How Is UPF Measured in Knitted Fabric?

Ultraviolet Protection Factor, or UPF, is measured by placing conditioned textile specimens in a spectrophotometer or spectroradiometer and recording how much ultraviolet radiation passes through at the wavelengths defined by the selected standard. Methods such as AATCC TM183 calculate UPF by comparing erythemally weighted incident UV with erythemally weighted UV transmitted through the specimen; the calculation also incorporates the solar spectral weighting specified by the method. Multiple measurements and specimens are required because a knitted surface is not uniform. The report must identify the method, specimen condition, sampling, wet or dry state, extension, preconditioning, and classification or labelling regime.

UPF is a weighted transmission factor

UPF is not simply the inverse of one broadband transmission reading. A laboratory measures spectral transmission, then weights each wavelength according to its relative contribution to erythema and the reference solar spectrum defined by the method. Lower weighted transmission produces a higher UPF. The resulting value describes the tested textile barrier under stated conditions; it is not a direct measure of the time any individual can remain in the sun.

UVA and UVB percentage blocking can also be reported, but those values are not interchangeable with UPF. A fabric can have a similar unweighted average transmission to another fabric while transmitting a different spectral distribution and therefore returning a different weighted result. Specification language should name the metric rather than treating “UV protection” as one undifferentiated number.

UPF also differs from SPF. SPF is used for sunscreen products applied to skin and is established through a different framework. UPF applies to textiles. Neither number removes the need for appropriate garment coverage, shade, and other sun-protection practices. The laboratory result should remain a material-performance statement, not be expanded into medical advice.

The instrument reads the ultraviolet spectrum

The specimen is positioned at the instrument port without wrinkles, unintended gaps, or uncontrolled tension. The equipment records direct and, according to its optical configuration, diffuse transmitted radiation across the ultraviolet wavelength interval required by the method. Calibration, reference measurements, aperture, wavelength increment, stray-light control, specimen-to-port contact, and software calculation settings must follow the standard and laboratory procedure.

Measurements are taken at several positions and across the required number of specimens. The calculation converts spectral data into the UPF result and any associated UVA or UVB transmission values. Because standards and markets can differ in sampling, preconditioning, calculation, rounding, classification, and labelling, a result should not be transferred casually from one regime to another. “Tested to AATCC TM183” and a claim prepared under a specific national labelling framework are related but not identical statements.

The raw transmission curve remains useful during development. It can show whether a change affects the spectrum broadly or only a limited region, while specimen-to-specimen variation reveals whether the knit is spatially stable. A single final class without the underlying condition and variability provides too little information for production correction.

A knit must be sampled across its real apertures

Knitted fabric presents loop openings, yarn intersections, fashioning marks, transfers, tucks, floats, ribs, and local density changes. The instrument aperture may cover many loops or only a small engineered zone, depending on scale. Sampling positions should represent the full repeat and any area likely to transmit more UV. Selecting only the densest section overstates the barrier; selecting an isolated hole without acknowledging the design can misrepresent the garment in another direction.

Technical face, wale direction, course direction, colour placement, and specimen orientation should be documented. Plated structures can expose different yarns on opposite faces. Intarsia, jacquard, marl, space-dyed, and colour-blocked fabrics may need separate controls where fibre, colour, cover, or finish changes. Trims and body structures should not be assumed equivalent because they share a yarn.

Production sampling must account for machine, batch, yarn lot, finishing route, and garment size where these influence density or extension. A development swatch knitted compactly on one machine does not guarantee that a bulk panel, larger size, or finished garment will preserve the same cover factor. Dimensional measurements and UPF data should be linked to the same conditioned specimen state.

Cover is central, but fibre and colour still matter

Cover factor describes how much of the projected area is occupied by yarn rather than open space. Shorter loop length, greater course and wale density, thicker or bulkier yarn, and more compact structures generally increase physical coverage. Pointelle, lace transfers, mesh, long float arrangements, and low-density jersey expose larger or more frequent apertures. These relationships are useful for development, but they do not replace measurement because yarn optics and finish can alter transmission through the covered area.

Fibre type, delustrant, pigment, dye concentration, optical brightener, UV-absorbing finish, yarn twist, hairiness, and cross-section can affect absorption and scattering. Darker or more saturated colours often transmit less UV within the same material system, but colour alone cannot predict a compliant UPF. A pale dense structure may outperform a dark open one; two apparently identical blacks can differ through dye recipe, fibre, and construction.

Thickness and mass per unit area can correlate with coverage, yet neither is a substitute for spectral testing. A thick lofty knit can still contain open channels. Likewise, a compact fine-gauge knit can provide substantial coverage at relatively low mass. The relevant question is not how substantial the fabric feels, but how the tested structure transmits the weighted UV spectrum.

Stretch, wetting, and care can change the result

Extension is critical in close-fitting knitwear. When a loop structure is stretched, yarn segments move apart, apertures enlarge, and the proportion of open area can rise. The relaxed panel result may therefore differ from the fabric worn across the body. Where the product programme requires an extended-state test, the extension level, direction, fixture, and basis for that level should be defined. A percentage pulled arbitrarily in the laboratory is not a substitute for measured end-use strain.

Wet testing addresses another plausible state. Water can change refractive relationships, flatten surface hair, alter yarn swelling, and redistribute openings. The direction and scale of change depend on fibre, construction, finish, and colour, so it is not technically sound to claim that wetting always raises or always lowers UPF. AATCC TM183 allows dry or wet specimen evaluation; the report must state which was used.

Laundering, simulated sunlight, pool water, abrasion, and repeated use can change colour, finish, dimensions, and cover. ASTM D6544 defines preparation exposures used before UV-transmission evaluation for materials intended to carry a UV-protective claim in its system. Conditioning should reflect the intended market and use. Dimensional relaxation after washing may increase cover in one construction, while fibre loss, finish depletion, or distortion may reduce performance in another.

UPF should not be inferred from colour fastness to light. Light-fastness methods assess the degree of colour change after exposure. UPF methods assess how much ultraviolet radiation passes through the textile. A dye can resist fading yet provide limited UV absorption, or contribute to low transmission while still changing shade under prolonged exposure.

A fabric value becomes a claim only through controlled translation

Labelling rules vary by market. The applicable standard defines how specimens are preconditioned, how the reported value or category is derived, and what wording or marking is permitted. ASTM D6603, for example, addresses labelling of UV-protective textiles within its scope. AS/NZS 4399 and European frameworks use their own requirements. Product teams should identify the destination market before approving artwork or copy.

The garment must preserve the conditions that made the fabric result possible. Pattern dimensions, negative ease, placement of mesh or pointelle, seam opening, button spacing, pocket or placket construction, and size grading can all change coverage. A test report for one fabric does not automatically extend to a second colour, a lower-density production lot, a contrast trim, or an altered silhouette.

Within technical knitwear development, useful checkpoints are yarn and colour trials, relaxed structural panels, conditioned and extended specimens where relevant, prototype fitting, care conditioning, and bulk verification. Selected work shows how structure and silhouette are developed together. Anne's remit is described in About; project-specific material and testing questions can be sent through the contact form.

Common questions about UPF in knitted fabric

Measure the condition that will be worn

UPF control in knitwear is an exercise in continuity. Spectral measurement supplies the number; cover, extension, moisture, care, colour, and garment engineering determine whether that number remains relevant from approved panel to finished product.