Transcript Digitization of books in Preservation Quality

Academy
Digitizing Books in Preservation Quality
Comparison between Scanners and Digital Cameras
Thomas Ingendoh, Image Access GmbH
©2013 Image Access GmbH
Definition of Terms Merriam-Webster
Definition
Scan or picture?
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Scan|ner: a device that scans a document line by line especially
for use or storage on a computer.
Di|gi|tal cam|er|a: a camera that records images as digital data
instead of on analog film.
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Different Document Scanners
Scanner
Sheet feed scanner
Flatbed scanner
Wide format scanner
DINA4 - DINA3, letter,
legal formats, single
sheets, automatic feeder.
DINA4 - DINA2
bound documents, books,
magazines, 3D objects.
DINA2 – DINA0+
engineering drawings,
maps, newspapers.
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Different Book Scanners
Book Scanner
Book scanner
Book scanner
Up to DINA1, glass plate.
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Book scanner
Up to DINA1, glass plate,
motorized book cradle.
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Up to A3, glass plate and
scan area reach to the
edge.
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CCD Image Sensor
Technology
Line sensor versa area sensor!
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All manufacturers of scanners use line sensors to capture
documents line per line, which is called scanning.
These are well known companies like Avision, Canon, Contex,
Colortrac, Epson, Fujitsu, Kip, Kodak, Microtek, Panasonic,
Zeutschel and others.
Line sensors and area sensors use the
same CCD technology.
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CCD Image Sensor
Technology
Line sensor versa area sensor!
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Line cameras have higher resolution, better color fidelity, less
noise and no pixel defects. This is why all scanner vendors use
the same well established technology.
A few companies use digital cameras to take pictures of
documents.
A picture of a document is no substitute for a scan of a
document.
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Technology
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Advantages of Line Sensors
for Book Scanning
Real RGB pixels instead of Bayer pattern artifacts.
Significantly higher system resolution. Entry level book scanners
can have 200 megapixels (Mp) compared to 50Mp currently
available on the most expensive digital cameras.
Larger pixel size means less noise.
Concentrated, high power illumination of only the scanning area
reduces the influence of ambient light.
Dynamically changing focus to follow the curvature of an open
book is impossible with area sensors.
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Technology
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Advantages of Area Sensors
for Book Scanning
Price?
The 50Mp chip KAF 50100 made by TrueSense (formerly Kodak)
has a price tag of $ 3,600 in quantities of >10. This does not
include the necessary electronic components and the PCB.
80 Megapixel digital cameras sell for $ 20,000 and more.
A huge price reduction is very unlikely due to the large amount
of silicon necessary.
Do the camera systems, that claim to be book “scanners“,
always ship with the megapixels listed in their brochures?
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Bayer Pattern
Bayer Pattern
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Line cameras scan line by line with red, green and blue sensitive
pixels to form a perfect RBG image.
Area sensors take a picture. The red green and
blue sensitive pixels lay side by side in a
Bayer pattern.
The algorithms used to interpolate the RGB pixels are optimized
for pictures. They fail if applied to high contrast printed text such
as what is commonly found in books.
Colored edges, jagged lines and other artifacts are common
among digital cameras.
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Raw data
50Mp digicam
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High Raw Data Quality Instead
of Interpolated Data
200dpi scan with a line sensor
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Interpolation
Digicams Must Interpolate
Each Pixel
200dpi scan raw data
50MP digicam raw data
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Size Matters
Pixel Size
The larger the „film“ the better the resolution!
Line camera with 22,500 (7,500 red,
green and blue) pixels covering an
area of 100mm². One scan = 225 Mp.
Area chip with 7,300 * 5,400 pixels,
each 36mm². One picture = 40 Mp.
Cell phone camera with 2,000 * 1,500
pixels of 2mm², One picture = 3 Mp.
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Noise Reduces Resolution
Pixel Size
Larger pixels collect more photons
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Digital cameras with their small pixel size can collect fewer
photons than line scanner cameras.
