Dave Plowman (News) wrote:

The problem is scanning old car wiring diagrams out of the handbook, etc.
If I do this in pure B&W - regardless of resolution and other settings, I
get a poor quality image. Scanning in grey gives excellent results -
except you can now see the paper 'colour' which often varies according to
how it has been exposed to daylight etc in use.

The scanner driver setting should allow you to select the gamma,
brightness, contrast and the threshold (the cut-off point between white
and black). Your photo-editing application should also allow these
parameters to be adjusted.

Photoshop certainly does. Try using the "levels" feature which permits
you to drag sliders around to set the white level, black level and the
midpoint. This will allow you to edit out the grey cast from the white
paper. Once you have done that you can use Photoshop to resample the
greyscale image into your desired B&W resolution. As a rule of thumb
resample B&W to about four times the resolution of the input greyscale
image.

However, I'd stick with greyscale, once you have mastered the tricks
used for tweaking the output you can get a much better image than you
can using B&W - for example on line drawings B&W can look sharper but is
more jagged. Greyscale can provide an anti-aliased image in which fine
lines and curves are reproduced well on a good colour printer.

You can also get PS to convert greyscale to a clean B&W image. It takes
a bit of playing about with and experience but it's fairly easy to do
once you have mastered editing the threshold and curves and have played
around with the colour mode a few times. I'd make a small test scan of
the sort of image and piddle about with that to get a feel for the
controls.