Inkjet Photo Paper

Definition

Inkjet photo paper is a coated printing substrate designed specifically to receive aqueous inkjet ink and reproduce photographic (continuous-tone) images at high resolution. Unlike plain office paper, in which ink is absorbed directly into a loosely bonded mat of cellulose fibers, photo paper carries a dedicated ink-receiving layer (also called the image-receiving or coating layer) applied over a prepared base. This layer is engineered to control precisely where and how fast the ink's liquid vehicle and colorant separate and set, so that individual ink droplets remain small and well-defined rather than spreading, feathering, or striking through the sheet.

The term is a family label rather than a single product. It spans a range of base materials, coating chemistries, surface finishes, and weights, unified by the goal of holding a colorant near the surface in a thin, uniform, high-density film. Because the medium is defined by its coating and construction, this page treats it as a material/media type. The machines and methods that deposit ink onto it are covered under inkjet printing and its drop-generation variants, and the colorants themselves under the ink-chemistry pages; those are referenced here, not repeated.

Construction: base and layer stack

Inkjet photo paper is a laminate structure. From the reverse toward the printed surface, most photo stock is built on one of two base types:

• Resin-coated (RC) base. A wood-pulp paper core is encapsulated between thin layers of polyethylene (PE), making the base effectively waterproof and dimensionally stable. Because waterborne ink cannot penetrate into the fibers, the paper resists the moisture-driven cockling and curl that plague uncoated stock, and the image quality is determined almost entirely by the coating applied on top of the PE. RC construction is the mainstream base for consumer and commercial glossy, satin, and luster photo papers.
• Fiber (fine-art) base. Higher-end and archival papers may use a fiber base of cotton rag or purified alpha-cellulose with no PE encapsulation. Some emulate the look of traditional darkroom prints by adding a mineral receiving layer over the fiber (for example a baryta-style, barium-sulfate-loaded layer), giving a matte-to-semigloss surface prized in fine-art work.

Over the base sits the ink-receiving layer (sometimes several sub-layers), optionally finished with a thin protective topcoat that improves scratch and gas resistance. High-gloss surfaces are commonly produced by cast coating, in which the wet coating is dried against a heated, highly polished chrome drum so that the surface takes on a mirror-smooth finish; matte and textured surfaces are produced with different pigments (such as silica) and drying methods. The base may also be brightened with pigments such as titanium dioxide and/or optical brightening agents to raise whiteness.

Ink-receiving layer types: swellable vs. porous

The single most important classification of inkjet photo paper is the mechanism its receiving layer uses to take up ink. Two broad families dominate, and many products blend or layer them.

• Swellable (polymer) coatings. The receiving layer is made chiefly of hydrophilic, water-swellable polymers (such as polyvinyl alcohol or gelatin). When aqueous ink lands, the polymer swells and the dye colorant dissolves into and is absorbed within the film, ending up encapsulated beneath the surface. This yields very smooth, high-gloss surfaces, high optical density, and — because the colorant sits inside a polymer that seals it from the air — good resistance to atmospheric gas (ozone) fading. The trade-offs are slower dry-to-touch time, lower immediate water resistance, and a greater tendency toward moisture-related curl, because the layer itself takes up and holds water.
• Microporous / nanoporous coatings. The receiving layer is a rigid, open network of fine inorganic particles (commonly fumed silica or alumina/boehmite) held together by a small amount of binder. The pore network draws ink in by capillary action, so the surface is dry to the touch almost immediately and is more water-resistant. Porous coatings accept both dye and pigment inks well. Their open structure, however, can leave dye colorant more exposed to airborne gases unless a protective topcoat is added, and the microstructure can slightly scatter light, which some porous glossy papers manage with additional surface treatment.

In short, swellable coatings tend to favor maximum gloss and gas-fade resistance at the cost of dry time and water resistance, while porous coatings favor instant drying, water resistance, and pigment compatibility. Because the two families respond differently to ink chemistry and humidity, the coating type is the property that most determines how a given photo paper behaves in practice.

Surface finishes and grades

Photo papers are marketed and cataloged by surface finish, which is a function of the coating's gloss and texture rather than a formal standard. Commonly encountered finishes, from most reflective to least, include:

• Glossy — a high-shine, highly reflective surface (often cast-coated) that maximizes perceived color saturation and black density but shows fingerprints and specular glare.
• Satin, silk, pearl, and luster — semi-gloss surfaces with a subtle sheen or fine texture that reduce glare and fingerprinting while retaining much of the density and gamut of gloss; the specific names are vendor conventions, not standardized grades.
• Semi-matte and matte — low-reflectance surfaces that diffuse light, giving a softer look with no glare, generally at the cost of maximum achievable optical density (Dmax) relative to gloss.
• Fine-art / textured — cotton-rag or alpha-cellulose papers, sometimes with baryta-style layers or canvas-like textures, used for exhibition and archival printing.

Because finish names are not defined by an external standard, they should be read as descriptive categories rather than precise, comparable specifications. Grade distinctions (for example "premium" versus "everyday" photo paper) similarly reflect coating quality, base type, and weight rather than any measured threshold.

How photo paper is measured (named standards)

The physical properties of photo paper are characterized with the same standardized pulp-and-paper methods used across the industry. This page names the methods; it does not assign any product a specific measured value, since results vary by product and are not universal constants.

