Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

acid: A chemical substance that, when dissolved in water, creates a hydrogen ion concentration in the solution greater than 10-7 moles/liter. Different acids react in different ways with their environments. Many acids can be reactive with photographic materials and storage enclosures, shortening their life span.
adhesive: A substance that causes two or more materials to bond together. The action of bonding varies among adhesive types (e.g., pressure-sensitive vs. glue).
alkali: A chemical substance that, when dissolved in water, creates a hydrogen ion concentration lower than 10-7 moles/liter. This term has also been used with paper products to imply the inclusion of an alkaline buffer.
archival: A term often used to imply that a material will be stable over time. The term has neither a recognized standard definition nor a quantifiable method for verification.
banding: Lines that appear on prints, typically either when a print head cartridge is misaligned (causing merging or bleeding of contiguous colors) or when a printer cannot reproduce a smooth transition between colors (producing a jump between one value and the next).
base: See alkali.
binder: The transparent layer on a photograph in which the final image is suspended.
binder’s board: A thick paperboard used to impart rigidity to book bindings, boxes, and photo albums.
bleed: Undesired migration of colorants through or across the surface of a material.
bleed-proof: A term used to describe an ink that does not migrate from its original point of application over time. This applies to both wet and dry environments.
bleed-through: The migration of a colorant from one side of a material, through the material, to appear on the reverse side.
blocking: The adhesion of a smooth surface, such as glass or plastic film, to the surface of a photograph. It is the result of the softening of gelatin under conditions of high humidity. In severe cases the two surfaces cannot be separated.
blur: Lack of sharpness, usually at the edges of light and dark areas of an image.
Bubble Jet: Canon U.S.A., Inc.’s trademarked name for their thermal, drop-on-demand ink jet printers.
buffered: A term used to describe material to which slightly water-soluble acid or base has been added in order to stabilize the pH. In paper, this is an alkaline substance (e.g., calcium carbonate), which is added to reduce the amount of acid that may build up either during the natural degradation of the paper fiber or through acid absorption from the environment.
chroma: One of the three main characteristics that describe a color (the others are hue and value). Chroma refers to how much a color diverges from a gray of the same value; sometimes referred to as saturation.
chromogenic print: Almost all consumer color photographic prints are of this type. In this process, the image-forming dyes are created during processing from colorless dye couplers included in the paper during manufacture.
cold extraction pH test: A standardized test method used to determine the pH of paper products. This method is preferred to surface pH tests (such as the use of a pH pen), because it measures the pH of both the interior and exterior of the sample.
cold storage: A refrigerated storage space usually with temperatures under 45ºF (7ºC). Low temperatures decrease the degradation rate of organic materials such as paper and photographs, resulting in significantly increased usable life spans.
color (or dye) coupler: A chemical within photographic papers that reacts during photo processing to create the image-forming dyes. Unreacted color couplers remain in the photo paper after processing and can potentially react with storage enclosures or atmospheric pollutants to stain the photograph.
colorant: A substance (dye or pigment) that imparts color to an ink.
continuous tone: Smooth tonal transition between image areas.
dark stability: The quality of a material that makes it resistant to degradation when stored in the dark.
digital image: Any image stored as numerical values on optical or magnetic media. This term is also used for any print created from such a set of stored numerical values.
Dmin: Minimum density. Usually the white area of a print or the clear area on a film. For color negative film it is the lowest density area of the orange mask.
Dmin stain/gain/yellowing: Increase in blue density in the non-image area of a film or print due to overall yellowing of the image.
dot gain: The tendency of ink dots to spread during printing as they interact with the substrate. The visual result of dot gain is a darker image.
dpi (dots per inch): The number of ink dots a printer can apply in one inch, either vertically or horizontally. It is a measure of printer resolution.
dry-time: The period from the moment the print is made until the ink’s solvent (usually water) has completely evaporated.
dye: A soluble colorant. Dyes are typically less stable than pigments but can produce a greater, more vivid, color gamut.
dye sublimation (also dye-sub): A printing technique that uses heat to diffuse dyes from a donor ribbon into the receiver layer of the medium. Dye-sub printers contain a roll of cellophane ribbon composed of three colored panels (yellow, magenta and cyan), and one clear panel of an overcoat material. The colors are applied to the medium one at a time. The intensity of the colors is controlled by variations in temperature on the print head. The final step is the application of the clear overcoat.
