Scanning systems: how to check the LIDE scanning line. Why do you need a white stripe in a scanner?

We continue the series of publications of “general educational” articles about the operating principles of various components of printers and MFPs. This article will discuss the system automatic calibration scanner.

The main element of modern MFP scanners is the scanning line CCD (Couple Charge Device, Charge Coupled Device, CCD). Accordingly, it is advisable to know the principles of operation of charge-coupled devices in order to understand where the problems that arise in scanners “grow from.”

Note that nowadays, in MFP scanners, a “contact image sensor” (Contact Image Sensor, CIS ), which is based on the same principle of charge coupling. Roughly speaking, CIS is a module that combines: a CCD scanning line with a length equal to the size of the scanned image; a line of short-focus lenses that replaces the system of mirrors and lenses; and an exposure lamp, the role of which is often performed by a line of LEDs.

The theory of operation of charge-coupled devices is well described in an article on the website StartCopy.net, so we will not repeat it here, but we recommend reading it.

The main theses arising from the theory:

At the current stage of technology development, the scanning line in any scanner has differences in the sensitivity of individual pixels. It's unavoidable.

If you do not take any measures to correct differences in pixel sensitivity, the scanned image will certainly be “striped.” Therefore, an automatic calibration system is used in all scanners. The most common terms for this system are A.G.C. (Auto Gain Control, Automatic Level Control) and Shading Correction(Shadow correction).

The presence of an automatic calibration system, in addition to solving the main problem of correcting different pixel sensitivities, also solves other problems:

• There is no need to control the brightness of the exposure lamp. Within a certain range, the system can compensate for over- or under-bright lamp light. Accordingly, the lamp control circuit is simplified, and such a concept as “adjusting the brightness of the exposure lamp,” which was one of the key ones in analog machines, becomes unnecessary and is replaced by digital processing of the scanner output signal.
• There is no need to have a lamp with uniform brightness along its entire length. The system can compensate for brightness differences in the same way as it can compensate for differences in pixel sensitivity. This allows the LED line to be used as a lamp.
• The system can compensate for lamp aging, and even dust on mirrors. Up to a certain limit, of course

Everything seems to be beautiful and magical, but there is a “weak link” - for the correct operation of the scanner automatic calibration system, you need reference white stripe with the same whiteness along its entire length. And in real operating conditions, this strip becomes dirty in places and loses its whiteness. This leads to the fact that for the pixels of the CCD line, onto which the non-white section of the calibration strip is projected, the system sets an excessive correction - the image is brightened.

Practical aspects related to the scanner:

If you clean the scanner optics, don't forget to clean the white calibration strip as well. It plays a very important role in the performance of the calibration system.

Typical white stripe position

Despite the fact that the automatic calibration system is capable of compensating for dirt on mirrors and lenses, they also need to be thoroughly cleaned :). At a minimum, the correction values ​​should not go beyond the range in which the system operates correctly. In most cases, a dry, lint-free cloth is sufficient to clean mirrors and lenses. If the dirt cannot be wiped off with a dry cloth, then it is better to refrain from using “vigorous chemicals” and try first “soft” optics cleaners such as “ScreenClene” from Katun.

Stripes of a lightened image on copies, parallel to the movement of the scanner carriage, during normal prints in printer mode, almost unambiguously indicate that the white calibration strip in some places has ceased to be white. Clean it and all other optics.

A completely faded copy with normal prints in printer mode can have several reasons:

• The white stripe in the scanner is no longer white along its entire length.
• Contamination of the optics has caused the increased correction values ​​set by the calibration system to become inaccurate, i.e. overcompensation has occurred.
• Someone screwed up the user and/or service image brightness settings.
• “The designer was overlooked,” i.e. The machine's firmware has an inaccurate algorithm for processing the scanned image, which lightens the print. Unfortunately, such cases are not uncommon.

The treatment for such pallor is simple and banal - check the machine settings, clean the white strip, clean all mirrors and lenses, including those that are not easy to reach.

