Epson 660 - Manual (Page 58)

Image 1 below is the original scanned image, while in Image 2 the threshold value is changed into 70.
This means that pixels in the image with a brightness level below 70 will all be cut into black, and those
with a brightness level above 56 are all turned into white. Therefore, you will find that by setting a lower
threshold value, some details are lost in the new image. However, if there are many unwanted dots in an
image, you can eliminate them.
To set a value you can drag the triangular slider or input a value in the text box.
Default parameters are:
Item Range Default value
Threshold 0~255 128
Image 1 Image 2