White ink is the layer that decides whether a print looks solid or looks like a print. It is also the layer most often approved by eye, in whatever light happened to be available, which is why the same job can pass inspection in one room and be rejected at the customer's desk.

This guide sets out how to test white ink opacity so the result can be repeated: the backing, viewing direction and lighting conditions to fix, the test chart and layer sequence, how to read transmission on clear material and hiding power on dark or coloured material, the registration tolerance that decides edge quality, and the approval and rework rules that turn an opinion into a specification.

Opacity is an agreement, not an impression

Opacity has no absolute value; it is a result under stated conditions. Two people can disagree about the same part and both be right if they are viewing it against different backgrounds in different light.

That is why the useful question is not "is this white enough" but "does this hide the background under the worst condition the product will meet". A printed panel viewed against a dark wall, a bottle held up to a window, a label on a coloured carton and a control faceplate in factory lighting are four different requirements, and they can demand different pass counts on the same material.

Fix the conditions first, then judge. Once the backing, the viewing direction and the lighting are recorded, the result becomes repeatable and comparable - and it can be handed to another operator without an argument. Print research bodies such as Fogra maintain published work on how viewing and measurement conditions affect perceived and measured print results, which is why those conditions belong in the record rather than in the notes.

Which backing, viewing direction and light conditions should be fixed?

Fix the backing, the viewing side and the light.

Fix three conditions: what is behind the part, which side is viewed, and what light falls on it. Change any one and the same print will be judged differently.

Backing matters most for clear and translucent material. A white print that looks solid against a white wall can look grey against a black surface, because the eye is influenced by transmitted light. Standard practice is to check against both extremes - a black backing and a white backing - and to record the result for each. For dark or coloured opaque material, the backing is the substrate itself, so the relevant variable is the substrate colour and the ink's ability to cover it.

Viewing direction decides whether you are judging front-surface or reverse printing. A print viewed through clear acrylic is judged from the unprinted side, and the layer order changes what that view shows. Lighting should be consistent enough to compare parts: the same position in the same room, with the same lamp type, is more useful than a theoretically ideal setup that nobody repeats. Where colour and appearance have to be discussed with a customer, referring to a published measurement standard is more productive than describing a shade in words.

The white ink test chart and layer sequence

Print a stepped chart on the production material: one, two and three white passes, a colour patch over each step, and a fine-line element to check edge behaviour. A single white square tells you nothing about where the requirement is met.

The chart should also contain the layer sequence you intend to use. White under colour is the common arrangement for opaque decoration on clear or dark material; white over colour creates different effects and different durability behaviour. Include the varnish layer if the product has one, because varnish changes both the appearance and the surface's resistance to handling, and its presence affects how the white is perceived.

Print the chart with the settings you expect to use in production: resolution, pass count and curing configuration. Where the machine uses standard industrial piezo printheads, the printed result depends partly on head geometry and channel configuration, so a chart printed on a different head configuration is not a valid comparison - the technical documentation from the component maker, such as Ricoh's industrial inkjet head documentation, describes those characteristics.

How is transmission read on clear materials?

Compare the colour patch over each white step, two backings.

Read transmission by comparing the colour patch over each white step against two backings. The correct pass count is the lowest one where the background stops influencing the colour.

Work in a fixed order. Place the printed area over a black backing, view it, then repeat over a white backing. If the colour appears to change between the two, the white layer is not yet hiding the background, and another pass is required. Record which step passed and which did not; the boundary itself is the useful information, because it tells the next operator how much margin the process has.

Judge from the side the customer will view. A panel printed on the reverse is inspected from the front face through the material, so the boundary between white and colour appears differently than it would on front-surface printing. Where the product will be lit from behind - illuminated signage, backlit displays - test with the intended backlight rather than in ambient light, because transmission under illumination is a stricter condition than reflection.

How is hiding power read on dark and coloured materials?

Check whether any substrate colour shows through.

On dark or saturated substrates, read hiding power rather than transmission: check whether substrate colour is visible through the colour layer and how the colour shifts as the white build increases.

Hiding power is a contrast problem. A dark substrate reflects less light, so the eye notices the moment the white layer becomes thin in a particular area, which usually happens first where the printed image is light or the ink spreads unevenly. Print the chart on the actual material, not a representative one, because the substrate colour is the variable being tested.

Include a colour patch, not just white, in the test. A white layer can look acceptable while the colour above it is subtly shifted, and that shift is what a customer notices when the part is next to a previously approved delivery. Where colour has to be agreed formally, test houses such as Intertek provide independent measurement services, and industry bodies such as Printing United Alliance publish guidance on how print appearance is specified and assessed.

White-to-colour registration and edge tolerance

Registration between the white layer and the colour layer decides whether edges look clean or ragged. Because white is printed first and colour sits over it, any misalignment shows as a light rim on one side of the artwork.

Define a tolerance in the units your operators can check, such as a millimetre deviation measured on the chart, and inspect it on the same test print as the opacity steps. Look at the edges of fine elements as well as large areas; the eye notices a light rim on a thin line long before it notices one on a panel. Where the artwork includes text or a logo with tight registration, include that element in the chart so the tolerance is tested against the part that matters.

Record the pass sequence that held the tolerance. If the colour was laid over an incompletely cured white layer, the edge can also appear smeared rather than offset, which points to a process question rather than a registration setting. Where the substrate is coated or film-protected, check that the film removal has not changed the surface in the printed area, since that also shows up as an irregular edge.

Visual and instrumented evaluation fields

Use visual judgment for the commercial decision and instrumented readings for process control. Record both, in the same place, against the same test print.

The visual fields are the practical ones: which pass step hides the background against each backing, whether any substrate colour shows through, whether the edge tolerance holds, and whether the finish matches the approved reference. The instrumented fields are the ones that allow comparison months later: measured values where an instrument is available, the lamp condition and viewing setup used, and the settings that produced the print.

Keep the approved test print as the physical reference, labelled with the record number and the settings version. Where the printed result depends on adhesion between layers, include an adhesion check in the same record: the cross-cut tape method described in ASTM D3359 is a widely used reference, and applying it to the tested chart keeps opacity and adhesion evidence in one document.

Approval, tolerance and rework rules

Approve against the recorded conditions, define what counts as a deviation, and write the rework rule before the first reject appears. Approval that depends on who is looking is not an approval.

Set three thresholds and keep them simple: the minimum pass count that hides the background, the maximum edge deviation allowed, and the colour difference that requires a reprint rather than a release. Then state what happens when a part falls slightly outside: rework by adding a layer, reprint, or reject. Adding a white or varnish layer over a marginal part changes its thickness and appearance, so if that is permitted it should be a documented rework route rather than an improvisation.

Record every rework decision against the job and keep the rejected part until the cause is understood. Where the same substrate generates repeated opacity rework, the answer is usually a qualification change - a different white ink, a different preparation or a permanent change in the pass sequence - rather than a tighter inspection standard.

Recording the approved opacity recipe

Record the approved result as a recipe: substrate and lot, preparation, white ink and batch, pass sequence, colour layer settings, varnish, curing configuration, backing and lighting conditions, and the approval date. Then attach the retained print.

This record is what makes the next delivery predictable. When a new material lot arrives, printing the same chart and comparing it with the retained print answers in minutes whether the lot behaves like the approved one. When a customer questions a delivery, the record shows which conditions were approved and what the part was judged against.

Keep the record with the qualification card for that material so the two documents stay together, and review the pass sequence when white ink or substrate changes. Opacity is the layer most sensitive to consumable variation, and the record is the only way to notice that the variation happened before the customer does.