Sometimes contamination of the optics and the white stripe leads to the fact that the machine, being unable to set adequate correction values, produces a scanner error (AGC error, exposure lamp error, “scanner warming up”, etc.). If these errors appear, do not rush to immediately change the CCD module, as is often required by the service manual. The repeatedly mentioned thorough cleaning of everything in the scanner saves you from these errors quite often, although, of course, not always.

Some machines have the ability to adjust the position of the scanner carriage under the white bar during auto-calibration. This adjustment is useful in cases where there is permanent damage or dirt on the strip that does not cover the entire width of the strip.

And lastly, the screws that secure the CCD ruler, lens and some other parts of the scanning module are almost always painted over. This was not done due to excess paint in production :) but means that you don’t need to unscrew these screws, even if you really want to. The likelihood that the module will work normally after unscrewing/screwing the screws is very low.

Fourth type - film scanners for digitizing images from transparent originals, their other names film scanners or slide scanners. The design of film scanners is similar to that of flatbed scanners. The differences are in greater resolution (that is, in a larger number of photosensitive elements in the scanning line) and in the smaller physical dimensions of the devices themselves. This type of scanning device is better suited than any other for converting analog photographs into digital format.

Rice. 7.4.

Finally, the fifth type of scanners - drum scanners. Depending on the design (namely, the location of the scanning ruler and illumination lamp inside or outside the drum), they are designed for scanning opaque originals, films, or both. Instead of a glass slide, scanners of this type use drum, on the surface of which the originals are fixed. Scanning ruler and backlight installed still. When scanning, the drum rotates at high speed, and digitization is carried out line by line with each revolution of the drum with a slight linear shift along the surface of the original. This design makes it possible to achieve a minimum scanning step and, accordingly, high resolution and quality of digitization. Drum scanners are used in the field of high-quality printing and are very expensive, tens and even hundreds of thousands of dollars.

In amateur photography, flatbed and film scanners are used. Continuous scanners used in paperless systems document flow and in business (although there is no obstacle to using them to digitize paper photographs).

Let's take a closer look at the device of a flatbed scanner. Depending on the design of the scanning line flatbed scanners are divided into two groups - devices CCD, in which semiconductor charge-coupled devices and devices are used as photosensitive elements CIS(Contact Image Sensor), where combined contact sensors serve as photosensitive elements.

Rice. 7.5.

In the body of the CCD scanner, under the glass slide, there is optical block, moveable transport mechanism along the surface of the original. Installed in the optical block Fluorescent Lamp illumination, the spectral composition of the light flux of which is as close as possible to the spectrum of sunlight.

The light reflected from the surface of the original through a system of deflecting mirrors hits the surface of the semiconductor photosensitive element. Photosensitive elements are located on the same line. To ensure accurate recording of the brightness values ​​of the reflected light beam (and the scanning process consists precisely in fixing the difference in brightness of the light flux reflected by the original image), each photosensitive element is equipped with a microlens that focuses the reflected light on its surface. Physical scanner resolution determined by the step of arrangement of elements on the ruler. This pitch is measured in pixels per inch. Standard range of resolution values flatbed scanners looks like this - 300, 600, 1200, 2400 pixels per inch. That is, on each inch (2.54 cm) of the scanning line there are 300, 600, 1200 or 2400 photosensitive elements arranged in a row. One can imagine the degree of miniaturization of modern electronics, and we are talking about mass-produced inexpensive devices.

The image scanning process can be schematically described as follows. Backlight illuminates original surface. Rays of light pass through a translucent deflecting mirror, reflected from the original surface, return, deviate work surface translucent mirror and focusing microlens, fall on the photosensitive surface of the semiconductor element. Accumulates on the surface of the element electric charge, the value of which depends on the brightness of the illumination. These variable signals are amplified and sent to an analog-to-digital converter ( ADC), where on their basis a digital code is formed - a sequence of logical zeros and ones. Then the computer driver program, according to the digital data, restores an image identical to the image on the surface of the original.

