Frequently Asked Questions

Understanding Scanner Mechanics, Scan Modes and Interpolation

To get the most out of your new Mustek scanner, you will need to understand some of the technical aspects of how it functions. Once you understand how the scanner works and what kind of data it sends to your computer, you will have a good foundation for building additional graphic and desktop publishing skills upon.

Scanner Mechanics - How They Work

For simplicity, we will discuss flatbed scanners in this document; however, the basic principles apply to hand scanners and sheetfed scanners also.

You begin by placing a document face down on the glass of your scanner and closing the lid. When you press the scan button, the scanhead inside your scanner's chassis begins to move. A fluorescent light on the scanhead shines light upward onto your document or picture. This light is reflected downward and hits a mirror in the bottom of the scanhead which reflects it towards a Charge Coupled Device (CCD) inside the scanner's chassis.

The CCD contains an array of photosensitive cells which read the intensity and/or color of the light that hits them. The CCD works with an analog to digital converter to convert the light to a level between 0 and 255 (0 being darkest and 255 brightest).

The CCD is what determines the Optical Resolution of your scanner. The most common scanners have a 300 dot per inch CCD. This means that for every inch of width in the image, the CCD can sample 300 individual dots of color. I will leave the detailed explanation of resolution issues to Document # 205. Here we are only attempting to grasp the basic concepts.

Scanhead Of A Flatbed Scanner

The scanner reads one line of data from your document at a time. If you are scanning a 6" wide photograph at 300 DPI, the scanner will send 6 x 300 or 1800 individual dots to the computer for the first line of data. The motor will then move the scanhead to the next line. If your photograph is 4" tall, the scanner will produce 4 x 300, or 1200 individual lines when scanning at 300 DPI. Each of these dots may be a different color and they are actually more like tiny squares than round dots.

If you were to take a large number of tiny ceramic tiles made in a wide variety of colors, you could arrange them in such a way that they would show a picture. When viewed from a distance, your tile mosaic would not look jagged at all. But if you came up for a closer look, you would see jagged edges between various colors. The image produced by your scanner is basically a tile mosaic made of many very small square dots. Using the example above of a 4 x 6 inch photograph, the scanned image would consist of 2,160,000 dots. This is the product of 1200 x 1800 which were calculated in the above paragraph. Your scanned image does not appear jagged because the dots are very small.

Below are images of a postage stamp that was scanned at 72 DPI. The picture on the left is the entire stamp. The picture on the right is an enlarged segment of the stamp. It has been magnified five times to show the mosaic dot pattern detected by the scanner. Notice that the stamp appears on your screen actual size. This is because computer monitors have a resolution of 72 DPI. (Hint: Scan at 72 DPI for Web Pages. If you scan higher than about 100 DPI, your images may not fit on your Web Page unless, of course, they're as small as postage stamps.)

These tiny squares of color are the data that a scanner sends to the computer. Once you get the dots into your computer, different software applications handle them in different ways. See Document # 203, Understanding File Formats, for more information about how different programs use this data.

Scanned at 72 DPI

Shown Actual Size

Enlarged Five Times

Note Mosaic Pattern

Scan Modes

Computers represent pictures in a variety of ways. The four methods that are most common are line art, halftone, grayscale, and color. Below are examples of each:

1-Bit Line Art

1-Bit Half Tone

8-Bit Greyscale

24-Bit Color

Line Art

Line art is the smallest of all the image formats. Since only black and white information is stored, the computer represents black with a 1 and white with a 0. It only takes 1-bit of data to store each dot of a black and white scanned image.

The diagram of a flatbed scanner mechanism near the top of this page was scanned in the line art mode. Line art is most useful when scanning text or line drawing. Pictures do not scan well in line art mode.

Halftone

While computers can store and show gray-scale images, most printers are unable to print different shades of gray. They use a trick called halftoning. Halftones use patterns of dots to fool the eye into believing it is seeing gray-scale information. For example, the patterns below represent a totally white dot, a 25% gray dot and a 50% gray dot.

