When people hold a modern traditional photographic image and a digital image to watch, as long as they pay attention to observation and analysis, they will be able to distinguish it. The image is realistic, full-fledged and soft, and the color is beautiful. The other one looks bright and colorful, with contrasting contrast and clear layers. Why are their respective advantages so distinct? This should be explained from their respective imaging principles and production processes.
1 The imaging principle and production process of modern traditional photographic imaging materials
Modern traditional photographic photosensitive materials include general art photographic film, photographic paper, telecine film, aerial photographic film, medical X-ray photographic film, film used in printing platemaking, certain microfilm films, and astrophotography dry plate films.
Generally, modern traditional photographic photosensitive materials cannot directly obtain images after photographic exposure. Usually, chemical treatments such as development, fixing, water washing, and drying are required to obtain visible and long-lasting images. Such a complicated imaging process is divided into three basic stages , Photographic exposure stage (formation of latent images), and development stage (formation of visible images).
1.1 Preparation stage of photosensitive material
The first is the preparation of photographic emulsions. The requirements for the use of each type of photographic film are different. The raw materials used in the preparation of photographic emulsions and the recipes and methods of production technology are also different. According to the recipes and technology of various tablets, the prepared potassium bromide (potassium halide, potassium iodide) and silver nitrate aqueous solution in the bright room are injected into the photographic gelatin aqueous solution with a certain temperature being stirred in the dark room by a double injection method. The potassium bromide and silver nitrate chemically react to form a silver bromide precipitate, which deposits evenly on a gelatin carrier and becomes a photographic emulsion.
This emulsion is subjected to a very complicated manufacturing process under dark lights, such as washing away potassium nitrate with ice water, adding chemical agents such as sensitizers, anti-fogging agents, leveling agents, defoamers, and hardeners. Become a formal photosensitive photographic emulsion that can be applied.
Coating and finishing stage: adjust the temperature of the above-mentioned photosensitive photographic emulsion to 5-8 ° C in a dark room, inject it into the coating tank of the coating machine, and then apply it to the film base or photographic paper carrier. Cut, punch, and package the product after passing the test, put it into the warehouse, and put it on the market.
1.2 Traditional film exposure stage
The photosensitive center of the silver bromide crystal in the film of the photosensitive material undergoes a chemical change due to the absorption of light energy during exposure, so that part of the silver bromide is decomposed to produce silver latent images and bromine. During exposure, the density of the latent image formed by the portion of the film irradiated by strong light is large, whereas the latent image center film is formed. They will affect the next development speed and the density of metallic silver particles after the film and the quality of the image (Figure 1).
Changes in the latent image after the exposure of the latent image and sub-latent image after strengthening
Figure 1 Schematic diagram of the change of photosensitive center when silver halide is exposed
1.3 Development stage of traditional photographic photosensitive material
The development is to use a developer to reduce the silver bromide crystal particles in the silver halide emulsion after exposure to chemical changes to generate metal silver particles. Therefore, to study the progress of development, you can also start by studying the development of individual silver halide crystals. The development of silver halide grains always starts from a certain point or a few points above. The reason why the developing effect starts from a certain point or certain points above is due to the effect of latent image. The latent image can prompt the silver halide around it to be quickly reduced to metallic silver. When these silver halides are reduced, they can prompt the reduction of the surrounding silver halides until the entire silver halide is reduced. The latent image plays a catalytic role in developing or accelerating the reduction of silver halide in the development process (see Figure 2).
Figure 2 Schematic diagram of the process from exposure to development
The structure of the metallic silver particles obtained by the development is not a solid mass, but a filamentous structure whose size is roughly equivalent to that of the original particles.
From the above, it can be seen that the particles of silver ions and metallic silver particles play an absolutely decisive role in the preparation of emulsions or the exposure and development of film or photographic paper. Because their slender particles are close to the size of nanometers, the fine particles are dispersed and aggregated into an image with beautiful colors, soft and delicate contrasts, and full layers, which makes people feel beautiful artistic enjoyment when watching.
The use of modern traditional photographic photosensitive material images is extremely wide. Such as fine art photography, tourist scenery, wedding photography, celebrity and great man standard portraits, paintings of famous painters, and images in activities such as culture, education, sports and health, industrial and agricultural production, scientific research and investigation, printing and electronic circuit intermediate plate making.