Photon noise is cut in half if the pixel size (edge length) doubles.
The recommended pixel size for preservation quality scanning is
8 x 8µm minimum.
Noise always reduces resolution. Digicams reduce noise via
clever software algorithms at the expense of resolution.
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Resolution is Not Always Resolution
Resolution
Advertised specifications can be misleading!
Picture from an area sensor, 200dpi
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Scan from a Bookeye 4V2 with 200dpi
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Overall System Resolution Counts
Resolution
System resolution is not optical resolution!
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This scan has the same
resolution in every part of the
image.
Resolution is not identical to
sharpness.
Comparative tests are
necessary.
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Determining the Resolution
Resolution
Nyquist was right!
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An ideal system with 400dpi optical
resolution can only resolve 400
pixels (200 line pairs) per inch.
Only 70% of this can be achieved in
reality due to aliasing and other
artifacts.
The value at which 5 lines can still be
counted multiplied by 70 is the
system resolution.
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Controlled Light, Good Results
Light
No movie set without lighting!
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Book scanners are open systems and thus need a high intensity
light source of good quality to overcome the influence of
uncontrolled ambient light.
All book scanners move a light bar synchronously to the line
sensor, either from left to right or top to bottom.
The exposure time per scan line is < 1/1000 of a second.
Digicams operate at exposure times up to 1s. -> Camera shake
Book camera systems either do not have a light source at all or
only a weak one which illuminates the whole scanning area.
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Controlled Light, Good Results
Light
Book”scanner” with digital camera
Picture taken in the morning
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Picture taken in the evening
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Controlled Light, Good Results
Light
Minimum requirements for a professional digitization project
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External light level should be kept below 500 lux.
No direct sunlight, no light from spotlights, flood lights or other
high intensity light sources allowed on the scanning bed.
Scanning light should come from high quality LEDs and should
be above 5,000 lux in the scanning region.
A white balance must be performed at the final destination of the
scanner.
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Dynamic Focus, Best Results
Depth of Field
Depth of field
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The depth of field is the variation of the distance to the scanned
object, in which the image appears equally sharp or in focus.
The lower the overall system resolution, the larger the depth of
field.
Fixed or autofocus digital cameras have a small depth of field.
If scanning books, a dynamically adjusted focal system yields
the best results.
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Dynamic Focus
Dynamic Focus Adjustment
During Scanning
Accurate book fold correction can
only be done optically!
Dynamic focus adjustment during
scanning via laser controlled
distance measurement achieves
sharp and crisp scans all the way
into the book fold.
This is only possible with real
scanners using a line sensor. It is
impossible with a digicam using an
area sensor.
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Dynamic Focus
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Dynamic Focus Adjustment
During Scanning
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Book Cradle, Book Holder
Book cradle
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Motorized book cradle with
glass flat.
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Up to 10cm, 4” thick originals.
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Gentle treatment of books.
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Ergonomic.
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Robust and built to last.
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Book Cradle, Book Holder
Book cradle
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Open book cradle without
glass flat.
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Up to 20cm, 8” thick originals.
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Scans flat or V-shape.
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Very gentle treatment of
books.
More ergonomical.
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Buzz Words
Marketing
Line sensor:
Area sensor:
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Trilinear sensor
Quadlinear sensor
CCD line sensor
RGB sensor
CCD
 Sensors for scanners
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Area sensor
Matrix
CMOS chip
Chip
One shot
 Sensors for digicams
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Conclusion
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Minimal Requirements for a High
Quality Digitization Project
Line sensor with at least 8 x 8µm pixel size.
High quality light source > 5,000 lux.
Optical resolution at least 400 dpi.
Total system resolution at least 5 lp/mm everywhere on the scan.
Book cradle for gentle treatment of valuable, fragile books.
Dynamic focus for scanning with and without glass flat.
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Digitizing Books in Preservation Quality
Thank you very much for
your attention!
Thomas Ingendoh,
CEO
Image Access GmbH
Learn more at
www.imageaccess.de
www.imageaccess.us
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