select image type through the mode from the screen of avision twain user interface for your own purpose. please see figure 44 as below:

   

    figure 4-4 image type

    each image type is described as follows:

    line art

    line art displays the image only in black and white.

    there are no intermediate shades of gray in between.

    halftone

    halftone simulates different shades of gray by varying the threshold value of a matrix.

    gray (1024 levels of gray scale internally)

    scanner recognizes each point in the image as a shade of gray (continuous tone). 10-bit gray gives more detailed information about the image.

    true color (30 bits)

    color image have continuous tones and a broader range of colors.

    256 colors

    the 256 colors image provides 256 levels of color hues.