A metamorphosis is taking place in today's photographic marketplace as traditional camera sales lag and digital camera sales explode. Digital camera users value the inherent control, convenience, and distribution advantages of digital photography, and a significant number of these users are choosing to print their photos using desktop inkjet printers instead of using traditional (or silver halide) photo processing. Recent breakthroughs in the quality and permanence of inkjet photo printing are helping to stimulate this ongoing conversion.
This report compares the color consistency and quality of photos printed using HP desktop inkjet printers with the color consistency and quality of prints, either film or digital, made using silver halide processing. It also discusses HP inkjet printer technological advantages and points out some of the potential problem areas in the traditional color film (or digital) and print development processes.
The public's long-standing attachment to the chemically complex silver halide photo print has caused millions of consumers to accept somewhat inconsistent color print quality and to mistakenly assume that their own lack of photographic abilities were to blame.
Most consumers do not have the knowledge or the opportunity to adjust the quality of their color prints during the silver halide development process. Consumers are usually at the mercy of high-volume print inspectors and computerized negative scans. Human inspectors decide what color adjustments should be made to the customer's photos based on a level of training and expertise that can vary from store to store as widely as the quality of the color prints they produce. Computer scans can be performed randomly or on each negative, but in either case can compromise photo quality and consistency.
Inkjet printers, on the other hand, use a digitally controlled "color pipeline" that automatically delivers consistent output. In addition, if they desire, inkjet consumers have the freedom to easily make custom adjustments to their images and the luxury to see those results immediately. In fact, independent research studies have shown noticeably more print-to-print color variation with silver halide processing than with HP inkjet prints from the same digital image.
The combination of digital cameras and digital inkjet printers has eliminated the intricate chemical reactions inherent in silver halide development in favor of digital processes that can be controlled for consistent color output. International consumer studies demonstrate that when inkjet prints are compared to traditional photo processing of film or digital photos, inkjet technology most often produces more predictable and more consistent photo color. Furthermore, inkjet technology generates these results print after print, for the life of the printer.
In 1995, Terrence W. Faulkner, the director of strategic planning for Eastman Kodak, said, "For more than 155 years, the photography industry has been based on the manufacture of sensitized silver halide materials. That technological base is now in the process of changing. Digital technology is enabling the convergence of imaging, information, and communications. This is dissolving old industry boundaries and we must accept the existence of an emerging imaging industry".1
The emerging digital imaging industry Mr. Faulkner refers to is now revolutionizing the photographic marketplace.
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What is it? |
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Color photography using silver halide processing
The primary element for light capture in traditional color photography is the silver halide crystal. Silver halide is a compound made of silver (Ag) atoms and atoms from the halide group (Cl, Br, I) of the periodic table.
In traditional color photography, when photographic film is exposed to light, the silver halide crystals in the emulsion of the film absorb light and form a latent image on the silver halide crystals. When the film is developed, the oxidized developer creates a colored image that registers with the silver that developed. When the paper is developed, a light source is used to transport the image from the negative to the photo paper.
Sensitivity to light, usually quantified by film speed, is one of the most important attributes of film emulsion. During film manufacturing, tiny amounts of sulfur and gold compounds are used in a heat treatment to enhance sensitivity. In color photography, organic sensitizing dyes are added and absorbed onto the silver halide surface during finishing to enhance the sensitivity of the silver halide emulsions in the visible spectrum. This is the major difference between color photography and black and white photography.
Color photography using inkjet printing
In inkjet printing, small drops of ink are placed onto paper to create an image. During inkjet printing, no light-induced or chemical-reaction processes are needed as the colorant is simply deposited directly onto the print. There are several varieties of inkjet printing systems, including thermal, piezo electric, and other hybrid systems. This report is based on Hewlett-Packard's thermal inkjet technology.
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How does it work? |
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This section compares and contrasts the processes in each stage of silver halide development and printing with those of digital photography using an inkjet printer.
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Silver halide: film development
When a customer delivers a roll of film to the photo processor, the film contains the latent images of the photo exposures. These latent images must be amplified and stabilized to make a color negative that can be printed and viewed by reflected light.
