14 DAY TRIAL // Yesterday, we've seen how digital pictures and text can be transformed into analog hard copies printed on paper and similar materials. Nowadays, there's this trend of migration from conventional photographic film to digital photographic processing. While printed images may still look better than their digital equivalents, what is to be done with all those heavy photo albums stuffed with past captured moments? You can try to save all these photos on your PC. But how to transform analog images into digital ones? This is where scanners come in handy. And it's not just about black-and-white photos that need to be stored somehow, think about scanning all kinds of photos for additional manipulation and improvements. Time to see how scanners work?
"A scanner darkly" Scanners are devices that can analyze two types of things: - Images, be these photographs, printed texts, or handwriting and drawings. - Objects (which would fit the scanning area, that is)- The scanner processes and converts all these into digital equivalents.
One may encounter several types of scanners, nowadays: a) Flatbed scanners (commonly known as desktop scanners) are the most versatile and frequently used scanners. b) Handheld scanners are basically a more portable version of the flatbed type. However, these ones rely on the user to move them on the surface of the object. This type of scanner is not popular anymore because it provides a poor image quality. It is only proficient at quickly capturing text. c) Sheet-fed scanners are similar to flatbed scanners except the document is moved by mechanisms in order to be scanned by an immobile head. A sheet-fed scanner resembles a small portable printer. d) Drum scanners are commonly used by the publishing industry to capture high-resolution images in less time. This type uses an improved technology called a photomultiplier tube (PMT). PMT implies that documents to be scanned have to be mounted on a glass cylinder. At the center of the cylinder there is a sensor that splits light bounced from the document into three beams. Each beam is sent through a color filter (red, green and blue, usually) into a photomultiplier tube where the light is changed into an electrical signal. A similar technology is to be found in rotary scanners, which are primarily used for high-speed document scanning. This is a derivation of the drum scanner, but it uses a CCD array instead of a photomultiplier.
We will now concentrate on the drum scanners and the flatbed ones.
Drums! Drum scanners may not be the most popular "breed," but they sure are their best in image processing. One of the unique features of drum scanners is the ability to control sample area and aperture size independently. The sample size is the area that the scanner's encoder reads to create individual pixels. The aperture is the actual opening that allows light to get into the optical bench of the scanner. The ability to control aperture and sample size separately is particularly useful for smoothing film grain when scanning black and white and color negative originals.
Drum scanners are capable of scanning both reflective and transmissive media, but they are presently less popular than quality flatbed scanners when it comes to this issue. Otherwise, film is where drum scanners continue to be the tool of choice for high-end applications. Film rolls can be wet mounted to the scanner drum and, due to exceptional PMT sensitivity; drum scanners are capable of capturing very subtle details in film originals.
Being less popular than flatbed models, drum scanners manage to remain in demand because of their capacity to produce scans which are superior in resolution, color gradation and value structure. In addition, since drum scanners are capable of resolutions of up to 12,000 ppi (pixels per inch), their use is generally recommended when a scanned image has to be enlarged. Drum scanners continue to be used in high-end applications, such as museum-quality archiving of photographs and print production of high-quality books and magazine advertisements. Moreover, due to the greater availability of pre-owned units, many fine art photographers are acquiring drum scanners.
Flatland The typical parts to be found in a flatbed scanner include:
* Charge-coupled device (CCD) array * Mirrors * Scan head * Glass plate (a.k.a flatbed) * Lamp * Lens * Cover * Filters * Stepper motor * Stabilizer bar * Belt * PC interface port(s) * Control circuitry
I placed the CCD first because it is considered to be the core component of a flatbed scanner. The CCD is the most common technology for image capture in scanners and it is basically a collection of minute light-sensitive diodes which convert light photons into electrons (electrical charges). The diodes are also known as photosites, which are sensitive to light. This is to mean that the brighter the light that hits a single photosite, the greater the electrical charge that will accumulate at that site. The light reflected from the scanned image reaches the CCD array through a series of mirrors, filters and lenses.
Let us now see how the flatbed scanner really works. These are the basic steps that are performed by all such scanners:
- The image source is placed on the glass plate and the cover is closed (although it is not necessary to do this). The inside of the cover in most scanners is flat white, although there exist some black ones, too. The reason to close the cover is that it provides a uniform background which the scanner software can use as a reference point for determining the size of the image source. However, many flatbed scanners allow the removing of the cover in order to scan a bulky object, such as a page in a thick book. - Furthermore, the lamp is used to illuminate the document. The lamp could be a cold cathode fluorescent lamp (CCFL) or a xenon lamp, while older scanners used to feature a common fluorescent lamp. - All the mirrors, lenses, filters and the CCD array constitute what is referred to as the scan head. This part is moved slowly across the document with the aid of a belt that is attached to a stepper motor. A stabilizer bar ensures that the scan head does not wobble or deviate in its complete scan pass. - The image of the document is reflected by an angled mirror to another mirror. Each of the mirrors is slightly curved to focus the image it reflects onto a smaller surface. - The last mirror reflects the image onto a lens. The lens focuses the image through a filter on the CCD array and the image begins to be processed. Present-day scanners use the single scan pass method. The provided lens splits the image into three smaller versions of the original. Each smaller version passes through a color filter (red, green or blue) onto a discrete section of the CCD array. At this point, the scanner hardware combines the data from the three parts of the CCD array into a single full-color image.
Lots of quality pixels Flatbed scanners read images in red-green-blue (RGB) color data from the CCD array. This type of data is then processed via a proprietary algorithm to correct for different exposure conditions and it is afterwards sent to the computer, via the scanner's input/output interface (usually SCSI or USB). Color depth varies depending on the scanning array characteristics, and for good quality results, it has to be of at least 24 bits. High quality models have 48 bits or more color depth. The other qualifying parameter for a scanner is its resolution, measured in ppi, sometimes more accurately referred to as samples per inch (spi). Instead of using the scanner's true optical resolution, the only meaningful parameter, manufacturers like to refer to the interpolated resolution, which is much higher and can be achieved with the help of software interpolation algorithms. Nowadays, a good flatbed scanner has an optical resolution of 1600-3200 ppi, while high-end flatbed scanners can scan up to 5400 ppi, and a good drum scanner has an optical resolution of 8000-14,000 ppi. Just keep in mind that equivalent resolutions higher than 1200dpi are oftentimes overkilled for the average color printers. One last important parameter for a scanner is its density range. A high density range means that the scanner is able to reproduce shadow details and brightness details in one scan pass.
The scanned result is at first a non-compressed RGB image which can be transferred to the PC memory. Some scanners compress and clean up the image using embedded firmware for improved results. Once stored on the HDD, the image can be processed and manipulated with a raster graphics program (such as Photoshop). Scanned pictures are usually stored by using image formats such as JPEG, TIFF, BMP, and PNG. Some scanners can also be used to capture editable text, so long as the text can be read by the computer in a discernable font via the Optical Character Recognition (OCR) engine.
This should be all the basic information that you need to know about scanners. The next article will present the primary source of digital pictures nowadays - the digital camera.