A European EPC contractor opens three flat plate collector quotations. Quote A says "high absorptivity black coating". Quote B says "selective coating, α = 0.95". Quote C says "BLUE CORE™ PVD coating, α ≥ 0.94 ±0.02 @AM1.5, ε ≤ 0.06 ±0.02 @80 °C, tested per ASTM E903 / E408". All three claim to be the best. Only one quote provides the data needed to predict useful heat output at 70 °C operating temperature. This article explains the difference between absorptance and absorptivity, why both must be paired with thermal emittance, and what α/ε numbers a buyer should actually require in the RFQ.

Source note: coating values, test methods, radiation-loss calculations and Soletks BLUE CORE / D-DOS product references below are tied to the standards and Soletks product references listed in the Sources section.

• Solar absorptance (α) is the measured fraction of incident solar radiation absorbed by a specific surface under stated conditions; solar absorptivity is the intrinsic material property used in heat-transfer analysis. In supplier datasheets, the two terms are often used interchangeably — buyers should require both the value and the test method.

• A credible selective absorber claim must include α (solar band 0.3‑2.5 μm) and ε (thermal infrared, typically @80 °C, 3‑50 μm). For modern selective coatings, expect α ≥ 0.94 and ε ≤ 0.06.

• At 800 W/m² and 2.0 m² absorber, raising α from 0.90 → 0.95 adds 80 W of absorbed power; at 80 °C surface and 25 °C ambient, lowering ε from 0.85 → 0.08 cuts radiation loss by ~667 W — emittance is usually the larger lever.

• Soletks BLUE CORE™ PVD coating (in-house magnetron sputtering line): α ≥ 0.94 ±0.02 @AM1.5, ε ≤ 0.06 ±0.02 @80 °C, lab-tested collector η = 0.813 (EFPC150).

• Soletks D-DOS spectrum selective coating (integrated solar water heater line): 93% solar absorption, used in ET-150 / 200 / 300 residential pressurized systems.

Source: optical values, ASTM test methods, radiation-loss calculations and Soletks coating specifications in this section are supported by the standards and product references listed in the Sources section.

A useful solar absorptance flat plate collector typical value is α = 0.94‑0.96 for modern selective absorber coatings, paired with thermal emittance ε = 0.04‑0.08. Buyers should judge α together with ε; high absorptance alone does not prove useful heat output.

For buyer screening, a 2.0 m² absorber at 800 W/m² irradiance and α = 0.95 receives 1,520 W of absorbed solar power. Dropping α to 0.90 lowers absorbed power to 1,440 W, a difference of 80 W before heat losses. Emittance matters even more at higher absorber temperature: at 80 °C surface temperature and 25 °C ambient, a simplified Stefan-Boltzmann radiation-loss screen is about 736 W at ε = 0.85 but about 69 W at ε = 0.08 over the same 2.0 m² surface.

For related Soletks pages, compare flat plate solar collector products, the high efficiency flat plate collector, the flat plate solar collector buyers guide, the durable flat plate collector selection guide, and the Soletks inquiry page.

Documentation that supports alpha and epsilon claims

Absorptance and emittance values should be read with model scope, ISO test references and Solar Keymark evidence.

Technical inspection file

Confirms the flat plate collector family and ISO-linked document trail before alpha and epsilon values are used in a design screen.

EN ISO 9806Model scope

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Solar Keymark file

Links collector performance documentation to EU-facing procurement review, incentive checks and model-list verification.

011-7S3254 FValid to 2029

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Where the coating data shows up in real products

Product photos stay as visual context, while alpha and epsilon remain the decision data in the table below.

Engineering flat plate array

BLUE CORE PVD coating matters here because large collector fields magnify small alpha and epsilon differences into useful heat-output differences.

BLUE CORE PVDField heat output

Integrated pressurized heater

D-DOS coating belongs to the ET residential hot-water line, so it should be evaluated separately from the BLUE CORE PVD flat plate path.

D-DOS coatingET hot water line

Absorptance is the fraction of incident radiation absorbed by a real surface under stated conditions; absorptivity is the intrinsic material property. In practice, flat plate collector datasheets often use the symbols interchangeably (α for both), but a precise RFQ should ask for the measured value at a stated test method.

