Short answer
In a report dated September 21, 2026, Surface Optics measured two Sun Home full-spectrum bulb heaters while energized, using emissivity-corrected infrared thermography. The redesigned double-bulb heater face averaged 144.3°C against 140.0°C for the single-bulb heater, 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. The report is linked below.
Read the full Surface Optics report (PDF, 15 pages). The headline comparison is the table on report page 1; the radiometric results are on pages 7 and 8.
Sun Home commissioned this study; Surface Optics Corporation performed the measurements and wrote the report. One specimen of each heater design was measured, in four matched passes.
The numbers at a glance
| Measurement | Single-bulb heater | Double-bulb heater |
|---|---|---|
| Directional thermal emittance, 20° (room temperature) | 0.978 ± 0.002 | 0.972 ± 0.001 |
| Effective emissivity in the thermal camera's band | 0.982 | 0.977 |
| Mean heater-face temperature | 140.0 ± 0.9°C (284°F) | 144.3 ± 3.8°C (292°F) |
| 95th-percentile heater-face temperature | 197.2 ± 2.3°C (387°F) | 203.0 ± 2.6°C (397°F) |
| Calculated surface-equivalent radiant exitance | 1.59 ± 0.02 kW/m² | 1.65 ± 0.06 kW/m² |
| Photographic red-incandescence index, same frame (single-bulb = 1.00) | 1.00 | 0.39 |
What this study measured that the first one could not
Our first Surface Optics study measured the optical properties of unpowered heater samples at room temperature. It established that both felt heater faces are very high-emittance surfaces, and it left the operating questions open: how hot the heater faces run, how much thermal radiation they emit, and whether the low-EMF double-bulb redesign gives anything up. This second study answers those questions with the heaters running.
The two questions are different. Emittance describes how effectively a surface can emit compared with a perfect blackbody at the same temperature. The amount actually emitted depends on operating temperature: for an emitting surface, total thermal radiation rises approximately with the fourth power of absolute temperature, so even a modest temperature difference produces a measurable difference in radiant output (report page 2). Two surfaces with nearly identical emittance, like these two heater faces, can still emit different amounts of heat if one runs hotter.
How Surface Optics measured our running heaters
The measurement program combined passive optical-property measurements with operating thermography, so that what the surface is capable of emitting is separated from how the assembled heater actually operates (report page 3). The processing sequence ran in four steps.
- Room-temperature reflectance. The ET-100 infrared reflectometer measured each surface in six wavelength bands from 1.5 to 21 µm at 20° and 60° incidence, three repeats per surface, with the heater system passive. Reflectance converts to emittance (emittance equals one minus reflectance).
- Camera-band emissivity. The long-wave ET-100 data were weighted over the thermal camera's response band, giving effective emissivity values of 0.982 for the single-bulb felt face and 0.977 for the double-bulb felt face. These are measured inputs rather than generic handbook values (report page 6).
- Radiometric thermography. A radiometric FLIR long-wave infrared camera recorded the energized heaters. A radiometric camera measures infrared radiance, not temperature; each heater's radiance field was corrected with its own measured emissivity to produce a surface-temperature field.
- Calculated radiant exitance. The corrected temperature field was combined with temperature-dependent hemispherical emittance using the Stefan-Boltzmann relation to calculate surface-equivalent radiant exitance, the thermal power emitted per unit of heater-face area into the outward hemisphere, reported in kW/m² (report page 3).
All primary comparison measurements were taken during continuous heating, before the sauna reached its 170°F control set point, so both heaters were continuously energized throughout the analyzed sequence. Four matched single-bulb and double-bulb measurement pairs were acquired within approximately 7 to 9 seconds of one another, and the ± values on this page are the standard deviation across those four passes (report pages 7 and 15).
Two rear-wall heaters were characterized: the left single-bulb heater and the adjacent center double-bulb heater. A third single-bulb heater appears in the visible photograph only as an independent same-exposure reference for the glow comparison; it was not part of the temperature or radiant-exitance averages (report page 5).