In developing film, reducing chemicals cause the exposed silver halide grains to develop to pure silver. This reaction produces an oxidized developer, which reacts with chemicals called couplers in each of the image-forming layers to form visible dyes, and amplifies the atomic silver to visible silver strands to cause the couplers to form a color that varies depending on how the silver halide grains were color sensitized. A different color-forming coupler is used in the red-, green-, and blue-sensitive layers. When the film is developed, the latent image in each of the different layers forms a different colored dye.
- Red-sensitive layers form a cyan-colored dye after development.
- Green-sensitive layers form a magenta-colored dye after development.
- Blue-sensitive layers form a yellow-colored dye after development.
When light-induced development is complete, the process is stopped by a stop bath, in which a fixing solution is used to remove the unexposed silver halide grains while bleaching chemicals are used to remove the silver. Finally, the negative image is washed to remove as much of the residual chemicals and reaction products as possible, and the filmstrips are dried.
The colors formed in the color negative film are based on the subtractive color formation system, which uses one color (cyan, magenta, or yellow) to control each primary color.
During film and print development, improper maintenance or replenishment of the development chemicals can negatively affect photo color consistency.
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Silver halide: print development
To make a color print, the negatives are used to expose high-quality, water resistant color print paper. The imaging surface of the color print paper is coated with light sensitive silver halide grains that are sensitized to red, green, and blue light.
A light source is used to transfer the image from the negative to the photo paper. Inconsistencies in the intensity of the light source, variations in the lamp types, or even the age of the lamp can affect the spectrum distribution of the light source and can adversely affect color image consistency.
In making color prints, subtractive color filters are added to adjust the color balance and to make the print more pleasing.
The process for having digital images developed by a traditional photo developer matches the development process of film images, which is described above in Silver halide: film development.
The following steps are used to develop the color print paper after it is exposed:
- Processing develops the latent-image sites and oxidized developer molecules combine with the color-forming couplers to create a silver image and a dye image. The reaction is stopped by a washing step.
- A combined bleach-plus-fix solution (called the BLIX) removes the silver image and any remaining unexposed silver halide.
- The print is then washed to remove residual chemicals.
- The print is dried.
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During print development, arbitrary image adjustments made by print inspectors and computer averaging of image attributes can negatively affect photo color consistency.
Generating digital images
A digital camera uses no film; instead it uses a sensor that converts light into electrical charges. The distinct advantages of digital cameras over traditional photography are a result of recording the images in digital form instead of on film.
Most digital cameras use a charge coupled device (CCD) image sensor. Some cameras use complementary metal oxide semiconductor (CMOS) technology. While CMOS sensors will almost certainly improve and become more popular in the future, they probably won't replace CCD sensors in higher-end digital cameras.
The CCD is a collection of tiny light-sensitive diodes (photosites) that convert photons (light) into electrons (electrical charge). The brighter the light that hits a single photosite, the greater the electrical charge that will accumulate at that site.
Digital cameras use specialized demosaic algorithms to convert the mosaic of separate color information into a mosaic with full color information.
Printing digital images
Thermal inkjet technology uses heat energy to vaporize a thin layer of ink to form a bubble that expels a small drop of ink through an orifice or nozzle. These ink drops form on the paper and compose the photographic images. The microscopic color ink drops are usually between 4 and 15 picoliters in volume. (A picoliter is one-trillionth of a liter.) As a reference, a three-picoliter drop of ink cannot be seen by the naked eye.
Once the ink drops are placed on the paper, the dots are usually between 50 to 70 microns in diameter. (A micron or micrometer is one-millionth of a meter.) The dots are accurately positioned, with resolutions up to 4800 x 1200 dots per inch (dpi). The dots combine different ink colors to create photo quality images.
Most often, customers decide what kind of camera to use and how to print their photos based on image quality. A customer's evaluation of image quality is strongly influenced by the perception of the color realism and richness in their photos.
