ASTM E408 Total Emittance

ASTM E408 Total Normal Emittance Testing by Inspection-Meter Techniques

ASTM E408 measures the total normal emittance of opaque surfaces at room temperature using portable inspection metering devices. This is an index that indicates how efficiently the surface emits heat relative to a blackbody having the same temperature as the former, on a scale of 0 to 1. MatX Lab performs ASTM E408 on thermal control coatings, anodized aluminum, metallic foils, tapes, and paint panels.

What Is ASTM E408?

ASTM E408 includes two methods for measuring the total normal emittance of surfaces by inspection-meter techniques. In Method A, a portable thermopile emissometer is placed on the specimen and radiation from the surface is compared with that of high-emittance and low-emittance reference standards. In Method B, the same property is measured using a hemispherical infrared reflectometer where the emittance of an opaque specimen is equal to 1 – reflectance.
Both methods express the result as a dimensionless number at room temperature. Polished aluminum foil has a value of about 0.03, a black anodized surface of about 0.85, while most organic paints range between 0.85 and 0.95 irrespective of color.

The measurement takes minutes and leaves the surface untouched, so it works as an incoming check and as a screening step before longer optical or calorimetric programs.

ASTM E408 Scope and Applications

The technique is applied to materials that are not transparent or translucent, but are reasonably flat and are at close to ambient temperatures.
E408 Applications:

1. Thermal control surfaces. Pairing emittance with solar absorptance measured under ASTM E903 to produce the α/ε ratio that drives radiative balance calculations.
2. Exposure and degradation studies. Measuring the same coupon before and after UV, humidity, thermal cycling, or handling to quantify how the surface changed.
3. Verification of low-emittance products. Testing whether foil-faced, radiant barrier, and reflective coating materials achieve the emittance value specified in the test.
4. Development of coatings. Evaluating pigment content, binder technology, and surface finishes during formulation.

Emittance at service temperature is another type of test measurement. Results vary with temperature; thus, testing a coating at 23 °C means little for performance at 500 °C, and this kind of test refers to ASTM E307. The hemispherical emittance test, where radiation measurements are performed in all directions and not just in the normal direction, can only be done according to ASTM E434.

ASTM E408 Test Procedure

The instrument work is quick. Specimen handling and calibration control the quality of the result.

Limitations. Thin-film samples on insulating substrates will give high readings if no heat sink is used, since the detector heats the sample much faster than heat dissipation through the substrate. Curved surfaces and small pieces may not lie flat inside the hole, and partial contact gives a meaningless reading rather than an inaccurate one. If the surface structure is coarse, this problem occurs on a smaller scale. In this case, you will measure normal total emittance, so directional surfaces like brushed metal and structured films will differ from hemispherical values in ways that matter for thermal calculations.

ASTM E408 Specimen Requirements and Test Conditions

Specimen size is set by the detector aperture rather than by the standard.

Ensure the coupons are sent in a way that protects the surface. Emittance is a surface property, and if the coatings have scratches, are tape-bound, or come in a covering that leaves residue, they no longer represent your products. Sheets and hard packaging would work great for this purpose.
Please tell us which surface needs to be measured if both surfaces are coated, and whether the specimen should be cleaned. We will not clean a specimen without an instruction since it is often the as-received surface that is being studied.

ASTM E408 Test Results and Reporting

Your report provides total normal emittance as a dimensionless number to two decimal places per reading position, as well as mean values.
The data is always provided with a description of the surface, its identification, the measuring technique used, the standards used, and the room temperature where measurements were conducted. If there are several readings per specimen, the range and mean are provided, with descriptions of how the surface appeared at each extreme position.
We quote uncertainty for each project, while reproducibility on properly prepared flat specimens is usually limited to several hundredths of an emittance unit. This technique cannot distinguish two coating materials with a 0.02 difference in emittance.
The turnaround is fast. Measuring a series of coupons takes just a few days, and it is mostly determined by preparation and conditioning, not the measurements themselves.

ASTM E408 FAQs