Grayscale

Grayscale images are the simplest of images for the computer to store. Humans can perceive about 255 different shades of gray. Computers represent grayscale information by storing a number from 0 to 255 in a single byte. When you view a grayscale image, it is equivalent to seeing a black and white photograph.

Color

Color images are the largest and most complex images to store. TVs and computer monitors mix the colors red, green, and blue to display all the colors visible to the human eye. If you were to look at your computer screen right now through a high powered magnifying glass, you would see that the white background of this page is actually made up of high intensity red, green and blue dots arranged like the diagram below. A pixel is a group of three dots, one of each color. Because the dots are very tiny, your eyes blend them together and you see white.

The monitor's internal electronics can vary the intensity of each color dot to 256 different levels of intensity. At the 0 intensity level, the dots are completely off and the screen appears black. If the red and green intensity is 0 and the blue intensity is 255, you see a rich blue color like the one above. By varying the intensity of each color dot between 0 and 255, there are 16.77 million different combinations. Each combination appears as a different color. If the intensity of each dot is set to an equal value, say 128, the monitor will appear as a grey shade. 128 would be the level of 50% grey. This is why there are only 256 greyshades when scanning in greyscale mode.

Computers use 8-bits (1 byte) to represent each of the color components (red, green, and blue). With 8-bits for each of the three colors there are a total of 24-bits to represent the entire color spectrum.

How to Determine File Size

If you're curious how to determine how large a scanned image

File Size = (Resolution x Horizontal Size) x (Resolution x Vertical Size) x Scan Mode

Where Scan Mode = 1/8 for line art and halftone, 1 for grayscale and 3 for color.

Here are the file sizes for a 4" x 4" photo at various scan modes and resolutions. Notice that the greyscale files are 8 times as large as the lineart files. The color files are 24 times as large as the lineart files and 3 times as large as the greyscale files.

4" x 4" image

100 DPI

150 DPI

300 DPI

600 DPI

Line Art/Halftone

19.5 Kb

44 Kb

175 Kb

703 Kb

Grayscale

156 Kb

352 Kb

1.37 Mb

5.5 Mb

Color

469 Kb

1 Mb

4.12 Mb

16.5 Mb

Interpolation

The resolution of a scanner is determined by the Optical Resolution of the CCD and the Stepping Speed of the scanner's motor. A 300 x 600 DPI scanner has a 300 dot per inch CCD and a motor that goes slow enough to scan 600 lines per inch as it travels the length of the bed. If you scan at 300 DPI on such a scanner, the motor runs twice as fast as it does when scanning at 600 DPI. If you scan at 600 DPI on such a model, the motor runs slower and the scanner's hardware interpolates the horizontal data from 300 up to 600 DPI. Basically, an integrated circuit chip in the scanner generates new data where there is none through an algorithm by averaging the color of adjacent dots and creating a new dot between them of the average color. This is Hardware Interpolation and it allows a 300 x 600 DPI scanner to produce a 600 x 600 DPI image.

Software Interpolation can increase the resolution even more than Hardware Interpolation. Software Interpolation is performed by the Twain driver within the Computer's CPU. This type of interpolation is quite misleading. It does not create sharper images. Image quality and sharpness is always limited by Optical Resolution. Software Interpolation merely increases the amount of data in a scanned image. It is roughly equivalent to scaling an image to make it larger.

The only good reason to scan at very high resolutions, such as 4800 DPI, is when you need to enlarge an image dramatically. If the postage stamp used above were scanned at 4800 DPI, the file size would be 19.1MB. This would increase to 76.4 MB at 9600 DPI. If you were even able to scan an 8.5" x 11" page in color at 9600 DPI, you would need a 12 Gigabyte hard drive to store the resulting file! (Obviously, your computer would crash if you tried this)