Image Scanning

Animated Tutorials

First Steps 4:28 90kbps
How to use a flatbed scanner to acquire an image. Watch this presentation to get the basics down.

The Web 6:26 90kbps
Scanning an image for use on the Web. Start thinking in pixels and not inches.

PowerPoint 7:03 90kbps
Scanning an image for use in PowerPoint. Scan to get the best quality but learn not to over-scan the image.

Print Publications 5:38 90kbps
See how to scan an image for use in a flyer, book, or any printed document.

B&W images for Printing 4:26 90kbps
Scan a chart, graph or other Black and White image for printing in a Word document or other file type.

B&W images for the Web 7:21 90kbps
Tricks to get the best quality Black and White image from paper to the Web.

Exposure Adjustment 4:17 90kbps
Tips for adjusting the shadows, mid-tones, highlights and color correcting.

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Scanning Tips

Introduction

Computers play increasingly important roles in our lives. Nowhere is this more evident than in our schools. Educators are now called upon to create web sites, computer-based presentations, and interactive CD-ROMs to engage the interest of their technology literate students. Often, prohibitively expensive printed course packet information is shifted to web sites where production and distribution costs are slight. Also, journal publishers are increasingly asking professors to provide research papers in digital form.

An essential ingredient in computer-based media is the visual image. Pictures and drawings clarify concepts and can promote a greater understanding of written material. We live in an image-driven culture. Successfully getting images into the computer is key to creating engaging web sites, in-class presentations, CD-ROMs and other computer-generated materials.

This article addresses strategies for transferring the best quality images into the computer. It covers having a Kodak Photo CD made as well as using slide and flatbed scanners to digitize images. It also outlines approaches for successfully scanning images for screen display (web sites, CD-ROMs) and for print output (handouts and posters).

The Kodak Photo CD Option

The scanning equipment found in most schools is no match for professional scanning equipment. One popular low cost scanning option to consider is having a Kodak Photo CD made. This is most economical if one has 50 or more 35mm slides or print negatives to be scanned and if the goal is to use these scans in a variety of ways (screen display and print output).

figure 1

Having a Photo CD made is simple. Bring 35mm slides, print negatives, or unprocessed film to a local camera store or discount film processor. They, in turn, will send the film to an authorized Kodak lab that uses special scanning equipment to create the Kodak Photo CD¾ saving each image at five different resolutions from low to high (figure 1). In other words, sending 100 slides yields 500 professional scans suited for use as web graphics and digital prints. The quality of the scans is very high and the CD is readable by both Macintosh and Microsoft Windows computers. The disc holds approximately 100 images and Kodak can add to discs that are not full at a later date. Expect to pay about one hundred dollars for a CD-ROM disc with 100 scans. This includes index prints that function as a visual table of contents. Because many film developers now offer their own scanning services, confirm that images will be scanned and returned as a Kodak Photo CD, not just scanned and saved on CD as JPG files¾ a less versatile format.

While the Kodak Photo CD option offers high quality scans, it is not suited to all situations. Having only a few slides to scan, an immediate deadline, flat work such as actual photographs or drawings, or limited funds, may contribute to the decision not to use the Kodak Photo CD approach. The next approach to consider is scanning the images oneself. However, before using a flatbed or slide scanner to bring images into the computer, one should first understand the basics concepts behind digital images.

Resolution - How big is a Pixel?

One way in which digital images differ from photographic ones is that they are made up of pixels¾ small squares assigned specific colors. Photographs are continuous tone images made up of an infinite gradation of tones. Digital photographs are simplified translations made up of a fixed number of colors appearing as a fixed number of squares or pixels. These pixels are arranged as a grid (figure 2).

figure 2

The pixel is the smallest indivisible unit of a digital image. An interesting characteristic of the pixel is that its physical size is dependent on the device that displays it. Different computer monitors display different resolutions of pixels. Older monitors show 640 pixels across by 480 pixels top to bottom. All images seen on such monitors are distributed over this 640x480 grid. New monitors can shift between many different resolutions ranging from 640x480 pixels to 1024x768 pixels and beyond. Obviously, switching to a higher monitor resolution does not change the physical size of the monitor¾ the size of the pixels simply gets smaller. Put another way, the grid of pixels increases (from 307,200 to 786,432 for example), but the increased number of pixels still fit into the same physical monitor space.