D2T2 (direct dye thermal transfer): See dye sublimation.
electrophotography: A printing process in which an image is formed with toner transferred to paper with variable areas of electrostatic charge and then fixed by heat or pressure. Used in photocopying machines and in laser and LED (light-emitting diode) printers.
enclosure materials: Manufactured or custom-made storage materials used to house photographs.
fading: The gradual loss of color density due to the breakdown of the colorants into invisible forms.
fiber-based print: A photographic print on a paper base that has not been laminated with polyethylene in manufacture. This type of print was made until the advent of polyethylene-laminated papers in the late 1960s. Prints of this type now being produced are typically black-and-white fine-art photographs.
gamut: The total set of colors possible from an image output process.
gas-fastness: The resistance of a colorant to fading upon exposure to atmospheric pollutants, most typically ozone.
gelatin stain: The discoloration, to yellow or brown, of the clear gelatin binder layer in black-and-white or color photographic prints. Additional stain may be created in color prints when environmental or enclosure pollutants react with residual dye coupler chemistry in the print.
giclée: A high-quality fine art print created with an inkjet printer.
glassine: A translucent paper storage enclosure most often used for photographic negatives. This material can become permanently embedded in gelatin under very humid conditions. ISO has determined that glassine is inappropriate for use as a photographic storage enclosure.
groundwood: Pulp created by the action of mechanically grinding wood. This low-cost pulp is chemically unstable and should be used only for short-term paper needs such as newspaper.
halftone: An image that has been reproduced through a screening process that converts it into a regular or irregular dot pattern. Shades of grays are achieved by varying the ink coverage in one of two ways: equally spacing dots of variable size (amplitude modulation screening), or using dots of equal size with variable spacing (frequency modulation screening).
hue: One of the three main characteristics that describe a color along with chroma and value. Hue refers to the attribute that categorizes a color as “blue”, “green”, “yellow”, “purple”, etc.
hue shift: Color change due to the unequal fade rate of one or more mixed colorants (e.g., yellow and cyan dyes mixed to make green) resulting in a shift from the original color toward the more stable of the colorants used in the original mixture.
humidity sensitivity: The tendency of a print or film to become damaged by high humidity, often seen as dye bleed or surface blocking.
hydrolysis: A chemical reaction by which paper fiber and color dyes degrade. It is driven by moisture and can be accelerated by acids, alkalis, and/or heat.
index print: A single sheet containing thumbnail images of the content of digital storage media.
inkjet printer: A non-impact printer that produces an image from digital data by propelling tiny droplets of ink onto a surface.
ISO (International Organization for Standardization): ISO is the world's largest developer of standards. It is a non-governmental organization composed of a network of the national standards institutes from 157 countries. The Central Secretariat is based in Geneva, Switzerland.
JPEG (.jpg): A standard data compression method for bitmapped images. JPEG stands for Joint Photographic Experts Group, the name of the committee that created the standard. This method is lossy.
kappa number: A numerical value indicating a paper’s relative content of lignin.
kiosk: A self-serve station that permits customers to print images from digital storage media or from a built-in scanner.
laser printer: A printer that uses an electrophotographic process to print out digital files. The image is produced by direct scanning of a laser beam across the printer’s photoreceptor. Toner is then applied and fixed by heat to the medium.
lightfastness: Resistance of colorants to fading or paper to yellowing upon exposure to light.
lignin: A chemically complex substance found in many plants, which bonds the cellulose fibers. Lignin can be largely removed during pulping; however, the cost of low-lignin papers is higher than that of high-lignin or groundwood papers. Lignin is known to contribute to the degradation of both papers and photographs.
liquid toner: See Toner.
lossless: A term describing a method of data compression in which smaller files are attained without the loss of any color or pixel data, thus permitting the reconstruction of original data from the compressed data (e.g., TIFF).
lossy: A term describing a method of data compression in which smaller files are attained by eliminating some of the original data; thus, decompression will not retrieve the original data (e.g., JPEG).
media: In digital imaging, media are either the electronic file storage materials (e.g., CD or USB) or printing substrates for digital output.
mottling: Uneven variations in optical density. The term is primarily used as a descriptor in the PAT.
out-gassing: The emission of volatile pollutants into the atmosphere. It is the method by which most harmful components of enclosure materials reach photographs.
oxidation: The chemical action of electron removal from one atom or molecule by another atom or molecule. This action is often cited as the cause of image fading in black-and-white photographs. Removal of an electron from an atom of metallic silver converts that silver to an ionic and invisible form. Oxidation is also implicated in the degradation of color images and paper objects.
PAT (photographic activity test): A test used to predict certain chemical interactions between enclosure materials and photographic images. This is one of many tests used to determine if a material is safe for use with photographs.
pH: The negative logarithm of the hydrogen ion concentration of an aqueous solution. It is the numeric scale used to indicate acidity/alkalinity. A pH of 7 is neutral; pH values below 7 indicate acid and above 7 indicate alkali or base.
pH testing pen: A pen containing a pH indicator dye that is used to assess pH. This is done by spreading a fine layer of the indicator dye on the object’s surface and then comparing the resulting color to a reference.
photographic activity test: See PAT.
photo-safe: This term has no standard definition, and there are no test methods for verification. It is often used in advertising to imply a product’s non-reactivity with photographic images.
pigment: A non-soluble substance used as a colorant. Pigments are generally more stable than dyes, but they produce a narrower color gamut.