Inconsistency in inkjet photo quality is primarily caused by variations in ink drop volume. To help alleviate this issue, Hewlett-Packard developed pen-to-pen drop volume calibration technology to closely monitor and control drop volume. Even HP inkjet products without this calibration technology produce more consistent photo quality than traditional silver halide processing.
In inkjet printing, the quality of the image is also influenced by the following:
- The resolution at which the image was captured by the digital camera
- The interaction of the ink and the paper
- Printer mechanics
- The appropriate media type and the consistency of that media type
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Factors influencing photo color quality |
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Traditional film development and print processing is a complex progression of chemical reactions. The arbitrary assessments made by a print inspector during the print development stage, the film development chemical replenishment process, and the computerized averaging of image attributes often used during the film development stage can manifest themselves individually or collectively in inconsistent, unpredictable, and undesirable color photo prints.
Print inspection
Humans, computers, or a combination of both are used to inspect traditional prints to help optimize photo characteristics such as color balance, contrast, graininess, and sharpness. Both human and computer inspection processes have inherent limitations.
Human print inspectors may view each print on the inspection monitor, cursorily fan the prints, or do no inspection at all. Human color adjustments are always the subjective, momentary opinion of the inspector and are dependant on the inspector's level of training. In addition, human inspections of color quality can vary significantly from day-to-day and from store-to-store.
Computerized negative scans during the film development stage can be random or performed on each negative. During random scans, averaged values are assigned to all negatives based upon the attributes in the negatives that were scanned. Even when all negatives are scanned, assessments are based upon algorithms that may not have the complexity to compensate for various picture-taking conditions. The by-product is compromised output and inconsistent results.
Inadequate chemical replenishment
To maintain consistency in photo color processing, the rate of chemical replenishment should be dependent upon the type of film being processed. Large-scale operations that require constant chemical and sensitometric monitoring usually pick an average calibration rate in order to maintain the color photo results within acceptable limits. The variations in chemical strength that are caused by calibration averaging and drift can lead to variations in photo color.
For the most favorable color quality, labs should compensate for the accumulation of contaminates and maintain the activity of the developer at a constant level. Because of monetary considerations, many labs try to stretch the life of their chemicals by not adding compensatory chemicals or by not changing the entire chemical mixture as frequently as they should.
Computerized attribute averaging
Until recently, print inspectors diligently tried to evaluate consumer prints during the print development stage. Today, many high-volume print developers strive for optimum cost efficiency by using computers to perform scans of a customer's negatives. This scan establishes parameters for 'average' critical characteristics in each batch of prints, such as exposure, color balance, graininess, tone adjustment, density, and contrast. The scan establishes these parameters based on the assumption that all the photo characteristics will fall into these ranges.
Assumptive computer averaging techniques can fail to properly compensate for pictures taken in a variety of different conditions, including indoor lighting conditions (tungsten-balanced or florescent-balanced) or outdoor lighting conditions (cloudy or sunny). Computerized batch averaging of various characteristics can be adequate for some photos but can greatly compromise the color accuracy, quality, and consistency of other photos.
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Inkjet printing: Advances in photo color quality |
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Printing photos from digital cameras using an inkjet printer saves time, provides instant gratification, and is breaking new ground in photo color quality and consistency. Furthermore, customers using inkjet printers can easily adjust their own photos and avoid relying on the uncertainties of traditional photo processing. The color consistency of HP inkjet printers means that inkjet customers can get high quality color prints without taking their film to a discount or drugstore chain and simply hoping for the best.
Worldwide independent consumer study
In 2002, SpencerLab Digital Color Laboratory, an independent consulting lab that specializes in digital imaging technology, conducted an international research study of silver halide and inkjet color print output. To get an accurate comparison, all prints from the traditional and digital cameras were created from the same digital files. (In traditional photography, photos are normally captured using an analog process. To print on an inkjet printer, photos must be digitized.)
Hundreds of study participants from the United States, Europe, and Japan ranked photo color print quality according to the following attributes:
- Richness (saturation)
- Realism (hue)
- Smoothness (lack of graininess)
- Sharpness (detail)
In general, inkjet output performed well in this study. For example, in all three regions, the research study demonstrated that photos printed on HP photo media by the HP Photosmart 1315, DeskJet 5550, and Photosmart 7550 inkjet printers were judged to have equal or superior print quality to silver halide photo prints.
