Technical Computer Peripherals Scanner

The Scanner

Simply put, a scanner converts light into 0s and 1s (binary). In other word, scanners convert analogue data into digital data.

All scanners work on the same principle of reflectance or transmission. The image is placed before the carriage, consisting of a light source and sensor, the light source could be the sun or artificial lights. Desktop scanners originally used fluorescent bulbs as light sources. Fluorescent bulbs have two distinct weaknesses, they rarely emit consistent white light for long, and while they're on they emit heat which can distort the other components. Most manufacturers have moved to 'cold-cathode' bulbs which differ from a standard fluorescent bulb in that there is no filament. They therefore operate at much lower temperatures and they are more reliable. Standard fluorescent bulbs are now, usually only found on low-cost scanners or older models.

To direct light from the bulb to the sensors that read light values, CCD scanners use prisms, lenses, and other optical components. A high-quality scanner will use high-quality glass optics that are colour-corrected and coated for minimum diffusion. Lower-end models will typically skimp in this area, using plastic components to reduce costs.

The amount of light reflected by or transmitted through the image and picked up by the sensor, is converted to a voltage proportional to the light intensity. The brighter the part of the image, the more light is reflected or transmitted, resulting in a higher voltage. Analogue-to-digital conversion (ADC) is sensitive and susceptible to electrical interference and noise in the system. In order to protect against image degradation, the better scanners on the market use an electrically isolated analogue-to-digital converter that processes data away from the main circuitry of the scanner. This adds costs to manufacturing, so many cheaper models include analogue-to-digital converters integrated into the scanner's own circuit board.

The sensor is implemented using one of three different types of technology. PMT (photomultiplier tube), CCD (charge-coupled device), used in desktop scanners and CIS (contact image sensor), which integrates scanning functions into fewer components, allowing scanners to be made smaller.

PMT sensor technology is used by expensive drum scanners. With PMT, light detected by the sensor is split into three beams which are passed through red, green and blue filters and then into photomultiplier tubes, where the light energy is converted into an electrical signal. PMT's have a much higher sensitivity to light and lower noise levels than CCD scanners. Drum scanners are capable of excellent tonal resolution, and are less susceptible to errors due to refraction or focus than flatbed scanners. Drum scanners are slow compared to CCD scanners and expensive and are generally used only for specialised high-end applications.

CCD technology has been in use for a number of years in devices such as fax machines and digital cameras. A charge-coupled device is a solid state electronic device that converts light into an electric charge. A desktop scanner sensor typically has thousands of CCD elements arranged in a long thin line. The scanner shines light through red, green and blue filters and the reflected light is directed into the CCD array via a system of mirrors and lenses. The CCD acts as a photometer, converting the measured reflectance into an analogue voltage, which can then be sampled and changed to exact digital values by an analogue-to-digital converter (ADC).

CIS scanners use dense banks of red, green and blue LEDs to produce white light and replace the mirrors and lenses of a CCD scanner with a single row of sensors placed extremely close to the source image. The result is a scanner that is thinner and lighter, more energy efficient and cheaper to manufacture than a traditional CCD-based device.

The technology used in a sensor mechanism is not the only factor that determines a scanners level of performance. The resolution, bit depth and dynamic range are equally important aspects of a scanners ability.

