COB led strip lumens per foot is a length-normalised output figure, and it is the least ambiguous brightness number a supplier can give you — provided you also know the wattage that produced it. On its own it is unfalsifiable. Paired with watts, it becomes efficacy in lumens per watt, and efficacy is the figure that separates a genuinely efficient strip from one that is simply drawing more power.
This guide covers what a lumen figure does and does not certify, why the light reaching your surface is well below the strip’s rating, why chip density does not create lumens, and how to state a brightness requirement for a COB LED Strip in terms a supplier cannot answer vaguely.
What Lumens Per Foot Actually Specifies
A lumen is total luminous flux — all the light the strip emits in every direction, weighted for how the eye responds to each wavelength. Lumens per foot divides that by length. The conversion is arithmetic: one metre is 3.28 feet, so a strip rated 1,000 lm/m is about 305 lm/ft, and a 400 lm/ft figure is about 1,312 lm/m.
What the conversion cannot do is tell you whether the original number was measured or estimated. Three claims commonly hide behind a lumen-per-foot figure:
- Chip flux summed rather than strip flux measured. Adding up the manufacturer’s ratings for every emitter ignores optical loss in the phosphor layer, the encapsulation and the strip itself. Summed figures run high.
- Measured at 25°C, not at operating temperature. LED output falls as junction temperature rises, which is why IES LM-80 lumen maintenance data is reported at defined case temperatures. Initial flux on a cold board is not what the run delivers after thirty minutes in a profile.
- Measured at nominal voltage with no drop. A figure taken at the feed point does not describe the far end of a 10 m run.
So the useful question about cob led strip lumens per foot is not “how many” but “measured how, at what temperature, at what input power”. A supplier who answers those three has given you a specification. One who quotes only the headline figure has given you a marketing number.
Efficacy Is the Test of a Lumen Claim
Any strip can be made brighter by driving it harder. That is why brightness alone proves nothing about quality. Divide lumens by watts and you get efficacy in lm/W, which is the figure that cannot be inflated without either better components or a physically impossible claim.
Work an example. A 12 W/m strip rated 1,200 lm/m is 100 lm/W. Another strip rated 1,400 lm/m looks brighter — but at 18 W/m it is 78 lm/W. The second strip is 17% brighter and 50% more power-hungry. Over a 30 m ceiling run that is 180 W extra load, which changes driver count, cable sizing and the heat the profile has to move. The brighter strip is the worse engineering choice in most installations.
Efficacy also caps what is credible. White LED efficacy is bounded by phosphor conversion loss, and warm CCTs and high CRI both cost output, because more of the blue pump light has to be converted and converted further into the red end of the spectrum. A quoted 3000K CRI 90 strip claiming the same lm/W as a 6500K CRI 80 strip of the same construction is describing something that does not happen. Background on efficiency claims is set out in the U.S. Department of Energy LED lighting guide.
The practical rule: ask for lumens and watts on the same line of the same datasheet, for the same CCT and CRI. If a supplier gives you a lumen figure at 3000K CRI 90 and a wattage from the 6000K CRI 80 version, the efficacy you calculate will be wrong in the flattering direction.
What Gets Lost Before the Light Arrives
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The strip’s rating describes bare tape on a bench. Every installation subtracts from it, and the subtractions compound rather than average out. This is the main reason a strip that measured correctly still looks dim on site.
| Loss | Typical effect | Why it happens |
|---|---|---|
| Opal diffuser | Removes roughly 20–35% | Scatters light, some of it back into the channel |
| Frosted or clear cover | Removes roughly 5–15% | Less scattering, so less hiding of the LED line too |
| Deep or narrow profile | Removes light to the channel walls | Emission outside the exit aperture is absorbed |
| Thermal derating | Several percent below initial flux | Output falls as junction temperature rises |
| Voltage drop at the far end | Visible before it is measurable | Lower voltage means lower current means less flux |
| Surface reflectance | Changes perceived result entirely | Dark timber returns a fraction of what white plaster does |
Stack the plausible middle of those and a 1,200 lm/m strip behind an opal lens in a warm profile is delivering closer to 800 lm/m at the aperture. Nothing failed. The specification was written against the wrong number.
