Using Kodak PhotoCD Technology for Preservation and Access:
A Guide for Librarians, Archivists, and Curators
Anne R. Kenney and Oya Y Rieger
Department of Preservation and Conservation, Cornell University Library for
New York State Education Department, Program for the Conservation and Preservation of Library Material

FORM 3
 

Part III: Image Quality Assessment Survey

Appendix A: Hardware and Software Required

1. A computer with minimum 36 Mb RAM (486 for PC, and 68030 or 40 for Mac) and a Photo CD compatible CD-ROM drive.

2. Ideally a 17" SVGA monitor set to1024 x 768 pixels that is capable of displaying 24-bit color and supporting a 72Hz refresh rate (you will need 3 Mb VRAM for 1024 x 768 pixels). If your monitor does not display 24-bit color, then drop the monitor setting to 800 x 600 pixels (you will need 2 Mb VRAM for 800 x 600 pixels).

3. An image viewing software program (preferably Adobe PhotoshopVersion 4.0 [1]) with the ability to:

Please note that when using Adobe Photoshop to view Kodak Photo CD images, You should use Kodak Photo CD Acquire Module plug-in so that Photo CD colors are mapped correctly. Directions on how to download and install this free plug-in are provided in Appendix C (Opening Images with Kodak Photo CD Acquire for Adobe Photoshop).

If you do not have access to Adobe Photoshop, you can locate another compatible viewing software through the following web sites:

Photo CD compatible freeware and shareware utilities:

http://www.public.iastate.edu/~stark/gutil_sv.html

Photo CD compatible commercial software:

http://www.kodak.com/digitalImaging/aboutPhotoCD/aboutPCD9a.shtml

4. Ideally a color viewing booth, such as The Judge from Macbeth [2] , for visual evaluation and comparison of original documents and prints from digital images. If you do not have access to a color booth, use natural daylight fluorescent bulbs [3] to control the lighting of your viewing environment.

5. Desired but not required is a dye sublimation printer such as Tektronix Phaser 440 or Kodak Digital Science 8650 Color Printer with minimum of 32Mb RAM


Appendix B: Recommended Viewing Conditions

An important pre-requisite for image evaluation is a controlled viewing environment. Please take into consideration the following recommendations during your image quality assessment.

The monitor and the original document need separate viewing environments. The original is best viewed in a bright surrounding, and the monitor in a dimmed surrounding. However, "dimmed surrounding" does not indicate a dark room. Viewed in the dark, the image on the monitor will appear to decrease in contrast.

To create an ideal setup:

The idea is not to have any light source overpower the brightness of the monitor, without making the room dark. With this in mind, you may want to preserve some distance between your viewing space for examining the original and the monitor. You should give your eyes some time to adjust when switching between the two environments. We also suggest that you eliminate any reflections on the monitor. When conducting your evaluation, it is ideal to wear gray colors.

If you are using a viewing booth (such as The Judge), the document should be tilted to eliminate reflections and glare from the lights in the booth. Set the lighting of the viewing booth at Noon Sky Daylight at 5000 Kelvin (ANSI standard) . If you do not have a viewing booth, place the original document in a position to minimize reflections and glare, preferably under a lamp using a natural daylight fluorescent bulb.


Appendix C: Opening and Printing Images Using Kodak Photo CD Acquire for Adobe Photoshop

To ensure correct mapping of Photo CD colors, it is necessary to install Kodak Photo CD Acquire Module plug-in, which allows you to select, customize, and import Photo CD images into Photoshop utilizing the KODAK Precision Color Management System (KPCMS). The utility of Acquire is further elaborated in Appendix D (Color Quality Assessment).

Kodak Photo CD Acquire Module plug-in for Adobe Photoshop can be downloaded free of charge for both Macintosh and Windows PCs at the following web site:

http://www.kodak.com/productInfo/technicalInfo/pcdAcquireDownload.shtml#acquire

We recommend that you use Version 1.0 for Windows and Version 2.2 for Macintosh. The later versions of the Acquire plug-in require the identification of source (type of film used in photography) and therefore introduce the characteristics of the film in displaying the image. During the image evaluation, our goal will be to match the image to the original document not to the film used in photography.

