Mask vs. Panel Testing: Why Optical Measurements Must Match Treatment Geometry

Table of Contents

• 1. Why the Same Number Means Different Things
• 2. Define the Measurement Question First
• 3. Make the Methods Comparable
• 4. What to Ask For, and How to Read a Thin Answer
• 5. Partner With Rainbow Technology for Custom LED Devices
• 6. References & External Links

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1. Why the Same Number Means Different Things

Two reports both say 40 mW/cm². One was taken on a panel at 15 cm with the sensor on the axis. The other was taken on a curved mask with the sensor placed where a cheek would be, and the figure is an average across eight points. Neither number is wrong. Placed next to each other in a comparison table, however, they describe different things, and the table now says something neither report claims.

This is the structural problem with comparing a mask to a panel. A flat measurement plane can sit at a constant distance from a flat source. It cannot sit at a constant distance from a curved device that is worn on a face — the distance changes across the surface by construction, and where you put the sensor decides what you measure. Symmetry of reporting is therefore not a formality; it is the only thing that makes the comparison valid at all.

The same applies within a single format. Two suppliers of “the same” mask can report figures that differ by a factor without either being dishonest, simply because one reports a maximum at the array centre and the other reports an area average.

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2. Define the Measurement Question First

Before asking a supplier for data, decide which of five questions you are actually asking. They are not interchangeable, and a report answering one will not answer another.

• Spectral information. Where the energy sits — peak wavelength, bandwidth, and anything outside the intended band.
• Irradiance at a point. A single figure at a defined position. Useful for verifying a specification, weak for describing a treatment area.
• A spatial map. How output varies across the surface. The only format that can show a bright centre and dim edges.
• An area average. A single summarising figure — meaningful only if the area it covers is stated.
• Uniformity. The relationship between the brightest and dimmest point, expressed in a stated way.

Whichever you choose, seven conditions travel with it: distance, angle of incidence, operating mode, warm-up period, instrument, calibration status and environmental conditions. For a curved or worn product there is an eighth: how the measurement points represent the intended treatment area. That last one is where a report either becomes usable or quietly stops being about your product.

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3. Make the Methods Comparable

The practical fix is procedural: issue one test protocol to every candidate and ask each one to report where it deviated. A supplier who documents a deviation is more useful than one who reports none — the first tells you what the comparison is worth.

What Differs	Why It Breaks the Comparison	What to Standardise
Maximum versus area average	A centre reading can sit well above the average across the treated area, so placing the two side by side overstates one device.	Ask for both, and state which figure supports which purpose in your own specification.
Sensor size relative to the source	A sensor aperture comparable to the emitter averages across it; a much smaller aperture samples a hot spot.	State the sensor aperture and its relationship to the illuminated area.
Where distance is measured from	The housing face, the LED surface and the outer window are three different reference planes — and three different numbers for the same rig.	Define the reference plane explicitly and draw it in the setup description.
Angle of incidence	An off-axis reading understates irradiance and can make a patchy array look even.	State the angle, and whether the sensor was normal to the emitting surface.
Warm-up and operating mode	Output drifts as the device heats, and pulsed versus continuous modes are not reported the same way.	Fix the warm-up period and the mode, and say whether the reading was taken at the start or end of a session.
How curvature is handled	A flat plane cannot hold constant distance from a worn device, so an unstated convention hides a real exposure difference.	Define the measurement points — or the mapping method — that represent the intended use area.
How readings are processed	Averaging, weighting or quietly dropping edge points changes the reported figure without changing the device.	Request the raw or spatial data, and the rule used to reduce it to one number.

One boundary is worth stating because it is so often blurred. A technically sound optical reading answers a measurement question; it does not establish a clinical or cosmetic outcome. If marketing intends to make an outcome claim, that wording needs its own evidence, about the finished product — a good irradiance report is not a substitute, and the two documents should never be filed as though they were one^[1].

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4. What to Ask For, and How to Read a Thin Answer

Eight items make an optical report reproducible by someone who was not in the room. If the method is not described well enough to reproduce or at least to understand, treat the result as incomplete evidence rather than as a weak data point.

• The test setup, with geometry. A description plus a drawing beats a paragraph every time, because it fixes the reference plane and the sensor position.
• Raw or spatial data where appropriate. Without it you cannot check how a headline figure was reached.
• Device revision, and the mode tested. A report that does not name the build cannot be tied to the product you are buying.
• Report date. Older than the last design change means it describes a different device.
• Instrument identification. Model and, where relevant, the sensor aperture.
• Calibration information. Status and date at the time of measurement, not “calibrated”.
• Acceptance criteria and result. The criterion should have existed before the test; a criterion written afterwards is a description.
• Stated limitations. What the setup could not represent — the curvature, the wearer, the second session.

In practice, the item that most often settles the question is the second one. A supplier able to send the spatial data has usually thought about the geometry; a supplier who can only send one number with no distance attached has usually not, and no amount of correspondence will turn the second into the first.

Standardising an optical test protocol across suppliers? Visit our
Custom Light Therapy Manufacturing Page.

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5. Partner With Rainbow Technology for Custom LED Devices

Geometry is where a supplier’s measurement discipline becomes visible. Rainbow Technology runs ODM and OEM programmes from our Shenzhen facility for both masks and panels, and reports optical data in a form that can be compared:

• A stated reference plane and sensor position for every measurement, drawn rather than described, so the figure can be reproduced.
• Point, area-average and spatial data as separate outputs, rather than one headline number that has to stand for all three.
• A stated convention for curved devices — which points were measured and how they represent the worn position — instead of leaving the reader to assume.
• Instrument, aperture and calibration status recorded with the result, plus the mode and warm-up period used.
• Sample and small-batch production from 500 units, with a stated capacity of 100,000 units per month; both figures depend on model, configuration, packaging and schedule, and are confirmed in a formal quotation.
• Compliance documentation matched to the exact model for your target market. Where a model holds a 510(k) record we provide the K-number so you can check its scope in the FDA’s public database; clearance is not the same as FDA approval, and the scope differs from model to model.

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6. References & External Links