Label legibility and readability testing

Short definition

Label legibility and readability testing asks whether intended users can perceive, find, distinguish and understand information on labels and packaging under real conditions. The formative readability test examines this by observation during development, using information tasks and controlled light variation, and does not replace a summative evaluation.

On this page

Definition and human factors relevance
The four levels of legibility
Regulatory framework
The formative readability test
Study design at a glance
Limits and distinction from summative evaluation
Frequently asked questions (FAQ)

Definition and human factors relevance

Label legibility describes whether the intended users perceive, find, distinguish and correctly understand the information on the label of a medical device or its packaging under the actual conditions of use. In this wiki, the term stands for the entire chain from perception to comprehension, not only for deciphering individual characters.

Legibility is not a property of the printed image alone. It arises from the interplay of design, printing and material, lighting conditions, viewing angle and viewing distance, the users’ vision, available time and prior knowledge. A label that is perfectly readable at a desk in good light can fail in a darkened room, under reflective film or under time pressure.

The human factors relevance is direct: The FDA expressly counts packaging, product labels and instructions for use among the interaction points of the user interface. According to the FDA, its definition corresponds to that of IEC 62366-1, which includes labeling and training as parts of the user interface. If an item of information is overlooked, confused or misinterpreted, it can lead to a use error. For the content of the information supplied by the manufacturer, see Labeling; for handling the packaging, see Packaging usability.

The four levels of legibility

For evaluation, separating legibility into four levels has proven useful. This is a structure used in this wiki, not a requirement of a standard. Anyone who observes the levels separately finds the cause of a problem more quickly.

Visual recognizability: Can the information be seen and deciphered at all under the conditions?
Findability: Do users find the relevant information in time, without turning or tilting the product for long?
Distinguishability: Can similar product variants, strengths or batches be reliably told apart?
Comprehensibility: Is the information, including symbols and abbreviations, understood as intended?

As an environmental condition, the FDA names that a low or high illumination level can make it harder to recognize displays and controls. In addition, several models of the same product in the room can make it harder to select the right one. Both directly affect recognizability and distinguishability.

Regulatory framework

The requirements for legible labeling arise from several sources (as of November 2026):

EU: According to Annex I, Section 23.1(a) of the Medical Device Regulation (MDR), the medium, format, content, legibility and location of the label and the instructions for use must be suited to the device, its intended purpose and the technical knowledge, experience and education of the intended users. According to Article 10(11), the information on the label must be indelible, easily legible and clearly comprehensible to the intended user or patient.
Process: IEC 62366-1 (with Amendment A1:2020) describes the usability engineering process, in which the label is considered as part of the user interface. According to the understanding of this wiki, it therefore belongs in the use-related risk analysis.
USA: The FDA human factors guidance (version of August 3, 2026) lists improving the recognizability or legibility of controls, labels and displays as a first-tier measure, inherent safety by design. According to the guidance, a design change is usually more effective than an adjustment of labeling or training, because labeling may not be accessible when it is needed.
Information supplied by the manufacturer: ISO 20417 sets requirements for the information to be supplied by the manufacturer, and ISO 15223-1 for the symbols that can be used for this purpose.
International: In 2024, the International Medical Device Regulators Forum (IMDRF) published the second edition of its principles of labeling for medical devices and in vitro diagnostic medical devices (IMDRF/GRRP WG/N52).

None of these sources specifies measured values for illumination, font size or contrast to the extent documented here. This article therefore gives no numerical values. How legibility can be demonstrated for a specific product is a task for usability engineering.

The formative readability test

In this wiki, the readability test is a method of formative evaluation that specifically examines the legibility of labels, packaging imprints and displays. The FDA defines formative evaluation as an evaluation intended to explore the strengths, weaknesses and unexpected use errors of the user interface. The goal is therefore not a pass or fail result, but finding and understanding weaknesses while the design can still be changed.

According to the guidance, a formative evaluation can take place with varying degrees of formality and varying sample sizes, with simple mock-ups or prototypes, and tailored to individual elements of the user interface or aspects of the environment. It may inform the design of labeling and training, which should be finalized before validation. Design changes are implemented iteratively and evaluated again.