plasticizer: A chemical added to plastics to increase their pliability. The most notable plasticized photo storage material is flexible PVC, which has been shown to exude its plasticizer onto the surface of enclosed images. ISO storage standards recommend against the use of PVC in photographic storage enclosures.
polyester: A plastic generally considered safe by ISO for use in storage enclosures for photographs. However, many plastics are treated with thin coatings, so polyester products should still pass the PAT in order to be considered safe. Polyester is also used as film for transparencies in inkjet printing systems and as a clear base for traditional photographic films.
polyethylene: A plastic generally considered safe by ISO for use with photographs. However, many plastics are treated with thin coatings, so polyethylene products should still pass the PAT in order to be considered safe.
polypropylene: A plastic generally considered safe by ISO for use with photographs. However, many plastics are treated with thin coatings, so polypropylene products should still pass the PAT in order to be considered safe.
porous printer media: Inkjet printing papers in which the ink receiving layer is formed by minerals (such as silicas or aluminas) bound by polymers to create tiny cavities in which the ink is absorbed. The inks remain in direct contact with the air, and thus have no protection from air pollutants.
pressure-sensitive adhesive: An adhesive that bonds on contact and whose bond strength increases with increased pressure and/or time.
PVC (polyvinyl chloride): A plastic generally considered by ISO as unsafe for use with photographs due to its plasticizer content and the potential for emission of hydrochloric acid upon its degradation.
rag paper or board: A paper or board with high long-fiber cotton content. Cotton fiber contains no lignin and has historically been considered the most appropriate material for use in photographic storage enclosures and framing packages. It is generally higher in cost than wood pulp papers. More recent research has shown that papers made from delignified, bleached wood pulps may be as safe for use with photographs as papers made from cotton pulps. It should be noted that despite the quality of pulp (wood or cotton) used in paper making, a variety of other paper additives could be reactive with photographs.
RH (relative humidity): A measure of moisture content in the air. Moisture is crucial to many of the chemical reactions that harm photographs. Reducing air moisture content can significantly extend the life of printed images on film or paper substrates. However, extremely low humidity can cause irreversible physical deformation and embrittlement of photographs. ISO has recommended that RH levels be kept between 30%- 50%. High humidity can lead to the growth of mold on photographs, dye bleed, or the bonding of prints to each other or surrounding smooth surfaces (see blocking).
reduction: The chemical action of electron addition from one atom or molecule to another atom or molecule. The addition of an electron from a reducing pollutant to an invisible silver ion converts that ion to a metallic and visible form. This reaction has been implicated in the formation of silver mirroring and red or gold spots on black-and-white images.
resin-coated paper (RC paper): A paper that has been coated with polyethylene film on both sides. Photographic papers that are resin-coated have a significantly lower processing time. All consumer color photographic prints are now printed on RC paper. Black-and-white prints can be printed on either RC or fiber-base paper.
resolution: In digital printing resolution is defined by the number of dots per inch (dpi) in an inkjet or laser print. A higher number of dpi, or higher resolution, implies that a higher level of detail (in spatial or color variation) can be distinguished in the image.
silver-gelatin print: A t raditional photographic print in which the final imaging material is metallic silver suspended in a gelatin binder.
silver halide: A chemical compound formed between silver and a halogen employed as the light-sensitive component in traditional photographic film and papers (usually silver bromide, silver chloride, and silver iodide).
silver image interaction: Any action that chemically alters metallic or ionic silver in a black-and-white photograph. This typically takes the form of image oxidation and silver ion reduction.
swellable print media: Inkjet printing paper in which the receiving layer is a moisture-sensitive polymer. The polymer swells in contact with wet ink and absorbs the colorant drops, conferring on them some protection from air pollutants.
thermal dye transfer: See dye-sublimation.
thermal stability: The decay rates of organic materials increase with temperature. Materials with a greater thermal stability will remain unchanged for a longer time.
TIFF (.tif): The standard lossless file format for bitmapped images. TIFF stands for tagged image file format.
toner: Dry or liquid coloring substance used in laser prints and photocopiers.
UV light: Electromagnetic radiation in the range of 300 to 400 nanometers. UV radiation is not visible and therefore “UV light” is a misnomer. UV radiation has been implicated in the degradation of photographs and papers. The life of a displayed image can be extended by reducing overall image exposure to UV radiation through the use of alternate light sources (daylight and fluorescent lights emit a high degree of UV radiation; tungsten emits a low degree), low light intensity, or special UV filters.
value: One of the three main characteristics that describe a color, along with hue and chroma. Value refers to the relative lightness (or darkness) of a color.
waterproof: Non-water-soluble. Some materials, like PVA glues, are initially water-soluble but become waterproof upon curing.
wet processing: The l iquid chemical procedure used to amplify and fix the latent image in a photograph.
Xerography: See electrophotography.