Short answer
Surface Optics Corporation, an independent optical laboratory, measured Sun Home's infrared heater surfaces on August 31, 2026 with an ASTM E408-conforming reflectometer (room-temperature measurement, values calculated to operating temperature). Our single-bulb full-spectrum heater grating measured 0.978 directional and 0.932 calculated hemispherical emittance; the double-bulb grating 0.972 and 0.923, against a perfect blackbody at 1.00. The report is linked below.
Read the full Surface Optics report (PDF, 15 pages). The headline values are in the ET-100 table on report page 3.
The numbers at a glance
| Surface | Directional, 20° | Directional, 60° | Calculated hemispherical, 300 K |
|---|---|---|---|
| Single-bulb full-spectrum heater grating (black felt) | 0.978 ± 0.002 | 0.955 ± 0.004 | 0.932 ± 0.003 |
| Double-bulb full-spectrum heater grating (black felt) | 0.972 ± 0.001 | 0.946 ± 0.003 | 0.923 ± 0.002 |
| Perfect blackbody (reference) | 1.000 | 1.000 | 1.000 |
Means of three repeat measurements, plus or minus one sample standard deviation. ET-100 infrared reflectometer, six bands from 1.5 to 21 µm, Planck-weighted at 300 K. Source: Surface Optics Corporation, August 31, 2026, report page 3; in-band values on page 6 and in Appendix A, page 14.
Source: full report (PDF), page 3.
What emissivity means, in plain terms
Emissivity is the ratio of thermal radiation emitted by a surface to that emitted by a perfect blackbody at the same temperature and under the same wavelength and angular conditions. It is not the fraction of electrical input converted to infrared. Radiant output also depends on surface temperature and area.
For a buyer, a measured emittance value helps describe a heater surface. It does not establish electrical efficiency, cabin uniformity or the infrared energy reaching your body. Those require additional measurements.
Emissivity is capped at 1.00. The two felt gratings had the highest emittance of the surfaces in this tested set. Operating temperature, uniformity and total radiant output remain separate questions.
How Surface Optics measured our heaters
Surface Optics Corporation is an optical measurement laboratory and instrument manufacturer based in San Diego. We supplied physical samples of every surface listed below, and the lab measured them, unpowered and at room temperature, with two instruments.
- ET-100 infrared reflectometer. The instrument illuminates the surface with its own modulated infrared source and measures the fraction reflected in six wavelength bands from 1.5 to 21 µm, at 20° and 60° incidence. Reflectance converts to emittance (emittance equals one minus reflectance), the six bands are weighted by the blackbody distribution at 300 K, and the near-normal result is converted to hemispherical emittance using the instrument's material-class correlation: dielectric for glass, wood and felt, metal for the aluminum wall. Surface Optics documents the ET-100 as conforming to ASTM E408 (Test Method C), with a stated accuracy of plus or minus 0.03 and repeatability of plus or minus 0.005.
- 410-Solar reflectometer. The instrument measures solar reflectance in seven bands from 335 to 2,500 nm, weighted to the ASTM G173 AM1.5 global-tilt solar spectrum. For the opaque aluminum wall, a solar reflectance index was calculated under ASTM E1980.
Each value in the ET-100 table is the mean of three repeat measurements at the same nominal location, reported with one sample standard deviation. These error bars describe repeatability of this measurement set, not full calibrated measurement uncertainty or variation across production units.
