Acceptance & QC

Accredited, or just printed: how to read a photometric test report

The nine fields that distinguish a usable LM-79 report from a document with a logo, why accreditation scope matters more than the accreditation, what measurement uncertainty does to a marginal claim, and the questions that expose a report describing a different product from the one on offer.

Written by Yilian Zhikong Technology (Shenzhen) Co., Ltd. · Last reviewed August 2026

Every tender return contains test reports. Very few of them are read past the summary table, and a surprising proportion do not support the claim they were submitted to support — not because anyone falsified anything, but because the report describes a sample, a condition or a configuration that is not what is being sold.

Start with what the report is for

An LM-79 report documents an absolute photometric and electrical measurement of a finished solid-state lighting product: total luminous flux, input power, power factor, chromaticity, colour rendering and the luminous intensity distribution. It is the evidence behind every performance figure on a datasheet, and the file it produces is the one your calculation software consumes — how to read that file is covered separately in reading an IES photometric file.

What follows applies to the report document, not the data file. The data file will load happily whether or not the measurement behind it was any good.

Nine fields to check, in the order they fail

1. The exact product identifier. Not the family, not the range — the ordering code, with its CCT, CRI, optic and driver variant. A report for the 4000 K CRI 80 wide-beam version does not evidence the 3000 K CRI 90 narrow one, and the differences between them are exactly the ones that matter.
2. The laboratory and its accreditation scope. Accreditation is granted for specific tests, not to an organisation in general. A laboratory accredited for electrical safety is not thereby accredited for photometry. Ask for the scope document, which is public wherever the accreditation body is a serious one.
3. The test method named and dated. "Tested to LM-79" without an edition, or "tested in accordance with internal procedure", are different claims from "LM-79-19". Internal procedures may be perfectly sound and are not comparable between suppliers.
4. The instrument. An integrating sphere and a goniophotometer measure different things. A sphere gives total flux; only a goniophotometer produces the intensity distribution your calculation needs. A report offering a distribution from a sphere measurement is offering something that was not measured.
5. Stabilisation. The criterion used and the time taken to reach it. A measurement made before thermal equilibrium overstates output, and the size of the error depends on the thermal design — which is precisely what you were trying to assess. See the Tc point for why this is the field that most often quietly matters.
6. Ambient temperature during the test. Usually 25 °C. If your application is a plant room at 40 °C, the report does not describe your installation and the difference is not small.
7. Input power measured, and the supply condition. Voltage, frequency and whether the measurement is true RMS. Efficacy computed from nominal wattage is not a measurement.
8. Sample size and sample selection. One unit is normal for LM-79 and is worth knowing about: the report describes that unit. Whether it represents production is a separate question, and it is the question incoming inspection exists to answer.
9. The date. Products get revised. A report predating a change of LED supplier, driver or optic describes a product that is no longer being made, and the change is rarely announced.

Uncertainty, and what it does to a marginal claim

Every measurement has an uncertainty, and a report from an accredited laboratory should state it. Photometric flux measurements carry an expanded uncertainty in the region of a few per cent, and colour quantities carry their own.

The practical consequence appears when a claim sits close to a threshold. A product reported at CRI 80.4 against an 80 requirement, with an uncertainty of ±2, has not demonstrated compliance — it has demonstrated that compliance is likely. Whether that matters depends on what the number is for. For a general office it does not. For a requirement that was written because a specific task depends on it, ask for the uncertainty and treat the interval, not the point, as the result.

The same reasoning applies in your favour when rejecting a delivery. A batch measured marginally outside a limit, on an instrument with a comparable uncertainty, is not a defensible rejection on that evidence alone — which is why the sampling and the instrument class in an acceptance measurement are worth fixing before the argument starts rather than during it.

What a report cannot tell you

Three things routinely get inferred from an LM-79 report that it does not contain.

• Lifetime. LM-79 is a snapshot. Lumen maintenance comes from LM-80 data on the package and a TM-21 projection, and neither is in this document — the distinction is worked through in L70, LM-80 and TM-21.
• Consistency. One unit measured once says nothing about the spread of a production batch.
• Behaviour when dimmed. Unless the report explicitly covers dimmed set points, everything in it belongs to full output. On a dimmed installation, flicker, chromaticity and efficacy at 10 % are separate questions with separate evidence.

What to write into the tender

1. Photometric performance evidenced by an LM-79 report from a laboratory accredited for photometric testing, scope document available on request, for the offered ordering code.
2. The report to state instrument type, stabilisation criterion, ambient temperature, supply condition and measurement uncertainty.
3. Reports dated within a stated period, and re-issued if the product is revised between tender and delivery.
4. Where a claim is within the measurement uncertainty of a specified limit, the supplier to state that explicitly rather than reporting the point value alone.