2 Digital photography imaging principles and uses
Digital cameras are equipped with optical lenses, framing frames, quick-release and other components in front of the body, and the operation method is similar to that of traditional cameras. The biggest difference is that there is a photoelectric charge-coupled conversion device CCD installed behind the digital camera. When shooting images, an optical lens is used to focus the scene on the CCD. Under the irradiation of light, the CCD pixel generates a current due to the change in charge distribution. The stronger the light, the greater the current. Store the binary code (image code) such as current conversion "0" or "1" in the memory, which is equivalent to the exposure process in a traditional camera. In order to produce a color image, the light must also pass through a set of red, green, The blue filter then falls on the CCD pixels respectively, and the color image is generated by signal combination. Obviously, the more the number of CCD pixels, the higher the resolution of the image, the larger the storage space occupied by each image. In fact, because the storage space occupied by each image is too large (usually each image is hundreds of KB to MB), so many digital cameras compress the image information and store it on the internal memory stick.
The next step is to transfer the image on the digital camera to the computer for editing and processing. After the user is satisfied, transfer it to the printer for printing, and finally obtain a beautiful image (digital photos can produce large images).
2.1 CCD photoelectric charge coupled conversion device
CCD, which is a photoelectric charge-coupled conversion device, is developed on the basis of metal oxide-semiconductor devices, and has now become a very mature solid-state image sensor. In addition to capture processing and image sensing, CCD can also be used for information processing and information storage, image analysis and processing. The two-dimensional CCD produced by the optical company is mainly used for still electrophotography. The ones developed by Kodak are:
2.1.1 Full-frame CCD image sensor
The CCD sensor is composed of many identical basic units (pixels). Each pixel has a photosensitive area and a storage area. The photosensitive area generates photoelectrons, and the accumulated charge is transferred to the storage area, and then output through the shift memory.
The full-frame CCD is an image capture device that combines the photosensitive unit and the storage unit. Its image capture process is divided into two parts, namely integration and readout. In the integration area, the electrons generated by the photons are accumulated in the imaging unit. The number of electrons accumulated in each CCD pixel corresponds to the illuminance of the image. The charge accumulated in each pixel in the readout area is transferred to the output circuit. This type of CCD is simple to produce, easy to use, and has a small signal, but it cannot see the light during reading. Therefore, a mechanical shutter is required in the camera, and the blue spectrum response is also low.
Each pixel of the sensor can be added with a color filter to form a color imaging sensor. The dye filter array made by this photographic method is called CFA. Because the human eye is most sensitive to green fine parts, the CFA is sensitive The number of green cells is twice that of red- and blue-sensitive cells.
When the illuminance is too large, when the charge generated in a pixel exceeds the capacity of the pixel, the excess charge will string into adjacent pixels, causing image blur, and the function of lateral overflow (COD) is to charge the excess charge before the charge is saturated Vent.
The CCDs used in amateur still photography are all equipped with COD, because the scene cannot be controlled. Cameras used in industry and science with controllable photographic conditions do not carry COD. It has high sensitivity and large pixel capacity.
2.1.2 6.4M pixel image sensor
Kodak professional digital camera DCS200C uses 1029 × 1526 to sequentially scan CCD to capture images, built-in hard drive to store images, megapixel electronic camera output color hard drive, the quality far exceeds the level of still video cameras.
A rotating filter wheel is added in front of the camera lens to enable the CCD sensor to receive red, green and blue images in sequence, so that only one monochrome CCD can capture color images. This color sequential capture method has high resolution and color reproduction it is good. However, due to vibration and other reasons, there is relative movement between the camera and the scene. When the red, green, and blue images form an image, the problem of inaccurate color registration will occur, so it is only suitable for shooting still scenes.
The most suitable for capturing still images is the full-frame CCD. The advantages of the full-frame structure are sequential scanning, high occupancy, low noise, wide dynamic range, and high pixel density.
The images taken or scanned into the computer can be processed with the cooperation of the corresponding image processing software. It is very convenient to process the images on the Windows platform. Because there are many kinds of image processing software developed under this platform and full functions, it can carry out all kinds of processing that people can think of. If you want to make the produced slides reach the level of photo quality, make the colors colorful, the images are exquisite, and the effect is extraordinary, Adobe Photoshop software is currently the best choice. Photoshop software is one of the first image software developed in the world, with powerful features, easy to operate and so on. The software can at least perform the following processing and trimming on image files.
2.2.1 Brightness and contrast adjustment: The brightness and contrast of the whole or part of the image can be adjusted arbitrarily, thus showing the effects of different image effects and traditional techniques.
2.2.2 Color adjustment: Adjust the color level, remove unwanted colors, intuitively adjust and correct the color cast and saturation of colors, and easily convert between positive and negative images.
2.2.3 Eliminate gray fog, spots, red eyes: using image processing software can easily remove the defects on the screen, and can adjust the image sharpness.
2.2.4 Change the texture of the image: make the image various deformations, so as to obtain unusual and unique innovative artistic images. There are many deformation formats, such as: displacement (displace)-make the pixels of the image in different directions Shift, squeeze deformation (Pinch)-squeeze the image inward or outward, coordinate transformation (Polar coordinates)-change the image coordinates from rectangular coordinates to polar coordinates or polar coordinates to rectangular coordinates. Square objects become round, three-dimensional text or objects produce a bulging effect and so on.
