How to Read an IES File



If you’ve ever wondered how Advanced Grow Lights actually verifies a manufacturer’s PPF claims, the answer is: we read the IES file. This guide explains what an IES file is, what’s inside it, and what manufacturers (and savvy buyers) can extract from one.

What an IES File Is

An IES file (extension: .ies; sometimes .ldt for the European equivalent) is a plain-text file format specified by the Illuminating Engineering Society (IES). It encodes the photometric data for a light fixture — specifically, how light is emitted from the fixture at every angle.

The format is dry and technical, but the file itself is just text. You can open one in Notepad or any text editor. It looks something like this:

IESNA:LM-63-2002
[TEST] LM-79 Test Report 2024-08-14
[MANUFAC] Example Lighting Co.
[LUMCAT] Model FXT-600
[LUMINAIRE] 600W Horticultural LED
[LAMPCAT] Integrated LED
TILT=NONE
1 1620.0 1.0 37 1 1 2 0.6 1.2 0.15
1.0 1.0 600.0
0 5 10 15 20 25 30 35 40 45
50 55 60 65 70 75 80 85 90 95
100 105 110 115 120 125 130 135 140 145
150 155 160 165 170 175 180
0.0
1620.0 1615.2 1601.4 1577.8 1545.0 1502.8 ...

Each section means something specific. Once you know what to look for, you can extract more information than the manufacturer’s spec sheet typically reveals.

The Header Fields

  • [TEST] — Identifies the lab and methodology. LM-79 (IES standard) and LM-80 (LED degradation) are the most common test standards. A reputable IES file lists an accredited testing lab.
  • [MANUFAC] — Manufacturer name.
  • [LUMCAT] — The fixture model number.
  • [LUMINAIRE] — A description of the fixture.
  • [LAMPCAT] — The light source type (LED, HPS, etc.).

If these fields are missing or vague, that’s a red flag. A serious manufacturer with a real LM-79 test will fill them in.

The Photometric Data Block

This is the meat of the file. After TILT=NONE (or a TILT specification for fixtures whose orientation matters), the file lists:

  1. Number of lamps and total lumens. For LED fixtures this is usually 1 lamp and a stated lumen value.
  2. Candela multiplier (almost always 1.0).
  3. Number of vertical angles and number of horizontal angles — the resolution of the candela distribution grid.
  4. Photometric type (Type C is standard for downlights and horticultural fixtures).
  5. Units (1 = feet, 2 = meters).
  6. Luminous opening dimensions (width × length × height in the chosen units).
  7. Input watts — the wall-power consumption used during the test.

Then the candela values at each angle — typically hundreds to thousands of data points describing exactly how much light is emitted in every direction.

What You (and We) Can Extract

1. Verified PPF

Lumens and candela in the file are based on the visible spectrum (human eye response). For horticultural use, we re-weight them using the published spectral power distribution of the LED (or HID) to compute photosynthetic photon flux. A separately provided SPD file (Spectral Power Distribution) is required for accurate PPF extraction — or a manufacturer can provide an IES file with PAR-weighted data directly (a horticultural variant some labs now produce).

Result: a verified PPF figure independent of the manufacturer’s marketing.

2. Verified PPFD at Any Mounting Height

The candela distribution tells us exactly how the light is emitted at every angle. From there, computing PPFD at any point under the fixture, at any mounting height, is straightforward inverse-square math.

This is how PPFD maps are generated. The manufacturer’s published PPFD map is one snapshot at one mounting height. Once you have the IES file, you can generate the map at any height — useful for evaluating fit in your specific room.

3. True Coverage Area

“Coverage area” claims are notoriously slippery. A 4×4 ft footprint at what PPFD? At the manufacturer’s stated mounting height, or at a more realistic one?

With the IES file, we compute the area where PPFD exceeds a threshold (typically 200 μmol/m²/s for veg, 600 μmol/m²/s for flower) at a reasonable mounting height. That’s the usable coverage area, not the marketing claim.

4. Uniformity

The ratio of min PPFD to max PPFD across the footprint indicates how uniform the distribution is. A uniformity ratio of 0.7 or better is excellent; below 0.5 means significant hotspots and dark edges.

5. Beam Angle

The angle at which intensity drops to 50% of peak (FWHM — Full Width at Half Maximum). Narrow beam fixtures concentrate light intensely in a smaller area; wide-beam fixtures spread light more uniformly over larger areas. The IES file makes this directly calculable.

What an IES File Cannot Tell You

Some things require additional data:

  • Long-term degradation. The IES file is a point-in-time snapshot. LM-80 testing (separate from LM-79 photometric) reports lumen maintenance over time. Both reports together give you projected lifespan.
  • Thermal performance under load. The test conditions for an IES file are typically a controlled lab environment. Real-world performance under continuous use in a hot grow room may differ — though the difference is usually small for well-designed LED fixtures.
  • Spectral details — unless paired with an SPD file. The IES file alone gives you total photometric output; the SPD tells you the wavelength distribution.
  • Driver efficiency — usually reported separately on the driver’s datasheet.

How AGL Uses IES Files

When a manufacturer submits a listing, we request their IES file through our reviewed email channel. (We don’t accept public uploads — security risk.) From there:

  1. We parse the file using open-source photometric libraries (luxpy, IES Parser).
  2. We compute fixture-level PPF and compare against the manufacturer’s claimed figure.
  3. We generate a verified PPFD map at the manufacturer’s stated mounting height.
  4. We compute usable coverage area and uniformity.
  5. If the file’s data backs up the manufacturer’s claims, the listing publishes with a Verified badge.
  6. If the manufacturer also provides DLC or third-party lab certifications, we add the Third-Party Lab Verified badge.

If we can’t verify a claim against the file, we contact the manufacturer with specifics. They can submit a corrected file, revise their published specs, or list without the Verified badge.

Where to Get IES Files (as a Buyer)

Most reputable manufacturers will provide IES files on request, especially for commercial buyers planning lighting layouts. Some publish them directly on product pages. If a manufacturer refuses to provide an IES file or claims they don’t have one, that’s a meaningful signal — either the photometric data doesn’t exist, or they don’t want you to see it.

Bottom Line

The IES file is the closest thing the lighting industry has to ground truth. It’s how lighting designers, engineers, and verification services check whether a fixture actually delivers what the marketing claims. Knowing what to look for in an IES file — or working with a directory like ours that does the audit for you — is the difference between buying a light based on data and buying one based on the cover photo.

Are you a manufacturer ready to submit your IES file? Start with our contact form and our team will walk you through the secure submission process.