Which scanners are better - CCD or CMOS? In this review we will try to answer this question by comparing the two technologies in several respects.

Linear charge-coupled device or CCD- this is analog integrated circuit, which consists of light-sensitive photodiodes capable of converting light energy into charge. Invented in 1969 by Willard Boyle and George Smith, the technology has gained widespread use thanks to Sony, which launched the production of CCDs for its cameras.

CMOS stands for Complementary Metal-Oxide-Semiconductor Structure. and is a photosensitive matrix consisting of a set of photodetectors. The technology originated in the 60s of the twentieth century, but gained popularity only in the early 1990s. The fact is that until that time, the gap in basic parameters with CCDs was so large that CMOS-based matrices did not receive noticeable development.

Use in scanners

The image processing process in a CMOS sensor begins with light rays that are reflected from the original hitting the surface of the scanning matrix. Further, during the exposure, charge is accumulated on the LEDs and the resulting parameters are subsequently read out. This occurs in any form and depends on the moment the photon hits the reading device.

By mixing red, green and blue, you can get any other color. One of the features of CMOS sensors is that each pixel only detects one of these colors. And in order to get the rest they use the so-called Bayer filter.

An important component of the operation of CMOS-based matrices is the use of a Bayer filter in the image creation process. The filter is used as a barrier in front of the matrix and allows only one color to pass through: green, red or blue. The final image is obtained by complex arithmetic averaging of information received from four (there are 2 times more green pixels, so not three, but four) neighboring sensors of different colors.

In contrast, scanners based on linear CCD sensors completely capture the red, green and blue lines, one after the other. The image is then projected onto a linear CCD sensor. The ruler moves and alternately captures red, green and blue elements. The computer then places the lines in in the right order, and generates an RGB image. The formation of an electronic image in full resolution occurs without a Bayer filter.

The Bayer filter factor directly affects the contrast and resolution of the scanned document. Under the same conditions of using CCD and CMOS, the dynamic range of the latter is often significantly inferior, since 2/3 of the color information is cut off by the filter.

Each subpixel in CMOS appears as a normal pixel, although it captures only one channel out of three. In a CCD system, each pixel receives all colors.

Resolution is the defining parameter for all digital images. Due to the “line-by-line” processing of elements, a linear CCD sensor is capable of forming the maximum number of pixels on each line.

On the other hand, the matrix “removes” the image in one step and is clearly limited in the number of dots per inch. The missing components are calculated by the processor based on data from neighboring pixels as a result of interpolation, which is not an increase in resolution, but only the assignment of ordinary color values ​​to nearby points. Therefore, the resolution of linear sensors, especially when scanning along the “narrow” side of the original, will be higher.

The most affordable planetary scanner with a three-line CCD in Russia is ELAR PlanScan A2B. Its price starts at 1.2 million rubles. A scanner of A2+ format with similar characteristics based on a 70 MP CMOS matrix - Book2net Kiosk starts from 2.5 million rubles.

In contrast to quality, matrices benefit in scanning speed, which, together with pixel processing, is 1-2 seconds, versus about 4-6 seconds of scanning with a CCD line.

Digitization processes

The specifics of working with stapled originals require the use of the V-shaped mode of the scanner cradle when the document is scanned with incomplete opening. This digitization process involves transferring the resulting image to a two-dimensional plane, or in other words, image alignment. In this respect, the matrix is ​​inferior to linear sensors. This is due to the fact that the entire image is captured, which necessitates focusing on the total scanning area, causing geometric distortions to occur at the edges of the electronic images and the need for V-mode alignment using a perspective compensation algorithm. Therefore, often CMOS scanners do not have V-mode or, in some models, it is implemented using two cameras for each half of the cradle.