Printers output pixels too. Printed images are measured in Dots Per Inch (DPI) where dots refer to pixels. Different printers can output the same pixel at different sizes. The pixels printed by a 300 DPI printer must be twice the size of those printed by a 600 DPI printer. In general, the smaller the pixel the higher the image quality of the printer.

Digital images are not fixed in size or quality. Instead, these two variables are inversely proportional. A digital image 600 pixels wide would print as very high quality at 600 DPI. The image would be one inch wide on paper. The same image would print at high quality at 300 DPI and be two inches wide (figure 3). However, a 12-inch wide print of the same image would make a low quality print (it would be only 50 DPI). Changing only the DPI of an image from 600 to 50 DPI does not change the file size or the total amount of raw pixels in the image¾ it only changes how the fixed amount of pixels is distributed when the image is printed.

600 DPI / 600 raw pixels across / 196K

300 DPI / 600 raw pixels across / 196K

figure 3

Considering the above, digital images are clearly only as good as the output devices that display them. Poor quality monitors limited to a 640x480 grid of 256 colors will make even the best digital image look bad. Likewise, an image that looks incredible on a monitor will not print well on a low quality 300 DPI inkjet printer. Understanding that pixels can vary in size and that digital images only look as good as the device that displays them helps one to understand why the same images look great under some conditions and poor under others.

Planning Before Scanning

The main question one should answer before making a scan of an image is, "How will the image be output?" Main uses of an image are either viewing it on screen or viewing it on paper. In some cases, both screen and printing outputs are required.

Scanning for the Screen

Images that will be displayed on monitors need to be scanned with pixels in mind; not units of measurement like inches. Inches do not exist for screen images as they do for print images. For example, suppose one is scanning an image for a public web page with the goal of making the image as large as possible¾ yet it should still fit on one computer screen with no scrolling. Since different monitors display different resolutions, keeping in mind the lowest common denominator is the best strategy. Older monitors display only 640x480 pixels. Scanned images should be made smaller than this pixel size to account for the web browser menu bars¾ 610 pixels wide x340 pixels high is a good maximum image size for a web image (figure 4).

figure 4 (as seen on 640x480 monitor)

Because browsers ignore all "inch" information stored within files, images are displayed on monitors in terms of raw pixels. An image 320 pixels wide is overlaid on the "pixel grid" of the monitor. On a 640x480 monitor, the image will stretch across half the screen. On a monitor set to 1024x768, the image will take up a little less than one third of the screen. In simplified terms, the same image will appear relatively larger or smaller depending on the resolution of the monitor used to view it.

Deciding on how many pixels wide or high to make a web site image is difficult because the public uses many types of monitors. It is considered bad form to make visitors scroll up and down and left to right to view web images. Keeping images below 600 pixels wide will guarantee that no matter what type of equipment your visitors use, they will not be forced to scroll horizontally.

Scanning images for specific on-screen uses is far easier. Suppose, for example, one is creating a PowerPoint presentation to show in class. First, find out ahead of time what resolution the equipment in the classroom operates at. If the equipment projects at 800 pixels across by 600 pixels up and down, scan your full screen images at this resolution and no more. Scanning images at higher resolutions than the display machine’s resolution will not make them clearer. Scanning more pixels than equipment can display will only slow a presentation down unnecessarily. For images to fill one quarter of the screen at 800x600, scan them at 400x300 pixels.

The same rules apply for multimedia CD-ROM creation. One might decide in advance that all users must have an 800x600 display or greater. All images should be scanned with this resolution in mind and the screens of the CD laid out at 800x600 pixels. The CD would display fine at resolutions of 800x600 and above, but not at 640x480.

