How to Read an LM-79 Report for LED High Bays

Demystifying an LM-79 report is one of the fastest ways to separate a spec-grade UFO high bay from a commodity fixture. LM-79 is not a logo or a pass/fail stamp; it is a standardized snapshot of optical and electrical performance. If you can read that snapshot, you can verify lumen output, efficacy, color quality, and whether the IES file you are using in your warehouse layout is trustworthy.

This guide walks contractors, facility managers, and specifiers through LM-79 reports with a focus on LED high bays, using the structure of the standard from the Illuminating Engineering Society (IES) and field-proven review habits.

Diagram-like scene showing how an LM-79 report connects lumens, watts, and light distribution for a warehouse high bay layout

1. LM-79 in Context: What It Is (and Isn’t)

1.1 Purpose of LM-79 for LED High Bays

ANSI/IES LM-79-19 is the current approved method for measuring the total luminous flux, electrical power, efficacy, color characteristics, and distribution of solid-state lighting products. According to Intertek’s overview of LM-79 testing, the method specifies how to measure lumens, watts, efficacy (lm/W), correlated color temperature (CCT), color rendering index (CRI), and power factor under controlled conditions.

For high bays, LM-79 is the basis for:

DLC (DesignLights Consortium) QPL qualification: Governs most utility rebates via the DLC V5.1 Technical Requirements.
Energy Code Compliance: Meeting minimum efficacy thresholds in codes like ASHRAE 90.1-2022 and the 2021/2024 IECC.
Photometric Accuracy: Providing accurate files (.ies, LM‑63 format) for tools such as AGi32.

1.2 LM-79 vs. LM-80/TM-21 and Safety Listings

A common misconception is that “it has an LM-79” means a luminaire is high quality. LM-79 alone does not address long-term lumen maintenance or safety.

LM-79-19: Measures integrated fixture performance at a specific point in time.
LM-80-21: Measures LED package lumen maintenance over time, as defined by the IES LM-80 standard.
TM-21-21: Uses LM-80 data to project long-term L70/L90 life per IES TM-21-21.
UL/ETL listings: Verify safety standards such as UL 1598 (2021) for luminaires and UL 8750 for LED equipment.

1.3 Test Conditions: The “Snapshot” Problem

LM-79 tests are performed at 25 °C ambient with the luminaire in thermal equilibrium. While this provides a consistent baseline, it rarely represents the actual environment of a high-ceiling warehouse.

The Thermal Reality: As documented in US DOE Solid-State Lighting studies (such as the CALiPER report series), LED efficiency is inversely proportional to junction temperature. When the case temperature (Tc) of a high bay rises from the 25 °C lab baseline to 40–50 °C (common in unconditioned warehouses), internal luminous flux typically drops by 5–15%.

Implication for you: LM-79 lumens are initial values. Designers must apply a Light Loss Factor (LLF)—often 0.70–0.85—to account for thermal droop, dirt depreciation, and LED aging.

2. How an LM-79 Report Is Structured

Most accredited labs follow a standardized layout. Below is a simulated breakdown of where to find critical fields.

2.1 Cover Page and Accreditation Details

Confirm the following to ensure the report isn't "self-certified" by a non-accredited factory:

Accreditation: Look for the NVLAP (Lab Code 200000-0 or similar) or IAS logo.
Report Date: Prefer reports within the last 3 years to ensure they reflect current LED binning.
Catalog Number: Must match your submittal exactly.

2.2 Simulated Data Block: What to Look For

Below is how the "Summary of Results" typically appears in a certified report.

Measured Parameter	Result	Standard/Requirement
Total Luminous Flux	21,450 lm	Matches Spec Sheet?
Luminous Efficacy	143.2 lm/W	Meets DLC V5.1?
Input Power (Watts)	149.8 W	Nominal ± 5%?
Power Factor (PF)	0.982	Must be ≥ 0.90
Total Harmonic Distortion (THD)	12.4%	Ideally < 15%
CCT / Duv	5021 K / 0.0012	Within ANSI C78.377 Quadrangle?
CRI (Ra) / R9	82.4 / 12	R9 > 0 is preferred for color tasks

2.3 Test Conditions and Stabilization

The report should explicitly state:

Ambient Temperature: 25.1 °C (standard).
Stabilization Time: Typically 30–60 minutes. If a fixture isn't stabilized, the lumen output will be artificially high because the LEDs haven't reached their full operating heat.

