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IES Files & LM-79 Reports: How to Evaluate Lighting Substitutions
An LM-79 report is one of the first documents commercial lighting buyers should check when evaluating an LED lighting substitution.
“Approved equal.”
Three dangerous words.
A contractor submits a replacement downlight that looks almost identical to the specified fitting, claims the same 25W input power, produces roughly the same total lumens and even carries the same 3000K/CRI 90 headline specifications.
So it’s equivalent?
Not yet.
When I review a lighting substitution, I don’t care whether the supplier can find a product with similar wattage and a close-looking housing; I want to know whether that fitting puts the required amount of light in the required directions, under reproducible test conditions, without quietly changing beam shape, intensity, glare or project illuminance.
That’s the test.
And the documents that start answering it are usually an LM-79 report and an IES photometric file.
The Illuminating Engineering Society updated the measurement standard again in 2024. ANSI/IES LM-79-24 covers reproducible electrical and photometric measurements of solid-state lighting products, including total luminous flux, electrical power, efficacy, luminous-intensity distribution and color-related quantities under standardized conditions.
That matters because a substitution should be compared with measured product performance.
Not brochure adjectives.
First, LM-79 and an IES File Are Not the Same Thing
I still see the terms mixed together.
A supplier says:
“We have IES LM79.”
What exactly do you have?
An LM-79 laboratory report?
An .ies photometric data file?
A PDF polar curve?
A lighting calculation?
Those are related.
They’re not interchangeable.
| Document / Data | What It Tells You | What It Doesn’t Prove Alone |
|---|---|---|
| LM-79 report | Measured electrical, photometric and color performance | Project equivalency |
| IES photometric file | Directional luminous-intensity distribution for calculation software | Test credibility by itself |
| Photometric plot | Visual representation of distribution | Complete numeric comparison |
| Lighting calculation | Predicted project performance using the luminaire data | Whether the source file matches production |
| Datasheet | Manufacturer’s stated product specifications | Independent measured performance |
| Sample | Appearance and physical construction | Full project photometric behavior |
DOE’s older but still useful technical explanation of LM-79 makes one point especially clear: solid-state-lighting testing uses absolute photometry, meaning the complete LED luminaire or integrated lamp is measured as the operating system rather than treating the source and fixture as separable photometric pieces.
That distinction matters enormously for substitutions.
Change the optic?
New photometry.
Change the output package?
Potentially new photometry.
Change the COB position?
Maybe new distribution.
Change the reflector?
Definitely don’t assume the old file still applies.
An IES File Isn’t a Product Brochure
However, the .ies file is where lighting designers start doing something useful with the measured distribution.
An IES photometric file contains structured luminous-intensity data—candela values at defined vertical and horizontal angles—so software such as AGi32, Visual, DIALux or other lighting-calculation tools can model what the fixture actually does inside a project.
This is why IES guidance tells designers to obtain manufacturer IES-format photometric files for products being considered and use those files both to compare luminaires and perform lighting calculations.
That’s the correct mindset.
Not:
“Both fixtures are 3,000 lumens.”
But:
“Where do those 3,000 lumens go?”
Much better question.
Same Lumens Can Produce Completely Different Lighting
Let’s compare two hypothetical 3,000-lumen commercial downlights.
Luminaire A
- 3,020 delivered lumens
- 30W
- 101 lm/W
- 24° beam
- high CBCP
- narrow distribution
Luminaire B
- 3,080 delivered lumens
- 30W
- 103 lm/W
- 60° beam
- much lower CBCP
- wide distribution
Total output?
Almost identical.
Project behavior?
Completely different.
Put Luminaire A above a merchandise display and you may get strong punch and contrast.
Put Luminaire B there?
Soft wash.
Now reverse the application and try to light a broad office workplane uniformly.
Different winner.
This is why I frankly believe lumen matching is one of the laziest ways to approve an “equal” luminaire.
Equal lumens don’t mean equal lighting.
Candela Is Where the Distribution Starts Talking
People love lumens because the number is easy.