Results: heater-face temperature
| Thermal metric | Single-bulb heater | Double-bulb heater | Double minus single |
|---|---|---|---|
| Mean heater-face temperature | 140.0 ± 0.9°C | 144.3 ± 3.8°C | +4.4°C |
| 95th-percentile temperature | 197.2 ± 2.3°C | 203.0 ± 2.6°C | +5.8°C |
| 99th-percentile temperature | 214.5 ± 0.9°C | 215.7 ± 0.3°C | +1.2°C |
The double-bulb heater was hotter in three of the four matched passes, and its upper-temperature distribution also shifted upward (report page 7). Both designs distribute thermal output over the full heater face. In the report's geometry-registered difference field, which normalizes the two faces to a common geometry, the double-bulb heater shows multiple regions that ran hotter than the corresponding regions of the single-bulb heater (report page 9).
These are surface temperatures of the heater face under an emissivity-corrected camera. They are not cabin air temperatures, and the report does not state the cabin temperature at the moment of each pass beyond the fact that it was below the 170°F set point.
Results: calculated radiant output
| Radiative metric | Single-bulb heater | Double-bulb heater | Difference |
|---|---|---|---|
| Calculated surface-equivalent radiant exitance | 1.59 ± 0.02 kW/m² | 1.65 ± 0.06 kW/m² | +3.7% |
| Relative heater-face result | 100% | 103.7% | +3.7 points |
Because the two heater faces have nearly the same emittance, the difference in calculated radiant exitance is driven by operating temperature rather than by the felt coating. Radiant exitance is thermal power emitted per unit of heater-face area; for comparable active heater-face area, it is directly proportional to the thermal power the observed front surface emits, which is why the report treats it as the physically meaningful basis for comparing the two architectures (report page 8).
For scale, 1.59 kW/m² is 1,590 watts per square meter of heater face. It is a calculated surface quantity, derived from the corrected temperature field and the temperature-dependent emittance, and it describes the heater surface only. The infrared energy reaching a person on the bench depends on distance and viewing geometry, which this study did not measure.
Results: visible glow is not infrared output
In a single visible photograph taken with common exposure and white balance, the report quantified the red glow of each heater with a photographic red-excess index, computed from the excess red-channel signal within equal-sized heater regions and normalized to the measured single-bulb heater: single-bulb 1.00, double-bulb 0.39, and the reference single-bulb heater 1.08 (report page 5). The index is not calibrated pyrometry or calibrated brightness; it is a relative indication of filament incandescence rather than a filament temperature.
The result cuts against a common shortcut. For incandescent elements of comparable construction, a lower visible red signal is consistent with a lower filament temperature, yet the emissivity-corrected camera shows the double-bulb heater face slightly hotter. How brightly a heater glows is not a measure of the infrared output at its face. The report attributes the measured behavior to the two-element architecture distributing thermal loading across additional emitting-element area while maintaining a strong infrared thermal state at the heater face (report page 12).
The other heater surfaces in the test
| Surface | Effective camera-band emissivity | Corrected active-region mean | 99th percentile | Calculated hemispherical emittance, 300 K |
|---|---|---|---|---|
| Full-spectrum front heater (glass) | 0.914 | 148.7°C (300°F) | 215°C | 0.854 |
| Floor heater (glass) | 0.944 | 55.7°C (132°F) | 65°C | 0.888 |
The front heater reached a corrected active-region mean of 148.7°C with a 99th-percentile temperature of approximately 215°C, a thermal state comparable to the bulb heaters. The floor heater operated at a much lower thermal state during the test, with a corrected active-region mean of 55.7°C. Carbon far-infrared panels were not part of this study.
What the double-bulb result does and does not show
We commissioned this study to answer a question buyers ask about low-EMF heater designs: does lowering EMF cost heat? In this comparison it did not. The double-bulb heater, which the report identifies as Sun Home's low-EMF design, matched the single-bulb heater's emittance to within a percentage point, ran slightly hotter at the face, and produced slightly higher calculated radiant exitance (report pages 1 and 12).
