1. Field of the Invention
The present invention relates to a document reader for optically reading a document to be converted into an electric signal and a method for adjusting its focus.
2. Related Art
A document reader is known which optically reads a color document by an image sensor of a storage-type charge transfer device such as CCD and color-decomposes the document thus read to acquire image data.
FIG. 13 is a schematic diagram of a conventional document reader in a mirror movement system. A document reader, generally 100' includes a reading unit 1 for optically reading a document to be electrically converted into multi-value (or binary) data, a data detecting unit 2 for detecting an output data from said read unit 1, a controlling unit 3 for receiving the detected data from the data detecting unit 2 to produce an image data and controlling the movement of a mirror carriage 12 and mirrors 15, 16 arranged in the reading unit 1, and an interface 4 for outputting the image data. The image data is supplied to an external image data processing unit 6 such as a PC (personal computer) through the interface 4 and a cable 5. Incidentally, in many cases, the controlling unit 3 is configured by an MPU (microprocessor having a CPU, RAM and ROM).
The reading unit 1 includes a document stand 11 on which a document or original is placed, a mirror carriage 12 which is a pair of a light source 13 and a mirror 14 used to scan the document optically; mirrors 15, 16; a light-gathering lens 17, a CCD 18 and a stepping motor 21 for driving the mirror carriage 12 and mirrors 15, 16.
In FIG. 13, assuming that the distance between a face of the document and the mirror 14 is a, that between the mirrors 14 and 15 is b, that between the mirrors 15 and 16 is c, and that between the mirrors 16 and the light-gathering lens 17 is d, and a+b+c+d=L, even when the mirror carriage 12 and mirrors 15, 16 move in a sub-scanning direction (direction of arrow 28) in document reading, the total L of the optical path from the document face to the light-gathering lens 17 is constant.
Specifically, after the mirror carriage 12 and mirrors 15, 16 have moved, the respective distances become a, b' c and d'. In this case, the mirrors 15, 16 move in the sub-scanning direction in synchronism with the movement of the mirror carriage 12 so that a+b'+c+d'=L, and the moving distance of the mirrors 15, 16 is equal to the moving distance of the mirror carriage 12.times.(1/2). For example, by setting a gear ratio for 2:1, the moving distance of the mirror is determined. Additionally, the light-gathering lens 17 and CCD 18 remain fixed and does not move in response to the scanning of the document.
As described above, the document reader 100' is designed so that the total L of the optical path is constant. In this case, L commonly contains a permissible error E. If the sum .delta. of errors is larger than the permissible error E, an adjustment was made in only fabrication so that .vertline..delta..vertline..ltoreq.E.
However, with an improvement of an image processing technique, a high resolution has been demanded for the color document reader so that the range of the permissible error E has been limited. Therefore, it has become difficult to absorb such an error only by techniques in design and fabrication inclusive of an improvement in material and accuracy of a motor of moving the mirrors and others. As a result, the requirement of high accuracy in the document reader has led to necessity of allowing for an increase in the production cost.
For example, in the case of the color document reader, the permissible error of E=2 mm or so has been traditionally required.
At the present time when the resolution has been improved, however, E.ltoreq.0.5 mm, preferably E.apprxeq.0 is required.
In the reader in which the document to be read is in contact with the document stand 11, the distance a between the document and mirror 14 has been regarded as constant, thus providing no problem.
On the other hand, in a flat bed type document reader as shown in FIG. 13 in which the document is a negative or positive film, the film, while it is held in a holder as shown in FIG. 5, is scanned.
In this case, a gap h (2-3 mm) due to the thickness of the holder is produced between the film and the document stand 11. This led to a problem of making it more difficult to realize the permissible error E=.delta.+.alpha..ltoreq.0.5 mm. If the thickness of the holder is constant, this problem can be solved in hardware. However, in many cases, unfortunately, there was an inconvenience that the thickness of the holder varies according to manufactures and/or kind of the film to be held by the holder.