Minilab scanning devices. Main characteristics of scanners: optical and interpolated resolution

To convert negatives or slides into digital format, a special device is used - a film scanner. It is different from regular scanner in that it is designed for processing small transparent images that have high resolution. Although many are equipped with special modules that allow you to scan slides, the resulting product is of low quality.

Only CCD (CCD) scanning elements can provide the required high-resolution images. That's why all film scanners are built using them. Some models have one CCD line. In this case, conversion to digital format requires three passes, which prolongs the scanning, but does not affect its result. Basically, a film scanner has a CCD matrix, and the image is digitized in one pass. Some models use multiple passes to reduce errors in the final image.

An important parameter that you should pay attention to when choosing a scanner is optical resolution. The width of the most common film is 35 mm, and the image itself is even smaller. Therefore the value optical resolution must be at least 2400 dpi (dots per inch). There are scanners that provide 4800 and 5400 dpi. And although the current level of technology allows us to achieve even higher values, this is impractical - the grain size of even a fine-grained film will be much larger than a pixel.

Particular attention should be paid to dynamic range or optical density. The higher the value of this parameter, the better the negative scanner can reproduce halftones and smooth color transitions. For high-quality film processing, the optical density value should be in the range from 3.2 D to 3.6 D. There is no point in purchasing models with any more, since the vast majority of films have exactly these values.

The quality of digitization is also influenced by the bit depth of light, which characterizes color rendition. A modern film scanner may have a 42- or 48-bit color representation, but processing in this format is used only inside the scanner and serves to reduce conversion “noise.” The final image has a standard 24-bit color encoding for computer technology.

In most cases, a slide scanner is connected to a computer via a USB interface. More expensive models can connect via SCSI-2 and (FireWire). In this case, quite often the kit includes a board with this controller.

A film scanner almost always has an image enhancement feature. These include Digital ICE, which allows you to remove specks of dust and scratches from an image without affecting the main image, and Digital GEM, which allows you to eliminate graininess, and Digital ROC, which allows you to restore colors in faded photographs, etc. Quite often, all these tools are combined in one package Digital ICE4 Advanced. The use of these technologies significantly prolongs scanning, but the result is excellent. Similar transformations in Photoshop will require much more time, and the result is by no means guaranteed.

At first glance, the idea of ​​creating flatbed scanner with an optical resolution of more than 600 ppi, not intended for working with transparent originals, seems rather doubtful - after all, for the vast majority of originals scanned in reflected light, 300-400 ppi is more than enough. However, we should not forget that a significant proportion of the originals scanned both at home and in the office are images printed using the printing method. Due to interference phenomena that occur when digitizing rasterized images, noticeable moire appears on the resulting image, which is quite difficult to combat without compromising the quality or size of the image. To combat such phenomena, special algorithms embedded in scanning control programs are used. As a rule, the operation of the moire suppression function is based on scanning the original with excess (that is, greater than the user-specified) resolution and further software processing of the resulting image. This is where the advantage of high-resolution scanners will be obvious in the truest sense of the word.

Main technical parameters of scanners

Resolution

Resolution, or resolution, is one of the most important parameters characterizing the capabilities of a scanner. The most common unit of measurement for scanner resolution is number of pixels per inch (pixels per inch, ppi). Ppi should not be equated with the more well-known unit dpi (dots per inch- number of dots per inch), which is used to measure the resolution of raster printing devices and has a slightly different meaning.

Distinguish optical And interpolated permission. The optical resolution can be calculated by dividing the number of photosensitive elements in the scanning array by the width of the tablet. It is easy to calculate that the number of photosensitive elements in the scanners we are considering, which have an optical resolution of 1200 ppi and a Legal tablet format (that is, a width of 8.5 inches, or 216 mm), should be at least 11 thousand.

Speaking about a scanner as an abstract digital device, you need to understand that optical resolution is sampling frequency, only in this case the countdown is not based on time, but on distance.

In table 1 shows the required resolution values ​​for solving the most common problems. As you can see, when scanning in reflected light, a resolution of 300 ppi is sufficient in most cases, and higher values ​​​​are required either to scale the original to a larger size, or to work with transparent originals, in particular with 35 mm transparencies and negatives.

Table 1. Resolution values ​​for solving the most common problems

Many manufacturers, in an effort to attract buyers, indicate in the documentation and on the boxes of their products an optical resolution value of 1200 * 2400 ppi. However, a figure twice as large for the vertical axis means nothing more than scanning with half a vertical step and further software interpolation, so in this case the optical resolution of these models actually remains equal to the first figure.

Interpolated resolution is an increase in the number of pixels in a scanned image through software processing. The interpolated resolution can be many times greater than the optical resolution, but remember that the amount of information obtained from the original will be the same as when scanning with optical resolution. In other words, it will not be possible to increase image detail when scanning at a resolution exceeding optical.

