1. Field of the Invention
The present invention relates to a film image information reading apparatus (ordinarily, referred to as a film scanner) for reading image information of a film by a photoelectric conversion element and, more particularly, to the adjustment of a gain of an output level of the photoelectric conversion element when the image of a negative film is read.
2. Related Background Art
Hitherto, there has generally been known a color scanner (color image information reading apparatus) in which various color information of a color document is separated into three primary colors of, e.g., R (red), G (green), and B (blue), and three kinds of image signals corresponding to the respective color separation images are obtained from the photoelectric conversion element, and the color information of the color document is reproduced by synthesizing those three kinds of image signals.
However, if a film such as photographic film, microfilm, or the like is used as a document, in general, the density range of the film is fairly wide to be above 3. In particular, if a self scan type sensor such as a CCD (charge coupled device) or the like is used as the photoelectric conversion element of the color scanner, the density range of the film is fairly wider than the dynamic range of the sensor. Therefore, the image signal read by such a sensor is largely adversely influenced and a problem occurs. Practically speaking, there is such a problem that the gradation of the portion having a high or low density of the film image is not reproduced.
On the other hand, if a color negative film is used as a document, as shown by film characteristic curves of FIG. 2, the density of each layer of R, G, and B in the unexposed portions is considerably large. Therefore, it will be appreciated that the color negative film is constituted in such a manner that the negative image is reproduced on the orange base portion. In other words, in the case of the negative film the mixed image information consisting of the true image information and the color information of the film base portion which is unconcerned with the image information is read as the color image signal. Therefore, the dynamic range of the sensor is not effectively used. In addition, in the case of the negative film, the negative image is formed. When the negative image is reversed to the positive image, the density of the base portion is reduced, so that the density information of the base portion is quite the meaningless image information. Further, the orange components of the base portion of the document slightly differs in dependence on the film maker, kind of film, or developing state of the film, causing a problem when the accurate color is reproduced.