The last row is the one buyers underestimate most. Reflectance is a property of the room, not the strip, and it can swamp every other decision: the same cove lighting a white plaster reveal and a dark stained oak reveal produces two completely different results from identical hardware. When a project’s finishes are dark, the answer is more light at the source or a shallower throw, decided at specification stage rather than discovered at handover.
Density Does Not Make Lumens
Chip count per metre is the specification most often mistaken for a brightness figure. It is not one. Density determines how the light is distributed along the strip; power determines how much light there is.
The mechanism is straightforward. Take a fixed 12 W/m budget. Spread it across 320 emitters per metre and each one runs at a given current. Spread the same 12 W across 528 emitters and each runs at roughly 60% of that current. Total flux is similar — in fact the denser strip is usually marginally more efficient, because LEDs are slightly more efficient at lower drive current, a relationship called droop. What changed is that the light now comes from more, dimmer points.
That is worth paying for, but for uniformity rather than output. Higher density means smaller thermal load per emitter, better spreading of heat along the copper, and a shorter distance between light sources — which is what lets a strip look continuous through a shallow diffuser. A 528 LEDs/m construction such as the COB LED strip on a 10 mm board at 24V earns its specification through even distribution and thermal headroom, not through a bigger lumen number than a 320 LEDs/m strip of the same wattage.
So when a supplier answers a brightness question with a density figure, the answer is off-topic. Density answers “will I see dots or a line”. Watts and efficacy answer “how much light will there be”. Both matter; they are not interchangeable.
The Number Your Specification Should Carry
A cob led strip lumens per foot figure is a product property. What a project is actually specified against is illuminance on a surface — lux, or footcandles — and the two are related by geometry, not by a constant.
The link is distance. Illuminance from a source falls with distance, so the same strip mounted 300 mm above a shelf and 1,200 mm above it delivers dramatically different lux on that shelf. A linear source spread along a line falls off less sharply than a single point does, which is exactly why continuous strip suits shelf and cove work, but the direction of the effect is the same. Mounting height is a brightness decision with as much weight as the strip selection.
This is why two questions have to be answered before a lumen target means anything: how far is the strip from the surface being lit, and what does that surface need. Lighting practice sets recommended illuminance by task rather than by product, and IES documentation is where those targets are established. Working backwards from a required lux value through the mounting distance and the diffuser loss gives a lumen-per-foot requirement that can be checked. Working forwards from “as bright as possible” gives an over-lit, over-powered installation that still has glare problems.
Colour appearance sits on top of this. At equal measured lux, a warmer CCT reads softer and often reads as less bright, because perceived brightness is not purely photometric. Specifying illuminance without specifying CCT leaves the visual result open — the reasoning is in the warm white cob led strip guide.
How Much Is Enough, by Application
Brightness requirements diverge because the job of the light diverges. In some applications output is the constraint; in others it is the thing that has to be held back.
Retail display and shelving
Here output has to compete with ambient store lighting, so under-specifying is the common error — a shelf lit to the same level as the aisle reads as unlit. But the strip is close to the merchandise and often in the customer’s sightline, so raw output has to be paired with a deep enough channel that the source is not visible. Colour rendering carries as much of the result as brightness does, since the point is accurate merchandise appearance. Project references such as retail display LED strip high CRI and retail shelf high CRI LED strip lighting deal with that combination, and the broader store-level planning sits in retail store LED lighting.
Gallery and display case work
The opposite constraint. Illuminance on sensitive material is limited on conservation grounds, so the specification is a ceiling rather than a target, and the whole problem moves to spectral quality and control at low output. Getting accurate colour at a restricted lux level is harder than getting it at full output — see high CRI COB LED strip gallery lighting and the high cri cob led strip guide for the R9 and spectral considerations behind it.