Candela is less friendly.
More useful.
Luminous intensity tells you how strongly the luminaire sends light in a particular direction.
Suppose two track heads both claim:
2,000 lumens
Fixture A has a center-beam candlepower of:
18,000 cd
Fixture B:
7,500 cd
Same lumens.
Different punch.
And on commercial track lighting, that difference can completely alter merchandise contrast, aiming strategy, spacing and mounting-height performance.
So when comparing substitutions, don’t stop at:
- lumens
- watts
- efficacy
- CCT
- CRI
Look at distribution.
Beam Angle Is Still Not Enough
“But both are 36°.”
I’ve heard that one too.
Beam angle usually identifies the angle between directions where intensity drops to a defined fraction—commonly 50%—of peak intensity.
Useful?
Yes.
Complete?
No.
Two nominal 36° distributions can have different:
- center-beam intensity
- field angle
- spill
- edge softness
- high-angle intensity
- asymmetry
- secondary rings
- cutoff
- glare characteristics
One gives a smooth commercial beam.
Another gives a hard hot spot surrounded by an ugly halo.
Both boxes say:
36°.
That’s why I want the photometric file.
The 2024 MaineDOT Specification Is What “Approved Equal” Should Look Like
Here’s a real example worth studying.
A November 22, 2024 MaineDOT highway-lighting plan didn’t tell contractors to submit a similar-looking luminaire and hope for approval.
For an alternative LED luminaire, MaineDOT required, at minimum:
- IES LM-79-19 absolute testing report
- LM-80 testing report
- TM-21 projected lumen-maintenance report
- manufacturer IES photometric file
- photometric plot overlaid on the actual lighting-zone layout
- light-loss-factor information
- optical specifications
- CCT
- driver details
- surge protection
- housing
- gasketing
That’s serious substitution review.
Notice something?
The alternative wasn’t being compared using one number.
It was being evaluated as a system.
Exactly.
Even More Interesting: LM-79-24 Now Exists
There’s a timing issue commercial buyers should understand.
Many 2024 project specifications still referenced LM-79-19 because that was the applicable version when the specification was developed.
But IES now lists ANSI/IES LM-79-24 as the current edition, superseding LM-79-19. The 2024 revision includes changes to spectral testing, airflow considerations, electrical supply issues and the test-report outline.
So when a supplier sends:
LM-79 report
check the edition.
Don’t automatically reject older valid test data—project specifications may explicitly call for a particular edition—but know what you’re looking at.
Standards move.
Tender documents lag.
That’s normal.
What an LM-79 Report Should Let You Verify
Depending on the product and report scope, I want to find data such as:
- tested luminaire identification
- electrical input power
- input voltage
- power factor where reported
- total luminous flux
- luminous efficacy
- luminous-intensity distribution
- chromaticity
- CCT
- color quantities/spectral data as applicable
- test conditions
- laboratory information
- test date
- report number
Then I match them against the quotation.
Exact model.
Not:
“Same series.”
Exact configuration.
If the report says:
DL-35W-4000K-36D
and the quotation says:
DL-30W-3000K-24D
you don’t have the same photometric product.
Obvious?
Apparently not always.
Scaled Photometry Deserves a Question
This one is more subtle.
Photometric data are sometimes derived or scaled from an existing measured configuration instead of physically testing every possible wattage, CCT or selectable setting.
That isn’t automatically fraudulent.
A 2024 Miami-Dade project document included commercial luminaire photometric data explicitly noting that certain reported results were scaled from IESNA LM-79-08 test data, with the test report and corresponding source data identified.
Transparent.
That’s what matters.
If your file is scaled, ask:
- scaled from which tested configuration?
- what parameter changed?
- is distribution unchanged?
- what assumptions were used?
- is the scaling permitted for the project/submittal requirement?
Don’t confuse derived data with physically measured data.
Know which you have.
A Supplier Can Edit an IES File
Here’s the ugly truth.
An .ies file is data.
Data can be edited.
Change the lumen multiplier.
Change header information.
Rename the file.