Source: definitions and measurement language in this section are aligned with ASTM E903, ASTM E408, ASTM G173 AM1.5 and ISO 9806 references listed in the Sources section.

For an opaque absorber surface, the conservation rule is α + ρ = 1 (transmittance τ ≈ 0). Higher reflectance means lower absorptance.

Kirchhoff's Law and Why "α at 80 °C" Differs from "α in the Solar Band"

By Kirchhoff's law of thermal radiation, at thermal equilibrium and at a given wavelength, absorptivity equals emissivity (α_λ = ε_λ). But the solar band (0.3‑2.5 μm, where the sun emits) and the thermal band (3‑50 μm, where an 80 °C absorber emits) barely overlap. A spectrally selective coating exploits this: high α in the solar band (≥ 0.94) and low ε in the thermal band (≤ 0.06). The same surface can be a strong absorber of sunlight and a weak emitter of infrared at operating temperature.

This is the physical foundation of every selective coating used in commercial flat plate collectors — and the reason "high absorptivity" alone is not a useful spec.

Typical absorber values should be read as α-ε pairs, not as absorptance alone. A coating with high absorptance and high emittance absorbs sunlight but also loses heat quickly at higher temperature.

Source: absorber ranges combine ASTM E903 / ASTM E408 measurement conventions, Soletks BLUE CORE PVD / D-DOS product values and Solar Keymark / ISO 9806 collector references listed in the Sources section.

Selective coatings matter because they combine high solar absorptance with low infrared emittance. Black surfaces absorb well, but high emittance leads to stronger radiation loss as the absorber gets hot.

A simple absorptance calculation shows why a small α difference changes collector input power. It also shows why absorptance alone does not prove useful heat output.

Source: the absorbed-power screen uses the standard irradiance × area × absorptance relation and Soletks / ASTM E903 coating values listed in the Sources section.

Formula: absorbed power = area × irradiance × α. Substitution Case A: 2.0 m² × 800 W/m² × 0.95 = 1,520 W. Substitution Case B: 2.0 m² × 800 W/m² × 0.90 = 1,440 W.

Over an annual yield of 500 kWh/(m²·year) at 2.0 m², the 5.3% absorptance gap translates to roughly 53 kWh/year of additional captured solar energy per panel — useful, but smaller than the savings achievable from low emittance at high operating temperature.

In one Soletks flat plate collector RFQ intake, the buyer supplied a 2.0 m² collector target, a coating report with measured α = 0.95 and measured ε = 0.08 per ASTM E903 / E408, and an operating temperature target of 70 °C; those values made the coating evidence more useful than the phrase "selective absorber" alone, and matched directly to Soletks BTE-2.0-2 BLUE CORE™ PVD coated absorber.

Thermal emittance changes useful output because a hot absorber radiates heat away. At higher absorber temperatures, emittance may be more important than a small absorptance difference.

Source: radiation-loss values use the Stefan-Boltzmann relation with ASTM E408 emittance context and Soletks BLUE CORE PVD values listed in the Sources section.

Formula: radiation loss = ε × σ × A × (T_surface⁴ − T_ambient⁴). Substitution high-emittance: 0.85 × 5.67 × 10⁻⁸ × 2.0 × (353⁴ − 298⁴) = 736 W. Low-emittance: 0.08 × 5.67 × 10⁻⁸ × 2.0 × (353⁴ − 298⁴) = 69 W.

This is why two absorbers with similar absorptance perform differently at higher temperature. The lower-emittance surface gives up less infrared heat, which leaves more energy available for the heat-transfer fluid.

Decision rule: the higher the operating temperature, the more emittance dominates. For DHW at 50‑55 °C, α and ε both matter. For district heating, process heat or space heating at 70‑90 °C, ε is the larger lever. The Soletks EFPC150 (max operating temperature 150 °C) uses BLUE CORE™ PVD with ε ≤ 0.06 specifically because higher temperature operation exposes the collector to dominant radiation losses.

Combining the two screens gives a more realistic picture of net useful heat at the absorber surface (before convection, conduction and glazing losses).

Source: this combined screen uses ASTM E903 / ASTM E408 coating values, the 800 W/m² irradiance assumption and the radiation-loss calculation introduced in the previous sections.

