film F. With regard to the spectral sensitivities for three colors of color photographic paper P, the spectral sensitivity for red covers a considerable part of the infrared region. On the other hand, the spectral sensitivity of a photodetector for measuring light passing through the film F, which is used for exposure control, does not cover the infrared region. Accordingly, if exposure control is effected with the spectral sensitivity difference between them left as it is, it is impossible to detect that the color photographic paper P is being exposed to infrared rays which cannot be detected with the photodetector. Therefore, exposure control cannot accurately be effected. In order to allow the spectral characteristics of the color photographic paper P and the photodetector to match each other, it is necessary to exclude infrared rays from the illuminating light source. For the above-described reasons, the infrared component is separated and removed by the cold mirror ML, etc.
In the conventional printer light source in which the infrared component is separated and removed from light emitted from the lamp S by using the cold mirror ML, as described above, the infrared radiation, which accounts for 80% or more of the energy emitted from the lamp S, is all dissipated as thermal energy. With regard to the remaining visible light also, only a part of it is used for printing. The transmitted infrared rays are absorbed by an infrared absorbing object (not shown) which is present around the reflector R, and thus escape in the form of heat. Accordingly, a fan for cooling the object around the reflector R is also required. For this reason, the conventional printer light source has been unavoidably large in size and costly.
The present applicant has proposed some techniques concerning diffusers in order to improve the light source efficiency, which has heretofore been inferior as described above, in Japanese Patent Application Unexamined Publication (KOKAI) Nos. 1-298337 and 2-278243, and Japanese Patent Application No. 6-66049.
Incidentally, there has been proposed a halogen lamp with an infrared reflecting film, in which, as shown in the sectional view of FIG. 7, infrared rays emitted from a filament FM of a lamp S are reflected back to the filament FM by an infrared ray reflecting film RL provided on the inner surface of a glass tube which constitutes the lamp S, thereby utilizing the reflected infrared rays for heating the filament FM (see "Nikkei Electronics", Jun. 24, 1991, P.145). The proposed arrangement enables infrared rays emitted from the lamp to be reduced by about 40%, and also makes it possible to reduce the consumed electric power by about 15%.
However, if a light source that employs such a halogen lamp with an infrared reflecting film is applied to a photographic printer, the following problems arise:
(1) The infrared reflecting film must be deposited on the inner surface of the tube of the lamp, and it is extremely difficult to produce a reflecting film having precise spectral characteristics on such a curved surface by vapor deposition, and it is also difficult to obtain the required spectral characteristics with satisfactory accuracy. PA1 (2) Since the infrared reflecting film is formed on the inner surface of the tube of the lamp, it is an integral part of the lamp itself. Consequently, when the lamp is to be replaced with a new one because its lifetime has expired, the infrared reflecting film must also be replaced together with the lamp. Thus, since the lamp which is to be replaced is per se costly, the lamp and the infrared reflecting film constitute in combination an expensive expendable article. Strictly speaking, the spectral characteristics of the reflecting film of the lamp replacing the old one are different from those of the former reflecting film. Therefore, the color components of light rays emitted from the old and new lamps are also different from each other.
Particularly, the problem (2) is fatal to a printer light source which needs to replace the lamp frequently.