Re: "Older" scanner better than new!

[Rob Morley <nospam@xxxxxxxxxxxx>]

On Mon, 28 Jul 2008 01:00:03 +0100
dave <dave@xxxxxxxxx> wrote:

> Just an observation more than anything else, but when my "old"
> parallel interface scanner packed up I replaced it with a Packard Bell
> usb one that worked fine. However, with my old scanner I could use it
> to scan decent pics of 3D objects. i.e. it was able to focus on parts
> of an object a cm or so above the top of the scanning glass. The new
> one however fails do this.

It depends on whether the sensor is CIS or CCD.  A CCD scanner is like
a digital camera (except its sensor array is only one pixel wide and
it uses mirrors to get the image to the lens) and as it has similar
optics it has depth of focus - objects not actually touching the glass
bed but within an inch or two of it still appear reasonably sharp.  CIS
doesn't use lenses, the sensor is directly below the glass bed and each
cell captures the light reflected from the object immediately above it
- as soon as the object is raised a little the adjacent cells also
catch some of that reflected light and the captured image becomes
blurred.