After downloading and installing the software, verify that the software files are located in the following directory (assuming that C is your hard drive):

C:\\Adobe Photoshop\Import\Plugins

Once you have downloaded and installed the Photo CD Acquire Module, start Adobe Photoshop and follow these steps to retrieve Kodak Photo CD images:

  1. From the File/Import menu, choose Kodak Photo CD Acquire.
  2. Kodak Photo CD Acquire menu will be displayed (Figure 1).
  3. Choose your CD-ROM drive and find the Photo CD directory that has the images (generally named photo_cd or image directory). Depending on the file structure of your Photo CD, you may need to click on the directory name to display the sub-directories and the files included in them. Note for Macintosh users: do not open the photos directory.
  4. Choose the image you wish to retrieve by highlighting the file name. A preview of your image will be displayed on the left bottom corner as seen in Figure 1.
  5. Select 16 Base resolution if you will be evaluating the resolution or printing the image. For color evaluation, you may prefer a lower resolution image (such as Base or 4 Base) to speed up image retrieval.
  6. Select from a series of image conversion parameters by setting the Metric option at Portfolio, which is the last option in the scrolling list.
  7. Click OK to accept the settings.

Note:
Sharpness and detail are often lost in the printing process and artificial sharpening is usually employed before printing to produce acceptable results. The Kodak Photo CD Acquire has a built-in Sharpening option for this purpose. However, as sharpening will affect your perception of image quality, do not sharpen your images during your image evaluation.

FIGURE 1: Photo CD Acquire Module, Version 1.0 for Windows


Appendix D: Measuring Resolution

Resolution is one of the indicators of evaluating image quality. As a general rule, the higher the resolution of an image, the greater its clarity and definition. The scanning array for Kodak Photo CD is 2,048 by 3,072. This translates to a scanning resolution of 2,238 pixels/inch for 35 mm film. Effective resolution achieved via Kodak Photo CD depends on two other factors:

If there is a perfect alignment (i.e., the aspect ratios of the document and the scanning array are identical and the full scanning array is used), effective dpi can be calculated by dividing one of the document dimensions into the corresponding pixel arrays [4]. For example, the effective dpi of a 30" x 20" document can be determined by dividing 30 into 3,072, which equates to a dpi of 102 [5]. However, in many occasions, perfect alignment is not possible if the full document needs to be captured and the aspect ratios of scanning device and document are not identical.

Therefore, we will ask you to calculate the effective dpi by relating the document's selected document dimension (SDD) to the pixel counts of the same dimension displayed on the monitor at one to one ratio (100% view). Recall that a selected document dimension is the width of a section of your document, such as the distance between the roof of a house and a street sign in a photograph, or the length of a line of text in a letter. This dimension will be used to correlate pixels to inches so as to determine the effective resolution (DPI) of the digital image.

To calculate effective dpi from a selected document dimension, display the image at 16 Base [6], and count the number of pixels composing the SDD by following the directions given for Adobe Photoshop:

Resolution targets [7] can be used to measure and evaluate digital resolution provided they have been photographed and scanned with the original or at the same setting. However, most of the resolution targets were developed for the micrographic or photographic industries and will not consistently predict the effective resolution due to sampling errors introduced in the digital representation of resolution test patterns. The most effective one for use in calculating the loss of resolution from the original is the RIT Alphanumeric Resolution Test Object. If you included resolution targets during your Kodak Photo CD project, please contact us for instructions on how to use them to evaluate resolution drop off.

Modulation Transfer Function (MTF) provides an objective measurement for digital resolution by providing a graphical representation of image quality. However at this point, the technique is not commonly used, and therefore not practical to utilize in our study.

FIGURE 2: Measuring Selected Document Dimension


Appendix E: Inspection of Legibility and Fine Detail

Please adhere to the following recommendations when you are inspecting legibility and fine detail to answer the questions presented in Section B of the image quality assessment survey.

Examine fine details in your document and the image to see if they are rendered in the light and dark portions of the image. Streaking and graininess are typical film attributes that might be evident in the displayed image. Banding (varying lightness and darkness) may be attributable to improper lighting. Newton rings (circular impressions) can be introduced during the scanning of transparencies. Additionally, images of halftone documents can exhibit a moire effect which will appear as a watered or wavy pattern.

Measuring Stroke and Detail Pixel Dimensions
Count the pixels that comprise your stroke by zooming in (use the zoom option of Photoshop's toolbox) until you can identify pixels as individual squares, and follow these directions:

If you zoom in until the individual pixels are visible, you may be able to count the pixels without using the Show Info function. After counting, evaluate whether the stroke has been adequately represented by following the magnification recommendations presented in the beginning of this appendix. If you have difficulty in evaluating image quality at the 1:1 (100%) view, zoom in until individual pixels are evident and re-evaluate rendering at this level of magnification. Repeat these instructions for evaluating the capture of your identified significant detail as illustrated in Figure 4.

