When setting up and investing in a suitable digitisation platform, the type of scanner system to be used is an instrumental part of the selection process. The purpose of this White Paper is to provide the sector with basic information, which should help you to understand and pin down the key functionalities and quality features of the digitisation systems and prevent you from making a bad investment.
The market basically falls into two categories of book scanning systems depending on the method used: The overhead scanner comprising a combination of line scan camera and light unit and the overhead camera as a scanner.
In the case of overhead scanner, line, optical system and illumination are intrinsically linked, moving over the document as a single unit. In the overhead camera, a digital camera is centrally mounted over the document to capture the image. Illumination is provided by a light source attached to the device, some devices are supplied without their own light.
Both systems almost exclusively use CCD (Charged-Coupled-Device) sensors. CCD sensors are sophisticated technological devices, their functionality has a proven track record in a variety of different image capture systems.
The main advantage of this sensor technology is its extreme high sensitivity to light, whereby disruptive interference, such as image noise, is kept to a minimum. Its structure is relatively simple, resulting in very few defective pixels in semi-conductor manufacture, whereby, owing to the lower overall number of pixels, line scan sensors are the only ones able to be delivered error-free.
Extract from White Paper – Book Scanner Quality – CCD line versus CCD array
The difference between the two scanning systems lies in the configuration of CCD chips as a line or an area sensor. Sometimes, the latter is also known as an array sensor. In the CCD line example, the individual light-sensitive cells are placed in a row. A two-dimensional image is created by the movement of the line across the document. This movement is designed in such a way that the sensor is advanced by exactly the distance of one pixel during the integration time. In order to produce colour images, pixels are typically arranged in extremely close proximity in three parallel rows. Each row has a colour filter – usually red, green and blue. This gives rise to a colour separation image.
The line length in standard overhead scanners is 7,500 to 10,680 pixels; in some instances, it can even exceed 20,000 pixels. In the case of a tri-linear colour line with, for example, a line length of 7,500 pixels, a line scan sensor has more than 22,500 discrete light-sensitive elements. Multiply this value by the sample rate x scan path to obtain the total resolution of the system. An important development in this connection: For the resolution proposed, overhead scanners – in contrast to overhead cameras – do not use interpolation methods to produce image content.
In the case of area sensors or array sensors, individual cells are arranged two-dimensionally. To obtain colour information, the cells are alternatively provided with R/G/B filters. In order to capture the full colour information of an individual pixel, either the whole array needs to be shifted and read several times or the missing colours must be interpolated from information contained in the neighbouring cells. The largest sensors on the market have up to 7,000 x 10,000 pixels, i.e. 70 megapixels, and are correspondingly expensive.
Expert advice: Volker Jansen, Technical Director, Zeutschel GmbH said: “The quality of digitised images depends on several different factors. From these, actual or real resolution, colour reproduction and homogeneous illumination are the most relevant.”
- Scanner systems with CCD line scan sensors support a much higher actual resolution than currently available systems with CCD area sensors. When processing an A2 document, for example, line scan sensors with a line length of 7,500 pixels offer a six times higher resolution than 14 megapixel area sensors.
- Although interpolation superficially increases the numerical value of the resolution, this does not apply to the quality of the raw data.
- ‘Colour interpolation’ often leads to image interference in the form of a ‘colour moiré”.
- Colour management according to ICC standards is essential. In terms of software, availability of ICC profiles and consistent support of ICC specifications are obligatory.
- Line scanners with LED lighting systems guarantee high levels of light efficiency and homogeneous illumination. Consequently, they are very good for use in a wide variety of different light conditions.
- Scanning systems with area sensors require optimal lighting conditions with low ambient light and, therefore, can hardly be put to use in open-access areas.
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