3. Reading the Key Numbers for High Bay Specification

3.1 Lumens and Efficacy: Beyond “Higher Is Better”

LM-79 efficacy tells you how efficiently the fixture converts power to light, but for high bays, the distribution of that light is the true performance driver.

A frequent myth is that the highest-lumen high bay always delivers the brightest work plane. In reality, as reflected in IES design guides like ANSI/IES RP‑7-21, a lower‑lumen fixture with a tighter beam can deliver higher illuminance on the task area.

Internal Simulation Case (AGi32 at 35 ft):

Fixture A: 30,000 lm, 120° beam → High total light, but high "spill" on upper walls.
Fixture B: 24,000 lm, 60° beam → 15–25% higher average foot-candles on the floor because the light is focused on the work plane.

3.2 CCT and Color Consistency

ANSI C78.377-2017 defines chromaticity quadrangles. If your report shows a Duv (distance from the blackbody curve) greater than ±0.006, the light may appear noticeably green or pink, regardless of the nominal CCT.

3.3 Power Factor and THD: Grid Stability

Modern energy standards like ASHRAE 90.1-2022 strictly regulate power quality.

PF < 0.90: May lead to utility penalties in large industrial facilities.
THD > 20%: Can interfere with sensitive warehouse automation equipment or Wi-Fi routers.

4. Photometric Distribution: Using LM-79 Like a Lighting Designer

4.1 IES Files and LM-63 Data

The LM-79 report is the "source of truth" for the .ies file. Always verify that the IES file name referenced in the report matches the file provided by the manufacturer. Using an outdated IES file can lead to layout errors that result in dark spots or excessive glare.

4.2 Spacing Criteria (SC)

Many reports provide Spacing Criteria (e.g., 1.3). This is a multiplier for mounting height.

Example: At a 30 ft height, an SC of 1.3 suggests a maximum spacing of 39 ft (30 x 1.3) to maintain uniformity.

5. Pro Tip: Avoiding Common LM-79 Misreads

5.1 LM-79 is Not a Pass/Fail Certification

A common pitfall is treating LM-79 as a "seal of approval." It is merely a measurement. A product can have a perfectly valid LM-79 report that shows it is inefficient, has poor color, or generates excessive glare. You must compare the data against the project requirements.

5.2 Check the Configuration

Ensure the test was performed with the actual lens you are ordering. A test performed with a clear lens will show ~5-10% higher lumens than a version with a frosted or prismatic lens.

6. Quick Comparison Framework

Use this table to triage candidate luminaires before running detailed layouts.

Decision Question	LM-79 Fields to Check	"Good" Benchmarks	When to Be Cautious
Rebate Eligible?	Efficacy, PF, THD	Efficacy ≥ 135 lm/W; PF ≥ 0.9	PF < 0.9; THD > 20%
Color Accuracy?	CRI (Ra), R9, Duv	CRI ≥ 80; R9 > 0; Duv < ±0.003	Negative R9; high Duv
Data Integrity?	Lab, Date, Catalog #	Accredited lab; < 3 years old	No accreditation; old report

Frequently Asked Questions

What is LM-79 in simple terms?

It is a standardized "lab report card" that proves a luminaire actually produces the lumens and uses the watts the manufacturer claims.

Does LM-79 prove the light will last 50,000 hours?

No. LM-79 is a "time zero" test. You need LM-80 and TM-21 reports to verify lifetime and lumen maintenance.

Why is my field measurement lower than the LM-79 report?

Field meters are often less accurate than lab goniophotometers, and real-world factors like high ambient heat (thermal droop) and voltage fluctuations typically reduce output by 10% or more compared to lab conditions.

Safety and Compliance Disclaimer: This article is for informational purposes. Lighting designs should be reviewed by a licensed professional engineer (PE) or a certified lighting designer (LC) to ensure compliance with local building codes, including Title 24 (2022) in California or ASHRAE 90.1 jurisdictions.

How to Read an LM-79 Report for LED High Bays