Unlike CMOS, a linear sensor forms an image gradually. Additionally, linear CCD scanners are equipped with “tracking” laser focusing systems, which allows continuous focusing on the extreme points of the document, despite differences in height from the edge of the page to the spine, thereby providing the processor with accurate information for calculating compensation for geometric distortions when scanning in a V-shaped mode. Therefore, page alignment for “linear” scanners is done more holistically.

Scanning a book in V-mode followed by alignment

In the RSL scanning department, priority is given to scanners based on CCD sensors. The main fleet of equipment consists of ELAR PlanScan A2B planetary scanners, the scanning system of which includes 3 CCD lines of 7500 pixels each.

Separately, it is worth considering the tasks of large-format scanning. Planetary scanners based on CMOS matrices from A1 format and larger are also mainly equipped with two cameras at once. This was done to improve the resolution of large originals, but this approach involves a number of compromises. The first of them is the need to programmatically stitch together two halves of the original, which, even taking into account the use of advanced algorithms, entails a violation of the integrity of the image, especially when dealing with high-quality digitization of illustrations or text spreads.

Second, but no less important, is reliability and service. Since both cameras operate alternately, the risk of one element breaking is exactly doubled, and maintenance costs also increase. Do not forget that the failure of any of the cameras completely stops the operation of the scanner.

The cost of producing CMOS sensors increases with sensor resolution. If an 18 megapixel matrix is ​​cheaper than almost any CCD line, then a 70 megapixel matrix is ​​already significantly more expensive. Therefore, planetary scanners with an optical resolution of 300 dpi based on CMOS matrices are more expensive than similar scanners based on linear CCD sensors.

Linear CCD technology is widely used in high-quality scanning systems, which create detailed images in formats up to 2A0 without the use of software stitching. Such scanners can generate large-format images without geometric distortion or loss of resolution. Depending on the model, the scanning system either moves itself during the digitization process or is driven directly by the scanning surface, which passes under the static block of the scanning system. Serial planetary scanners based on CCDs are capable of achieving significant resolution levels - up to 600 dpi on A0+ format (with an original size of 914 mm x 1524 mm).

Along with this, a single scanning element reduces the number of nodes and Supplies in the scanning system, which increases the overall reliability of the design and reduces service costs.

A common problem with CMOS sensors is overheating. Since the matrix in the scanner is constantly in working condition, often during long-term operation the electronics heat up and lead to overheating of the pixel structure, which increases the digital “noise” of scan images. Linear CCD sensors, on the other hand, only turn on during scanning, so this problem is eliminated with linear sensors

Comparative testing

To clearly demonstrate the operation of scanning systems, we conducted a number of tests on key parameters. The German Microbox Book2net Kiosk based on a 70 MP CMOS matrix and the domestic ELAR PlanScan A2B with a three-line CCD sensor were selected as test subjects.

1. Color rendition

Scan color targets under the same ambient lighting conditions and resolution settings. Test digitization involves disabling all processing functions.

Both images of the test object meet professional standards, but the result of the CCD sensor differs in color accuracy with big amount distinguishable tones. This factor does not play a leading role when scanning text originals, but will be very significant when working with illustrations.

2. Uniformity of lighting

The operation of CMOS-based scanner illuminators is not related to the technological implementation of the scanning system - these are, as a rule, always independently mounted lamps. As a result, the system is highly sensitive to external factors, and the power of the light flux, forced to dissipate throughout the scanning area, is not enough to uniformly distribute light on the document. Therefore, one of the main features of CMOS scanners is the shading of the edges of the image. This is clearly visible on the test sample.

A scanner based on a CCD sensor shows a more uniform distribution of light output. Due to synchronous LED lighting, which passes along the original gradually, greater uniformity of illumination of the document is ensured.