Suddenly a casual buyer sees the project model and assumes it came directly from a goniophotometer.
So I don’t treat possession of an IES file as proof of laboratory testing.
I want traceability:
IES file → LM-79 report → exact tested model → laboratory → production configuration
That’s the chain.
Break the chain and confidence drops.
Check the IES Header
Open the IES file.
You don’t need to be a programmer.
Look for identification information such as:
- manufacturer
- luminaire catalogue number
- test laboratory
- test report number
- test date
- lamp/source description
- number of lamps/sources
- dimensions
- photometric type
- units
Exact contents vary.
But if the file says one model and the quotation gives another, ask why.
If the manufacturer says:
“Same optics.”
Maybe.
Show the relationship.
The Distribution Type Matters More Than Procurement Thinks
Outdoor and roadway buyers know this already.
Type II.
Type III.
Type IV.
Type V.
Inside buildings, distribution language may be less formal, but the principle remains.
A substitution can produce enough lumens and still put them in the wrong places.
This is why DOE’s June 2023 federal exterior-lighting acquisition guidance doesn’t simply talk about wattage. It defines performance in terms of luminaire efficacy and references recognized photometric measurement standards for product evaluation.
Performance needs measurement.
The “Better Efficacy” Trap
Supplier B says:
“Our fixture is better. 140 lm/W.”
Specified product:
110 lm/W
Impressive.
Maybe.
But did you need more lumens per watt?
Or did you need a certain distribution?
Suppose:
Specified luminaire
- 2,500 lm
- 22W
- 114 lm/W
- controlled wall-wash distribution
Proposed substitute
- 2,800 lm
- 20W
- 140 lm/W
- symmetric flood distribution
Supplier wins the efficacy spreadsheet.
Loses the wall.
A substitution isn’t automatically better because one metric improves.
The project has to work.
Lighting Calculations Are Where Substitutions Should Be Judged
Take the manufacturer’s IES file.
Build the actual room.
Use:
- real dimensions
- actual mounting height
- fixture locations
- reflectances
- workplane
- maintenance/light-loss assumptions
Then compare:
- average illuminance
- minimum illuminance
- maximum illuminance
- uniformity
- vertical illuminance
- wall illuminance
- relevant glare metrics
- power density
- fixture count
That’s lighting substitution evaluation.
Not lining up two catalogues.
This is also why SENLUX project support should start with the project drawing and photometry rather than only the specified luminaire’s appearance.
A substitute should solve the same lighting problem.
2024 MaineDOT Went One Step Further: Overlay It on the Project
I like the MaineDOT requirement because it forces the proposed alternative out of the catalogue and into the real geometry.
The contractor had to provide a photometric plot overlaid on the lighting-zone layout for the proposed alternative.
That’s exactly what I’d ask for on a meaningful project.
Because the statement:
“Our photometry is similar.”
means very little.
Show me the spacing.
Show me the minimums.
Show me the maximums.
Show me uniformity.
Show me the actual layout.
Now we can compare.
A 2024 NYC Project Shows the Same Procurement Discipline
And this isn’t just highway lighting.
A New York City construction specification issued in April 2024 for a Bellevue Hospital project required independent laboratory photometric data conforming to IES procedures for lighting submissions, while also requiring approved samples and stating that luminaires supplied for the work should be identical to those approved samples.
That’s the second half of the substitution problem.
Photometry proves performance.
Sample approval controls what actually arrives.
You need both.
The Sample Has to Match the File
Imagine this:
You approve an IES file for Reflector A.
Factory sample looks good.
Production starts.
Reflector A is delayed.
Purchasing installs Reflector B.
Same beam-angle label.
Same diameter.
Nobody recalculates.
Now the file describes one optic.
The shipment contains another.
Your photometric approval?
Meaningless.
This is why SENLUX manufacturing and quality control has to connect approved optics, LED source, drive current and mechanical configuration to the controlled BOM.
Photometric data belong to a configuration.
Not a model name forever.