Study design at a glance

The following is a suggestion for practice. For formative evaluations, the FDA describes what the protocol typically specifies: purpose and objectives, the part of the user interface being evaluated, scenarios and tasks, participants, data collection, analysis and the use of the results. The following points fill in this framework for legibility.

Context of use: Derive from the context analysis and the use-related risk analysis which information is critical and under which use environment factors it is read.
Information tasks: Set concrete tasks, for example finding an expiration date, distinguishing two variants or stating a dosage, instead of asking whether something is legible.
Controlled light variation: Derive the lighting conditions from the context of use, for example reference light, reduced light, directional light with reflection or alternating bright and dark conditions. For validation, the FDA expressly names dimmed lighting as an environmental factor to be built in.
Behavior instead of opinion: Observe how long the search takes, whether the product is turned or tilted, whether the distance is shortened and whether mix-ups occur. Self-reports supplement the observation.
Participants: Include representative users, also with vision aids and protective equipment. Manufacturer employees may take part in formative tests, but their performance can be misleading if they know the product or are not representative.
Discussing causes afterward: For validation, the FDA describes open, neutrally worded follow-up discussions in which difficulties such as not noticing or not understanding a label are discussed and use errors are examined for their cause. The principle can be transferred to formative tests. See root cause analysis.
Documentation: Record the test conditions reproducibly, that is, light sources, distance, angle and the state of the prototype, plus findings, causes and the design changed as a result.

According to the FDA, thinking aloud is useful in formative evaluations but not acceptable in validation. For legibility tasks, consider whether it changes the natural search and reading behavior.

Limits and distinction from summative evaluation

A formative readability test does not replace a summative evaluation. The FDA notes that the results of formative tests cannot be transferred directly to the final design and that the frequency of problems can be underestimated or overestimated, for example because participants are unusually careful or the prototype differs from the final state. The test therefore does not prove that a label is reliably legible under all conditions.

For validation, the following applies: The labeling used must correspond to the final design, including the labels on the product, the packaging and the package labels. If use errors in critical tasks show up there, a reference to a subsequent change to the instructions for use or another labeling element is not sufficient according to the FDA unless additional test data demonstrate its effectiveness. Legibility problems should therefore be found and eliminated early. Findings and the changes made as a result should remain traceable in the usability engineering file.

In brief

Label legibility arises from the interplay of design, printing and material, light, viewing angle and distance, vision, time and prior knowledge. It comprises four levels: recognizability, findability, distinguishability and comprehensibility.

The formative readability test observes user behavior on concrete information tasks under deliberately varied lighting conditions, discusses causes afterward and documents the conditions. It provides design guidance, but no proof for the final design.

Proof to authorities remains a matter for the summative evaluation or human factors validation with the final labeling.

Frequently asked questions (FAQ)

Is a sufficient font size the same as legibility?

No. Legibility also depends on contrast, material, surface, light, viewing angle and distance, vision, time and prior knowledge. The EU Medical Device Regulation (MDR) requires that legibility be suited to the device, its intended purpose and the intended users. This article deliberately gives no numerical values, because no primary source could be evaluated for them.

When is a readability test conducted?

During development, as long as the design of the label, the packaging imprint or the display can still be changed. The FDA describes formative evaluations as part of ongoing development. Labeling and training should be finalized before human factors validation.

Does the readability test replace the summative evaluation?

No. According to the FDA, the results of formative tests do not apply directly to the final design. For proof, the final labeling is tested in the summative evaluation or human factors validation.

Which lighting conditions should be tested?

Those that actually occur in the context of use and are relevant for critical information. The FDA names a low or high illumination level and dimmed lighting as examples of environmental factors. Which levels make sense for a product follows from the context analysis and the use-related risk analysis.

Is it enough to change the label after a finding in validation?

Not without further steps. According to the FDA, a reference to a changed instruction for use or another changed labeling element is acceptable only if additional test data show that the change effectively reduces the risk.

Do you want to ensure the legibility of your labels and packaging? We support you in planning, conducting and documenting readability tests as part of your usability engineering.

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