Last updated: November 4, 2026
At a red light therapy factory, an integrating sphere helps measure how much optical power an LED or device emits and how that power is distributed across wavelengths. To find out how much light reaches a user 15 cm from a panel, the lab runs a separate irradiance test at that distance. Both measurements belong in an optical test program because they describe different aspects of the same device.
How an integrating sphere works
Cutaway diagram showing integrating sphere interior coating, ports, and detector
An integrating sphere is a hollow chamber lined with a highly reflective, diffuse coating, often made from PTFE or barium sulfate. Light reflects repeatedly off the walls. A baffle shields the detector from direct light, so the detector samples the mixed light inside the chamber. Once calibrated, the system can relate that signal to the amount of light collected. A spectroradiometer adds wavelength information. [1, 2]
The mounting arrangement determines what the sphere collects. With a source mounted inside a sphere in a 4π setup, the measurement can capture emission in all directions. With a forward-emitting LED or module mounted at a suitable port in a 2π setup, the measurement covers the forward hemisphere. For a large panel, the lab has to check that the port and mounting arrangement capture the intended output. If the setup covers only one module or a portion of the panel, the report should identify exactly what was measured. [1, 2]
The device and its fixture also absorb some of the light reflected inside the sphere. Labs can assess this effect, known as self-absorption, with an auxiliary lamp and apply a correction where appropriate. Port size, coating condition, detector response and the match between the calibration and test geometries all affect uncertainty. A large sphere can make a panel easier to mount, but the lab still needs to validate the setup. [2, 3, 19]
What the sphere measures and what requires a separate test
Annotated integrating sphere test report showing flux, SPD curve, peak wavelength, and irradiance with distance
| Measurement | Unit | What it tells you |
|---|---|---|
| Radiant flux | W | Optical power collected in the specified geometry. This is the useful output metric when comparing red and near-infrared channels. |
| Spectral radiant flux | W/nm | How that optical power is distributed across wavelengths. A calibrated spectrum can show peak wavelength, bandwidth and output integrated over a defined band. |
| Luminous flux and luminous efficacy | lm and lm/W | Visible-light output weighted for human vision, and its ratio to electrical input power. These measures are useful for lighting products. [17] Near-infrared power is best reported in radiometric units. |
| Irradiance | mW/cm² | Optical power arriving per unit area at a specified location. Measure it separately with a suitable calibrated detector at the intended distance. |
A sphere-coupled spectroradiometer can show whether a device has peaks near 660 nm and 850 nm. If the channels can be controlled separately, record each channel on its own as well as in combined mode. This makes it easier to check peak wavelength and compare channel output across batches. If a spec sheet gives a ratio such as 1:1, find out what was counted or measured. An equal number of red and near-infrared emitters can still produce different optical power in the two bands. Peak height and integrated band power can differ too.
For white-light products, the measured spectrum can also be used to calculate chromaticity, correlated color temperature and color rendering metrics where applicable. For a red and near-infrared panel, the spectrum and radiant output by band are usually more informative. [1, 2, 5, 6]
Measuring irradiance at the distance people use the panel
Split diagram showing same LED panel measured at two distances with different irradiance values
The center reading tells you how much light reaches one point. To see how evenly a panel covers an area, use a fixed measurement plane and record readings across a grid. Report the center, average, minimum and maximum over the defined area, along with the grid spacing and any uniformity formula used. A large panel can give the same center reading as a compact lamp while covering a much wider area.
For a close-range panel test, record where the distance starts, how the detector faces the panel, which channels are on, the output setting and how long the device has warmed up. Six inches is 15.24 cm. Even when two distances are nearly the same, readings can differ because of detector response, sampling position or operating mode. A large LED array also behaves differently from a point source at close range, so measure it at each distance you intend to publish. [4, 12]
For example, REDDOT's EST-T1 product page gives a solar-meter reading of 34 mW/cm² at 6 inches. A separate REDDOT sales page lists 35 mW/cm² at 15 cm. The figures illustrate why the source page and test setup matter. If you are comparing models for an order, request the report for the exact configuration and use the same detector method, operating mode and sampling plan. [13, 14]
Where sphere testing fits into factory quality control
Lab technician at large integrating sphere system with red light therapy panel at entry port
During incoming inspection, a calibrated sphere and spectroradiometer can help check LED samples for wavelength and output differences between lots. In development, the same setup can compare an LED board before and after a lens or diffuser change, provided the lab keeps the drive settings, temperature and collection geometry consistent. On a finished panel, a separate irradiance map shows how that design change affects coverage at the use distance.
