> Jim> Why does my scanner provide resolutions up to 9600 dpi when
> Jim> the default resolution for photos seems to be 100 and OCR
> Jim> 300? What use is 9600 - scanning postage stamps?
> FWIW, most consumer level scanners on the market scan with a maximum
> optical resolution of 300-600dpi. Many scanners are marketed with
> such claims as "scans at up to 9600dpi (interpolated)". This is a
> joke, false advertising, and ought to be illegal.
Yep. The problem probably is, that most law-folks do not understand
the technical details ...
> Interpolation is a process of creating a new pixel based on the pixels
> surrounding it. Interpolation can look ok at low levels, but it is
> not adding any real, captured information to your scanned image. I'm
> always surprised that the marketing folk stop at 9600dpi, they might
> as well advertise "scans at 1billion dpi (interpolated)", since
> besides disk and memory space, there's no limit to how many
> interpolated pixels can be added to a scan.
Well - maybe someone with a clue about physics might jump in, then.
There is actually a hard limit on achievable _real_ (not interpolated)
resolution. Except for some paranoid exceptions (SNOM for those that
are interested), any optical system is limited by diffraction.
That means, that you cannot resolve structures below the wavelength
of the light you are using to look at it.
Visible light ranges at around 400-700nm. Let's have a look at the
red component (longest wavelength) and assume we look at a bright red
Now there is a simple calculation:
MaxDPI = 1 Inch / MinimumDotSize
MaxDPI = 25.4 mm / 620nm = 25.4e-3 / 620e-9 = 25.4/620 * 1e+6 = 40968.
That means the marketing folks have only two numbers left, if they don't
want to advertise beyond physical limits of real achieveable resolution :
19200 and 38400 (assuming they keep going up in *2 increments).
So I assume the 9600 is chosen, because it has the nice "9" as the
first digit (you know - something beginning with a 9 must be pretty
high ... :-).
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