Optic Substitution Is an Engineering Change
I would treat changes to these as potentially photometrically significant:
- reflector
- TIR lens
- diffuser
- COB position
- LED source
- drive current
- aperture
- shielding
- louver
- housing geometry affecting optical path
Do they all require full LM-79 retesting every time?
Not necessarily.
But they require engineering review.
That’s the point.
“Equivalent lens” shouldn’t be a purchasing decision made five minutes before the assembly line starts.
Lumens Should Be Delivered Lumens
Be careful here.
LED package output?
Not the same as luminaire output.
Suppose the COB is nominally capable of:
3,500 lm
The finished fitting delivers:
2,650 lm
Why?
Optical loss.
Thermal conditions.
Drive current.
Reflector.
Lens.
Diffuser.
Shielding.
That’s normal.
So when comparing fixtures, use finished-product measured output.
LM-79 is useful precisely because it measures the SSL product as configured.
Don’t Compare System Lumens to LED-Chip Lumens
This is an old trick.
Supplier A:
“3,000 lumen lamp.”
Supplier B:
“3,500 lumen.”
Better?
Then you discover Supplier B quoted theoretical LED-chip flux.
Actual luminaire output:
2,700 lm.
Marketing lumens.
I hate those.
For a commercial downlight, ask:
“Is this LED-source lumen output or LM-79 measured luminaire output?”
Simple.
Useful.
CBCP Matters for Accent Lighting
For narrow-beam downlights and track heads, Center Beam Candlepower (CBCP) can matter much more than total lumens.
Imagine two 20W track lights:
| Product | Lumens | Beam | CBCP |
|---|---|---|---|
| A | 1,900 lm | 15° | 32,000 cd |
| B | 2,050 lm | 24° | 16,500 cd |
Which is brighter?
Bad question.
Where?
At the center of the accent beam, A may deliver dramatically more intensity.
Across a wider surface, B may be more useful.
Again:
Distribution.
What I’d Compare in an IES File
For a genuine substitution, I’d review:
Total output
Are delivered lumens comparable?
Intensity distribution
Do the candela curves have similar shape?
Beam and field characteristics
Is the optical spread actually equivalent?
Peak intensity
Especially important for accent, high-bay and high-mount applications.
High-angle intensity
May affect glare.
Upward light
Relevant for some suspended/direct-indirect systems.
Asymmetry
Critical for wall washers and asymmetric optics.
Luminaire dimensions
Photometric center and luminous opening can affect calculations.
Power
Is the energy claim actually improved?
Color data
Does CCT/CRI/chromaticity meet the project?
That’s a real comparison.
“Or Approved Equal” Doesn’t Mean Clone the Housing
This is especially important for OEM manufacturers.
A tender may list:
Brand X – Model ABC – or approved equal
Some buyers assume that means the replacement must look identical.
Not necessarily.
“Equal” generally concerns project requirements as defined by the specifier.
The key questions are:
- Is appearance architecturally controlled?
- Are dimensions constrained?
- Is cutout fixed?
- Is trim geometry important?
- Is photometric performance defined?
- Does the specification explicitly require matching finishes?
- Does the installation require compatibility with existing infrastructure?
Sometimes appearance matters enormously.
Sometimes photometric equivalency matters more.
Sometimes both.
That’s why OEM lighting development should begin by separating must-match criteria from performance criteria before anyone starts copying another manufacturer’s housing.
A Supplier’s “Equivalent” Spreadsheet Isn’t Enough
You’ve seen these.
| Parameter | Specified | Ours |
|---|---|---|
| Wattage | 30W | 30W ✓ |
| Lumens | 3000 | 3050 ✓ |
| CCT | 3000K | 3000K ✓ |
| CRI | 90 | 90 ✓ |
| Beam | 36° | 36° ✓ |
Five green ticks.
Approved?
No.
Where are:
- IES files?
- candela distribution?
- photometric plots?
- UGR where applicable?
- minimum lux?
- uniformity?
- CBCP?
- high-angle intensity?
- project layout?
Those five ticks prove only that five headline numbers are similar.