The results: every surface we tested
| Surface | Class | Directional, 20° | Directional, 60° | Calculated hemispherical, 300 K |
|---|---|---|---|---|
| Single-bulb full-spectrum heater grating (black felt) | Dielectric | 0.978 ± 0.002 | 0.955 ± 0.004 | 0.932 ± 0.003 |
| Double-bulb full-spectrum heater grating (black felt) | Dielectric | 0.972 ± 0.001 | 0.946 ± 0.003 | 0.923 ± 0.002 |
| Floor heater (glass) | Dielectric | 0.942 ± 0.002 | 0.902 ± 0.011 | 0.888 ± 0.002 |
| Full-spectrum front heater (glass) | Dielectric | 0.909 ± 0.002 | 0.851 ± 0.016 | 0.854 ± 0.001 |
| Interior glass window | Dielectric | 0.912 ± 0.001 | 0.847 ± 0.001 | 0.858 ± 0.001 |
| Exterior glass window | Dielectric | 0.913 ± 0.005 | 0.851 ± 0.010 | 0.858 ± 0.005 |
| Interior wood panel | Dielectric | 0.904 ± 0.006 | 0.870 ± 0.006 | 0.850 ± 0.006 |
| Exterior aluminum wall sample | Metal | 0.896 ± 0.017 | 0.859 ± 0.008 | 0.827 ± 0.017 |
Surface Optics Corporation, ET-100, August 31, 2026, report page 3. Glass, wood and felt were treated as dielectrics and the aluminum wall as a metal for the hemispherical conversion.
Source: full report (PDF), page 3; band-by-band ET-100 reflectance in Appendix A, page 14.
The two felt gratings are samples from single-bulb and redesigned double-bulb heater assemblies. They are the highest-emittance surfaces in the tested set. The glass-front and floor-heater samples had calculated hemispherical emittance of 0.854 and 0.888 at 300 K. The double-bulb calculated hemispherical value shown here is 0.923 from the page 3 data table. Pages 1, 5 and 13 print 0.922; that discrepancy has not been reconciled. The September 21, 2026 report repeats the split: its pages 4 and 6 print 0.923 and its Appendix A prints 0.922.
Why we are publishing the numbers that measured lower
Our earlier spec said 0.99. Here is how the measurement refines it.
Our product pages have carried a blanket 99 percent emissivity figure. That figure is not established by this study. The single-bulb sample reaches an in-band emittance of 0.990 in the 1.5 to 2.0 µm band at 20° incidence, while its Planck-weighted directional and calculated hemispherical values are 0.978 and 0.932 at 300 K. These are sample-specific values, not a replacement specification for every product.
The double-bulb heater measured slightly lower emittance, and that is not the story.
The page 3 table gives the double-bulb grating a calculated hemispherical value of 0.923, compared with 0.932 for the single-bulb grating. This difference is not a measured performance advantage for the single-bulb design. In the follow-up study reported September 21, 2026, Surface Optics measured one specimen of each heater while energized, in four matched passes: the double-bulb heater face averaged 144.3°C against 140.0°C, with calculated surface-equivalent radiant exitance of 1.65 against 1.59 kW/m² and a photographic red-incandescence index of 0.39 against 1.00. Full results are on our heater thermal output testing page.
What the lab excluded, and why.
This study excluded the energized heater and LED-panel readings from its quantitative results because self-emission interfered with the passive measurements. The report calculates estimates at selected higher surface temperatures by reweighting the room-temperature spectra. These estimates assume the spectral reflectance remains unchanged; they are not measurements of energized heaters or proof of actual in-use emittance. The energized measurements were completed in the second study, using a radiometric thermal camera corrected with the emittance values on this page.
Doesn't emittance need to be measured at sauna temperature?
Room-temperature reflectance measurements provide useful emittance data for the tested surfaces. The report calculates estimates at selected higher surface temperatures by reweighting the room-temperature spectra. These estimates assume the spectral reflectance remains unchanged; they are not measurements of energized heaters or proof of actual in-use emittance.
The ET-100 illuminates a surface with its own modulated infrared source, measures reflectance and derives emittance. Its six bands cover 1.5 to 21 µm. The calculation shifts the blackbody weighting to the selected temperature while retaining the measured room-temperature reflectance.
Page 5 provides these calculated estimates at selected surface temperatures:
| Surface temperature | Single-bulb grating | Double-bulb grating | Full-spectrum front heater (glass) | Floor heater (glass) |
|---|---|---|---|---|
| 100°C (212°F) | 0.936 | 0.927 | 0.859 | 0.888 |
| 200°C (392°F) | 0.940 | 0.932 | 0.865 | 0.888 |
| 300°C (572°F) | 0.943 | 0.935 | 0.871 | 0.888 |
Calculated from the measured room-temperature spectra by shifting the blackbody weighting; not heated measurements. Source: Surface Optics Corporation, report page 5. Room-temperature values are in the table above.
