How to Verify CRI Claims with an LM-79 Test Report
Marketing copy can say “90+ CRI” all day. However, the only way to verify what a luminaire actually delivers is to audit the IES LM-79-19 report behind that claim.
This guide provides a step-by-step technical framework for contractors, facility managers, and lighting designers to verify Color Rendering Index (CRI), R9, and spectral data before committing to a fixture.
1. LM-79 in One Page: The "Performance Transcript"
LM-79 is the Illuminating Engineering Society (IES) standard for the electrical and photometric measurement of solid-state lighting. Unlike a "chip-level" test, LM-79 measures the entire fixture as a system.
- Whole-Luminaire Integrity: Testing includes the driver, optics, and diffusers. Lenses and diffusers can shift color and reduce CRI; therefore, an LED package test (LM-80) is not a substitute for a luminaire test (LM-79).
- Standardized Methodology: According to ANSI/IES LM-79-19, labs must report measurement uncertainty (typically ±1.5% to ±2% for color metrics).
- Accreditation Requirement: Data is only as good as the lab. Valid reports come from labs accredited by bodies like NVLAP or A2LA.
2. Interactive Inspection Checklist (Machine-Ready)
Use the following checklist to triage any report provided by a vendor. You can copy this list into your project management software to ensure compliance.
2.1 The "Red Flag" Filter
- [ ] Exact SKU Match: Does the report list the full model number (including CCT and lens options)?
- [ ] Accreditation Seal: Is the NVLAP or ILAC-MRA logo present on the header?
- [ ] Test Method: Is "IES LM-79-19" (or the 2008 version) explicitly cited?
- [ ] Ambient Temp: Was the test conducted at 25°C ± 1°C? (Deviations here affect color stability).
- [ ] Stabilization: Does the report confirm the luminaire reached thermal stability before measurement?
2.2 Downloadable Template: Luminaire Comparison Worksheet
Copy the table below into Excel or Google Sheets to compare test data against your project specifications.
| Parameter | Project Requirement | LM-79 Tested Value | Delta (Difference) | Acceptance Status |
|---|---|---|---|---|
| CCT (Kelvin) | e.g., 5000K | Pass if within ±150K | ||
| CRI (Ra) | e.g., 90 | Pass if within -2 pts | ||
| R9 (Deep Red) | e.g., 50 | Pass if within -5 pts | ||
| Duv (Tint) | ±0.006 | Pass if within range | ||
| Luminous Efficacy | e.g., 130 lm/W | Pass if within -3% |
3. Anatomy of an LM-79 Report: Where the Data Lives
A standard 15–20 page report can be overwhelming. To verify color, skip directly to the Colorimetric Report or Spectral Power Distribution (SPD) pages.
3.1 Sample Data Excerpt (Anonymized)
Based on common accredited lab outputs, a compliant color table should look like this:
Test Results: Color Rendering Indices
| Index | Value | Index | Value | Index | Value |
|---|---|---|---|---|---|
| Ra | 91.2 | R6 | 93.4 | R11 | 91.0 |
| R1 | 91.5 | R7 | 90.1 | R12 | 72.4 |
| R2 | 95.2 | R8 | 81.3 | R13 | 92.6 |
| R3 | 97.4 | R9 | 54.8 | R14 | 98.2 |
| R4 | 91.1 | R10 | 88.5 | R15 | 87.4 |
| R5 | 90.8 |
Note: Ra is the average of R1 through R8. R9 (saturated red) is calculated separately and is the most common point of failure for "High CRI" claims.
3.2 Reading the SPD Curve
The Spectral Power Distribution (SPD) graph shows the radiant power at each wavelength.
- The "Blue Pump": Most LEDs show a sharp peak at ~450nm.
- The "Cyan Gap": A deep valley between 470-490nm often indicates a lower-quality phosphor mix, which can make colors look "flat" even if Ra is high.
- Red Tail: For CRI 90+, the curve should remain elevated past 620nm.