What that establishes has limits. The comparison involved one specimen of each design and four matched measurement pairs. The 3.7 percent exitance difference sits within the overlap of the two specimens' pass-to-pass variation (1.59 ± 0.02 against 1.65 ± 0.06 kW/m²), the double-bulb temperature mean carries a ± 3.8°C spread, and the double-bulb heater led in three of the four passes rather than all four. Overlapping variation does not establish a difference between the designs, and it does not establish equivalence either. What the data show is comparable observed performance in this setup, with the double-bulb specimen slightly higher on every reported metric. The report's own framing is that the double-bulb architecture preserves, and in this test modestly increases, heater-face infrared thermal performance while reducing visible incandescence.
This is not an EMF measurement. The study did not measure electromagnetic-field magnitude; our EMF record is the independent Vitatech survey summarized on our safety and testing hub and explained in how our EMF testing works. The report's conclusion about the low-EMF design combines its thermo-optical results with those separately established EMF measurements (report pages 1 and 12). And it is not a wall-plug efficiency figure: efficiency is an output-to-input ratio, and this study quantified the output side of the heater face only (report page 3).
What the study did not measure
- EMF. Electromagnetic-field magnitude was outside the scope of this measurement program (report page 12).
- Wall-plug efficiency. A percentage would require synchronized electrical input power and calibrated total radiant output at a defined distance (report pages 3 and 12).
- Infrared energy at the bench. The results describe the observed heater face; dose at a bathing position depends on distance and viewing geometry.
- Internal components. The bulb envelope, filament or heating element, and curved reflector were not measured separately; the results describe the assembled front-surface response (report page 6).
- Warm-up curves and integrated radiant watts. Both were proposed in the first report's plan; neither appears in this report.
- Cabin air temperature at each pass. The report states only that the sauna had not yet reached its 170°F control set point.
- A mapping to production models. Which models, hardware editions and shipping dates use each tested heater has not been established here.
A note on one number in the report
The report prints the double-bulb heater's calculated hemispherical emittance at 300 K as 0.923 in its main tables (pages 4 and 6) and as 0.922 in Appendix A (page 13). The same split appeared in the first report and has not been reconciled. Neither value changes the operating results on this page, which rest on the camera-band emissivity of 0.977 and the temperature-dependent emittance interpolated at each measured local temperature.
Exterior surfaces
The report also repeats the exterior solar-optical measurements of the matte black aluminum wall and the exterior glass (pages 10 and 11). Those results, and what they do and do not say about cabin heat retention, are covered on our heater emissivity testing page.
What this means if you are shopping for an infrared sauna
Use operating measurements the way we do here: as a verification question. Five things to ask any brand:
- Has the heater's operating surface temperature been measured with an emissivity-corrected instrument, or has only the emissivity been measured?
- Is radiant output reported in a physical unit, such as W/m², and over what heater area?
- Who performed the test, and is the full report public?
- If a heater is marketed as low-EMF, has its thermal output been measured against the standard design, or only asserted?
- Are photographs of glowing heaters being offered as evidence of infrared output?
This page answers those questions for the two tested heaters. The infrared energy reaching a bathing position remains a separate measurement, and cabin temperatures are published per model on each product page.
Bottom line
In emissivity-corrected thermography by Surface Optics Corporation, reported September 21, 2026, the double-bulb heater face averaged 144.3°C against 140.0°C for the single-bulb heater, 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 in the same frame. In this comparison of one specimen of each design across four matched passes, the low-EMF double-bulb heater showed comparable heater-face thermal performance, slightly higher on each reported metric. These results describe the two tested heaters; EMF, wall-plug efficiency and infrared dose at the bench were not measured.
- Read the full Surface Optics report (PDF)
- Read the emissivity study
- EMF, VOC and materials testing
- Explore full-spectrum infrared saunas
Frequently asked questions
How hot do Sun Home's infrared heaters get?
In the Surface Optics thermography reported September 21, 2026, the single-bulb heater face averaged 140.0°C (284°F) and the double-bulb heater face averaged 144.3°C (292°F), with 95th-percentile temperatures of 197.2°C and 203.0°C, across four matched passes on one specimen of each design. These are heater-face surface temperatures measured with an emissivity-corrected camera, not cabin air temperatures; cabin temperatures are published per model on each product page.
Does a heater that glows less put out less heat?