FIGURE 3: Measuring Stroke


FIGURE 4: Measuring Significant Detail


Appendix F: On-screen Color Quality Assessment

Color and tone reproduction relates to replication of colors and tones in the digital images to reproduce the same appearance of color and tone ranges as in the original scene. Unlike some of the document attributes (such as dimension, detail, stroke), color is not an easily measurable property. Furthermore, color and tone assessment may be highly subjective and changeable according to the viewing environment and to the characteristics of monitors and printers. When you are viewing a Photo CD image, many factors can cause color and tone shift on the monitor, including:

One can attempt to control these factors by utilizing the following techniques and tools:

Color Management Systems (CMS)
The main source of difficulty behind accurately reproducing colors stem from the fact that the total set of colors (color gamut) of input/output devices might be different from each other because of lack of uniform color space. Color consistency from one component to another is best achieved using CMS software, which attempt to produce consistency in the representation of color in image files across image capture, display, and output devices. Examples of CMS software include Colorific, CorelDraw, Apple ColorSync, and Kodak Precision Color Management System (KPCMS). Adobe Photoshop (Version 2.5.1 and 3.0) includes KPCMS as an extension to accurately reproduce Kodak Photo CD color space. In addition, Acquire Module plug-in takes advantage of the KPCMS and provides a color-managed conversion to various output devices.

Improvements in managing color digitally may be forthcoming from an international consortium of industry leaders working to develop an electronic pre-press industry standard. Their "International Color Consortium (ICC) Profile Format" is intended to represent color consistently across devices and platforms. The ICC's work is important because if vendors can agree on the ways to define and interpret a device-independent color space, it will become more feasible to maintain colors across distributed systems.

Monitor Calibration
A common problem when evaluating images is that they may appear differently when viewed on different monitors. Calibration is the process of adjusting your monitor's color conversion settings to standardize the display of images on different monitors so that the images look the same with different monitor/video card combinations.

The ideal method of calibrating a monitor is to utilize monitor calibration hardware and accompanying software. However, if you do not have access to the necessary hardware and software, you may want to use your application program's calibration tools. For example, Adobe Photoshop includes a basic monitor calibration tool, which can be used to: eliminate color cast in your monitor display; ensure that your monitor grays are as neutral as possible; and standardize the display of images.

The keys to calibrating a monitor are to set the gamma and white point. You can optimize your monitor at the following settings suggested for image evaluation:

gamma = 2.2
white point = cool white (5000 Kelvin)
color depth = millions of colors (24 bit)

During our Kodak Photo CD evaluation, monitor calibration is highly desirable. You can check the settings of your monitor by following these steps for Adobe Photoshop:

You can set the color depth to millions of colors (24 bit) through the display settings of your monitor. The directions will vary depending on your platform and operation system. If you can not sustain 24 bit color at 1024 x 768 resolution, drop to 800 x 600. If you still cannot support 24 bit color, you will not be able to evaluate color appearance.

Color Quality Control Instruments and Software
Color quality control instruments (densitometer, colorimeter, spectrophotometer) and software provide full spectral measurement and matching of colors from a wide range of objects, reflective surfaces, and monitors. However, color quality control software (such as Colortron and X-Rite) were not used in this study.

Grayscale and Color Targets
It is highly desirable to include grayscale and color targets during photography to provide a level of control over color and tone shifts. Good color-matching requires not only the information on the color scale, but also the gray scale control of shadows, mid-tones, and highlights. These targets can either appear at the beginning of each film roll [10] or where possible, photographed with the individual document. They are instrumental in color-matching images during photography, digitization, on-screen viewing, and printing.

Set Your Neutrals Right!
Don Brown, one of our project consultants from Kodak, says "if you get your neutrals right, most of your color management work is done." Following his statement, we recommend that if you do not use any of the above measures suggested to control your colors, follow the easily-applicable approach described below.

Kodak provides a reference image to evaluate the color calibration of your monitor and/or image viewing software used. It can be downloaded free from the following sites:

For Macintosh computers: ftp://ftp.kodak.com/pub/photo-cd/general/referenceImage.sea.hqx
For PCs: ftp://ftp.kodak.com/pub/photo-cd/general/referenceImage.exe

This grayscale reference image (Figure 5) was created through purely digital means and did not involve scanning film on a Photo CD imaging workstation. The patches represent ideal neutral patches in PhotoYCC color space and can be used to evaluate grays in your image and to adjust colors of the image (not the monitor).

