Image reading apparatus capable of eliminating moire on images

Abstract

An image reading apparatus which includes a primary scanner for scanning an original X-ray image obtained using a grid and recorded on a recording medium with light in a primary scanning direction. A clock signal generator generates clock signals having a frequency N times as high as a frequency of pixel clock signals, wherein the pixel clock signals correspond to a desired pixel size, and wherein N is a positive integer. A converter converts a light beam from the recording medium into electric signals by a scanning operation of the primary scanner, and converts the electric signals into digital signals based on the clock signals. A low pass filter is provided which has at least one set of filter characteristics having a cut-off frequency between 1/3 and 2/3 of the frequency of the pixel clock signals for filtering the digital signals converted by the converter. A thinning mechanism thins out the filtered digital signals to 1/N in the primary scanning direction, and a memory stores a plurality of the filtering characteristics of the low pass filter. The plurality of the filtering characteristics includes the cut-off frequency of 1/3 of the frequency of the pixel clock signals, and the low pass filter has a filtering characteristic which is selected in relation to a grid density of the grid and at least one of the desired pixel size and the frequency of the pixel clock signals.