The matte black paint loses 47% of absorbed energy to radiation alone at 80 °C, while BLUE CORE™ PVD loses only 3.4% — even before convection and conduction are added. This is why "high absorptivity black paint" is not equivalent to a true selective absorber, even when α values appear comparable.

A credible coating claim should state α, ε, test method, temperature basis and durability exposure. A single "high absorptivity" phrase is not enough for procurement.

Durability matters because UV exposure, humidity, thermal cycling and stagnation change optical properties over time. A coating that starts at α = 0.95 but degrades rapidly gives weaker lifecycle value than a slightly lower initial α with documented stability.

The coating manufacturing process determines optical stability over a 20+ year service life. Three main routes are used in commercial flat plate collectors:

Source: process-level α, ε and service-life ranges are based on ASTM E903 / ASTM E408 measurement conventions, ISO 9806 durability references and Soletks BLUE CORE PVD / D-DOS product data listed in the Sources section.

Soletks operates one of the first fully automated magnetron sputtering production lines for high-weather-resistant selective absorber coatings, with proprietary IP across the entire chain — film-system development, vacuum coating equipment design, process optimization, stable operation, and product testing. This in-house capability is the reason Soletks can guarantee α and ε tolerances directly on the absorber datasheet rather than referencing a third-party coating supplier.

Coating choice changes lifecycle cost through heat output, aging, cleaning interval and replacement risk. The strongest RFQ compares optical values with service conditions.

Source: lifecycle ranges and aging drivers are supported by ISO 9806 durability references, ASTM coating measurements and Soletks coating specifications listed in the Sources section.

The lifecycle mechanism is optical degradation. Dust increases reflection, coating damage changes absorption, and higher emittance increases heat loss. All three reduce useful heat even when the collector frame remains intact.

Official references: ISO 9806, ASTM E903, ASTM E408, ASTM G173, NREL review of solar selective absorber materials.

A useful RFQ asks for absorber coating data in the same package as collector performance data. Send Soletks:

• project country, city and application;

• collector area in m² and target quantity;

• target operating temperature in °C (DHW 45‑60 °C, space heating 60‑80 °C, process heat 80‑150 °C);

• required coating type or α-ε target;

• requested solar absorptance value, test method (ASTM E903) and reference spectrum (AM1.5);

• requested thermal emittance value and test temperature (ASTM E408 @80 °C);

• fluid type, flow rate and working pressure;

• stagnation temperature or overheat protection requirement;

• climate risks: dust, salt air, humidity, freeze, high summer heat, UV index;

• documents required: datasheet, ISO 9806 test report, Solar Keymark certificate, coating α/ε measurement report, pressure test, warranty and packing list.

Send these RFQ details through the Soletks inquiry page and request a coating-data comparison. For a formal RFQ, request a quote from Soletks that compares α, ε, collector efficiency curve, operating temperature, durability and lifecycle risk against Soletks BLUE CORE™ PVD (EFPC, BTE) or D-DOS (integrated ET-series) options.

The final specification should include α, ε and the collector test basis.

Optical values, calculations, lifecycle ranges and degradation rates in this article are for early RFQ planning. Final selection should use coating test data, ISO 9806 collector data, Solar Keymark certificates, supplier datasheets, operating temperature, local climate, cleaning plan and installer requirements. Soletks coating specifications (BLUE CORE™ PVD α ≥ 0.94 ±0.02 @AM1.5, ε ≤ 0.06 ±0.02 @80 °C; D-DOS 93% absorption) reflect 2024‑2026 published datasheets and may be updated; confirm current values via the Soletks inquiry page before finalizing specifications.

Authority references used in this article include ISO 9806 (collector testing), EN 12975 (collector reference), Solar Keymark (EU certification), ASTM E903 (solar absorptance / reflectance / transmittance), ASTM E408 (total normal emittance), ASTM C1371 (emissometer field method), ASTM G173 (AM1.5 reference spectrum), ISO 22975-3 (absorber durability) and ISO 9001 (quality management). Soletks product references: EFPC115 / EFPC150 (peak efficiency 0.79 / 0.81, lab-tested η = 0.813, 1.0 MPa, 150 °C max), BTE-2.0-2 (1.87 m² aperture, 0.78 optical efficiency), integrated ET-150 / 200 / 300 (D-DOS 93% absorption, 7 bar), in-house BLUE CORE™ PVD magnetron sputtering production line.