Graphic Overlay UV Tests: Hours or Radiant Exposure?

Read the clock and the complete unit

Start with the time definition. Total elapsed time can include light-on exposure, lamp-off conditioning, handling and interruptions. Ask which intervals the reported counter includes. A lamp-off stage may have an important temperature or moisture purpose even though it contributes no lamp radiation under the worksheet's zero-irradiance assumption. Do not remove it from the program simply because it is absent from a radiation calculation.

CIE's public vocabulary defines spectral irradiance as irradiance per wavelength interval, expressed in W/(m²·nm). A reported value at 340 nm therefore needs that wavelength, the instrument's measurement basis and the full denominator. It does not describe the entire source spectrum. A report of broadband irradiance in W/m² instead needs its integration band; the two numbers cannot be compared by dropping “per nm.”

CIE expresses radiant exposure, energy per area, in J/m², equivalently W·s/m². For the spectral bookkeeping used below, integrating spectral irradiance over time retains the wavelength denominator: J/(m²·nm). State the basis whenever an exposure value is compared, rather than treating every quantity called a “dose” as interchangeable.

Worked ledger: three reports labeled 120 hours

Hypothetical HYR editorial calculation—not measured product data or an ASTM-prescribed cycle. Assume ten invented 12-hour cycles with no interruptions. For the calculation only, irradiance is constant during each light-on interval and zero while the lamp is off. All spectral values use the same stated 340 nm measurement basis, geometry and bandwidth. Temperatures, moisture, full spectra, specimen response and acceptance are not supplied.

For a constant spectral irradiance Eλ, the spectral radiant exposure Hλ is Eλ multiplied by light-on time in seconds. In the units used here: Hλ [kJ/(m²·nm)] = Eλ [W/(m²·nm)] × light-on hours × 3,600 / 1,000. If irradiance varies, use the appropriate time-integrated record; multiplying a nominal setpoint by an uncertain counter does not establish the actual exposure.

Report A uses six light-on hours and six lamp-off hours per cycle, with an assumed Eλ of 0.50 W/(m²·nm). Its 120 elapsed hours contain 60 light-on hours. The illustrative calculation is 0.50 × 60 × 3,600 / 1,000 = 108 kJ/(m²·nm), not the 216 obtained by incorrectly treating all 120 hours as light-on time.

The matrix compares two alternative ledgers with A. Report B reaches the same calculated 108 with a different irradiance and light-on duration. Report C has more calculated spectral exposure. Neither comparison ranks the resulting damage or establishes that one program can replace another. The numbers audit the stated assumptions, not the unknown material response.

Do not turn an equal number into an equivalent test

Q-Lab's technical introduction distinguishes the effects of radiation, heat and water, and warns that proportional irradiance-for-time substitution generally cannot be assumed. This substitution is often called reciprocity: doubling irradiance and halving exposure time would preserve the calculated exposure, but it need not preserve a material's response. The A-to-B arithmetic is not evidence that reciprocity holds for an overlay.

The same 340 nm value also leaves the rest of the spectrum unresolved. Lamp type, optical filters and the service environment matter to the comparison. Do not equate a fluorescent-UV report, a xenon-arc report and an exposure behind glazing from a matching number at one monitoring wavelength. Ask the laboratory what evidence supports the proposed comparison for the identified material and property.

Keep lamp-off temperature and moisture stages visible beside the radiation ledger. A shorter, brighter exposure may change those durations or the thermal history as well. If the buyer wants a shortened program, treat it as a proposed test-plan change requiring a documented technical basis—not a unit conversion performed by purchasing.

Tie the exposure to the actual overlay and property

Define the tested construction: exact film and thickness, coating and exposed side, printed layers, texture, window zones, formed features and adhesive or backing where relevant. Identify whether the record concerns an unprinted film, decorated coupon, finished overlay or mounted membrane interface. A supplier film report does not automatically cover a changed print stack or an installed assembly.

Choose observations against the project's concern. They may include legend color or contrast, yellowing, cracking, surface appearance, adhesion, window haze or transmittance. Record the method, measurement locations, specimen support, baseline, post-exposure condition and acceptance rule. These are proposed review fields, not claims that any particular construction will show those changes or pass those checks.

Keep optical and functional decisions separate. An acceptable color comparison does not itself establish display readability, key operation, bond integrity or enclosure protection. If a mounted check is required, identify its housing, viewing or operating conditions and acceptance owner. Retain the same specimen identities through the report so a later result cannot silently inherit another sample's exposure history.

Five reporting failures to resolve before comparison

Ask for a specific correction when one of these gaps appears. A more impressive hours total cannot repair a missing exposure definition.

• Elapsed hours are multiplied as if the lamp was always on: reconstruct the actual light-on intervals and retain the lamp-off stages separately.
• The wavelength or “per nm” denominator is omitted: obtain the complete spectral or band-integrated measurement basis before calculating.
• A nominal setpoint is presented as measured exposure despite interruptions or drift: request the retained irradiance and timing record with deviations.
• Equal calculated exposure is used to approve a brighter, shorter program: require a material- and property-specific technical comparison.
• A film report or photograph becomes a finished-overlay life claim: restore the tested construction, measured property and actual conclusion boundary.

Keep outdoor life separate from the RFQ's test decision

ASTM G151-26's public guidance cautions against a generic conversion from laboratory hours or radiant exposure to exterior months or years. It ties interpretation to the particular materials, degradation behavior and relevant outdoor evidence. This article supplies none of that field-correlation data; the ledger cannot become a multiplier for a service-life promise.

Before comparing quotations, issue one controlled requirement covering the construction, method and edition, source and measurement basis, exposure cycle, temperatures, moisture, specimen allocation, observations and acceptance. Request exceptions explicitly. Agree whether the work is screening, a specified qualification task or part of a broader correlation study so the report answers the decision actually purchased.

For an HYR review, send the existing report and overlay drawing rather than only a target hours figure. Include the installation's direct or filtered sunlight, orientation, protective glazing and maintenance context, plus the property that must remain acceptable. That lets engineering identify missing component evidence without promising outdoor life or approving the complete equipment.