2.2.5 Produce various painting styles: The image can be processed to obtain painting effects of different styles, mainly including the soft focus effect (diffuse)-making the image look like a blurry haze effect through frosted glass (making the picture rich Poetic); embossed effect (emboss)-used to make the objects in the image embossed or recessed on the image background; embossed effect (extrude)-convert the image into a series of three-dimensional cubes or cones, which can be used Change the image or generate a special three-dimensional background. The size and height of the cube or cone sector can be selected manually; findges—make the image look like it has been outlined by a pencil; tiles effect (tiles) )-Process the image into the effect of building block stitching; wind effect--add some small horizontal lines to generate wind effect, the direction and strength of the wind effect can be chosen by anyone; pointillize--point an image Divided into random points to produce an effect like a stippling work; engraved gravure effect (mezzotint)-generates the engraved gravure image that makes the original image the closest; fragmentation (fragment)-makes the image unfocused Used for the rapid formation of focus background of the above kind can be attached to the catching protrusion like kind.
2.2.6 Others: such as simulating the shooting effect of various substances and simulating the processing effect of traditional darkroom techniques.
3 Printing process
First of all, there must be a qualified inkjet machine and computer software using the inkjet machine, including the speed of the image drum required by the user to print the image, the speed of the special drawing paper conveyor, the temperature of the heating roller, and the various colors of the inkjet nozzle The edited software such as ink sequence, speed, amount, evaporation drying speed after image printing is input into the computer.
Before printing, we must strictly clean the printing machine and check whether the preparations are in place.
Use the inkjet image software instructions of the inkjet machine to start the inkjet machine program for inkjet printing. According to the performance of different inkjet printers, Beijing Yunyuan Digital Company has a complete set of software for using inkjet printers to provide users with options for use.
There are many types of inkjet printers. The inkjet printers produced by various production companies have their own characteristics and patents. The sizes and sizes are different, but generally include the various components shown in Figure 3.
3.1 The inkjet print head on the inkjet printer
The inkjet print head on the inkjet printer has different structure designs, as shown in Figure 4, Figure 5 and Figure 6. Different inkjet printers have different inkjet printheads. The nozzle diameter is about 4 μm. Generally speaking, according to pixels, for example, Epson color stylus is 1440 × 1440dpi, Lexmark 7000 is 1200 × 1200dpi. There are many pixels in the nozzle of the inkjet machine. The nozzle density of the inkjet printer is large, the nozzle diameter is small, the ink droplets sprayed are small, and the pixels lost by the image printed are few, and the produced image is naturally fuller, and the admiration is much more real.
The reason why the image printed by digital photography is distorted is that the ink droplets splashed on the carrier paper reflect that each pixel point is a molecular-level particle size, and there is a certain distance between the particle point and the particle point. Many pixels cause the image to have high contrast, distinct layers, not soft and full, and the human eye looks uncomfortable and distorted, which is why it cannot be compared with the beauty of traditional photographic images in quality.
3.2 Special printing paper
If you use ordinary paper inkjet printing, the inkjet ink splashed onto the paper is shown in Figure 7. The ink dot spreads the paper fiber to the surroundings and to the deep layer of the paper, and the image wetted by the ink dries slowly, which affects the printing speed. In order to prevent the penetration of ink and accelerate the evaporation and drying speed of ink droplets, a layer of coating is generally added to the surface of the paper. The coating contains a polymer binder and titanium dioxide (TiO) or talc (SiO2). The thickness of the coating is matched with the ink. It helps to absorb some of the ink's moisture (as shown in Figures 8 and 9), which can promote the rapid drying of the image, shorten the time for spray painting, and reduce costs.
4 Advantages and uses of digital photo images
4.1 The advantages of digital photography images are that the methods used to produce them are diverse, the images are true, the colors are bright, the layers are clear, and the contrast is clear.
4.2 Uses: It can be used almost wherever traditional photographic images can be applied, and it is widely used in CT, MR, DSA, ECT, US and other digital medical image inspection devices. It is also most suitable for making large-format images. Such as various policy promotional images, merchandise advertisements, light box patterns, etc.
Its only defect is that the images reproduced with it are not as realistic as those printed with traditional photographic elevation materials, beautiful colors, full layers, and soft contrast.
5 Conclusion
Judging from the above advantages and uses of the traditional photographic images and digital photographic images, as far as the current quality level of digital photographic images is concerned, it cannot yet completely replace the application of traditional photographic images. People will take advantage of their respective strengths and avoid their shortcomings, and they will also coexist with each other for a period of time.
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