Bit depth

Bit depth, or color depth, determines the maximum number of values ​​that a pixel color can take. In other words, the higher the bit depth when scanning, the large quantity shades may be contained in the resulting image. For example, when scanning a black and white image with 8 bits, we can get 256 shades of gray (2 8 = 256), and using 10 bits we can get 1024 shades (2 10 = 1024). For color images, there are two options for the indicated bit depth - the number of bits for each of the basic colors or the total number of bits. The current standard for storing and transmitting full-color images (such as photographs) is 24-bit color. Since when scanning color originals the image is formed according to the additive principle from three basic colors, each of them has 8 bits, and the number of possible shades is slightly more than 16.7 million (2 24 = 16,777,216). Many scanners use a higher bit depth - 12, 14 or 16 bits per color (the total bit depth is 36, 42 or 48 bits, respectively), but for recording and further processing of images this function must be supported by the used software;

otherwise, the resulting image will be written to a 24-bit file. It should be noted that a higher bit depth does not always mean more Images. When indicating 36- or 48-bit color depth in documentation or promotional materials, manufacturers often remain silent about the fact that some of the bits are used to store service information.

Dynamic range (maximum optical density)

As you know, darker areas of an image absorb more light falling on them than lighter areas. The optical density value indicates how dark a given area of ​​the image is and, therefore, how much light is absorbed and how much is reflected (or passed through in the case of a transparent original). Typically, density is measured for a standard light source that has a predetermined spectrum. The density value is calculated using the formula:

where D is the density value, R is the reflectance coefficient (that is, the proportion of reflected or transmitted light).

For example, for a section of the original that reflects (transmits) 15% of the light incident on it, the density value will be log(1/0.15) = 0.8239.

The higher the maximum perceived density, the more dynamic range of this device. Theoretically, the dynamic range is limited by the bit depth used. Thus, an eight-bit monochrome image can have up to 256 gradations, that is, the minimum reproducible shade will be 1/256 (0.39%), therefore the dynamic range will be equal to log(256) = 2.4. For a 10-bit image it will be slightly more than 3, and for a 12-bit image it will be 3.61.

Essentially, this means that a scanner with a higher dynamic range can better reproduce dark areas of images or simply dark images (such as overexposed photographs). It should be noted that in real conditions the dynamic range is less than the above values ​​due to the influence of noise and crosstalk.

In most cases, the density of opaque originals scanned for reflection is less than 2.0 (which corresponds to an area of ​​one percent reflection), and a typical value for high-quality printed originals is 1.6.

Slides and negatives may have areas of density greater than 2.0.

Light source

1. The light source used in the design of a particular scanner greatly affects the quality of the resulting image. There are currently four types of light sources in use: Xenon gas discharge lamps . They are distinguished by extremely short switching times, high radiation stability, and long service life. But they are not very efficient in terms of the amount of energy consumed and the intensity luminous flux, have a non-ideal spectrum (which may cause a violation of color rendition accuracy) and require high voltage(about 2 kV).
2. Hot cathode fluorescent lamps. These lamps have the highest efficiency, a very smooth spectrum (which can also be controlled within certain limits) and a short warm-up time (about 3-5 s). The negative aspects include not very stable characteristics, rather significant dimensions, a relatively short service life (about 1000 hours) and the need to keep the lamp constantly on while the scanner is operating.
3. Cold cathode fluorescent lamps. Such lamps have a very long service life (from 5 to 10 thousand hours), low operating temperature, smooth spectrum (it should be noted that the design of some models of these lamps is optimized to increase the intensity of the luminous flux, which negatively affects the spectral characteristics). For the listed advantages, you have to pay for a rather long warm-up time (from 30 s to several minutes) and higher energy consumption than hot cathode lamps.
4. Light emitting diodes (LED). They are usually used in CIS scanners.

Color LEDs have very small dimensions, low power consumption and do not require time to warm up. In many cases, three-color LEDs are used, which change the color of the emitted light at high frequencies. However, LEDs have a rather low luminous flux (compared to lamps), which reduces scanning speed and increases the noise level in the image.

A very uneven and limited radiation spectrum inevitably entails a deterioration in color rendering.

Scanning speed and warm-up time During testing, the time required for a “cold” start and recovery from power saving mode was measured. To evaluate the performance of the tested scanners, we measured the time required to complete several of the most common tasks. The countdown began from the moment you pressed the Scan (or similar) button in the application from which the scan was performed, and ended after

Even the highest resolution will not be able to produce a high-quality image if the digital values ​​obtained during scanning do not adequately reflect the colors of the original image. For correct color rendering, two scanner characteristics play an important role.

Firstly, this is color depth, i.e. the number of bits used to encode the color of each digitized pixel.