My Lighting Substitution Verification Table
| Item | Specified Product | Proposed Substitute | What I Want |
|---|---|---|---|
| Input power | Compare | Compare | Same or improved |
| Delivered lumens | LM-79 | LM-79 | Project appropriate |
| Efficacy | lm/W | lm/W | Evaluate, don’t overvalue |
| CCT | Measured | Measured | Within spec |
| CRI / color | Measured | Measured | Meets requirement |
| Beam angle | Compare | Compare | Similar where required |
| CBCP | Compare | Compare | Application appropriate |
| Distribution | IES | IES | Photometric equivalence |
| High-angle intensity | IES | IES | Glare check |
| Project lux | Calculation | Calculation | Meets design |
| Uniformity | Calculation | Calculation | Meets design |
| Fixture quantity | Layout | Layout | No hidden increase |
| Warranty | Compare | Compare | Commercial equivalence |
| Certification | Verify | Verify | Market compliant |
| Physical dimensions | Drawing | Drawing | Installation compatible |
That’s a substitution review.
Not price first.
Watch for the “More Fixtures Required” Trick
Supplier proposes a cheaper fitting.
Unit price:
20% lower.
Wonderful.
Then photometric recalculation shows you need:
15% more fixtures
to maintain minimum illuminance and uniformity.
Add:
- more luminaires
- more wiring
- more installation labor
- more control nodes
- more maintenance points
Still cheaper?
Maybe.
Maybe not.
This is why fixture price alone is a terrible comparison metric.
Project cost wins.
IES Files Can Reveal a Fake “Same Beam”
A polar plot gives you a quick visual.
If specified product has a tight symmetric curve and substitute has huge wings at high angles, you already know something changed.
Maybe the substitute provides more ambient fill.
Maybe it produces glare.
Maybe spacing can increase.
Maybe it ruins the intended contrast.
Don’t instantly label the difference good or bad.
Interpret it in the application.
For Wall Washers, Distribution Is Everything
Wall washers are where bad substitutions become obvious quickly.
A true wall-wash optic is designed to send useful light vertically across a wall while managing hot spots near the ceiling and falloff lower down.
Replace it with a generic asymmetric flood?
The datasheet may still say:
30W / 3000 lm / 3000K / CRI90
The wall says otherwise.
For applications like this, the commercial lighting application-design process should evaluate vertical illuminance, setback and spacing using the actual IES data.
No shortcuts.
For Offices, Glare Re-enters the Conversation
Suppose the substitute delivers identical horizontal lux.
Good.
But its high-angle intensity is higher.
Now UGR changes.
You’ve preserved illuminance and degraded visual comfort.
That’s why our earlier discussion of commercial downlight glare control matters when substitutions involve office downlights.
Lux equivalence isn’t necessarily experience equivalence.
For Retail, Contrast Ratio Can Change
Specified accent fitting:
tight 20° beam.
Replacement:
36°.
Total lumens?
Same.
Now the product display receives less peak intensity.
Background receives more.
Contrast ratio collapses.
Retail lighting suddenly looks flat.
Again:
No photometric match.
What I’d Request From an OEM Supplier
For a meaningful project substitution:
- Exact product datasheet
- LM-79 report
- IES/LDT photometric file
- Photometric test report number
- Test laboratory identification
- Product drawing
- Exact optical configuration
- LED source
- Driver
- CCT/CRI data
- Project lighting calculation
- Comparison against specified product
- Sample
- Controlled BOM
- Change-control commitment
Fifteen items.
Sounds heavy?
MaineDOT’s 2024 substitution requirements weren’t exactly casual either.
Serious projects require evidence.
Red Flags I’d Challenge Immediately
“Same lumens.”
Where do they go?
“Same beam angle.”
Show the candela distribution.
“We have IES.”
For which exact model?
IES filename doesn’t match product.
Explain.
Photometry scaled but not disclosed.
Ask for the source test.
LM-79 report uses different optic.
Wrong evidence.
Supplier changed reflector after sample approval.
Recheck.