Source: full report (PDF), page 5.
These selected temperatures are calculation inputs. They do not establish the actual operating temperatures or material capability of a production heater.
In this study, energized readings were excluded because the heater's own emission interfered with the passive instrument. The follow-up study reported September 21, 2026 addressed the operating question directly, with emissivity-corrected radiometric thermography of the energized heaters rather than contact probes; its results are on our heater thermal output testing page.
How to compare emissivity claims
Compare values only when the surface, wavelength range, angle, temperature weighting and method are stated. A single in-band value is different from a Planck-weighted directional value or calculated hemispherical emittance. The full report lets you check those distinctions for our tested samples.
What the exterior aluminum and glass samples measured
Surface Optics also measured exterior aluminum and glass samples supplied by Sun Home. The optical measurements describe those surfaces; they do not establish alloy grade, patent applicability or performance of every Luminar edition.
| Surface | Solar reflectance (AM1.5G) | Solar energy absorbed | Thermal emittance | ASTM E1980 SRI (low / medium / high wind) |
|---|---|---|---|---|
| Matte black aluminum wall | 0.051 ± 0.001 | About 95 percent | 0.83 | -6 / -4 / -2 |
| Exterior glass | 0.040 ± 0.0005 | Not determinable from reflectance alone (transmitting glazing) | 0.86 | Not applicable |
Solar reflectance weighted to the ASTM G173 AM1.5 global-tilt spectrum. SRI is defined for opaque surfaces only. Source: Surface Optics Corporation, August 31, 2026, report pages 10 to 11.
Source: full report (PDF), pages 10 to 11; passive 410-Solar band data in Appendix B, page 15.
What it means. The opaque aluminum sample reflects about 5 percent of incident sunlight and absorbs about 95 percent. Its calculated thermal emittance is approximately 0.83. Under the medium-wind ASTM E1980 model, these properties correspond to about 84°C (183°F), approximately 1.5°C warmer than the reference black surface. This is a comparative material index, not an observed wall temperature or a prediction for a vertical sauna wall.
What it does not mean. These are exterior-surface measurements. Cabin warm-up, cool-down and heat retention depend on the full wall assembly and insulation, which this study did not measure.
A practical note. Strong solar absorption does not establish a safe-touch temperature. This study did not measure the exterior temperature of an installed sauna in sunlight.
What emissivity cannot tell you, and what we measured next
Emittance is a surface property. Two surfaces of the same material, at the same emittance, produce very different radiant output if one runs hotter, because radiative heat transfer rises with the fourth power of absolute temperature. That is why the double-bulb result above was a starting point rather than a verdict, and why no emissivity figure, ours included, tells you how much infrared reaches the bench.
The second phase has now been reported. Surface Optics corrected a radiometric thermal camera with the camera-band emissivity derived from the values on this page (0.982 for the single-bulb face, 0.977 for the double-bulb face) and measured both heaters under continuous heating, before the sauna reached its 170°F control set point. Of the outputs proposed for this phase, the report delivers:
- emissivity-corrected surface temperature maps (report page 7)
- mean, 95th-percentile and 99th-percentile operating temperature: 140.0°C, 197.2°C and 214.5°C for the single-bulb heater; 144.3°C, 203.0°C and 215.7°C for the double-bulb heater (page 7)
- normalized local radiant-output and temperature-deviation maps across the heater face (page 9)
- a geometry-registered difference field, double-bulb minus single-bulb (page 9)
- calculated surface-equivalent radiant-exitance maps and heater-face values: 1.59 and 1.65 kW/m² (pages 7 and 8)
Warm-up curves and integrated radiant output in watts, also proposed, do not appear in the report. As with the emittance values, the operating results describe the visible heater surface; they do not establish the infrared dose reaching a user without the relevant viewing geometry and distance. Read the results on our heater thermal output testing page.
