Paper Optics & Testing

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Pad reads 89 ISO but the D65 number is three points higher, the OBA is carrying it.

  TranslationThe brightness measured on an opaque stack under standard C-level UV is 89, but with daylight-level UV it reads about three units higher, so the optical brightener's fluorescence is doing much of the work.

• Whiteness flatlined at 160 and the tint went plus two, we're greening, back off the tetra.

  TranslationCIE whiteness has stopped rising and the color has shifted green, a sign of brightener overdose, so reduce the tetrasulfonated optical brightener.

• Opacity is down a point since they cut the basis weight, so either bump the TiO2 or live with the show-through.

  TranslationThe lighter sheet hides less, so add titanium dioxide pigment or accept that printing on the back will be visible from the front.

• Look-through is wild on the drive side and the dry line is crooked, that's a headbox problem, not pulp.

  TranslationFormation is blotchy on one edge and drainage on the forming table is uneven, which points to flow from the headbox rather than a change in the pulp.

• Two reels off shade after the grade change, b-star is yellow, keep them on hold and they go to broke if QA says so.

  TranslationTwo finished rolls missed their color target, leaning yellow, after switching products; hold them and repulp them if quality assurance rejects them.

• Gloss is fine at 75 degrees, but the solids have no snap because the coating is drinking the vehicle.

  TranslationBare paper gloss meets spec, but printed areas are dull because the coating absorbs the ink's oil before it can level and shine.

• UV cal is overdue on the spectro, so don't release any brightened reels off those numbers.

  TranslationThe instrument's ultraviolet calibration has lapsed, so brightness and whiteness readings on papers with optical brightener cannot be trusted for releasing product.

Blue reflectance, whiteness formulas, tint and the yellowing that erodes them.

• Blue-whitealso cool white
  Trade description of a white shaded and brightened toward blue so it reads cool and very white. Shop talk
• Brightnessalso blue reflectance
  The blue-light reflectance of pulp or paper, read at an effective wavelength of 457 nm and reported as a percentage.
• Brightness padalso brightness sheet, pulp pad
  Thick, flat test disc formed from pulp slurry so brightness can be read on an opaque surface.
• Brightness reversionalso reversion, color reversion
  Loss of brightness as pulp or paper yellows with age, heat or light exposure.
• CIE whitenessalso CIE W, CIE whiteness index
  Formula-based whiteness index computed from luminance factor Y and chromaticity, under D65 or C illumination.
• D65 brightnessalso ISO 2470-2 brightness
  Diffuse blue reflectance at 457 nm measured with the instrument's UV content tuned to daylight illuminant D65.
• ISO brightnessalso ISO 2470 brightness, diffuse brightness
  Diffuse blue reflectance factor at 457 nm, measured with d/0 geometry and UV content matched to illuminant C.
• PC numberalso post color number, post-color number
  Post color number: a Kubelka-Munk measure of how much a pulp yellows during an aging test.
• Pointsalso point
  Mill shorthand for units of brightness, opacity or whiteness on their percentage or index scales. Shop talk
• R457also blue reflectance factor
  Notation for diffuse blue reflectance factor at an effective wavelength of 457 nm, the quantity behind brightness.
• TAPPI brightnessalso GE brightness, directional brightness
  North American directional brightness measured with 45/0 geometry at 457 nm, widely called GE brightness.
• Tintalso CIE tint, tint value
  Companion value to CIE whiteness that tells whether a white leans toward green or toward red.
• Whiteness
  How white a surface looks to the eye, judged across the whole visible spectrum rather than one blue band.
• Yellowness indexalso YI, E313 yellowness
  Single number that rises as a near-white sample shifts toward yellow, used to track aging and lignin effects.

How a sheet hides what is behind it, and the reflectance math that explains why.