3. Resolution

For visual inspection and evaluation of optical resolution, it is recommended to use the TO-2 test object (GOST 13.1.701-95), which is scanned in turn on each device at an angle of 45 degrees. The test object contains a set of perpendicularly located groups of pairs of lines with different frequencies. For evaluation, it is necessary to select a group whose elements are distinguishable and readable. Value 9.0 is comparable to 600 dpi resolution

According to the results of testing on objects obtained from a CMOS sensor, lines up to 6.3 are clearly readable and differ up to 7.1. Indicators 9.0 are indistinguishable. What, despite a large number of megapixels in the matrix (70 megapixels), does not correspond to the optical resolution indicators declared by the manufacturer, namely, it does not provide 600 dots per inch on an A2 format image.

On the test object of a scanner with a linear CCD, the 8.0 mark is clearly readable and the lines are distinguished up to a value of 9.0, which fully corresponds to the optical resolution of 600 dpi on the A2 format.

Scanning systems based on CIS (Contact Image Sensor) contact image sensors have become extremely popular among manufacturers of scanners, copiers, multifunctional office devices (MFPs), and faxes. And therefore, when repairing and diagnosing all these devices, it often becomes necessary to make sure that the scanning line, which is sometimes also called the scanning head, is in good working order. The author suggests familiarizing yourself with one of the diagnostic options for this most important part of scanning devices.

LIDE (Light Indirect Exposure) technology, developed by CANON, is one of the types of contact image sensors (CIS), called CIS in the literature. In contact image sensors, a line of photodetectors, most often phototransistors, are used to read a line. The number of photodetectors corresponds to the number of points in the scanned line, i.e. Each photodetector perceives one point (one pixel) of the scanned image. Each photodetector has its own focusing lens, which allows the light flux reflected from one pixel of the original image to be collected and focused on the surface of the photodetector. General principle scanning an image using CIS is shown in Fig. 1.

Rice. 1. Principle of image scanning using CIS

As can be seen from the figure, the line of light-sensitive sensors occupies the entire width of the scanned line, and at the same time fits as tightly as possible to the scanner glass. Since scanning is carried out at a 1:1 scale, there is no need for a complex optical system, which is the main advantage of CIS technology.

Main feature LIDE technology is an original scanning lamp design. In general, there is no lamp as such. Instead of a lamp, three LEDs are used, located on the side of the scanning head, and a specially shaped plastic light guide (Fig. 2). This light guide ensures that the light flux emitted by the LED is distributed over the entire length of the line and redirected to the scanned image.

Rice. 2. Scanning head light guide design

The internal structure of the LIDE scanning line is shown in Fig. 3. Thus, the scanning head contains three LED “lamps” with different colors glow, and these lamps must have independent control (Fig. 4).

Rice. 3. Internal structure of the LIDE scanning line

Rice. 4. LED lamp control circuit

When scanning color images, the original must be illuminated alternately with three different colors of light: red (R), green (G) and blue (B). When scanning in full color, these "lamps" switch at a fairly high frequency, resulting in the illusion that the document is being scanned with white light, which, in fact, is not true.

Development of cooperation in the office equipment production industry and peripheral devices led to the fact that the same LIDE head can be used in the most various devices different manufacturers. For example, in scanning devices entry level The CANON CIS head labeled CLG-60216G has become very widespread (Fig. 5). This LIDE module can be found in scanners from CANON and BENQ, in MFPs and copiers from CANON, in MFPs produced by Samsung, Xerox and HP. Such widespread use of this LIDE head has positive aspects, because service specialists have the opportunity to replace compatible modules from seemingly completely different and incompatible devices. For example, the author of this article successfully replaced the CIS module in the Samsung SCX-4100 device, and the replaced module was taken from the BenQ 5250C scanner.

Rice. 5. CANON CIS head marked CLG-60216G

So, if enough malfunctions of scanning devices occur, you can observe a situation where the scanning lamps do not turn on, and the scanner does not enter the ready mode, but goes into a fatal error state. There may be several reasons for this behavior of the scanner:

LIDE module malfunction;

Malfunction of the control microprocessor;

Malfunction of the motor moving the LIDE carriage;

Malfunction of the scanning carriage initial position sensor (if present).