What this means if you are shopping for an infrared sauna
Use emissivity as a verification question, not a tiebreaker. Five things to ask any brand:
- What is the emissivity figure, and is it directional or hemispherical?
- What instrument measured it, over what wavelength range, and at what angle?
- Was the actual heater surface tested, or is the figure a material-class value from a table?
- Who performed the test, and is the full report public?
- What is the heater's measured operating temperature and radiant output? Emissivity alone cannot tell you how much heat you will receive.
This page answers the measurement questions for the tested samples. Operating-temperature and radiant-output results for the two bulb heaters are published on our heater thermal output testing page.
Bottom line
In passive room-temperature testing by Surface Optics Corporation on August 31, 2026, the single-bulb felt heater grating sample measured 0.978 directional thermal emittance at 20°; its calculated hemispherical thermal emittance was 0.932 at 300 K. These results describe the tested sample, not every heater or production model; carbon far-infrared panels were not included in this study. The double-bulb sample is shown separately in the tables, with the report discrepancy disclosed. Operating-temperature and radiant-output measurements were completed in a second Surface Optics study reported September 21, 2026: heater-face means of 140.0°C for the single-bulb heater and 144.3°C for the double-bulb heater, and calculated surface-equivalent radiant exitance of 1.59 and 1.65 kW/m², from four matched passes on one specimen of each design.
- Read the full Surface Optics report (PDF)
- Read the heater thermal output results
- Explore full-spectrum infrared saunas
Frequently asked questions
What emissivity did the tested heater surfaces have?
In passive room-temperature testing by Surface Optics Corporation on August 31, 2026, the single-bulb felt heater grating sample measured 0.978 directional thermal emittance at 20°; its calculated hemispherical thermal emittance was 0.932 at 300 K. The double-bulb sample measured 0.972 directional at 20° and 0.923 calculated hemispherical at 300 K in the page 3 table. The glass-front and floor-heater samples had calculated hemispherical values of 0.854 and 0.888. The double-bulb calculated hemispherical value shown here is 0.923 from the page 3 data table. Pages 1, 5 and 13 print 0.922; that discrepancy has not been reconciled. The September 21, 2026 report repeats the split: its pages 4 and 6 print 0.923 and its Appendix A prints 0.922.
Why did Sun Home previously publish 99 percent?
Our product pages carried a blanket 99 percent emissivity figure. This study does not establish that figure for every heater. The single-bulb sample's 0.990 in-band value applies to 1.5 to 2.0 µm at 20°; its weighted directional and calculated hemispherical values are 0.978 and 0.932 at 300 K.
Is 0.93 hemispherical emittance good?
A perfect blackbody is 1.00. The single-bulb felt grating's calculated hemispherical value of 0.932 was the highest in the tested set. Emittance alone does not establish cabin performance or delivered infrared energy.
Does higher emissivity mean a hotter sauna?
Emissivity is the ratio of thermal radiation emitted by a surface to that emitted by a perfect blackbody at the same temperature and under the same wavelength and angular conditions. It is not the fraction of electrical input converted to infrared. Radiant output also depends on surface temperature and area; a hotter cabin does not follow from emittance alone.
Were the heaters tested while running?
Not in this study; its quantitative results are passive room-temperature measurements, and its energized readings were excluded because self-emission interfered with the reflectometer. A second Surface Optics study reported September 21, 2026 measured the heaters while running, with a thermal camera corrected by their measured emissivity; see our heater thermal output testing page.
What did the follow-up thermography find?
Under continuous heating, across four matched passes on one specimen of each design, the double-bulb heater face averaged 144.3°C against 140.0°C for the single-bulb heater, its calculated surface-equivalent radiant exitance was 1.65 against 1.59 kW/m², and its photographic red-incandescence index was 0.39 against 1.00 in the same photograph. The differences are small and the pass-to-pass variation overlaps, so the data show comparable observed heater-face performance in that setup, with the low-EMF double-bulb specimen slightly higher on each reported metric; they do not establish a large advantage. EMF was not measured in that study. Details are on our heater thermal output testing page.
