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Will digital cameras eventually replace line scanner technology?

The following technical excursus will explain:

•The technology of reduction line sensors, contact image sensors and area sensors used in digicams.

•The applications for the various technologies.

•The benefits of each technology from the view of the scanner world.

•Why the question “Chip or CCD” is nonsense.

•Why some vendors call a digicam on an expensive frame a “Scanner”.

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The abbreviation CCD stands for Charge Coupled Device. A charge coupled device is nothing more than a long analog shift register. It can be charged in parallel with the accumulated charge of many photo-sensitive elements, which were previously exposed to light.

•A CCD is used to transfer parallel accumulated electrical charges or voltages in a serial fashion to one or more outputs.

•Every optical sensor has a CCD shift register in it, whether it is a line camera or an area camera.

•It works simply and very effectively.

•It does not use much space on the silicon.

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A line sensor has between one and four lines of photo sensors aligned in rows. B&W line sensors use one line of photo sensors; tri-linear sensors have three: one each for red, green and blue. Some sensors use a fourth line of photodiodes to capture an RGB image and a B&W image at the same time.

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• Line sensors capture lines of 5.000, 7.500 or 10.000 pixels per exposure.

• To capture a 2D image either the CCD or the scanned object have to be moved in sync with the exposure.

• RGB data is shifted to the outputs via three or more CCD shift registers.

• External electronics convert the analog voltage into the digital domain.

An area sensor has an array of photodiodes with a relation 4:3, 16:9 or 16:10. They come in sizes of 12-50 megapixels. The pixels capture only one color each, typically there are one red, one blue and two green per cell. This pattern is called the “Bayer” pattern.

•Area sensors capture a full image per exposure.

•RGB data is shifted to the output via three or more CCD shift registers.

•External or internal electronics convert the analog voltage into the digital domain.

•Software interpolates the missing two colors per pixel.

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Line Sensor Area Sensor

Application: Cameras for full frame pictures with short exposure time when movement of objects cannot be controlled.

•Color accuracy reduced by interpolation.

•Limited resolution due to Bayer pattern.

•Small sensor area <10µm² produces noise.

•Cost of sensors very high, proportional to the square of the resolution in dpi.

•A 50 Megapixel area sensor has an MTF or resolution of not more than 30,000,000 pixels compared to a line sensor.

•High f-stop necessary to achieve some focal depth for 1D curved surfaces.

Application: Scanners, inspection cameras and others when movement of object or camera is controlled.

•Color accuracy very high, no interpolation.

•MTF and resolution very high.

•Large sensor area >70µm² reduces noise.

•Cost of sensors fairly low, proportional to the resolution in dpi.

•A 7500 tri-linear line sensor which scans 10,000 lines has effectively 75,000,000 red, green and blue pixels.

•Line sensor equipped scanners can follow a 1D curvature like a book fold.

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The abbreviation stands for Contact Image Sensor. CIS modules have the photo sensors, the LED illumination and a 1:1 lens build into one solid block. They have the most compact design and can be made longer than one meter.

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• Our CIS sensors capture 12” at 600dpi or 7.224 pixels at a time.

• To capture a color image, the red, green and blue LEDs illuminate the object one after the other at three times the vertical resolution.

• RGB data is shifted to the output via three or more CCD shift registers.

• External electronics convert the analog voltage to digital.

Line Sensor CIS Sensor

Application: Medium speed scanners which must have contact to the scanning surface.

•Color accuracy somewhat reduced due to limited quality of LED light.

•Low focal depth. Less than 0,5mm. MTF and resolution limited by multiple exposures.

•Cost of CIS sensors low due to low cost Selfoc lens and high integration of all parts.

•Large chips and three exposures for color limit speed.

•Large sensor area >1500µm² reduces noise.

•Insensitive to mechanical tolerances, easy 2D adjustment.

Application: High speed scanners, inspection cameras etc. some distance away from object.

•Color accuracy can be very high if white LED illumination is of high quality.

•Focal depth fairly large depending on dpi. MTF and resolution very high.

•Cost of overall camera system high because of high quality lens and mirrors necessary.

•Small chips and three independent sensors for RGB allow very high speeds.

•Small sensor area <70µm² produces noise.

•Sensitive to mechanical tolerances and tricky 3D adjustments due to long track length.

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• All sensors are chips and they all use CCD shift registers. Chip versa CCD is not a meaningful statement.

• Line sensors are best for highest quality scanners. Image Access uses line sensors in all and all but one scanners.

• Only scanners equipped with a line sensor can follow a 1D curvature like all Bookeye 3 and Bookeye 4 scanners do.

• High quality dual light CIS sensors are the second choice if price is the driving force. These are found in the new cost effective

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• To call a “digicam on an expensive frame” a “Scanner” like Microbox and I2S currently do is misleading. It only makes sense, if two cameras are used with a V-shaped cradle like Kirtas -- but this is obviously very costly.

• To be able to follow the curvature of a book, one must scan horizontally with a line camera. Scanning vertically like Zeutschel needs a high f-stop, low aperture, a lot of light and still does not allow for 1D optical curvature correction.

• There is no high quality scanning without high quality, controlled light. Scanners that claim to be able to scan with “no light” like I2S rely on the unreliable ambient light.

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Thomas IngendohCEO

+49 202 270 5812ingendoh@imageaccess.de

www.imageaccess.de

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