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Digital Conversion Methodologies


Quality Control
Histograms
File Formats
Hardware/Software

Digital Conversion Methodology: Quality Control

To assure high quality digital files, each image will be evaluated twice. The scanning technician will examine the histogram at the time of capture and creation of the master file. At random, the master files are examined by the curator to assure that all files meet the following criteria:

File Format: Uncompressed TIFF files. A TIFF file is currently the most flexible file format for long-term storage of digital images. TIFFs can be turned into any other file formats you require, for example, JPEGs or GIFs for World Wide Web viewing.

Resolution: The initial scans will be 800ppi uninterpolated, although resolutions as high as 3175ppi uninterpolated are available. These higher resolutions are most helpful for very small originals and images that are heavily used and most important intellectually.

Histogram:

Clipping & Spiking: Histograms offer the best view of tonal range captured by the scanner and will be examined for clipping and spiking as well as for the correct setting of white and black points.

Clipping and spiking result when the white and black points are not set on true white and black during the set up of the scan. If white and black are improperly set, everything above or below those points is "clipped" or registers as the same tone.

Spiking on the ends of the histogram usually indicates clipping. This problem also shows up in the image itself as blockage and pixelization in the shadows and blowouts in the highlights. Acceptable spikes can occur if the edge of the original negative has lost emulsion, for example, or the sky holds no detail and is one tone in the original. Such instances, however, are rare.

Histogram:

White & Black Points: Optimum placement of whites and blacks is best observed through the histogram, although the image itself will be examined without the histogram on the screen. It is important to look at the number value assigned to the brightest highlight and the darkest shadow. Highlights should not read a number value higher than 247 and shadows should not be less than 7 or 8. If these numbers are exceeded the scan must be redone. This is particularly vital if the original image has a short dynamic range. The white and black points must not be set on 0 and 255, as this will stretch the dynamic range of the image, creating gaps in the histograms, and thus unusable scans.

Files failing to meet the above criteria will be rescanned. No adjustments in PhotoShop™ or other image manipulation software will be allowed to correct mistakes made in the scanning process.

Once the master files pass quality control, the thumbnail and reference files will be created. The technician will need to make some adjustments to contrast and possibly cut back frame edges when creating these trimmed-down files. Because of the manipulations necessary in PhotoShop™ to create snappy images for on-screen viewing, the thumbnails and reference files will undergo a more limited set of quality control procedures. The technician will examine the histogram for excessive gaps and check the shadow areas in the image for pixelization. When the technician is satisfied that the files will pass quality control, the curator will examine a random selection and then mark the files for copy onto the final storage media, CD-ROM.

 

Digital Conversion Methodology: Format

The initial scan captured from the original materials, called the master file, will be an uncompressed TIFF with a resolution of 800ppi, and approximately 35 MB in size for a grayscale scan of a 6x8 inch glass plate negative and 100MB for an RGB scan 6x8 inch color originals. Trimmed files will be JPEGs. File names will remain constant with extensions differentiating thumbnails, reference, and master files. The reference file will be comprised of approximately 600 pixels wide with a resolution of 300ppi. The thumbnail will be approximately 150 pixels wide and have a resolution of 72ppi.

Master: The file created from the initial scan of the original material. These digital masters will be burned on CD-ROMs and put on a refreshment cycle of three years. The archival files will be used to create the reference files and thumbnails.

Minimum Resolution: 800 pixels per inch
Format: uncompressed tif

Reference: The full-screen view of the image created from the archival file. This file will be accessed after a user selects the thumbnail. The reference files will be used for electronic access only on the World Wide Web and on the Nebraska State Historical Society's in-house patron computers.

Resolution: 150ppi (600 pixels wide)
Format: high quality jpg

Thumbnail: The smallest image file created from the archival file allowing the user to view several images on the screen at one time. Selecting a thumbnail prompts the access of the reference file for that image.

Resolution: 72ppi (150 pixels long dimension)
Format: high quality jpg

 

Digital Conversion Methodology: Production

Hardware:

Creo/Scitex EverSmart Supreme™ Scanner: The EverSmart Supreme™ is a high resolution flatbed scanner that utilizes an 8000 element, anti-blooming CCD. It scans on both the X and Y axis enabling extremely high resolution over the entire image bed.

Resolution: up to 5,600 ppi optical resolution uninterpolated
Dynamic Range:
4.0
Maximum Density:
4.3
Tonal Depth: 48 bits per pixel [16 bits per channel, RGB]
Input Modes:
Reflective, transparency, negative, positive, color, B&W, line art, printed material up to 12 x 17 inches.
Original Thickness:
Transparencies up to 3/16 inch, reflectives: unlimited.

Creo/Scitex EverSmart Pro™ Scanner: The EverSmart Pro™ is a high resolution flatbed scanner that utilizes an 8000 element, anti-blooming CCD. It scans on both the X and Y axis enabling extremely high resolution over the entire image bed.

Resolution: up to 3,175 ppi optical resolution uninterpolated
Dynamic Range: 3.7
Tonal Depth: 42 bits per pixel [14 bits per channel, RGB]
Input Modes: Reflective, transparency, negative, positive, color, B&W, line art, printed material up to 12 x 17 inches.
Original Thickness: Transparencies up to 3/16 inch, reflectives: unlimited.

Phase One FX Digital Camera: The Phase One FX is a high end digital scan back camera ideal for capturing digital images from materials too large for the EverSmart Pro™ image bed or too fragile to be placed upside-down on the flatbed scanner. The FX fits on a Toyo 4x5 view camera.

Resolution: up to 10,500 by 12,600 pixels, totalling 396 million color pixels
File Size: uninterpolated files of 400 MB
Tonal Depth: 36 bit, RGB

Power Macintosh G4™ Workstation:

400 MHz PowerPC 604e Processor
512 MB RAM
58 GB Internal Hard Drive
56 GB External Firewire Hard Drive
CDRW drive
Internal Zip Drive
OS X

Monitors:

Apple Studio Display - 17 inch Flat Panel
ViewSonic PT810™ - 21 inch

Software:

Adobe PhotoShop 7™ Imaging software. This will be used primarily as an evaluation tool, but also for slight contrast changes or trimming of reference files and thumbnails for screen viewing only.

Roxio Toast 5 Titanium CD mastering software. Used to create two sets of master CDs, as well as backup CDs for the reference and thumbnail files.


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Last updated 22 May 2003

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