Not in this comparison. The double-bulb heater registered a photographic red-incandescence index of 0.39 against 1.00 for the single-bulb heater in the same photograph, yet its emissivity-corrected face temperature averaged 4.4°C higher and its calculated surface-equivalent radiant exitance was about 3.7 percent higher. The index is a relative photographic measure, not calibrated brightness, and visible filament brightness is not a direct measure of heater-face infrared output.
Did the low-EMF double-bulb design reduce heat output?
Not in this comparison. Across four matched passes on one specimen of each design, the double-bulb heater face averaged 144.3°C against 140.0°C and 1.65 against 1.59 kW/m² of calculated surface-equivalent radiant exitance. The differences are small and the pass-to-pass variation overlaps, so the data show comparable observed performance in this setup, with the double-bulb specimen slightly higher; they do not establish a difference or an equivalence between the designs. EMF itself was not measured in this study.
How many heaters were measured, and how many times?
Two: one single-bulb heater and one double-bulb heater on the rear wall of the tested sauna, characterized in four matched late-stage measurement pairs acquired within about 7 to 9 seconds of one another during continuous heating. A third single-bulb heater appears in the visible photograph only as a same-exposure reference. The ± values reported are the standard deviation across the four passes.
Was EMF measured in this study?
No. The study did not measure electromagnetic-field magnitude. Sun Home's EMF record is the independent Vitatech survey summarized on our safety and testing hub, and the report's conclusion about the low-EMF design considers its thermo-optical results alongside those separately established measurements.
Is this an efficiency measurement?
No. Electrical efficiency is an output-to-input ratio. This study quantified the output side of the heater face; a wall-plug efficiency percentage would additionally require synchronized electrical input power and calibrated total radiant output at a defined distance.
Were the heaters actually running during the measurements?
Yes. All primary comparison measurements were acquired during continuous heating, before the sauna reached its 170°F control set point, so the heaters remained energized throughout the analyzed sequence. The radiometric data were corrected using each heater's measured emissivity before temperatures and radiant exitance were calculated.
Which Sun Home models use the tested heaters?
The report identifies a single-bulb heater and a double-bulb heater on the rear wall of the tested sauna, plus a glass-faced front heater and floor heater. A mapping from those heaters to specific production models, hardware editions and shipping dates has not been established here. Carbon far-infrared panels were not included in this study.
How does this relate to the emissivity study?
The first study measured what the heater surfaces are capable of emitting, at room temperature and unpowered. This study measured how the assembled heaters actually operate: emissivity-corrected face temperature, calculated radiant exitance and a photographic glow index. The first study's emittance values are the inputs that correct the thermal camera in the second.
Can I read the full report?
Yes. Read the complete 15-page Surface Optics report. See also our heater emissivity testing page and our safety and testing page.
Sources and verification
- 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: executive summary and primary finding, page 1; fundamentals of thermal emission, page 2; measurement and data-reduction method, page 3; room-temperature surface properties, page 4; heater identification and visible incandescence, page 5; single- versus double-bulb emittance and camera calibration, page 6; emissivity-corrected thermography, page 7; derived radiative performance, page 8; spatial distribution, page 9; additional heater surfaces and exterior solar-optical properties, pages 10 to 11; conclusions, page 12; temperature-weighted hemispherical emittance, page 13; ET-100 band reflectance, page 14; 410-Solar and primary radiometric data, page 15.
- Instruments: ET-100 infrared reflectometer (six bands, 1.5 to 21 µm, 20° and 60° incidence, three repeats per surface); radiometric FLIR long-wave infrared camera, corrected with the measured camera-band emissivity of each surface; 410-Solar reflectometer (seven bands, 335 to 2,500 nm) for the exterior surfaces.
- Method basis as described in the report: Kirchhoff's relation for converting reflectance to emittance on opaque surfaces (page 2); temperature-dependent hemispherical emittance from Planck weighting of the room-temperature spectrum (pages 2 and 13); Stefan-Boltzmann calculation of surface-equivalent radiant exitance from the corrected temperature field (page 3).
- First study: our heater emissivity testing page and the Surface Optics optical characterization report (PDF), measurements performed August 31, 2026.
- Related Sun Home testing: EMF, VOC and materials testing, how our EMF testing works and Sun Home technology, explained.
Questions about this report or the measurement method? Contact us.