The short answer
Use lumens when you need the total light output stated for a source or assembly, lux when you need the light arriving at a defined plane, and cd/m² when you need the visible luminance of a sign face from a defined direction. “Nits” is commonly used for cd/m², but record the SI unit in project documents. Do not convert one figure into another unless the emitting area, distribution, distance, angle, optics, face material and measurement conditions are known.
1. Start with the decision, not the biggest number
Ask what the measurement must decide. A module datasheet may state luminous flux, a workshop check may measure illuminance on the inside of a face, and a client may care about the finished face luminance from the normal viewing direction. Those are three different layers of the system.
The CIE defines luminous flux as a photometric quantity measured in lumens, illuminance as flux per unit area measured in lux, and luminance in candela per square metre. NPL likewise treats illuminance and luminance as separate calibrated photometric measurements. A value is useful only when its quantity, unit and conditions are clear.
Watts, chip brand, module count and colour temperature are not substitute brightness measurements. They may be useful design inputs, but none alone proves how a built sign face will look.
2. Use this measurement-purpose comparison table
Choose one row before selecting equipment or writing an acceptance criterion. Keep the original unit rather than relabelling a lux reading as lumens or a module-lumen figure as sign luminance.
| Quantity | Unit | Practical question | Minimum conditions to retain |
|---|---|---|---|
| Luminous flux | lumen (lm) | How much total visible light does the stated source or assembly emit? | Exact product and variant, test method, temperature, drive condition and tolerance |
| Illuminance | lux (lx = lm/m²) | How much light arrives at this defined plane? | Meter plane, distance, angle, measurement grid, ambient light and complete sample geometry |
| Luminance | candela per square metre (cd/m²) | How bright does this emitting or reflecting surface appear in the measured direction? | Face material, viewed area, measurement direction, distance/focus, drive state, ambient condition and grid |
| Electrical input | watt (W) | How much electrical power is used? | Input voltage/current, operating state and instrument method; do not present it as a photometric value |
3. Understand why there is no universal conversion
Lumens can be distributed into a narrow or wide beam. Lux changes with the size and location of the measurement plane, while luminance depends on the emitting surface, its optical behaviour and the observation direction. A translucent acrylic face can scatter and absorb light; cabinet depth, module pitch, internal reflectance and graphics can change uniformity and face output.
Even a mathematically valid point-source approximation is not automatically valid for a close, extended LED array inside a sign. Do not use an inverse-square calculation as a shortcut where the geometry does not meet its assumptions.
If a supplier gives only a single number, request the test conditions and the exact variant. Glowfy product pages currently provide selected product data, but they do not publish one common lumen, lux and face-luminance dataset measured across every module under identical sign geometry. This guide therefore does not rank K2, K8 or other modules by an invented conversion.
4. Control the sample and measurement conditions
For a fair comparison, build equal-size sample boxes with the same depth, face material and finish, internal colour, graphic coverage, module layout rule, supply voltage at the sample and thermal stabilisation time. Change only the variable being assessed.
Record instrument make, model, serial or asset ID, calibration status and measurement range. Fix the sensor position and orientation with a jig where practical. Record ambient light and subtract or control it only according to the chosen method; do not mix readings taken in a bright workshop with readings taken in darkness.
Use a grid rather than one favourable centre point. Retain every reading, the average only when the method defines it, and the minimum-to-maximum spread or another agreed uniformity indicator. A phone camera can document appearance, but automatic exposure, tone mapping and white balance make it unsuitable as an unqualified photometer.
- Same sign dimensions, face, graphic and internal finish.
- Same electrical operating state and measured supply voltage.
- Fixed meter plane, direction, distance and grid.
- Recorded ambient condition, warm-up time and instrument details.
- Raw readings retained, including edges and low points.
5. Worked example: compare two module layouts without inventing results
A fabricator wants to compare two proposed module layouts for a 300 × 300 mm opal-acrylic sample. Supplier A states a lumen value per module; Supplier B provides a different module-lumen value. Adding the catalogue lumens may support a source-level check, but it does not predict the finished face cd/m² because the beam, spacing, depth, face transmission and internal reflections remain part of the result.
Build both samples to the same approved depth and face specification. Set the same electrical conditions, allow the agreed stabilisation time, then measure face luminance on a 3 × 3 grid from the normal direction. If the project also needs internal incident-light evidence, place the illuminance sensor on one documented plane and repeat the identical grid for both samples.
The decision record should state the actual readings, uniformity result, visible defects, electrical input and pass criterion. If the face or geometry changes, the result no longer represents the original comparison. A result of “sample again” is better than transferring a number to a different construction.
6. Reject common comparison errors
Do not compare a bare-module lumen figure with a lux reading at an undisclosed distance. Do not treat the highest point reading as whole-sign performance, and do not claim that twice the module lumens will give twice the face luminance.
Avoid “nits” without a defined measured surface and direction. A display-panel specification and a fabricated sign-face measurement may share the same unit but not the same method or optical construction.
Do not infer photometric performance from a chip-family name. The guide to Philips 2835 chips explains component context; the LED-module comparison guide considers broader product choices. Neither replaces a controlled measurement of the final sign construction.
7. Complete the brightness acceptance worksheet
Before testing, agree the sign face, viewing direction, operating state, ambient condition, instrument, grid and pass criteria. Identify whether each criterion is stated in lumens, lux or cd/m² and why that quantity answers the project question.
After testing, attach the sample drawing, photographs, raw readings, calculations and deviations. Mark unverified catalogue data as supplier-stated and do not merge it with measured project results.
For a production sign, repeat the checks required by the project plan after installation. Glowfy can help review module options and the sample schedule, but the responsible designer or test provider must approve the method and acceptance limits.
- Decision and photometric quantity selected.
- Exact products, variants and drive conditions recorded.
- Geometry, face construction and ambient conditions fixed.
- Instrument and complete measurement grid retained.
- Pass, hold or resample decision signed with deviations.
Frequently asked questions
Are lumens and lux the same thing?
No. Lumens measure luminous flux, while lux measures illuminance at a surface and equals lumens per square metre. The lux result depends on how the light is distributed and where the measurement plane is placed.
Is one nit equal to one candela per square metre?
Yes, “nit” is commonly used for one candela per square metre. For specifications and test records, cd/m² is the clearer SI expression, together with the measured surface, direction and conditions.
Can I calculate sign-face nits from module lumens?
Not reliably from lumens alone. You also need the emitting geometry, optical distribution, cabinet depth, module layout, face material, graphics and a valid model or measurement method. A controlled finished-face sample is normally the safer comparison.
Can a phone lux-meter app compare LED signs?
It may reveal a rough repeatable change on the same phone and setup, but it is not a substitute for a suitable calibrated photometer. Phone sensors, optics and software are not normally documented for traceable project measurements.
Technical references
Use the current datasheet for the exact product you install. These manufacturer and standards resources explain the general principles.
Define the measurement before comparing modules
Share the sign geometry, face material, module options and proposed measurement grid so Glowfy can help review a repeatable sample schedule.