REFRACTION IN SCANNING
Definition
Importance
to Scanning
  • Refraction is the bending of light as it travels from one medium to another with a different refractive
    index.  (Fig 1)
  • Refraction is what makes photography and the photographic lens possible, but is also a disadvantage
    when it occurs as an unwanted add-on to the scanning process. The images of light striking the convex
    and concave lens are examples of refraction with a purpose.
Effects of
Unwanted
Refraction
  • As an unwanted add on in scanning Refraction causes the image light rays to de-collimate or so  to
    speak, to spray out slightly losing sharpness.
  • Film scanners and Film-Flatbed Dual scanners have an inherent technical advantage over the
    ordinary flatbed scanner.  This disadvantage of the flatbed scanner compared to the Dual counterpart
    is compensated by the fact that the optics of the flatbed are not subject to dust. The drawer of Dual
    scanners exposes the optics to dust every time it is open to insert film.
Effect on the
Image
  • Refraction degrades the image since not all light rays are perpendicular to the glass, and instead of
    pinpoint landing on the sensor as a perpendicular light ray would, it is scattered over several sensors.  
  • This corruption of the original image impairs resolution, contrast and saturation. .
REFRACTION
AND THE
FLATBED
SCANNER
  • With flatbed scanners the image travels through the glass bed and is refracted by it, on the way to the
    sensors; such refraction is unavoidable.  
  • However, additional refraction caused by the glass used to make fluid mounts need not involve
    additional refraction. If the glass in the fluid mount faces the light rather than the sensors, additional
    refraction is avoided.
  • Epson V 750 scanners are provided with a wet mount tray which require that the film be placed on top
    of the glass when wet mounted and incur double refraction because the glass is in the path to the
    sensors. The In additional chunk of glass of the Epson tray effectively subtracts some of the benefits
    gained by fluid scanning.
  • ScanScience's simple answer to the problem is to provide an adapter that avoids glass between the film
    and the sensors.  The fluid mounting plate faces the lamp, with the film being placed under the glass.
    The image going to the sensors is untainted by additional refraction from the fluid mounting plate.
  • In Flatbed scanners like the Epson V750, the convenience of use plus the high resolution for Medium
    and Large format film is why many artists use it in place of drum scanners.
THE FILM
SCANNER
Avoids
Refraction
  • Film Scanners have the inherent advantage over flatbeds that they are not refraction limited.
  • The absence of the extra glass, potentially gives film scanners an edge.
  • This advantage is retained with fluid scanning because the placement of the glass on the side of the
    light NOT of the sensors.
  • The inherent advantage of the Film Scanner does not help it overcome one of its shortcomings: film
    curling, film unflat-ness and loss of focus.
Resolution
and
Sharpness
in Flatbed
Scanners
    Flatbed scanner resolution is specified as fixed horizontal and variable vertical.
    The ability of a scanner's optical system to resolve in both directions is fixed of course. The optical
    system consists of a light source, lens and sensor unit or CCD . So why is the vertical resolution greater
    than the horizontal for most flatbed scanners ?

    Manufacturers take liberties with resolution figures, so instead of real resolution, the figures really mean
    how many pixels /unit area the unit is set to deliver. In other wors a pixel taken is a pixel resolved, would
    be the creed. Not so. The ' stepping motor ' is part of the pixel tking but it is blind, and cannot resolve it.
    It can stop for a pixel being taken as it moves the sensor in a vertical direction. A shot can be taken at
    various vertical distance intervals, thus, the more frequently it stops to take a reading, the more pixels
    will be produced.  This greater number of pixels can result in much larger files, but  the scans thus
    produced may not be sharper, albeit it may be argued there are technical reasons why they should be
    smoother.

    Users frequently compare scanner's resolution based on the figures specified by the scanner
    manufacturer, and that offers manufacturer the incentive to hype the figures.

    Based on the manufacturer's resolution figures a user may misguidedly rate the resolution of a flatbed
    scanner as greater than that of a film scanner.  However, the 4000 dpi given for a film scanner are of
    real 4000 resolved pixels, while those given for the flatbed scanner are for pixels only,  resolved or
    otherwise.
    The reality is that if a flatbed scanner can achieve a resolution of 3200 dpi or even 2400 dpi on a MF or
    LF film this is excellent.
Courtesy of WikiSpaces. The excellent
treatment of the subject and quality of the
images deserves full recommendation.
Thanks to the educator that posted this
wonderful teaching tool.
Link:
https://chemicalparadigms.wikispaces.com/Un
it+2+Refraction+of+light
Fig.-1-
Fig. -2-
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Fig. -4-
3-Tech-Refraction
4-Tech-Newton
ScanScience
Fluid Scanning Technology
UNWANTED REFRACTION IN FLUID SCANNING