4. How to Compare Catalog Claims vs. Test Reality
4.1 The "±2 Point" Rule
In our experience reviewing thousands of photometric files, a report showing 88.4 CRI for a product marketed as 90 CRI is generally acceptable. This falls within the standard measurement uncertainty of most labs. However, if the report shows 84 CRI, the product is mislabeled.
4.2 CCT and Duv (The "Green/Magenta" Factor)
CCT (e.g., 5000K) only tells you how "warm" or "cool" the light is. Duv tells you the tint.
- Positive Duv: The light has a green tint.
- Negative Duv: The light has a magenta tint.
- Standard: ANSI C78.377 defines the "bins." If the Duv is greater than ±0.006, the color shift will be visible to the naked eye.
5. Advanced Verification: TM-30 and CSV Extraction
While CRI (Ra) is the industry standard, it is limited. It only uses 8 pastel colors to calculate the average.
5.1 TM-30-18 Metrics
Modern LM-79 reports often include IES TM-30 data, which uses 99 samples.
- Rf (Fidelity): How accurately the light renders colors (0-100).
- Rg (Gamut): How saturated the colors look. A value >100 means colors are more vivid; <100 means they are more muted.
5.2 Extracting Data from CSV Files
If you are managing a large-scale project, ask for the .CSV or .SPDX spectral data file. You can import this into the IES TM-30 Calculator (Excel-based tool) to verify the manufacturer's claims independently.
6. Verifying SKU and Application Match
6.1 The "Component" Trap
Ensure the report is not an LM-80 report.
- LM-80: Tests the LED chip for 6,000+ hours. It does not tell you the CRI of the finished fixture.
- LM-79: Tests the finished fixture.
6.2 Hot vs. Cold Performance
Most LM-79 tests are "Cold Start" (stabilized at 25°C). In high-heat environments like foundries or high-ceiling warehouses, the CRI and CCT can drift as the phosphor heats up. If color is critical, ask if the lab performed "Hot State" testing or refer to the manufacturer's thermal derating curves.
7. Step-by-Step Approval Process for Project Leads
- Request: Demand the full PDF report (not a summary) and the .IES file.
- Scan: Check the model number against the submittal.
- Audit: Use the Worksheet in Section 2.2 to log the Ra, R9, and CCT.
-
Contextualize:
- Warehouse: CRI 80 is sufficient.
- Manufacturing/QA: CRI 90+ and R9 > 50 are required.
- Retail/Art: TM-30 (Rf > 90, Rg 95-105) is recommended.
- Contractualize: Include a clause in your Purchase Order: "All luminaires must perform within ±2 points of the submitted LM-79 CRI value upon third-party field sampling."
8. Frequently Asked Questions
Can I use a handheld spectrometer for verification?
Handheld devices (like those from Sekonic or Hopoocolor) are excellent for field audits, but they lack the controlled environment of an integrating sphere. Use them to identify outliers, but use the LM-79 report for official disputes.
Why does my "90 CRI" light look green?
Check the Duv in the LM-79 report. If the Duv is +0.004 or higher, the light has a yellow-green cast. This is common in high-efficacy LEDs where color quality was sacrificed for lumens-per-watt.
9. Key Takeaways
- Whole-Fixture Only: Only accept LM-79 reports for the complete luminaire SKU.
- R9 is King: For high-quality light, Ra (CRI) is not enough; check that R9 is positive (for 80 CRI) or >50 (for 90 CRI).
- Tolerance Matters: Allow for a ±2 point variance due to lab uncertainty.
- Check the Lab: Verify the lab's NVLAP accreditation to ensure the data isn't "marketing math."
Editorial Team & Review Process This guide was authored by our Senior Technical Support Engineering Team in collaboration with third-party lighting laboratory consultants. All technical thresholds (±2 point Ra variance, ±0.006 Duv) are based on ANSI/IES LM-79-19 standards and industry heuristics for field acceptance. Content is reviewed annually to align with updated IES and DLC (DesignLights Consortium) technical requirements.
Safety & Compliance Disclaimer This article is for informational purposes only. Lighting specifications should be verified by a licensed Professional Engineer (PE) or a Certified Lighting Consultant (CLC). Always refer to local building codes and the latest IES standards for project-specific requirements.