[filmscanners] Re: HP PhotsSmart - questions

["" <wbgilloolyjr@charter.net>]

Laurie,

You've got things oversimplified here, or maybe disconnected.

Yes, stray light inside the scanner (from having artwork not covering
the entire flatbed) is an issue and affects (reduces) contrast (but may
improve shadow detail), however this is NOT controlled by AR coatings on
the lens or flatbed.  It should be controlled by masking the sensor
(like a lens shade) and shielding lens (really like a lens shade).

If your scanner manufacturer did not pay enough attention to this, you
can completely avoid the problem by laying a black sheet of paper over
any reflective artwork (print) covering the entire flatbed.  If you are
scanning transmissive artwork you can cut a small hole in the middle of
the black sheet and place your slide/transparency under the hole.

You can do a test to see whether this is necessary in your particular
scanner by scanning something small with and without the black masking
material using exactly the same scanner settings.  If your dark areas
(from positive artwork, not negative) are lighter without the black
mask, your scanner does have an issue with light scanner inside.

The good news is that even if it does, a piece of construction paper
costing a few cents will solve the problem.

If you need to scan multiple items you will usually get less light
scanner if you stack them parallel to the movement of the scanner (so
they get scanned one at a time) than parallel to the lamp (so they get
scanned in a batch).

Mr. Bill


Laurie Solomon wrote:
>>Anti-reflection coating on the flatbed of a scanner has
>>nothing to do with uncovered parts of the flatbed and
>>everything to do with the fact that anytime the image forming
>>light hits a glass-air surface there is the potential for a
>>reflection (or backscatter) which will reduce the contrast of
>>the image (non image-forming light hitting the unexposed
>>parts of the CCD/CMOS).
>
>
> I beg your pardon.  Are you saying that the light shinning through the
> uncovered portions of the glass scanner bed does not tend to bounce off the
> flatbed bed cover and scatter so as to create the backscatter you are
> speaking of? If it does, which I have always been told was the reason why
> one should mask the uncovered portions of the bed, then it does have
> something to do with the problem and the reason why an anti reflective
> coating might be used in large commercial grade flatbed scanners.  I never
> said that it was the only reason or factor and agree that there may be many
> such factors.  But I think that your articulation is a little extreme.

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