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The Debate of CIS vs CCD Is Over: The 6 Essential Elements When Selecting a Document Scanner　

1

When discussing the image quality of a document scanner, it is nonsense to merely assert the good or bad points based on an evaluation of only the technology and performance of individual image processing modules. It is important to evaluate the total image quality provided by the entire system of the document scanner by integrating individual modules and furthermore by fusing hardware processing with software processing.

CIS (Contact Imaging Sensor) and CCD (Charged Coupled Device) are representative examples of image processing modules, but the technology of CIS has dramatically improved over the last 10 years. Today, a comparison of CIS with CCD in image quality doesn't have any significant meaning. For most document scanners, CIS provides the highest quality images with advanced technology and delivers the most reliable performance.

This article takes Panasonic's document scanner as an example of the following 6 elements that must be considered by users.

Executive Summary

1. Colour reproducibility

2. Scanning speed

3. Focal length depth of field

4. Magnification

5. Resolution and clarity

6. Power consumptions

--- P2

--- P3

--- P4

--- P5

--- P6

--- P7

2

Whether the technology used for image processing is CIS or CCD, both sufficiently reproduce the colour spectrum.

Since the colour space of the document scanner is generally converted to a colour space that is suitable for the display monitor, the colour gamut and gradation become narrower than the document. However, both CIS and CCD have sufficient colour gamut and gradation to reproduce images on a display monitor. As a result, the colour reproducibility of CCD is not a superior technology over CIS. In addition, Panasonic uses the following IT8 colour chart to achieve more faithful colour reproducibility.

Colour reproducibility1

IT8 chart reproduced with CIS

3

Differences between a CIS module and a CCD module do not cause any difference in scanning speed. The scanning speed depends on the performance of the entire system of the document scanner, including mechanical, hardware and software capabilities.

While the structure of a CCD module cannot split lines into parallel data output, a CIS module consists of multiple small pixel chips that are connected together for the entire reading width. Each chip independently reads out pixel data. As a result, the more chips there are in the parallel data readout, the faster the scanning speed. Panasonic has developed and employed hardware for parallel processing of the image correction that was previously processed by the later stage, for higher speed throughout the entire system.

For document scanners in recent years, as sophisticated image processing performance, such as OCR and barcode recognition, are required, the scanning speed depends on the performance of the entire system, i.e., not only the device performance but including the software.

Compared with CIS, in order to achieve higher speed with CCD, a major investment has been required in die shrink fabrication process. Also, since a high-quality A3-type CIS module has become available at relatively low cost, Panasonic believes that even higher-quality, higher-speed products will evolve employing CIS.

Differences in CCD and CIS

CIS realizes higher speed with parallel processing

Scanning speed2

Scan direction

Scan direction

Glass plate Glass plate

CCDLens

LEDMirror Photodetector

Light source

Data readout

Hardware image processing

Image Output

Light guideErect life-size lens

CCD CIS

4

Compared with the SELFOC lens used on a CIS module, the lens used on a CCD module has a larger depth of field because it shrinks and records the image.

However, thanks to the remarkable improvements in the SELFOC lens capabilities, a depth of field of up to about 0.25 mm is now available. Since almost all paper thickness is less than 0.1 mm, generally speaking, the depth of field doesn't cause any problems in daily document scanning.

In addition to this, Panasonic overcomes the performance difference between CIS and CCD modules with its own technology. Panasonic's duplex scanners using a CIS module in recent years adopt Panasonic's own system, which always keeps the reading surface within the focus range by moving one of the opposing CIS modules up and down in order to be able to scan even thicker documents, such as plastic cards (2.4-mm thick) and passports (4-mm thick). This contributes to obtaining a clear image without distortion.

Movable feeder mechanism

Focal length depth of field3

Thin paper

CIS movable unit

CIS fixed unit

CIS movable unit

CIS fixed unit

Thick paper

Unit moves down

Vertical movement of unit maintains focal distance constant

Unit moves up

5

With a CIS module, the magnification from the paper surface to the sensor is 1:1 and distortion is prevented as the magnification is consistent for the entire reading width of the document.

In the case of a CCD module, distortion may occur at the time of imaging because a reduction lens is used and the magnification is not necessarily always consistent.

Comparing distortion in CCD and CIS imaging

Magnification4

Document Document

Lens

Lens

Sensor

SensorLens

Lens

Sensor

When the document floats When the document floats

Sensor

Scanning image

Scanning image

Document Document

Vertical line reproduction

CCD

Distortion occurs

Distortion increases near

both ends

Entering the interior of the readout area

No distortion even when the

document floats

CIS

CCD CIS

6

A SELFOC CIS lens offers consistent sharpness or focus over its entire document length. There is no difference in the resolution or sharpness of an image at the edges and the centre of the document because the magnification from the paper surface to the sensor is 1:1. On the other hand, as one of the natural characteristics of the CCD module, sharpness decreases due to differences in the focal position between the center and the edge of the reduction lens.

In addition, as an optical characteristic of the CCD reduction lens, "limb darkening," which is an effect whereby the light intensity passing through the lens reduces the farther that it travels from the centre. This is capable of influencing the dynamic range at both edges of the image, and the signal-to-noise ratio.

Spherical aberration of the CCD reduction lens

Resolution and clarity5

Spherical lens

The bokeh effect occurs due to differences in the focal position of the light outer

periphery and centre vicinity of the lens

7

Since a CCD module has a long optical path, generally a brighter light source is required than CIS. This possibly causes the power consumption of the device to increase. On the other hand, there is no doubt that a CIS module will deliver equivalent image quality and throughput performance at a power that is much lower than is required for a comparable CCD module as the distance from the paper surface to the sensor chip is very close and a sufficient output level can be ensured.

Another element concerning the CCD reduction lens system is that it requires more light intensity as it has to illuminate a fairly small pixel site through the reduction lens. Therefore, the CCD module generally consumes more power than the CIS module.

Optical path length comparison

Power consumption6

CCD CIS

Light source Light sourceMirror

Imaging device(3 rows)

Lens

Lens

Imaging device

Longoptical

path

Short optical

path

8

When selecting a document scanner, rather than concentrating on the single aspect of which image processing module is best, CIS or CCD, we need to consider the 6 elements: 1. Colour reproducibility, 2. Scanning speed, 3. Focal length depth of field, 4. Magnification, 5. Resolution and clarity, and 6. Power consumption, in order to find a superior product as a total system.

Panasonic document scanners are highly recommended for the excellent balance that they achieve in these elements.

Conclusion

1.6.

2. 5.

3.4.

Colour reproducibility

6elements

Powerconsumption

Scanningspeed

Resolutionand clarity

Focal lengthdepth of fieldMagnification

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