• Black cavityalso black body, light trap
  Light trap placed behind a single sheet so nothing reflects back from the backing during an R0 reading.
• Kubelka-Munk theoryalso K-M theory, KM theory
  Two-flux light model linking a sheet's reflectance and opacity to its scattering and absorption coefficients.
• Light-absorption coefficientalso absorption coefficient, k
  Per-grammage measure of how much light a paper absorbs, written k and given in square meters per kilogram.
• Light-scattering coefficientalso scattering coefficient, s
  Per-grammage measure of how strongly a paper redirects light, written s and given in square meters per kilogram.
• Opacity
  A sheet's ability to hide what lies behind it, expressed as a percentage from readings over black and white backings.
• Padalso opaque pad, stack
  Lab talk for a stack of sheets thick enough to be opaque, used for R-infinity, brightness and color readings. Shop talk
• Printing opacityalso ISO opacity, paper-backing opacity
  Opacity calculated as single-sheet reflectance over black divided by the reflectance of an opaque pad of the same paper.
• R-infinityalso R∞, intrinsic reflectance factor
  Reflectance factor of a pad so thick that adding more sheets no longer changes the reading.
• R0also single-sheet reflectance, R zero
  Reflectance factor of one sheet measured over a black cavity, the starting point for every opacity calculation.
• Reflectance factor
  How much light a sample sends back compared with an ideal white diffuser lit and viewed the same way.
• Show-through
  Print on the back of a sheet, or on the page beneath, visible from the front because too much light passes through.
• Strike-throughalso bleed-through
  Penetration of ink or its oil vehicle through a sheet far enough to darken or stain the reverse side.
• TAPPI opacityalso contrast ratio, C89
  Contrast ratio opacity: single-sheet reflectance over black divided by reflectance over an 89 percent white backing.

Spectrophotometers, geometries, illuminants, calibration and the color numbers mills report.

• 45/0 geometryalso 45/0, 0/45
  Measurement setup with the sample lit at 45 degrees and viewed straight on, or the reverse.
• b*also b-star, b value
  Yellow-blue coordinate in CIELAB, positive toward yellow and negative toward blue, watched closely on white papers.
• Cal tilealso the tile, working standard
  Lab shorthand for the white or colored ceramic tile used to check or standardize an instrument each shift. Shop talk
• Chasing shadealso chasing color
  Making repeated dye or brightener adjustments to pull the paper's color back onto target after an upset. Shop talk
• CIELABalso L*a*b*, Lab
  Color space describing a color by lightness L* plus a* for red-green and b* for yellow-blue.
• d/0 geometryalso d/0, diffuse/normal geometry
  Measurement setup with diffuse illumination of the sample and viewing perpendicular to its surface.
• Delta Ealso dE, total color difference
  Single number for the total color difference between a sample and its target, usually in CIELAB units.
• Gloss trapalso specular trap, specular port
  Black absorbing port in an integrating sphere that removes mirror-like surface reflection from a measurement.
• Illuminant Calso CIE illuminant C
  Standard CIE light distribution approximating average daylight, with relatively little ultraviolet.
• Illuminant D65also D65, CIE illuminant D65
  Standard CIE daylight at about 6500 K, with enough ultraviolet to excite optical brighteners strongly.
• Integrating spherealso sphere, Ulbricht sphere
  Hollow sphere lined with a highly reflective matte white that bathes a sample in diffuse light from all directions.
• Luminance factoralso Y, Y value
  Tristimulus Y: the overall lightness of a sample as the eye sees it under a given illuminant and observer.
• Metamerismalso illuminant metamerism, metameric failure
  Two samples matching in color under one light but not under another, because their reflectance curves differ.
• Off shadealso off-color, out of shade
  Describes paper whose color has drifted outside the tolerance of its approved standard. Shop talk
• Reference standardalso calibration standard, certified standard
  Certified material with assigned reflectance values that transfers a measurement scale to a working instrument.
• Shadealso color
  The exact color of a paper grade measured against its approved standard, for whites as well as tinted sheets.
• Spectrophotometeralso spectro
  Instrument that measures reflectance across the visible spectrum and computes brightness, whiteness, opacity and color.
• Standard observeralso CIE standard observer, CIE observer
  CIE model of average human color vision, defined for a 2-degree or a 10-degree field of view.
• UV calibrationalso UV adjustment, UV content adjustment
  Setting an instrument's ultraviolet content so fluorescent samples read as they would under the specified illuminant.
• UV cut filteralso 420 nm filter, UV-excluding filter
  Filter that blocks wavelengths below about 420 nm so a sample can be measured without its fluorescence.

