Does an IES or LDT file prove CCT and colour rendering in a DIALux project?
Separate photometric distribution from CCT, CRI and spectral evidence so a DIALux calculation does not become an unsupported claim about a luminaire's colour quality.
The short answer: a usable photometric file is not a colour-quality certificate
An IES or LDT file used in a DIALux project does not, by itself, prove the luminaire's correlated colour temperature, colour rendering or measured spectrum. It supplies photometric information for the calculation, and product records may carry declared colour values. Those declarations still need to match the exact supplied variant and supporting manufacturer or laboratory evidence. A passing lux result answers a different question from a verified colour-quality claim.
DIAL's DIALux evo support guidance explicitly says the IES and LDT formats do not contain spectrum information and describes choosing a predefined spectrum in the software. That is an important limit: selecting a spectrum for a model is not measuring the delivered luminaire. Keep the calculation inputs, declared product values and test evidence distinct in the project record.
1. Ask which kind of data the file actually provides
Photometric distribution describes how light is emitted in different directions. Combined with the installation geometry and calculation assumptions, it supports evaluation of illuminance and other quantities within the agreed scope. Spectral data describes how optical radiation is distributed across wavelengths. The two kinds of information are related to the same product but are not interchangeable.
An LDT record or a manufacturer's catalogue may include a stated colour temperature or colour-rendering value. Read that as a declaration that must be checked against the product schedule, not as a substitute for a measured spectral data set. The IES publishes TM-27 specifically as an electronic format for transferring spectral data; having an IES photometric file does not mean that spectral evidence has also been supplied.
2. Match the colour declaration to the exact product variant
Request the full product code, optic, output option, driver or operating condition and the declared CCT and colour-rendering specification. A product family can offer several variants, so a generic family brochure may not establish the properties of the selected item. The lighting schedule, quotation, data sheet, photometric file and any test report should refer to a consistent variant or clearly document an authorised equivalence.
For a distributor comparing two nominally similar office luminaires, equal wattage and a shared CCT label do not establish that all optical and colour characteristics are the same. Ask the supplier to resolve missing identifiers before substitution. Do not rename a file or change a colour field to make different records appear consistent; a correct-looking schedule cannot repair unsupported source data.
3. Treat CCT and colour rendering as separate requirements
CCT is a description of the light's colour appearance, while colour rendering concerns how the source presents object colours relative to the applicable reference and method. A CCT label alone is not a colour-rendering result. Likewise, a high declared CRI does not establish every aspect of appearance or suitability for a particular material-inspection task.
The CIE's published colour-rendering method uses test-colour samples in its evaluation. When the brief asks for CRI, clarify whether it means the general index Ra or additional information, and request evidence under the named method. If another metric or a task-specific visual appraisal is required, have the responsible designer define it explicitly. This guide sets no universal CCT, CRI or colour-quality threshold.
4. Document any spectrum selected inside DIALux
DIAL's support article describes selecting a luminaire in the lamp tool and changing its colourimetric data by choosing from predefined spectra. If that workflow is used, state which spectrum or model assumption was selected and why. A generic selection may help represent a concept, but it should not be labelled as the manufacturer's measured spectrum unless the source and software workflow justify that claim.
A warm-looking or cool-looking rendering also depends on display, exposure, materials and viewing conditions. It is useful for communication, but cannot certify how real samples will appear under the installed fitting. Use the required product evidence and, where appropriate, an agreed physical sample evaluation. Do not infer CRI from the colour of a screenshot or mistake a false-colour illuminance map for an assessment of object colour.
5. Recheck photometry when the colour variant changes
If procurement changes the CCT or colour-rendering variant, ask whether the supplied photometric distribution, lumen output and other relevant data still describe that exact product. Do not assume either that nothing changes or that a universal correction factor applies. Obtain the correct file or an explicit supplier-supported basis for the proposed variant, then recalculate affected results when the calculation input changes.
For example, a consultant may accept the layout concept while still waiting for evidence on the proposed colour option. Record those as separate decisions. A favourable illuminance calculation does not close an unresolved product-specification question, and confirmed colour declarations do not establish that the installation meets the lighting criteria. Both sides of the review need their own traceable evidence.
6. Issue a report with a clear evidence boundary
Identify the exact luminaire variants, photometric file names and revisions, modelled output, installation geometry, calculation surfaces, maintenance basis and project criteria. State declared CCT and colour-rendering values with their source, and identify any colourimetric assumption used in the software. List unresolved spectral or product evidence rather than presenting a software field as a measured property.
A professional calculation can support layout evaluation, product comparison, procurement or submission. It does not certify the product, measure the delivered batch or grant client, consultant or authority acceptance. The responsible project team should decide which product tests and visual checks are required and who confirms the supplied equipment after installation.
A product-data decision checklist
Before calculation, confirm: (1) full product and variant identifiers; (2) matching IES, LDT or other supported product data; (3) declared CCT and named colour-rendering requirement; (4) the source of those declarations; (5) relevant test or spectral evidence where the brief requires it; and (6) whether a physical material or sample appraisal is part of the scope.
Before issue, verify: (1) product codes agree across all documents; (2) any selected software spectrum is labelled accurately; (3) colour declarations are not inferred from lux results or screenshots; (4) variant changes have been checked with the supplier; (5) recalculation follows changed photometric inputs; and (6) outstanding evidence and acceptance responsibility are explicit.
Prepare a useful enquiry for LUXGRID
Send the plan, mounting information, product schedule and exact photometric files, together with the colour requirements and supporting data sheets or test reports. Explain whether the project needs an illuminance layout, a product-option comparison or coordination with a specialist colour assessment. That lets the calculation scope address the correct decision without implying that one report covers every product property.
The official references below support the distinction between basic photometric data, a chosen software spectrum, spectral-data transfer and colour-rendering evaluation. The examples in this guide describe a review process, not test results from a completed LUXGRID project.
Official technical references
Frequently asked questions
Can an LDT file list a colour temperature or CRI value?
A product record may include declared colour information, but a declaration is not a measured spectrum or colour-quality certificate. Verify the exact variant and supporting evidence. DIAL states that IES and LDT formats do not contain spectrum information.
If I change the spectrum in DIALux, have I verified the real luminaire?
No. Selecting a predefined spectrum changes a model assumption. It does not measure or certify the supplied product. Document the selection and use the manufacturer or laboratory evidence required by the project.