Cove, cabinet and architectural lines
Indirect installations lose the most to geometry, because the light is bounced off a surface before it reaches the room. A cove washing a ceiling has to overcome both the diffuser and the reflectance of that ceiling, which is why cove runs are frequently specified brighter than the room’s target lux suggests. Cabinet lighting is the reverse case: short throw, close viewing, and glare is the failure mode rather than insufficiency.
Voltage Drop Is a Brightness Problem
A lumen rating is a single figure, but a long run does not have a single brightness. Current flowing through the strip’s own copper drops voltage along the length, so the far end runs at a lower voltage, draws less current and emits less light than the feed end.
The visual threshold is low. A gradient of a few percent along a run is generally imperceptible; the eye is poor at judging absolute brightness but good at detecting a difference across a continuous line in one field of view. That is why the working limit is about 10% drop at the far end, and why a continuous COB line is less forgiving of it than a dotted SMD strip — the uninterrupted line gives the eye a reference to compare against along its whole length.
Voltage choice therefore has a brightness consequence, not only an electrical one. At a given power, current is inversely proportional to voltage: 12 W/m draws 1 A/m at 12V but 0.5 A/m at 24V, and since drop scales with current, the 24V run holds its output roughly twice as far. This is the practical case for a 24v cob led strip on any run beyond a short cabinet section.
Where a single feed cannot hold the limit, the answer is layout rather than a bigger driver. Feeding from both ends roughly quarters the worst-case drop, because each half carries half the current over half the distance. A larger driver does not help at all — it raises available current without changing the resistance of the copper the current has to travel through.
How to Measure It and What to Ask For
Brightness is the easiest strip property to test yourself, because it needs no laboratory. A loose sample on a desk tells you almost nothing; the same sample in the intended profile tells you most of what you need.
- Mount the sample in the actual profile with the actual diffuser, at the actual distance from the surface being lit.
- Take a lux reading on that surface, then take a second reading with the diffuser removed. The difference is your real diffuser loss, not a catalogue figure.
- Leave it running for at least half an hour and read again. The drop between the first and second reading is thermal derating in your mounting condition.
- If the sample is a long run, read at the feed end and at the far end and compare.
- Read against the project’s actual finish material, not a white sheet of paper.
On the supplier side, a brightness enquiry that lists only a model number cannot be answered usefully. Send the mounting distance, the surface material and finish, the profile and diffuser, the target illuminance if one is specified, the run length and voltage, the required CCT and CRI, and the ambient temperature at the strip position. Those inputs let a lumen-per-foot requirement be calculated rather than guessed.
Ask in return for lumens and watts at your specific CCT and CRI on one datasheet line, whether the flux figure is measured on a finished strip or summed from chip data, the temperature the measurement was taken at, and the colour tolerance in SDCM so that repeat orders match. Measurement and colorimetry conventions behind those figures are maintained by the International Commission on Illumination.
FAQ
How do I convert lumens per metre to lumens per foot?
Divide by 3.28. A 1,000 lm/m strip is about 305 lm/ft. The conversion is exact, but it does not validate the original figure — how that number was measured matters more than the unit it is expressed in.
Is a brighter COB strip a better COB strip?
Not by itself. Any strip can be made brighter by driving it harder, which raises power draw and heat. Divide lumens by watts: a 1,200 lm/m strip at 12 W/m is more efficient than a 1,400 lm/m strip at 18 W/m, and usually the better choice.
Does higher LED density give more lumens?
No. At the same wattage, output is roughly the same and the denser strip is marginally more efficient. Density buys uniformity and spreads heat across more emitters; wattage and efficacy determine how much light there is.
Why does the installed run look dimmer than the datasheet?
Because the rating describes bare tape at room temperature. A diffuser removes 20–35%, thermal derating removes several percent more, and a dark surface returns a fraction of what white plaster does. Specify against light at the surface, not light at the strip.
Send the Surface and the Distance
A brightness requirement can be calculated once the geometry is known. Send the mounting distance to the surface, the surface finish, the profile and diffuser, the run length and voltage, the CCT and CRI, and any specified illuminance target — density, wattage and lumen-per-foot requirement all follow from those. Contact Senfey with the geometry and we will work the losses through it and specify against light at the surface.