Shine, smoothness, coating, calendering and how ink sits on the sheet.

• 20-degree glossalso 20 gloss
  Specular gloss measured at a steep 20 degrees, used to separate very glossy papers, boards and prints.
• 75-degree glossalso TAPPI gloss, 75 gloss
  Specular gloss measured at 75 degrees from the normal, the standard angle for coated and most uncoated papers.
• Calenderalso calender stack
  Stack of hard rolls that presses a paper web to make it smoother, glossier and more even in thickness.
• Coat weightalso coating weight
  Mass of dry coating applied to paper per unit area, usually stated per side in grams per square meter.
• Glossalso specular gloss, sheet gloss
  The mirror-like shine of a paper or print surface, measured as specular reflectance at a defined angle.
• Ink holdoutalso holdout
  The ability of a paper surface to keep ink and its vehicle on top instead of absorbing them.
• Mottlealso mottling
  Uneven, blotchy appearance in a print or coating, seen as cloudy patches of density or gloss.
• Print glossalso ink gloss
  Gloss of the printed ink film on a paper, as distinct from the gloss of the bare sheet.
• Smoothness
  How flat a paper surface is at a fine scale, usually measured by air leaking between the sheet and a reference land.
• Snapalso gloss snap, print snap
  Pressroom word for the visual contrast between glossy printed areas and the surrounding paper. Shop talk
• Supercalenderalso supercal, super
  Off-machine stack of alternating steel and resilient filled rolls that gives paper high gloss and smoothness.

Optical brighteners, dyes, fillers and the furnish that sets a sheet's optics.

• Black lightalso blacklight, UV lamp
  Handheld or bench UV lamp used to check quickly whether a paper contains optical brightener. Shop talk
• Bluingalso blueing
  Mill talk for adding blue or violet dye to make a white sheet look whiter by masking its yellow cast. Shop talk
• Carrieralso OBA carrier, brightener carrier
  Water-soluble polymer such as PVOH that boosts optical brightener efficiency in coatings and size-press starch.
• Filleralso loading, loading material
  Fine mineral powder added to papermaking stock to raise opacity, brightness and smoothness and to cut cost.
• Finesalso fine material, fines fraction
  Fiber fragments and small particles that pass a fine screen and have an outsized effect on sheet optics.
• Fluorescencealso fluorescent component
  Light a sample emits after absorbing UV, measured in paper as the gap between UV-included and UV-excluded readings.
• Furnishalso stock recipe
  The recipe of pulps, fillers, dyes and chemicals that make up a paper grade's stock.
• Greeningalso OBA greening
  The greenish cast a sheet takes on when optical brightener is dosed past its effective range. Shop talk
• OBAalso FWA, OWA
  Optical brightening agent: a dye that absorbs UV light and re-emits it as blue, making paper look whiter.
• PCCalso precipitated calcium carbonate
  Precipitated calcium carbonate: a synthetic filler, often made beside the mill, that raises brightness and opacity.
• Quenchingalso OBA quenching, fluorescence quenching
  Loss of optical brightener response caused by chemicals or materials that suppress its fluorescence.
• Shading dyealso hueing dye, toning dye
  Small dose of blue or violet dye used to cancel the yellow cast of a white sheet and steer its tint.
• Tetraalso tetra OBA, tetrasulfo
  Shorthand for a tetrasulfonated stilbene optical brightener, the general-purpose OBA type. Shop talk
• TiO2also titanium dioxide, tie-oh-two
  Titanium dioxide: the most powerful white pigment for adding opacity, used where every point of opacity counts.

Formation, sidedness, weight, thickness and the flaws testers hunt for.

