Lineart vs Greyscale scans for text

When faced with creating artwork a black and white book from a previous print that has no digital artwork available, the fastest and cheapest option is to scan the book. This assumes that there are no changes to the text, that the book is for print only, and that the client has allowed the spine to be cut off of the book so that the pages can scan through a document feeder. If the book is text only with no halftone, I would recommend the scans end up as 1200 dpi linearts. This is fine if the book is text only, but if the text contains images such as photographs, then two scans are required – one for the text (linearts), and one for the images (300dpi greyscales). The two sets of scans then have to be combined by placing the images into InDesign.
One might ask “why not use greyscale scans at 1200dpi”? Apart from the filesize when printing – the text will look terrible. To understand why high resolution doesn’t always equal better quality, the answer lies in one process: Rasterising.
Regardless whether a greyscale scan is 300dpi, 600dpi or 1200dpi, the scan will still have to go through the rasterising stage on the copier where a filter is applied to make the shades of grey that the artwork may contain. This is usually done with a halftone filter in the copier’s RIP software. This is where the images are converted into halftone dots – measured in LPI – Lines (of dots) Per Inch.
Using a 150lpi halftone filter, here is what happens when the same images are rasterised (click on the image to see at correct size):
The 600dpi and 1200dpi images do look better than the 300dpi, but the type is still not sharp, and looks bumpy. This is because of the halftoning that is occurring in the RIP. Despite the resolution the text was scanned, the bumps on each image are in the same spots in each scan (though the severity of the “bumps” is different with the dpi)
Lineart images are different in that there are no shades of grey, and do not have halftone applied to them. So once passing through a 150lpi halftone filter, here is how the images look after being rasterised (click on the image to see at correct size):
Here, the difference in dpi does matter, as the 1200dpi lineart is sharper than the 600dpi lineart, which is definitel­y sharper than the 300dpi lineart
Here is the side-by-side comparison (click on the image to see at correct size):
So by creating the book with separate scans for the images and type, the quality will be greatly improved, but will take longer to set up.