Was the emittance measured at sauna operating temperature?
The report calculates estimates at selected higher surface temperatures by reweighting the room-temperature spectra. These estimates assume the spectral reflectance remains unchanged; they are not measurements of energized heaters or proof of actual in-use emittance.
Does emissivity change with temperature?
The page 5 calculations estimate single-bulb hemispherical emittance of 0.936 at 100°C and 0.943 at 300°C. The double-bulb estimates are 0.927 and 0.935; the floor-heater glass remains 0.888. These calculations assume unchanged spectral reflectance and do not prove actual in-use values. The double-bulb calculated hemispherical value shown here is 0.923 from the page 3 data table. Pages 1, 5 and 13 print 0.922; that discrepancy has not been reconciled. The September 21, 2026 report repeats the split: its pages 4 and 6 print 0.923 and its Appendix A prints 0.922.
Does the black aluminum exterior make a sauna heat up faster or hold heat longer?
This study measured exterior optical properties, not cabin warm-up or retention. The opaque aluminum sample absorbs about 95 percent of incident sunlight and has calculated thermal emittance of about 0.83. Whole-wall and insulation performance require separate evidence.
Which Sun Home models use the tested heater surfaces?
The results identify the single-bulb and double-bulb felt gratings and the glass-front and floor-heater samples tested by Surface Optics. A mapping from those samples to specific production models, hardware editions and shipping dates has not been established here. Carbon far-infrared panels were not included in this study.
Can I read the full Surface Optics report?
Yes. Read the complete 15-page Surface Optics report. The follow-up thermography report is linked from our heater thermal output testing page. See also our safety and testing page.
Sources and verification
- Surface Optics Corporation, "Optical Characterization of Infrared Sauna Components" (PDF), prepared for Sun Home Saunas, measurements performed August 31, 2026, 15 pages. Page map: summary, page 1; ET-100 emittance table, page 3; temperature-weighted estimates, page 5; grating in-band data and excluded energized measurements, page 6; planned thermography, pages 7 to 10; exterior solar reflectance and SRI, pages 10 to 11; LED panel and conclusions, page 13; band reflectance appendices, pages 14 to 15. Instruments: ET-100 infrared reflectometer (six bands, 1.5 to 21 µm, 20° and 60° incidence, ASTM E408 Test Method C) and 410-Solar reflectometer (seven bands, 335 to 2,500 nm). ET-100 repeat measurements: three per surface.
- Surface Optics Corporation, "Thermo-Optical Performance Characterization of Infrared Sauna Heaters" (PDF), prepared for Sun Home Saunas, dated September 21, 2026, 15 pages. Page map: summary and primary finding, page 1; method, page 3; room-temperature surface properties, page 4; visible incandescence, page 5; camera calibration, page 6; emissivity-corrected thermography, page 7; radiant exitance, page 8; spatial distribution, page 9; glass-faced heaters and exterior solar-optical properties, pages 10 to 11; conclusions, page 12; appendices, pages 13 to 15. Results are summarized on our heater thermal output testing page.
- ASTM G173, standard tables for reference solar spectral irradiances (AM1.5 global tilt), used for solar-reflectance weighting. ASTM E1980, standard practice for calculating the solar reflectance index of horizontal and low-sloped opaque surfaces, used for the aluminum wall's SRI at the 5, 12 and 30 W/m²K convection conditions.
- Temperature independence of reflectance, and the use of room-temperature reflectance to calculate emittance at higher temperatures: Surface Optics Corporation, ET-100 product documentation (FAQ); temperature-weighted values from report page 5.
- Sun Home's earlier blanket 99 percent emissivity specification is not established by this sample-specific study.
- Related Sun Home testing: EMF, VOC and materials testing and how our EMF testing works.
Questions about this report or the measurement method? Contact us. Measurements performed August 31, 2026.