THE BASICS
When scanning it is possible to break down the types of images available to three basic categories. Each one of these images has it’s own set of problems and solutions in the scanning process. An important factor to remember in scanning is bit depth. Line art is 1 bit scanning so the computer program only keeps track of black and white. This creates a smaller computer file than if the image was scanned as a gray scale. Gray scale images are usually 8-bit images this means that there are 256 different levels of gray this creates a larger computer file than a 1-bit image as in line art. Color scanning creates an even larger file. Standard color scans in the past have been 24-bit images, so this leads to an image that can be as much as 3 times larger than a gray-scale image. This happens because the scanner and computer programs must now keep track of the red, green and blue channels (primary colors of light). Scanner manufacturers are constantly improving the specifications of their scanners. Now it is not unusual to see 30 bit and 36 bit scanners. These high bit scanners are capable of capturing a larger range of colors, creating better scans.
File size is always an important consideration. This affects how fast the computer processes the information. The larger the file size, the more numbers the computer has to crunch and the slower the computer will process. When scanning it is always a good idea to scan at the optimum settings in order to keep the file size as low as possible, without sacrificing image quality.
When scanning we often refer to resolution. Resolution is how many samples are taken from an image on a per inch basis. It is often referred to dpi as in dots-per-inch or as ppi, which is pixels-per-inch. The standard scanning resolution for the Internet is 72 dpi. This is the average screen resolution of most personal computer screens. This resolution works well because it is relatively low, it keeps the file sizes low and makes downloading of web pages faster. When it comes to print, the standards for scanning are a little more involved. When scanning for print it is necessary to consider what kind of image and what kind of print device you are using. These factors contribute to the options available. The main thing to remember; the higher the resolution the finer the detail, the larger the file size.
Line Art
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| Fig. 1 Line Art 600dpi |
Fig. 2 Line Art 72dpi |
This image consists of only black and white, no grays. It is also referred to as a bit map or 1 bit scanning. This type of image requires a high resolution scan because there is no transition of grays to smooth the edges. The resolution should be set to match the maximum dots-per-inch (dpi) of the output device. So, if your printer prints 600 dpi then the image should be scanned at 600 dpi to get the maximum quality print (Fig. 1). A minimum scan resolution for line art is 300 dpi below this you will get the jaggies. Jaggies are a stair-stepped edge on line art where there should be a smooth edge (Fig. 2).
Gray Scale
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| Gray Scale 200dpi |
Gray scale scanning is used primarily for photographs. It is also useful for scanning some line art images that are difficult to scan at the line art setting, such as pencil drawings with shading or line art with very fine lines that tend to blur when scanned as line art.
The advantage of scanning gray scale images is in the resolution. Because there is a gradual value change between extreme values of white and black, the image may be scanned at a lower resolution thus reducing your final file size. However, due to the scanner having to keep track of multiple levels of grays the final file size will still be larger than a line art image. Scan resolution for gray scale can be as low as 200 dpi for printing to a laser printer or ink jet printer. If you are planning to send this image to a professional pinter for offset printing the standard resolution is 300 dpi. When scanning in gray scale the image may be improved some by sharpening the image.This is an option available with most scanning software and is a filter in Adobe Photoshop. Sharpening increases contrast in an image along the edges of objects. This gives the illusion of a photo having greater focus. It makes good photos look even better, but it will not fix an out of focus photograph and too much sharpening can create unnatural halos around objects.
Color
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| Color 200dpi |
The resolution settings are the same for color as they were for gray scale. The only difference between the two are the bit depth settings. As stated previously, the bit depth for color scanning is 24 bits and above. When using scanning software the settings are often seen as true color, rgb color or color photo.
When using Photoshop to scan, it often works well to adjust the colors right after you scan. This can be reached by going to image > adjust > auto levels or auto contrast. In most cases, either of these commands will improve an image after it has been scanned.
SAVING YOUR SCANS
Once you have scanned your photos and illustrations you then need to decide what is the best file format for saving your document. It all depends on what is to be the final use. If you are using a Windows computer and are sending photos to someone with a Macintosh and the photos are to be printed, a tiff is your best choice.
TIFFs (Tagged Information File Format) are designed specifically for scanned images and are extremely flexible when having to cross over from Windows machines to Macintosh and visa versa. Macintosh computers also have software built into the operating system so they may read PC disks. However if you are giving someone on a Windows machine a disk and you are on a Macintosh, you need to save the file to a PC disk. The Macintosh can read and write to a PC disk without any problem and you can even format a disk for a Windows computer on your Macintosh without any additional software. The only downfall to tiffs are their large file size. This makes them unsuitable for e-mailing.
If you are sending photographs over the Internet, through e-mail or posting them to a website, then your best option is a JPEG (Joint Photographic Experts Group). These files are great for e-mail because they are easily read by many programs, and they have an extremely small file size as compared to tiffs. However, the small size comes with a few drawbacks. JPEG is a lossy compression. What this means is that some of the data is removed in order to make the files smaller. Lossy compression leads to images that have a lesser quality and can be unsuitable for printing to a laser printer. Additionally, if these scans are to be printed by a professional printer, they must be converted to tiff files otherwise they will not print from an imagesetter (prints negatives for the offset printing process) correctly.
Another file format designed for the Internet is the GIF (Graphic Interchange Format). This file format is perfect for any image with a limited color range. The gif format is only capable of displaying 256 colors.When using programs like Adobe Photoshop you can reduce that number and by reducing the number of colors you reduce the file size, which is very important when dealing with the Internet. Gif files work well in for graphic, flat colored work and also have the ability to have transparent backgrounds.
The tiff, jpeg and gif are not the only file formats to choose from, but they are the most compatible. Other formats you may see are the bmp (Windows Bitmap), pcx (Windows Paintbrush), pict (Macintosh Quickdraw) Another is the PNG. This format was created after and intended to be a replacement for the gif. It is more flexible with photographic data than the gif but it still has some compatibility problems.
STEPS TO SCANNING
In order to keep this document general and nonspecific to any one scanner or scanning software, I am listing the steps required to scan that are common to all scanners and scanning programs. These steps should be helpful to anyone using any number of scanners.
All scanners come with their own proprietary software. The directions for scanning come with those scanners and can be very specific. Scanners also come with a piece of software called a TWAIN. This is a plug-in that allows you to scan directly into Adobe Photoshop. It is not necessary to do this. However, Photoshop has so many different tools for improving photos that most graphics professionals do this.
Step 1: Place the image to be scanned on the scanner.
Step 2: Open scanning software.
Step 3: Choose image type (line art, gray scale, color).
Step 4: Choose resolution based on image type. Often this is set when the image type is selected.
Step 5: Preview the image. Some scanners do this as soon as you activate the software. The order is not important.
Step 6: Crop the image. It is very important to select only the area you want in your final scan. Any extra area scanned adds to the file size and is a waste of disk space. The additional file size also adds to the processing time and printing time.
Step 7: Resize the photo to your desired final size. Photos should always be scanned the size they are going to be used. If you scan the photo too small and then later try to make it larger, the resolution is reduced and the image quality will suffer.
Step 8: Scan. If you are using a photo manipulation software like Adobe Photoshop, you can make any number of adjustments to the photo after this step.
Step 9: Save
Updated 9.10.2007