When scanning a pure black and white image (not grayscale) for display on a web page, a slightly different strategy is needed. Suppose the goal is to make the image 400 pixels wide so it fits comfortably on a web page. Try scanning it as a pure black and white image at double the final width (scan 800 pixels across). Next, use an image editing software program to change the pure black and white image to a grayscale image. Finally, reduce the image to 400 pixels across. Most image editing programs will automatically soften up the black lines by adding shades of gray (anti-aliasing), making it more acceptable (less jagged) as a web graphic. This strategy is only one suggestion. Getting the best scan of an image often means experimenting.

Keep in mind that quality and file size are inversely proportional. Since it takes one second to download 3.6KB on a 28.8Kbps modem under perfect conditions, it is important to keep the image file size small if it is to be displayed over the web. The goal is to save the scanned image at the lowest quality possible without compromising the appearance of the final image. As a general rule, save photographic images for the screen as JPEGs. (Use image editing software to accomplish this if the scanning software does not offer this format as an option.) The JPEG format is good for images with a continuous tone color. JPEG’s compression method supports 16,777,216 colors (24-bit) while keeping the image very small. The medium quality choice for JPEG usually works out to be the best selection, but one may need to experiment to determine the best quality setting for an image. Save simple graphic images as GIFs rather than JPEGs. The GIF format is good for images with large areas containing few color variations. The GIF method compresses graphic images optimally, but because it allows a maximum of only 256 colors for an image, it is poor choice for photographic images.

Scanning for Print Output

When the goal of scanning an image is to print it, other questions must be asked. One must take into account the resolution of the printer and the desired size of the image on the printed page. When scanning for print output, inches do matter.

Typically, one should scan color images such as photographs at about half the resolution of the output printer. For example, if printing to a 600 DPI printer, the scanned image should be around 300 DPI. Experiment with this general guideline to find the optimal scanning resolution for specific printers. One thing is sure¾ scanning photographic images to match a printer’s resolution will not significantly improve the quality of the output image, but will create large files that may be difficult to work with.

When scanning a pure black and white image with no gray-scale for re-printing (for example, a simple graph from a journal article), it is best to scan it as a "sharp black and white drawing" to match the resolution of the output printer. For example, if printing to a 300 DPI printer, the scanned pure black and white image should be scanned at 300 DPI. Pure black and white drawings need this much resolution because they are the hardest to render due to their high contrast. A black and white image scanned at less than half of the printer resolution (i.e. 100 DPI) will show very jagged edges along any curved or diagonal lines (figure 5). Because a pure black and white image does not contain any color, scanning it at a higher resolution will create a much smaller file than a similarly-sized color image.

(100 DPI / 5K)

(300 DPI rabbit / 4K)

figure 5

After determining the DPI of the printer to be used (for this example, a 600 DPI printer) and deciding on the resolution to scan the image at (for this example, 250 DPI for a photographic image), one must next consider how large the image should appear on the page. Suppose one wishes to scan an 6x8 inch photograph and place it in a newsletter in a 3x4 inch space. By setting the scanning software to scan at 250 DPI, and scaling the image to 50%, the final scan will be a 250 DPI image that will print at 3x4 inches (figure 6). In raw pixels, it will be 750x1000 pixels (good for printing, but much too large for a web graphic). As a general rule, save images to be printed as TIF files. Don’t use web graphic formats for printing such as JPEG or GIF because they use compression methods that do not print well.

figure 6

Scanning for Multiple Uses

Many scans are intended for multiple uses. Teachers may want to scan images for a web site and for placement on a written exam. The best advice under these conditions is to have a Kodak Photo CD made. The next best option is to scan the image twice, once for the web and once for print. However, in the interest of time, these two options may not be feasible. When making a single scan for both print and the web use, scan for the most pixels needed¾ usually the print image. Initially, save these images as TIF files or some other uncompressed format. The goal is to scan the most pixels needed for printing and save the image in an uncompressed format. The scanned image can be resampled down to a smaller size later and compressed for web use. Reducing an image size once it has been scanned and saving in a compressed format may be accomplished using an image editing software package and is covered below.

Scanning Guidelines

All scanning hardware comes with unique software tools to control it. This article touches on elements common to most scanning software. The examples provided are from Deskscan II, the HP flatbed scanning software. While other products vary in appearance and features, most allow similar adjustments. Please read the manuals that came with your particular hardware and software if you have questions.