Secondly, this is dynamic range, i.e. the range of shades in the original that the scanner can distinguish, from completely transparent to completely opaque.

About color depth

Much of the modern software that comes with a scanner produces a 24-bit color file. However, the scanner's internal analog-to-digital conversion can specify color values ​​with 30, 36, or even more bits. This implementation is adopted because the 16 million colors available at 24 bits per pixel (8 bits for each of the primary colors - red, green and blue) may be unevenly distributed in the image. Most often, shades are lost in the shadows and in the lightest areas.

We must not forget that for anyone semiconductor device characteristic is the presence of noise - photosensitive elements (CCD and CDI) are no exception. The circuits of the analog-to-digital converter also introduce a certain error into the analog signal.

With a very high bit depth, and therefore accuracy, of analog-to-digital conversion, it is quite easy to “catch” signals that are very similar to noise. Hardware circuits and software modules can simply discard (filter) this noise-like information. This leaves a wide enough range of values ​​to process and save in the final 24-bit file. By software scanners determine those 24 bits out of, for example, 30, which correspond to the best reproduction of light and shadows. Thus, increasing the bit depth of analog-to-digital conversion leads to “stretching” the color depth at the scanner output to a full 24 bits.

Unfortunately, the color depth characteristic cannot judge whether all these bits actually contain visual important information. The sensitivity of the sensors and the quality of the analog-to-digital circuit, as well as some other factors, play a significant role in the quality of the final image. However, on average, we can assume that the higher the bit depth of the scanned image, the higher the quality of the picture, although according to many assurances the human eye is not “designed” for a color depth of more than 24 bits.

About dynamic range

This characteristic is extremely rarely indicated for scanners belonging to the lower class, but it is very important for professional work with images, and, first of all, when working with films. Optical density is inextricably linked to the dynamic range characteristic.

Optical density is a characteristic of the original. It is calculated as the decimal logarithm of the ratio of light incident on the original to light reflected from the original (for opaque originals) or transmitted through the original (for slides and negatives). Minimum possible optical density value 0.0D– this is a perfectly white (transparent) original. Meaning 4.0D corresponds to the extremely black (opaque) original. When applied to a scanner, its range of optical densities characterizes the scanner's ability to distinguish nearby shades (this is especially critical in the shadows of the original). The maximum optical density of the scanner is the optical density of the original, which the scanner also distinguishes from “complete darkness”. The scanner will not be able to distinguish all shades of the original “darker” than this border. In practice, this means that an “office” scanner can lose all the details, both in the dark and light areas of even an ordinary photograph, not to mention scanning a slide, and especially a negative.

So, for example, if the scanner has a dynamic range equal to 2.5D, then it will be able to adequately digitize photographs, but will not be able to work with negatives having an optical density of more than 3.0 D, i.e. that the scanner will not perceive the darkest areas of the image and will produce an incomplete scan.

A typical film has a minimum density of about 0.3 (50% transparency) and a maximum density of up to 3.3 (99.5% opacity): the range is around 3.0, although some slides range as high as 3.6. If the slide has maximum density ( ) 3,3 Dmax 3,0, , and the scanner operates with values ​​only up to 3,0 then the color details are denser

, most likely, will turn out to be black.

Inexpensive flatbed scanners have dynamic range 2.0-2.7D, good 36-bit 3.0-3.3D, latest models - 3.6D. The range of optical densities of the scanner is determined, first of all, by the quality, type and bit capacity of the ADC, CCD matrix and the operating algorithm of the scanner controller, i.e. built-in scanner software. Mathematical dynamic range limit for a scanner with a 30-bit ADC - 3.0D, and for a 36-bit scanner - 3.6D(decimal logarithm of the number of possible gradations for each color, which is equal to 2 to the power of the number of digits per color).

It is worth understanding that it is impossible to scan a negative with acceptable quality using a regular 30-bit flatbed scanner, even if a slide module is sold for it. Even a 30-bit scanner with best-in-class real dynamic range can scan color slides tolerably, but don't expect acceptable results with artistic black-and-white negatives shot by a professional photographer. For negatives you need a different class of scanner.

Comparisons between density ranges should be done with caution. There are no standard procedures for measuring and recording the density range. Some manufacturers may perform tests to measure the actual, practical range. Others only give theoretical limits for their scanners. You cannot make a decision about choosing one or another model only on the basis of the stated characteristics - it is better to perform several test scans.

It should be noted that slide scanners with densities above 3.4 cost over $10,000. This is certainly expensive, but flatbed scanners with a comparable density range, such as Agfa's SelectScan Plus, Linotype-Hell's Topaz and Scitex's Smart 340, cost more than $30,000.

You always have to pay a considerable price for quality.