Resolution relates to the detail that a scanner can accomplish, and is measured in dots per inch (dpi). The more dots per inch a scanner can resolve, the more detail the scanned image will have. A typical A4 flatbed scanner has a CCD element for each pixel, so for a desktop scanner claiming a horizontal optical resolution of 600dpi (dots per inch), there would be an array of over 5,100 CCD elements in the scan head. The scan head is mounted and transported across the source image. The head moves a fraction of an inch at a time, taking a reading between each movement. A flatbed scanner uses a stepper motor which turns a set amount each time.
The number of elements in a CCD array determines the x-direction sampling rate, and the number of stops per inch determines the y-direction sampling rate. Although these are conveniently referred to as a scanner's ‘resolution', the term is not strictly accurate. The resolution is the scanner's ability to determine detail in an object and is defined by the quality of electronics, optics, filters and motor control, as well as the sampling rate.
Scanners typically offer resolutions of 2,400dpi, 4,800dpi and 9,600dpi. Scanners simply aren't capable of picking up this level of detail. The actual optical resolution of the CCDs in most modern scanners is 600 x 1,200dpi at best and all higher figures are based on interpolation. Basically, an integrated circuit chip in the scanner generates new data by taking the dots the scanner actually sees, and calculating where the dots in-between would most likely fall, using an algorithm to 'guess' the colour of the new dots by averaging the colour of adjacent dots. Software interpolation can increase the resolution even more than hardware interpolation. It is performed by the PC's processor under the control of the scanner's TWAIN driver software. Interpolated images will always seem too smooth and slightly out of focus.
Colour scanners have three light sources, one for each of red, green and blue primary. Some scanning heads contain a single fluorescent tube with three filtered CCDs, while others have three coloured tubes and a single CCD. The former produce the entire colour image in a single pass, the target being illuminated by the three rapidly changing lights, while the latter have to go back-and-forth three times. Colour scanners use one of two methods for reading light values Beam splitter or coated CCDs. When a beam splitter is used, light passes through a prism and separates into the three primary scanning colours, which are each read by a different CCD. This is generally considered the best way to process reflected light, but to bring down costs many manufacturers use three CCDs, each of which is coated with a film so that it reads only one of the primary scanning colours from an unsplit beam and produces results that are difficult to distinguish from those of a scanner with a beam splitter.

Bit-depth is a measure of how much information a scanner records about a pixel. When a scanner converts something into digital form, it looks at the image pixel by pixel and records what it sees. That part of the process is simple enough, but different scanners record different amounts of information about each pixel. How much information a given scanner records is measured by its bit-depth. Black and white scanning is known as a 1-bit scanner because each bit can only express two values, on and off. In order to see the many tones in between black and white, a scanner needs to be at least 4-bit (for up to 16 tones) or 8-bit (for up to 256 tones). The higher the scanner's bit-depth, the more accurately it can describe what it sees when it looks at a given pixel. This, in turn, makes for a higher quality scan.
Most modern colour scanners are at least 24-bit and reads 8 bits of information for each primary scanning colour. A 24-bit unit could capture over 16 million different colours and is near-photographic quality. 30-bit and 36-bit scanners, can capture billions of colours. Software program that can read a 30-bit or 36-bit image, can use the extra data to correct noise in the scanning process and other problems that effect the quality of the scan. Scanners with higher bit-depths tend to produce better colour images.

Dynamic range is similar to bit-depth and measures how wide a range of tones the scanner can record. Dynamic range is measured on scale from 0.0 (perfect white) to 4.0 (perfect black), and the single number given for a particular scanner tells how much of that range the unit can distinguish. Most colour flatbeds have difficulty recording the subtle differences between the dark and light colours at either end of the range, and tend to have a dynamic range of about 2.4. High-end units are usually capable of a dynamic range between 2.8 and 3.2, and are suited to demanding tasks like a print press. Drum scanners frequently have a dynamic range of 3.0 to 3.8, and deliver a very high colour quality.

A 24-bit scanner should offer an 8-bit range (256 levels) for each primary colour which is accepted as being indiscernible to the human eye. A few of the least significant bits are lost in noise, while any post-scanning tonal corrections reduce the range still further. It's best to make any brightness and colour corrections in one go from the scanner driver before making the final scan itself. 30 or 36 bit scanners have a much wider range to start with, offering better detail in the shadow and highlight areas, allowing tonal corrections. A 30-bit scanner collects 10-bits of data for each of the red, green and blue colour components while 36-bit scanners collect 12-bits for each. The scanner driver allows control over which 24 of the 30 or 36 bits are kept and which ones are discarded by changing the Gamma Curve.

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