Thus, a specialist diagnosing such a device needs to determine whether the LIDE head is working properly, or whether there is a problem in another unit.

Let's look at how to check the serviceability of the LIDE head using the example of the above-mentioned and widely used CANON CLG-60216G head.

Full check Checking the serviceability of the LIDE head is quite labor-intensive and requires, at a minimum, equipment such as an oscilloscope, multimeter, laboratory power supply and generator. The author proposes to consider a simplified version of diagnosing the CIS head, which consists of checking only its backlight module. Such diagnostics will make sure that all three lamps of the LIDE module are working properly.

The module in question has a 12-pin connector, which is used to connect it to the main board of the scanner via a flat cable (see Fig. 5). The pin assignments of this connector are shown in the table, and the location of pin 1 is shown in Fig. 6. Based on the information provided, the method for checking LIDE module lamps suggests itself.

Rice. 6. 12-pin module connector CLG-60216G

To turn on each lamp, you only need to apply the appropriate voltage to it. Therefore, for diagnostics you will only need one device - an adjustable power source capable of generating at its output constant pressure in the range 0...3.5 V.

The module testing procedure is as follows:

1. Turn on the power source and set its output voltage to about 3.3 V.

2. The “plus” of the power supply is applied to pin 8 (VLED).

3. The “minus” of the power supply is applied to pin 11 (RLED). As a result, the red LED “lamp” should light up. A change in the supply voltage should lead to a change in the brightness of the lamp.

4. Next, the “minus” of the power supply is applied to pin 10 (GLED). As a result, the green “lamp” should light up. Its brightness should change in proportion to the change in voltage at pin 8 (VLED).

5. Similarly, apply a blue lamp to pin 9 (BLED) of the power source.

Thus, having checked all three light sources, we can confidently say that the LIDE head backlight module is fully operational. General scheme diagnostic stand for checking the CIS module is shown in Fig. 7.

Rice. 7. Diagram of a diagnostic stand for checking the CIS module

When carrying out this testing procedure, you may encounter an interesting feature. The fact is that the red LED is the brightest and lights up even when a voltage of 2.5 V is applied to it, while the green and blue LEDs light up when the voltage on them is more than 3 V.

Sometimes there are LIDE heads with a 16-pin connector, but for them everything described above is absolutely true. The fact is that a 16-pin connector has four last contact are not used, and the purpose of the first 12 contacts completely coincides with what is described in the table.

Table. Assignment of connector pins on the CLG-60216G module

Contact no.	Designation	Description
		Data transmission line read by photodetectors
		"Earth" for photodetectors
		Photodetector supply voltage
		Reference voltage for photodetectors
		The starting pulse determines the moments when information is read by photodetectors
		Clock frequency for transmitting data read by photodetectors
		Supply voltage for LED lamps
		Blue control signal LED lamp(active low)
		Green LED control signal (active low)
		Red LED Lamp Control Signal (Active Low)

Of course, this technique is not complete and does not allow you to check the serviceability of photodetectors, but, nevertheless, it is very visual and informative, allowing you to make sure that the LIDE head is working “in principle”. This method Diagnostics are convenient to use in situations where there are suspicions about the serviceability of the control controller and the scanner connecting cable. The malfunction of these elements, as well as the LIDE head, manifests itself in the absence of glow of the scanning lamps, as well as in the beating of the scanning carriage against the edge of the copy table at the initialization stage when the scanner/MFP/copier is turned on.

In conclusion, I would like to draw your attention to the fact that you can check the LEDs of the LIDE module with the most ordinary tester in the “diode check” mode. To do this, you should “ring” the diodes between the VLED and RLED, GLED, BLED contacts. When testing, it is necessary to change the polarity of connecting the probes of the device in order to ensure the open and closed states of the module LEDs being tested. In this case, the glow of the “lamps” will not be intense (perhaps the lamps will not light up at all), and it will be impossible to control it. But, nevertheless, it is quite possible to get an answer to the question about the serviceability of the LEDs.