After downloading the reference image, click on the file name to expand (decompress) the image. Retrieve the reference image following the directions presented in Appendix C (Opening Images Using Kodak Photo CD Acquire for Adobe Photoshop) and view the image at 1:1 (100%). For best viewing results, be sure to warm up the monitor for 30 minutes before viewing the image.

If any of these conditions are not met, then you may have a problem with either your monitor set up or your method of acquiring Photo CD images. As a short-term solution, you may want to adjust your reference image by using the color and tonal adjustments of your image viewing software. If you are using Adobe Photoshop, you can use the Levels and Curves color correction tools to map existing ranges of color values to new ranges. If you wish to make adjustments to match the appearance of the image colors to the original document's, refer to the Adobe Photoshop user guide and the following article:

Fraser, Bruce, "Picture Fixes." Adobe Magazine, July/August 1996, Vol. 7, No. 6, pp. 61-65.

If your images were filmed/scanned with a grayscale target, compare it to the Kodak reference image to determine whether there is a color cast to the image. By using the Windows/Show Info option of Photoshop, confirm the values for color shift and compare information in the highlights, neutrals, and shadows. Examine color shift especially in the highlight and midtone ranges because the eye is more forgiving of difference in the dark areas than light areas.

FIGURE 5: Kodak Grayscale Reference Image


Footnotes:

(1) Note that the Adobe Photoshop directions included in the appendices are based on Version 4.0 for Windows platform. If you are using another version and platform, refer to your user guide to find the corresponding commands for the tasks mentioned. (Return to Text)

(2) The Judge color viewing booth provides metered daylight and balanced light levels to provide a reliable and controlled viewing environment to evaluate color. It is used for matching colors, evaluating color quality, and appraising color uniformity under specified lighting conditions. It costs $1,300 and can be purchased from Macbeth (1-800-MACBETH). (Return to Text)

(3) Natural daylight fluorescent bulbs can be purchased from lighting supply stores. When you are purchasing the bulb, be prepared to answer the following questions: what length (usually 2 feet to 4 feet) and what temperature (D5000 for daylight). (Return to Text)

(4) The aspect ratio (AR) offers a means of relating one dimension to another. In the case of a document, the aspect ratio relates the width to the height. In the case of a digital camera, the aspect ratio relates one pixel dimension to another.

AR = larger dimension divided by smaller dimension.

AR for Kodak Photo CD:

AR =3,072 divided by 2,048 = 1.5 (Return to Text)

(5) Effective dpi can be calculated by using the following formula:

If document AR £ scanner AR, then calculate the dpi using the following formula:

dpi = scanner's smaller pixel dimension divided by document's smaller dimension.

If document AR > scanner AR, then calculate the dpi using the following formula:

dpi = scanner's larger pixel dimension divided by document's larger dimension (Return to Text)

(6) Kodak Photo CD technology provides 5 levels of display based on pixel dimensions, from 128 x 192 to 2,048 x 3,072 pixels. When you are calculating the effective resolution, use 16 Base image (2,048 x 3,072 pixels). (Return to Text)

(7)The most commonly used resolution targets are RIT Alphanumeric Resolution Test Object
(716-475-2739), AIIM Scanner Test Chart (301-587-0202), and IEEE Std 167A.1-1985 (URL:http://stdsbbs.ieee.org/product/167A.1-1995.html). (Return to Text)

(8) Two of the most common color models are RGB (red, green, and blue) and CMYK (cyan, magenta, yellow and black). RGB is the default mode for Photoshop images and it is commonly used in video display devices. CYMK is the color space for printing. (Return to Text)

(9) Diagnosis and correction of inappropriate monitor color temperature and color balance is beyond the scope of this study. However, we suggest that you turn on your monitor for approximately 30 minutes before your evaluation and set the background color of your monitor to gray. (Return to Text)

(10) Provided that the same lighting and focus are used throughout filming and the film is processed in the same batch. (Return to Text)

(11) You can determine the color values of any part of this image by using Photoshop's Info palette (Windows/Show Info). After you open the Info palette, position the pointer over any part of the image, and the color value under the pointer will be displayed at the top left column of the palette. For example, when you place the pointer over the gray frame of the reference bar, all the color values will be 96, indicating pure gray. When you are viewing the reference bar, your image should be displayed in RGB mode. You can verify this by making sure that the left column reads RGB, not CYMK. Refer to the Adobe Photoshop manual to interpret these color values. (Return to Text)