For routine retesting, record the sample or batch ID, instrument and calibration status, mounting geometry, warm-up time and channel settings. Compare results taken under the same conditions. If a report gives a flux or irradiance tolerance, ask how the lab handles measurement uncertainty when a result lands close to the acceptance limit. ISO 13485 covers medical-device quality management systems. ISO/IEC 17025 covers laboratory competence. When you commission outside testing, check that the lab's accreditation scope covers the specific measurement you need. [7, 8, 12]
ANSI/IES LM-79-24 and CIE S 025/E:2015 describe optical and related measurements for solid-state lighting products within their respective scopes. They are useful references for understanding how light output is measured and reported. For a therapy panel, specify the optical and safety tests that apply to that model and its intended use. Keep the supporting documents separate. An ETL listing identifies the model and applicable standards in Intertek's directory. Where CE marking applies, the manufacturer identifies the applicable EU requirements, prepares technical documentation and issues the EU declaration of conformity. [5, 6, 10, 11]
Optical output and photobiological safety
Photobiological safety document beside integrating sphere SPD output curve showing hazard classification input
IEC 62471 covers photobiological safety evaluation for lamps and lamp systems within its scope. The relevant exposure measurements can include spectral irradiance or spectral radiance, depending on the hazard and test geometry. A sphere spectrum can identify emission bands and help plan the assessment. The exposure evaluation uses measurements appropriate to the hazard, with conditions such as viewing distance, apparent source size or exposure duration specified where relevant. [9]
For non-laser equipment intended for therapeutic, diagnostic, monitoring, cosmetic or aesthetic use, IEC 60601-2-57 may apply to basic safety and essential performance. Ask for the report that covers the device's intended use, exact model and operating modes. The total watts from a sphere test describe the device's optical output. Exposure measurements describe how much radiation reaches the eye or skin under the conditions evaluated. [9, 15]
What buyers should request from a supplier
Spec sheet comparison showing verified vs unverified irradiance data for red light therapy devices
- A spectrum for the exact model and channel settings. Ask for the actual peak wavelengths, the measurement range and whether the spectrum represents total output or a reading at one position.
- A flux report if total optical power is part of the specification. Check the mounting geometry, what portion of the device was measured, how the system was calibrated and whether the result is in optical watts.
- An irradiance map at the intended use distance. Check the distance reference, detector type and wavelength response, area covered, measurement grid and center and average readings.
- Model-specific safety and conformity documents. Match the model and hardware revision to the scope of each report or listing. If a lab's accreditation is cited, check that it covers the measurement.
- Consistent conditions for a side-by-side comparison. Use the same distance, operating mode and measurement method across devices. Treat a handheld reading as a spot check unless the detector is calibrated for the wavelengths and the setup matches the report.
FAQ
What is the purpose of an integrating sphere?
It collects light within a defined geometry and mixes it through diffuse reflections. A calibrated detector can then measure the collected flux. Coupling the sphere to a spectroradiometer lets the lab measure how that flux is distributed by wavelength.
How much does an integrating sphere cost?
A quote depends on the sphere diameter, port configuration, wavelength range and whether the package includes a spectroradiometer, reference source, calibration and fixtures. When comparing quotes, give suppliers the dimensions of your largest sample and the wavelength bands and output levels you need to measure.
What else are integrating spheres used for?
Labs use them for LED, lamp and module characterization. A sphere can also measure a material's reflectance or transmittance when configured to collect scattered light from a sample. The fixtures and calibration depend on the measurement. [1, 16]
How is an integrating sphere calibrated?
The lab measures a traceable reference in the relevant configuration and uses it to establish the system's response. For absolute spectral output, the calibration must cover the required wavelengths. Check the reference and test geometries and assess self-absorption when the source or fixture changes. Recheck the setup when the coating, detector or optical coupling changes, and follow the lab's documented calibration schedule. [2, 3]
References
- CIE. Measurement of LEDs (CIE 127:2007).
- NIST. Calibration and Transfer Standards for Total Spectral Radiant Flux.
- NIST. Detector-Based Integrating Sphere Photometry.
- NIST. Spectral Irradiance Calibrations (Special Publication 250-89).
- Illuminating Engineering Society. Optical and Electrical Measurements of Solid State Lighting Products (ANSI/IES LM-79-24).
- CIE. Test Method for LED Lamps, LED Luminaires and LED Modules (CIE S 025/E:2015).
- ISO and IEC. General Requirements for the Competence of Testing and Calibration Laboratories (ISO/IEC 17025:2017).
- ISO. Medical Devices Quality Management Systems (ISO 13485:2016).
- IEC. Photobiological Safety of Lamps and Lamp Systems (IEC 62471:2006).
- Intertek. ETL Listed Mark Directory.
- European Commission. CE Marking Responsibilities for Manufacturers.
- REDDOT LED. Hardware Testing and Optical Retesting.
- REDDOT LED. EST-T1 Desktop Red Light Therapy Panel Specifications.
- REDDOT LED. LED Therapy Devices and T1 Specifications.
- IEC. Safety and Essential Performance of Non-Laser Light Source Equipment (IEC 60601-2-57:2023).
- NIST. Reference Integrating Sphere for Spectral Reflectance.
- CIE. Measurement of Luminous Flux (CIE 084:1989).
- REDDOT LED. About REDDOT LED.
- NIST. Improved Photometric Standards and Calibration Procedures at NIST.
Related guides
Related guides on integrating sphere uses and red light therapy testing