Original type. Scanning can be done in transmitted light (for originals on a transparent backing) or reflected light (for originals on an opaque backing). Scanning negatives is particularly challenging because the process is not simply about inverting the color gradations from negative to positive. To accurately digitize color in negatives, the scanner must compensate for the color photographic veil on the original. There are several ways to solve this problem: hardware processing, software algorithms for transitioning from negative to positive, or lookup tables for specific types of film.

Optical resolution. The scanner does not take the entire image, but line by line. A strip of light-sensitive elements moves along the vertical surface of the flatbed scanner and captures the image point by point, line by line. The more photosensitive elements a scanner has, the more points it can remove from each horizontal stripe Images. This is called optical resolution. It is usually calculated by the number of dots per inch - dpi (dots per inch). Today, a resolution level of at least 600 dpi is considered the norm.

Speed ​​of work. Unlike printers, the speed of scanners is rarely indicated, since it depends on many factors. Sometimes the scanning speed of one line is indicated in milliseconds.

Color depth measured by the number of shades that the device is able to recognize. 24 bits corresponds to 16,777,216 shades. Modern scanners are produced with color depths of 24, 30, 36, 48 bits.

Dynamic range characterizes what range of optical densities of the original the scanner can recognize without losing shades in either the highlights or shadows of the original. The maximum optical density of the scanner is the optical density of the original, which the scanner also distinguishes from complete darkness. The scanner will not be able to distinguish all shades of the original darker than this border.

Batch processing - This is scanning multiple originals at the same time, saving each image in separate file. Program batch processing allows you to scan a certain number of originals without operator intervention, providing automatic switching of scanning modes and saving scanned files.

Zoom range - This is the range of amounts of original rescaling that can be done during scanning. It is related to the resolution of the scanner: the higher the maximum optical resolution, the greater the magnification factor of the original image without loss of quality.

By interface type scanners are divided into only four categories:

Scanners with a parallel or serial interface connected to an LPT or COM port. These interfaces are the slowest. Problems may arise due to a conflict between the scanner and the LPT printer, if there is one.

Scanners with a USB interface Cost a little more, but work much faster. A computer with a USB port is required.

Scanners with a SCSI interface, with their own interface card for the ISA or PCI bus, or connected to a standard SCSI controller. These scanners are faster and more expensive than the representatives of the two previous categories and belong to a higher class.

Scanners with a modern FireWire interface (IEEE 1394) specially designed for working with graphics and video. Such models have been introduced to the market relatively recently.

Optical resolution - measured in dots per inch (dpi). A characteristic showing that the higher the resolution, the more information about the original can be entered into the computer and subjected to further processing. A characteristic often cited is “interpolated resolution” (interpolation resolution). The value of this indicator is questionable - this is a conditional resolution to which the scanner program “undertakes to count” the missing points. This parameter has nothing to do with the scanner mechanism and, if interpolation is still needed, then it is better to do it after scanning using a good graphics package.

Color depth

Color depth is a characteristic that indicates the number of colors that a scanner can recognize. Most computer applications, excluding professional graphics packages such as Photoshop, work with 24-bit color representation (total number of colors - 16.77 million per pixel). For scanners, this characteristic is usually higher - 30 bits, and, for the highest quality flatbed scanners, - 36 bits or more. Of course, the question may arise - why does the scanner recognize more bits than it can transfer to the computer. However, not all bits received are equal. In scanners with CCD sensors, the top two bits of the theoretical color depth are usually “noise” and do not carry accurate color information. The most obvious consequence of “noise” bits is the insufficiently continuous, smooth transitions between adjacent gradations of brightness in digitized images. Accordingly, in a 36-bit scanner, the “noise” bits can be shifted far enough, and in the final digitized image there will be more pure tones per color channel.

Dynamic range (density range)

Optical density is a characteristic of the original, equal to the decimal logarithm of the ratio of light incident on the original to reflected light (or transmitted - for transparent originals). The minimum possible value is 0.0 D - a perfectly white (transparent) original. A value of 4.0 D is a completely black (opaque) original. The dynamic range of a scanner characterizes the range of optical densities of the original that the scanner can recognize without losing shades in either the highlights or shadows of the original. The maximum optical density of the scanner is the optical density of the original, which the scanner also distinguishes from complete darkness. The scanner will not be able to distinguish all shades of the original darker than this border. This value very well separates simple office scanners, which can lose detail in both dark and light areas of the slide and, especially, the negative, from more professional models. As a rule, for most flatbed scanners this value ranges from 1.7D (office models) to 3.4 D (semi-professional models). Most paper originals, be it a photograph or a magazine clipping, have an optical density of no more than 2.5D. Slides usually require a dynamic range of more than 2.7 D (usually 3.0 – 3.8) for high-quality scanning. And only negatives and x-rays have higher densities (3.3D - 4.0D), and buying a scanner with a higher dynamic range makes sense if you will be working mainly with them, otherwise you will simply overpay.