• Ash contentalso ash, ash level
  Mineral residue left after burning a paper sample, used to estimate filler and coating pigment content.
• Basis weightalso ream weight, basis
  Weight of a 500-sheet ream cut to a grade's basic sheet size, used in North American paper trade.
• Bulkalso specific volume
  Volume per unit mass of paper, the inverse of density, usually given in cubic centimeters per gram.
• Calender blackeningalso blackening, calender darkening
  Dark, translucent spots or streaks produced when calendering crushes a sheet that is too wet or under too much nip load.
• Caliperalso thickness, sheet thickness
  Thickness of a single sheet of paper, measured under a set pressure and given in micrometers or thousandths of an inch.
• Conditioningalso sample conditioning
  Bringing paper samples to a standard temperature and humidity before testing so results can be compared.
• Dirt countalso dirt, speck count
  Measure of visible dark specks in pulp or paper, reported as equivalent black area per unit area.
• Felt sidealso top side, FS
  Face of the sheet that was away from the forming wire, also called the top side.
• Flocalso fiber floc
  Clump of entangled fibers in stock or in the finished sheet, the building block of poor formation.
• Formationalso sheet formation, formation uniformity
  How evenly fibers are spread through a paper sheet, judged by small-scale variation in mass.
• Grainalso grain direction
  The main direction in which fibers in a paper sheet are aligned, almost always the machine direction.
• Grammagealso gsm, g/m2
  Mass of paper per unit area in grams per square meter, the base for most optical calculations.
• Handsheetalso lab sheet, test sheet
  Laboratory-made paper sheet formed from a pulp sample under standard conditions for testing.
• Look-throughalso look-through test
  Visual check of formation made by holding a sheet up to a light and judging how even or cloudy it looks. Shop talk
• Shivealso fiber bundle
  Small bundle of fibers that were never separated during pulping, seen as a splinter-like speck in paper.
• Two-sidednessalso sidedness
  Difference in properties between the two faces of a paper sheet, such as color, brightness, gloss or texture.
• Wild formationalso cloudy formation, flocky formation
  Very uneven, blotchy formation with obvious flocs and thin areas when the sheet is held to the light. Shop talk
• Wire sidealso bottom side, WS
  Face of the sheet that lay against the forming wire while the sheet was formed.

Paper machine sections, crew roles and the slang of the machine floor.

• Back tenderalso backtender
  Paper machine crew member who runs the dry end, handling reels, samples and threading the sheet. Shop talk
• Breakalso sheet break, web break
  Tear of the running paper web on the machine, which sends everything to broke until the sheet is rethreaded. Shop talk
• Broke
  Paper made on the machine that will not be sold as is, and is repulped and fed back into the stock. Shop talk
• Cansalso dryer cans, dryers
  Crew name for the steam-heated cast iron cylinders in the dryer section of a paper machine. Shop talk
• Couchalso couch roll, suction couch
  Suction roll at the end of the forming section where the wet web is dewatered and leaves the wire.
• Dry endalso back end
  Back section of a paper machine, from the dryers through calender and reel, where the sheet is finished. Shop talk
• Dry linealso water line, wet line
  Visible line on a Fourdrinier table where the wet sheet's surface turns from glossy water to dull matte. Shop talk
• Feltalso press felt
  Endless fabric that carries the wet sheet through the press nips and absorbs the water squeezed out of it.
• Fourdrinieralso Fourdrinier machine, flat wire
  Paper machine design in which stock drains on a single, horizontal moving wire to form the sheet.
• Grade changealso changeover, grade swap
  Switching a paper machine from one product to another while it keeps running. Shop talk
• Headboxalso flow box, HB
  Pressurized box that spreads stock in a thin, even jet across the full width of the forming wire.
• Jumboalso parent roll, jumbo roll
  Shop name for the full-width parent roll wound at the paper machine reel before slitting. Shop talk
• Reelalso Pope reel, parent reel
  Winding station at the end of the paper machine, and the full-width roll of paper wound there.
• Reel samplealso turn-up sample
  Strip of paper torn across the full width of a finished reel for lab testing before the reel is released. Shop talk
• Sheetalso web
  Papermakers' word for the continuous paper web on the machine, and for the grade as a product. Shop talk
• Tailalso threading tail
  Narrow strip cut from the edge of the sheet and threaded through the machine first after a break. Shop talk
• Turn-upalso turnup, reel change
  Transfer of the running sheet from a full reel spool onto a new empty one at the paper machine reel. Shop talk
• Wet end
  Front section of a paper machine, where stock is formed into a sheet and most of the water is drained and pressed out. Shop talk
• Wirealso forming fabric, fabric
  Endless woven plastic fabric on which the paper web is formed and drained.

Frequently asked questions

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