1.  Field of the Invention
The present invention relates to an apparatus and a method that are provided to control a radiographic apparatus configured to capture a radiographic image. 
2.  Description of the Related Art
Generally, radiographic imaging apparatuses including an image-pickup unit configured to pick up a radiographic image has been used to perform medical radiography, industrial nondestructive radiography, and so forth. 
For using the radiographic imaging apparatus while maintaining the initial performance capabilities thereof, the characteristics of the image-pickup unit should be calibrated, where the characteristics are changed over time.  Usually, a calibration operation has been performed by a user (e.g., a radiologic technologist working in a hospital).  Further, the frequency of performing the calibration operation has been determined under an operation rule established for each facility under present circumstances.  That is to say, the calibration operation has been performed at the opening time every day, or once every half a year and/or year under the operation rule established for the facility. 
In the following case, a correction image used to correct gain variation is acquired, as an exemplary calibration operation.  For acquiring the above-described correction image, the user arranges the radiation source (an X-ray tube) and the image-pickup unit at appropriate positions in the first place.  When the radiation source and the image-pickup unit are arranged at the appropriate positions, the entire image-pickup unit is irradiated with a radiation emitted from the radiation source (the X-ray tube), for example.  Next, the user sets a tube voltage and/or a tube current related to the occurrence of radiation to an appropriate value, and actually irradiates the image-pickup unit with radiation.  At that time, the irradiation is performed without placing any object (subject). 
The image of the dose of radiation detected during the above-described irradiation is picked up as image data and subjected to image processing.  Consequently, the correction image used to correct the gain variation is acquired. 
Here, an image captured without placing any object (subject) is generally referred to as a gain image and/or a white image (hereinafter referred to as the gain image). 
A calibration method used for the above-described image-pickup unit is disclosed in Japanese Patent Laid-Open No.  2001-351091.  Namely, when capturing and acquiring the gain image without placing any object (subject) at the calibration time, a plurality of the gain images (approximately four gain images in most instances) is acquired, so as to reduce random noises included in the gain images.  Namely, the plurality of gain images have been acquired and averaged so that the random noises included in the gain images are reduced.  The reduction of the random noises, which is attained by the averaging, should be performed to generate a correction image with high precision. 
The above-described correction image is used as a correction image used to perform the gain correction when the image of an object (subject) is captured under normal conditions, for example.  More specifically, the captured images are omitted by using the correction image, so as to correct the gain variation between the captured images.  Therefore, when the gain correction is performed for a captured image by using a correction image with low precision (including many random noises), artifacts are left in the corrected captured image.  In that case, therefore, it becomes difficult to obtain an appropriate captured image.  Accordingly, a correction image with high precision should be generated at the calibration time. 
Thus, according to known technologies of generating the correction image, the random noises have been reduced by acquiring and averaging a predetermined number of captured gain images.  However, for generating a correction image with higher precision, consideration should be given to image-capturing mode used at the calibration time (e.g., the radiation dose), because the amount of random noise included in the gain image varies with image-capturing modes.  According to the known technologies, therefore, the number of times the image capturing is performed becomes unnecessarily large, for acquiring the correction image, so that the life of the radiographic apparatus is often reduced.  Further, the number of times the image capturing is performed often becomes so small that appropriate correction precision is obtained with difficulty. 
Further, the number of times the image capturing is performed to obtain four gain images becomes four or around under normal conditions.  However, the time number four is often manually set by a user at the calibration time.  However, the user does not know whether the time number four is sufficient for reducing the random noises included in the gain image, and what should be used as a guide to set the time number.  In the past, therefore, the time number has been determined and set under empirical rule established by the user.  Accordingly, the present invention has been achieved to set the number of times the image capturing is performed, the time number being appropriate to perform calibration for the radiographic apparatus.