A scanner is a device that digitises an image, making it possible to store and manipulate a replica on a computer. The picture produced by a scanner is in bitmap form, having a specified resolution, which causes a small but inevitable loss of detail. In addition, the pixels in the digital image can become apparent if the image is magnified or otherwise resized. So, you should try to scan your original at a size and resolution to suit the printer that eventually reproduces the picture.
The quality of a scanner is determined by its bit depth, resolution and dynamic range, as described in the following sections.
Bit depth, also known as colour depth, tells you how many shades of colour or greyscale that a scanner can extract. It’s value is specified as a number of bits. A typical colour scanner produces 24, 30, 36, 42 or 48-bit data, normally using 8, 10, 12, 14 or 16-bit data for each of the three colour channels employed in red, green and blue (RGB) colour coding. The following table shows the most common standards:-
The optical resolution of a scanner indicates how much detail can be obtained from a picture. This is set by the number of scanning elements or pixels in the mechanism and is normally measured in pixels per inch (ppi) or dots per inch (dpi). In many scanners, the resolution in both horizontal and vertical directions is the same, although some models have a greater vertical resolution, thereby ensuring accurate reproduction of the serifs in textual characters.
The following list shows typical resolutions for scanners, measured in dpi:-
|300 ||400 ||667 ||600 ||600 |
|700 ||800 ||800 ||800 ||1000 |
|1200 ||1220 ||1250 ||1400 ||1600 |
|2000 ||2400 ||2400 ||2500 ||3048 |
|3200 ||3200 ||4000 ||5760 |
where the first figure in each instance is the horizontal resolution.
Optical resolution shouldn’t be confused with a higher figure known as interpolated resolution, where extra pixels of values ‘guessed at’ by software are interposed between the pixels produced by the scanner. Subjectively, this can be quite effective, since ‘blocks’ of pixels are replaced by smooth shades, although no actual details are added to the image.
In reality, the highest possible resolution isn’t always necessary. Scanning at 300 dpi is perfectly acceptable, especially if you don’t enlarge the image. Remember, most high-quality magazines are printed at a resolution of only 300 dpi.
This figure, also known as density range or maximum density (dMax) indicates the range of brightness that a scanner can extract from an image. This can vary from 0.0D to 4.2D, with a high number indicating a greater range. A typical modern scanner should gives a figure of 3.5D or higher.
This kind of scanner is used by publishing professionals and is, generally speaking, outside the league of mere mortals. Its sophisticated mechanism ensures precise scanning without distortion of the image.
This usually gives good results, although it may be too large for use with a portable computer. Fortunately, an expensive flat-bed scanner can give results approaching those produced by a drum scanner. And such a device is as easy to use as a photocopier: you simply place the original material on the glass panel and activate the scanning software.
Such scanners often use a cold cathode discharge tube as a light source, which is pulled across the original image by an accurate stepper motor. The light reflected from the original is picked up by a row of charge-coupled device (CCD) sensors, all fixed in position. A scanner with a resolution of 600 dpi needs around 5,200 sensors to cover the full width of the page.
This type of scanner is ideal for use with a portable computer: you insert a sheet of paper and it’s automatically scanned. Unfortunately, you can’t scan from a book or from a sheet attached to another document, although you could photocopy an original and put the copy into the scanner.
This device lets you scan photographic slides. Fortunately, some high-resolution flat-bed scanners can also be fitted with a transparency adaptor, which does the same thing. A typical device takes 35, 46, 70 or 120 mm film, usually on a slide, although some scanners accept Advanced Photo System (APS) film when fitted with a special cartridge holder.
Transparencies must be scanned at a higher resolution than paper images, typically
800 × 1600,
1000 × 2000,
1200 × 2400,
1600 × 3200 or
3048 × 3048 dpi. Other common resolutions include 2438, 2700, 2820, 2900, 4000 and 9600 dpi.
To reproduce an original 35 mm film image on an A4 sheet, you’ll need to use a resolution of at least 1800 dpi. And to get it to A3 size you must use 3600 dpi or better. Fortunately, larger film formats can be scanned at correspondingly lower resolutions. For example, a 60 mm film image can be scanned at 1200 or 3000 dpi for A4 and A3 respectively.
This kind of scanner is only viable for intermittent use with a portable computer. The traditional variety of hand-held device, which can scan both text and graphics, has a wide scanning head, attached to an interface box that’s connected to your computer. Modern scanning pens, which only work with text, are self-contained, battery powered and incorporate optical character recognition (OCR) capability, only requiring a connection to the computer for downloading the text.
The scanning head must be moved slowly and steadily across the document, whilst avoiding sudden movements. Scanners with a wide head have to be moved down the page, whilst a scanning pen is moved along each line of the required text.
This unusual approach uses a special scanning head that temporarily replaces the printer head in a printer. The material to be scanned is inserted in the printer, as if you were printing on it, and the scanning software is activated. This controls the printer, extracting data from the special head as it moves. Unfortunately, the novelty of this soon wears off, especially after you’ve swapped the printer heads several times. In addition, you can’t scan from a book or from paperwork that’s attached to some other form of document, although you could photocopy the original first.
The earliest device of this kind is Thunderscan, designed for Apple’s original ImageWriter printer. Some types of Canon printer can also be fitted with a special scanning cartridge.
A scanner often comes with software, such as a ‘lite’ version of a painting application for modifying your scanned images.
Ideally, when reproducing an image at its original size, you should scan an image at the same resolution as your printer, or at a multiple of the printer’s resolution. To adjust the scale of the image or use half-toning you should use one of the procedures described in the following sections.
Line art doesn’t contain shading, which means that your printer doesn’t need to use dither or half-toning to create shading. Instead, the graphical content is usually in black and white. The scanning resolution for this kind of material is given by:-
So, for example, if you want your final image to be printed at 60% of its full size on a 600 dpi printer you’d use:-
Contrary to expectations, you can get a resolution for line art higher than the resolution of the scanner. To do this, you’ll need an application such as Photoshop. To obtain a 1200 dpi image from a 600 dpi scanner you should proceed as follows:-
Image Size: enter 1200 to resample the image at 1200 dpi
Filter ➡ Sharpen: set Amount to 500%, Radius to 1, Threshold to 5
Adjust ➡ Threshold: set Threshold to 128 (half of 256)
Mode ➡ Bitmap: press the Threshold button and enter 50%
Printing equipment can create shading by using half-tones within numerous squares across the paper. Although effective this reduces the actual resolution of the image. Typically, each square contains 25 pixels, constructed 5 pixels high and 5 pixels wide. The density of these squares is measured in lines per inch (lpi), where each ‘line’ is really a line of squares.
A printer that uses half-toning employs a set number of lines per inch, related to its resolution by the following equation:-
A typical 300 dpi printer works at 60 lpi with 5 pixel high squares. Higher resolution printers often use 60 lpi for every 300 dpi of resolution. Hence a 1200 dpi printer might work at 240 lpi.
When scanning material for half-tone printing you might be surprised that you don’t have to scan at the highest possible resolution. In fact it’s best to use the following relationship:-
For example, suppose you want your final image to be printed at 60% of full size on a 600 dpi printer. In this instance 120 lpi is used and the scanning resolution should be:-
©Ray White 2004.