Only catalogue comparison provided.
Not enough.
Fixture price is lower but layout wasn’t recalculated.
Dangerous.
Project calculation uses the specified brand’s IES file with the substitute’s name typed over it.
Yes, that deserves suspicion.
FAQ
What is an LM-79 report?
An LM-79 report documents standardized electrical and photometric measurements of a complete solid-state lighting product, including applicable quantities such as total luminous flux, electrical input, efficacy, luminous-intensity distribution and color characteristics, allowing buyers and specifiers to evaluate measured luminaire performance rather than relying solely on manufacturer catalogue claims.
The current IES edition is ANSI/IES LM-79-24, which superseded LM-79-19.
What is an IES photometric file?
An IES photometric file is a standardized digital file containing a luminaire’s directional luminous-intensity data and related product information, allowing lighting-design software to model how the luminaire distributes light within a room, roadway or other project rather than representing performance only through total lumen or beam-angle specifications.
Designers can use IES files to compare products and calculate application performance.
Is an IES file the same as an LM-79 report?
No, an IES file is not the same as an LM-79 report: the LM-79 report documents measured electrical and photometric test results under standardized conditions, while the IES file packages directional photometric data into a machine-readable format used by lighting calculation and visualization software.
For substitution verification, I normally want both.
How do you compare two LED lighting substitutions?
Compare LED lighting substitutions by reviewing measured output, wattage, efficacy, CCT, color quality, luminous-intensity distribution, beam characteristics and exact IES data, then run both luminaires through the same project geometry to compare illuminance, uniformity, glare, fixture count and other application requirements before declaring them equivalent.
Catalogue specifications alone are rarely enough.
Can two luminaires with the same lumens perform differently?
Yes, two luminaires with identical total lumen output can perform very differently because their optical systems may distribute those lumens across different angles, producing different center-beam intensity, beam width, high-angle light, uniformity, glare and illuminance on the target surface even when wattage and efficacy appear nearly identical.
Total flux tells you quantity.
Distribution tells you usefulness.
Can an IES file be used to verify an approved-equal luminaire?
An IES file is one of the most useful tools for evaluating an approved-equal luminaire because it allows the proposed product’s intensity distribution to be modeled in the actual project layout, but buyers should also verify that the file is traceable to the exact product configuration and supported by credible photometric testing.
A file alone isn’t laboratory proof.
Should a lighting substitution have an LM-79 report?
A commercial LED lighting substitution should normally have appropriate photometric evidence such as an LM-79 report when the project specification requires standardized SSL testing or when measured performance must be verified, although the exact documentation requirement depends on the tender, jurisdiction, product category and approval process.
A 2024 MaineDOT project explicitly required LM-79 testing for proposed alternative LED luminaires.
What should buyers check inside an LM-79 report?
Buyers should check the tested manufacturer and model, configuration, input voltage, wattage, delivered lumen output, efficacy, photometric distribution, color data, laboratory, report number, test date and applicable LM-79 edition, then confirm those details correspond to the exact luminaire, optic, CCT and driver being quoted for production.
The model match is where mistakes often appear.
“Equal” Needs Evidence
The phrase I’d stop accepting on its own?
Equivalent product.
Equivalent how?
A real verification chain looks more like:
Exact luminaire → LM-79 report → traceable IES file → optical comparison → project calculation → physical sample → controlled production BOM
That’s defensible.
And public-sector specifications already operate this way. In November 2024, MaineDOT required an LM-79 report, IES file and project-specific photometric overlay for proposed alternative LED luminaires; a 2024 NYC hospital specification separately required independent laboratory photometric data and approved physical samples before installation.
So when somebody tells you:
“Our fixture has the same watts and lumens.”
I wouldn’t reject it.
I’d say:
“Send me the IES file.”
If you’re evaluating an approved-equal or alternative commercial LED product, SENLUX can compare photometric files, optics, output, project layouts and OEM configurations before mass production. Explore our commercial LED lighting products, use SENLUX technical support, or contact SENLUX Lighting to review a substitution or project specification.