In reflecting on the study, SpencerLab president David Spencer said, "In our recent research in the U.S., Europe, conventional silver halide prints from major discount and drug stores showed noticeably more print-to-print color variation than HP inkjet prints of the same digital image. In addition to store-to-store variation, we also found color differences when later reprinting the same image at the same retailer, while prints from HP inkjet printers were generally consistent from day-to-day. As some of our test participants said, conventional print quality 'all depends on the laboratory that I send it to or the people (doing the) developing,' and 'sending the same photo to the same laboratory over a period of two weeks gives you two different results. There is just no consistency,' and 'it depends on the colors, sometimes you get the right colors [from a lab], sometimes you don't'." 2
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Ensuring consistent color print quality for customers |
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To ensure consistent color reproduction, HP adheres to industry-leading tolerances and practices in the following categories:
Pen positioning and movement accuracy
Accurate positioning and movement of the ink cartridge in relationship to the paper as it advances is crucial for consistent color and image quality. HP carefully specifies the dimensions of all parts and motors associated with these properties and uses diagnostic prints made at the end of the assembly line to check the accuracy of assembled printers.
Drop formation and ejection consistency
In the design of the inkjet cartridge, operating parameters associated with drop formation and ejection are carefully optimized. However, real-world manufacturing processes always create slight variations in the drop ejection characteristics of a given inkjet cartridge. HP has developed technology that allows each print head to be tested and encoded during manufacturing so that the printer can automatically compensate for differences between the pens. HP is working to extend this technology more broadly across the inkjet product line.
Color mapping (predicated on a specific media)
The rendering/color mapping process essentially describes how a digital image file is transformed into a set of specific commands that fire the inkjet cartridge to deposit the correct amount of drops in the correct locations. The firing of the inkjet cartridge in a specific alignment pattern creates a digital gamut of colors. Digital inkjet printing has the advantage that all inputs (the source file) and outputs (the firing pulses applied to the ink cartridge and depositing drops on the paper) are completely digital and can be controlled at every step of the transformation process.
Successful color mapping requires a thorough knowledge of the inkjet cartridge characteristics including carriage velocity, the signature on the smart chip, and the characteristics of the photo paper, including whiteness and dot gain. Thus, optimization of an inkjet printer requires specific inkjet cartridges and photo paper. Inserting a different inkjet cartridge or paper into the printer can compromise the color accuracy of the end result. Many HP printers use automatic media sensors to determine the category of the print media so the printer can compensate for the signatures and characteristics of the consumables. HP photo paper is designed for HP inks whereas third party photo papers are typically designed for broad compatibility including non-HP inks. Printing on third party paper with HP inks often reduces the quality of the output.
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For decades, customers have unquestionably accepted traditional silver halide photo processing as the only way to get their film and photos developed. Today, millions of consumers are realizing the genuine control, convenience, and distribution advantages of inkjet photo processing over the unpredictable and inconsistent color quality produced by discount and chain store silver halide processors.
HP has developed and implemented many unique technologies to further improve upon the inherently strong color consistency of inkjet printing. HP's synchronized ink, paper, and printer system technologies are the reason that digital images printed on HP inkjet printers are more consistent than silver halide versions of the same digital images.
Photographic development is no longer a "hope-for-the best" or "take-what-you-get" proposition. Digital photography and HP inkjet printers have opened up a new world of photographic fun, instantaneous convenience, and consistent color quality to consumers around the world.
For more information |
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This paper touches on only some of the many advantages of HP inkjet technology. For more information on this paper or related subjects, visit the following Web sites:
1Faulkner, Terrence W. "Testimony before the Federal Trade Commission." 19 Oct. 1995.
2David Spencer is the president and founder of SpencerLab, an independent testing and evaluation laboratory in Melville, NY. (http://www.spencerlab.com/)
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Printable version