• Sheet size. The A-series and B-series trimmed sizes are defined by ISO 216 (built on a √2 aspect ratio in which each size is half the area of the next). North American cut sizes follow separate conventions, and photographic prints are also sold in traditional imperial cut sizes (for example 4×6 in, 5×7 in, 8×10 in) that are market conventions rather than a single governing standard.
• Grammage (weight). Mass per unit area in grams per square metre is determined by ISO 536; North American practice also expresses "basis weight" in pounds. Photo stocks are typically much heavier than plain office paper; glossy photo papers commonly fall in a range on the order of roughly 180–300 g/m², though exact weights vary by product and are not universal.
• Caliper (thickness). Sheet thickness (and derived bulk) is measured under standardized methods such as ISO 534, usually reported in micrometres or mils. Weight and caliper together are discussed on the dedicated paper-weight-and-caliper reference.
• Brightness. Diffuse blue reflectance is measured as ISO brightness under indoor daylight per ISO 2470-1, and as D65 brightness under outdoor daylight per ISO 2470-2.
• Whiteness. CIE whiteness — reflectance across the full visible spectrum, sensitive to optical brightening agents — is determined for outdoor daylight (D65/10°) by ISO 11475 and for indoor illumination by ISO 11476.
• Opacity. Diffuse-reflectance opacity is measured per ISO 2471 (and the analogous TAPPI T 425), the property most relevant to whether a duplex or reverse image shows through the sheet.
• Gloss. Specular gloss of paper is measured at fixed geometries (commonly 75° or 20°) under methods such as ISO 8254 and TAPPI T 480.

Color reproduction is additionally characterized through media-specific ICC profiles used in a color-managed workflow, which is treated under the color-management pages rather than here.

Role in printing and print quality

The receiving layer is what turns a stream of ink droplets into a photographic image, and it drives most of the observable quality attributes of an inkjet print:

• Ink absorption and dry time. By controlling how quickly the vehicle separates from the colorant, the coating sets how fast the print is dry to the touch and handleable. Porous coatings dry near-instantly; swellable coatings dry more slowly.
• Dot definition and dot gain. Confining each droplet to a small footprint keeps printed dots sharp and prevents the lateral spread that enlarges tone values. This is the substrate side of dot gain, and it is a primary reason photo stock resolves fine detail that plain paper cannot.
• Color gamut, density, and Dmax. A good receiving layer holds a high concentration of colorant in a thin, uniform film near the surface, producing higher optical density, deeper blacks, and a wider color gamut than the same ink on uncoated paper.
• Water and smudge resistance. Porous coatings and RC bases generally resist water better than swellable coatings on absorbent bases.
• Permanence. Display and dark-storage permanence of reflection prints are evaluated with accelerated-aging methods — for example indoor light stability under ISO 18937, tied together by the reflection-print image-permanence framework — as predicted ratings under stated conditions, not fixed constants. In general, swellable coatings protect dye from ozone/gas fading better, while porous coatings may need a topcoat for the same protection; optical brightening agents can also fade over time, shifting whiteness. Actual longevity depends on the ink, the media, and display and storage conditions together.

Interaction with ink chemistry and print defects

Photo paper does not act alone; its behavior is coupled to the ink it receives and to several substrate-related defects documented elsewhere on the encyclopedia.

• Ink chemistry pairing. Swellable coatings are optimized around dye-based ink, whose dissolved colorant integrates into the swelling polymer for maximum gloss and gas-fade resistance. Microporous coatings handle both dye and pigment-based ink and are the more natural match for pigment, whose insoluble particles are trapped in the pore network for instant dry and water resistance. On some glossy media, pigment can exhibit gloss differential or bronzing at heavy ink loads — an ink-and-media interaction described on the pigment-ink page and not repeated here.
• Curl and cockle. Because swellable layers and absorbent bases take up water from the ink, they can bend or pucker the sheet; RC and fiber-reinforced bases resist this. The mechanisms, diagnosis, and remedies for these moisture-driven deformations are covered on the paper-curl-and-cockle page.
• Show-through and strike-through. Heavy, opaque photo stock with high grammage and a sealed base resists the reverse image becoming visible; the optical and penetration mechanisms, and their measurement, are covered on the show-through page.
• Feeding and handling. Heavy, smooth, coated sheets place different demands on the paper transport than plain stock, and are moved by the same feed rollers described on their own page. This reference does not give device-specific media-handling settings.

Relationship to adjacent concepts and reference scope

Inkjet photo paper sits within a broader family of substrates and processes documented separately:

• Coated vs. uncoated paper. Photo paper is a specialized, heavily coated medium; the general distinction between coated and uncoated stock, and its effect on ink holdout, is the umbrella concept.
• Brightness, whiteness, and opacity. The optical properties measured above are shared with all paper and are the levers that make a photo sheet appear bright and neutral.
• The printing process. Inkjet printing (and its thermal-inkjet and piezoelectric variants) is the method that deposits ink onto this medium.
• Distinct photographic media. Dye-sublimation media and thermal-transfer materials reproduce images by entirely different transfer physics and are not the same as inkjet-receptive photo paper, even when marketed for photo output.

Scope note: this is a neutral technical reference describing the medium, its types, and how its properties are measured. It is not a buying guide or compatibility list. It names measurement standards but quotes no standard's numeric limits, states no prices, and lists no brand product names, product lines, or model-specific media recommendations; where a specific setting or profile is needed, consult the printer and media manufacturer's own documentation.