Previewing the Image

All scanning software allows for a preview of the material to be scanned. A quick preview is made of the flat work or slide so that one can use the software controls to crop out elements not needed (figure 7). After setting cropping marks, set the image type and scaling to meet your particular needs (figure 7). Most scanning software allows one to toggle between pixels and inches as well as other units of measure (figure 8). This will be necessary as one scans for print and web graphic purposes. The scaling slider controls how many pixels are captured during the final scan and may indicate the final size of the file once scanned and saved (figure 9).

figure 7

figure 8

figure 9

Identifying the Image Type

Most scanning software allows one to choose the type of image to be scanned (figure 10). Correctly setting this option will insure the best scan possible. For flatbed scanning of photographs, choices such as "sharp black and white photo" or "sharp millions of colors" work fine. If however, one is scanning a photographic image from a book or magazine, sharpening is not advisable. Images appearing in print publications have been reproduced on a printing press and are made up of small dots of ink. Sharpening the image’s dot pattern will call attention to it and away from the image itself. For these types of images pick a choice that does not including sharpening.

figure 10

Setting White and Black Points

Tools to set the white (highlight) and black (shadow) points of a scan are common to most software packages. Usually appearing as crosshairs or eyedroppers, these tools are used in conjunction with the initial preview image (figure 11). Click on the highlight tool and then mouse over the lightest part of the image and click. Next, use the shadow tool to identify the part of the image that should be black. This will not only effect the highlight and shadow areas, but it will help to adjust the color balance of the image. For example, if a photograph has a green tint because it was made under fluorescent light setting the white and black point while scanning can correct this problem. Although most scanning software packages do not permit setting the gray (midtone) point, it is still possible to set a gray point after scanning using an image editing software package such as PhotoShop. This can also be helpful in correcting the color balance of an image.  

figure 11

Brightness, Contrast and Color

Most scanning software also allows for the adjustment of brightness, contrast (figure 12), and color balance (figure 13). Use these controls to effect the preview image, making it appear as correct as possible on the screen. It is better to error on the side of too little contrast rather than too much contrast. One should attempt to capture as much information and detail as possible when scanning. For example, capturing predominately black and white tones and not middle grays by setting the contrast too high means that information is not captured in the scan and can never be retrieved.  

figure 12

Likewise, any color correction should be done at the time of the scan. Most scanning software allows for simple adjustments. Any corrections you can do at the time of scanning will only make for a better starting point if you choose to make further adjustments with a program such as PhotoShop.  

figure 13

Image Editing Software

Image editing software packages such as PhotoShop can be useful in further enhancing images once scanned. These packages have a variety of advanced features for modifying an image which are not available through standard scanning packages. It is best to make as many corrections at the time of scanning as possible using the scanning software provided. Should an image need additional adjustments after scanning, it may be brought into an image editing software package for further modification.

Image editing tools such as Photoshop allow one to resize an image, making it smaller (figure 14). After resizing an image, it is usually necessary to sharpen it using a filter. In Photoshop, The Unsharpen Mask filter is a versatile tool that allows fine control over image sharpening via slider controls (figure 15). Try experimenting with this filter (or others like it) to make a resized image appear sharper. Although image-editing software allows one to increase the size of an image by adding pixels, it is not recommended. Programs make "best guesses" when increasing an image’s size and these guesses are no substitute for a proper scan. Trying to increase an image’s size by adding pixels will result in a large file that is blurry¾ and no amount of sharpening will fix it. Making a large file small allows the computer to throw away information rather than make up information¾ a task it is more successful at. Again, if you have a scanned image, but need more pixels, you should rescan the image rather than attempt to add pixels using software.

figure 14

figure 15

Conclusions

This article has addressed effective strategies for transferring the best quality images into the computer. Having a Kodak Photo CD made is one good way to move images into the digital realm, but using slide and flatbed scanners to digitize images can be equally effective when used appropriately. Working with images takes time and patience, but with care and experimentation, anyone can produce stunning digital images.

This guide made possible through an SBOE grant