Gidsen 

What lux and lumens leave out

Photometry measures light using a standardized model of visual sensitivity. Radiometry measures optical radiation without that human-vision weighting. NIST distinguishes these approaches because radiation can be present even when it makes little or no contribution to visible brightness. [1]

Lumens express luminous flux: optical output weighted for vision. Lux expresses illuminance: incident luminous flux per unit area. One lux is one lumen per square metre. A lumen specification alone does not establish illumination at a particular surface, let alone its radiometric exposure. [2] [3]

The weighting changes with wavelength. CIE defines illuminance through the spectral irradiance distribution and the spectral luminous-efficiency function. Consequently, a lux value does not identify a unique irradiance unless the relevant spectrum is also known. There is no universal lux-to-mW/cm² factor for every red-light source. [3]

Near-infrared makes the gap especially clear

Consider two hypothetical sources with identical visible red output at the measurement surface. One also emits near-infrared radiation outside the photometric response range. An ideal photometric measurement would give the same lux value, although the second source delivers additional radiant power there. This is a consequence of the measurement definitions, not a test of two devices. [1] [3]

A low lux value therefore cannot show that near-infrared output is absent. Nor does a red glow establish how much near-infrared is present. The visible and invisible contributions need appropriate spectral information.

Which units describe incident skin exposure?

For constant incident irradiance, radiant exposure follows this unit relationship:

Radiant exposure (J/cm²) = irradiance (mW/cm²) × time (seconds) ÷ 1,000

The divisor converts milliwatts to watts. If output varies, the calculation needs the irradiance integrated over time, or a correctly measured average over that same interval. This describes energy arriving at a surface; it does not calculate how much a particular tissue absorbs. [4] [5]

A number in mW/cm² also needs a measurement location and a stated spectral range. A single reading at the centre does not establish exposure everywhere in the treatment area. Ask whether the reported value represents a point, a spatial average or a maximum. These are interpretation questions, not instructions to change device positioning.

Even a complete optical report does not supply a treatment prescription. The American Academy of Dermatology (AAD) notes that red-light studies use different devices, durations and research questions, and that the safest and most effective amount for home use remains an area needing research. [9]

Why does a SAD light box advertise lux?

The same phrase, “light therapy,” is used for different purposes. The National Institute of Mental Health describes bright-light boxes for winter-pattern seasonal affective disorder (SAD), including a 10,000-lux light box. That is a description of bright-light treatment for a mood disorder, not a skin-LED exposure target. [6]

The shared label does not make these devices or their exposure instructions interchangeable. A SAD-box lux figure cannot be copied into a red/NIR skin routine, and a skin device's J/cm² figure does not establish that it is suitable for SAD treatment. This comparison explains terminology; it provides no mood-treatment protocol.

Why a phone brightness app cannot certify exposure

An app's displayed number needs a defined measurement method. In its developer FAQ, the phyphox physics-experiment project explains that a phone light sensor's lux reading and a camera-based brightness reading concern different optical quantities. A camera image is not automatically an illuminance measurement. [7]

For an app reporting lux, the spectral-information gap described above remains. For an app reporting irradiance, check how that value is obtained: direct measurement, an assumed spectrum or a conversion from another quantity. A radiometric unit on the screen does not itself validate the result.

Certification would require evidence for the actual phone, sensor or accessory, software method, spectral range and measurement conditions. This article has not tested any app and makes no accuracy claim about a particular one. NIST's spectral-irradiance calibration reference describes a measurement scale with traceability and stated uncertainties; its abstract does not validate phones or provide a home-testing method. [8]

Consumer instruments are not automatically invalid

A properly characterized lux meter can be useful for illuminance. A radiometer can be useful for irradiance if its detector response, calibration and measurement range suit the source. The relevant question is whether the instrument is appropriate for the quantity and spectrum being measured, rather than whether it is sold to consumers.

For red/NIR mixtures, ask how the instrument accounts for each wavelength band. A calibration performed for one spectrum does not, by itself, establish accuracy for another. The checklist below asks for the evidence needed to assess that suitability; it is not a claim that every instrument fails.

What should an optical measurement report disclose?

Use these questions when interpreting a report or requesting clarification. They combine the CIE definitions with NIST's calibration principles; they are an editorial verification checklist, not an international treatment standard. [4] [8]

• Quantity and units: Is the value illuminance, irradiance or radiant exposure?
• Spectrum: Which wavelength bands are included? Are visible red and NIR reported separately or combined?
• Geometry: Where is the measurement plane, and what distance, orientation and treatment-area sampling were used?
• Operating state: Which mode and setting were tested? Does the reading represent constant, peak or time-averaged output?
• Instrument suitability: Which detector and calibration were used, and how was its spectral response accounted for?
• Uncertainty: How much uncertainty accompanies the result, and what supports the stated traceability?

If a report supplies only “brightness,” lumens or lux, label the skin-exposure question unresolved. Missing information is a reason to seek clarification, not a reason to increase duration or move closer.

FAQ

Can lux ever be converted to irradiance?

Yes, under defined conditions. With a known spectrum and a suitable illuminance measurement, the photometric relationship can support a spectrum-specific conversion. A measured spectrum differs from assuming that every source labelled “red” has the same distribution. The conversion also does not establish a treatment target. [3]

Would two meters agreeing prove the reading is accurate?

Agreement alone cannot establish traceability or uncertainty. Both instruments could rely on the same unsuitable spectral assumption. Ask what calibration and validation support the readings, rather than treating agreement as certification. This is a measurement inference from the need for a documented calibration basis. [8]

Does multiplying lux by time solve the problem?

No. That gives a photometric exposure expressed in lux-seconds, retaining the visual weighting. CIE distinguishes luminous exposure from radiant exposure. It does not become J/cm² merely by adding a time value. [5]

The useful next step

When the question is skin exposure, request a radiometric report with spectral, spatial and timing context. Then assess its relevance to the specific device instructions and intended use. Brightness alone cannot complete either step.