How to read LED photometry: curves, lux, UGR and R9 | Factorled

David Torres

2026-10-09 11:22:04

How to read LED photometry: curves, lux, UGR and R9 | Factorled

Blog · Technical guide · Photometry · Regulations 2026

9 October 2026 · Reading time: 14 min · Category: Technical guides · Designers · Installers · Architecture

In the design of professional spaces, light quality completely transforms the perception and functionality of an environment. Choosing the right luminaire goes far beyond looking at the watts or the design of the housing: to guarantee the visual success, comfort and legal compliance of a project, it is essential to know how to read professional lighting photometry. In this guide you will learn to interpret the key data of a photometry using real laboratory reports, you will get to know the standards that govern technical projects in 2026 and you will be able to apply this information to select everything from COB LED strips to panels for demanding clinical use.

UGR < 19Glare limit for offices, consulting rooms and laboratories (EN 12464-1)

R9 ≥ 50Minimum saturated red required by the most demanding healthcare specifications

< 3 %Recommended Flicker Percent in healthcare environments and workplaces

1 lx = 1 lm/m²The relationship that separates what the luminaire emits from what reaches the surface

Quick answer: what to look at first in a photometry?

A professional photometry is read in five layers: (1) the distribution curve (polar and isocandela) to know where the light goes and at what angle; (2) the luminous flux in lumens and the illuminance in lux that reaches the surface; (3) colour rendering, going down from Ra to R9; (4) glare (UGR) in the row of the table that matches your room; and (5) the flicker (flicker) of the luminaire + driver as a whole. Then everything is cross-checked against the standard EN 12464-1:2021.

1. What is a photometric study and why does it define the success of your installation?

Photometry is the science that measures visible light as perceived by the human eye. In the lighting industry, a photometric file (usually in .ies or .ldtformat) or a photometric curve in a data sheet represents how the light emitted by a luminaire is distributed and behaves in three-dimensional space.

If you are faced with extensive technical catalogues or download luminaire files for calculation software such as Dialux or Relux, it is normal for the data to feel overwhelming. However, understanding these graphs allows you to tell a mediocre product from a high-performance technical solution, prevent eye strain problems and ensure that the investment in lighting pays off in the long term.

Header of a goniophotometer report of an 18 W LED floodlight with flux, efficacy and maximum intensity — **Figure 1.** Header of a real goniophotometer report (Everfine laboratory) of an 18 W LED floodlight. At a glance you get the essentials: total flux 1,173 lm, efficacy 61.6 lm/W, maximum intensity 774 cd and 99.4% of the light emitted downwards (CIE class "Direct"). Everything else in the report develops these figures.

The impact of a correct reading: a real example

Imagine you buy 3CCT spotlights (with selectable colour temperature) for a retail store, or solar floodlights for the outside of an industrial warehouse. If you do not read the photometric curve, you could install a spotlight with a beam that is too narrow (creating hard shadows) or too wide (losing intensity where it is really needed). Knowing how to interpret the photometry ensures you place the exact light in the precise spot.

Comparison of a 24-degree LED spotlight versus a 60-degree one with the same 800-lumen flux — **Figure 2.** Same flux (800 lm) with two beam angles: the 24° spotlight concentrates ~390 lx in a 1.3 m circle at a height of 3 m; the 60° one spreads barely 67 lx over 3.5 m. Neither is "better": the first is for highlighting product, the second for general lighting. Indicative values.

Cone diagram with average illuminance and illuminated diameter according to the mounting height of the LED floodlight — **Figure 3.** Cone diagram (AAI) of the 18 W floodlight in the example: with an 83° beam angle, at 1 m it lights a 1.77 m circle with an average of 414 lx; at 3 m the circle grows to 5.3 m but the illuminance drops to 46 lx; at 10 m only 4 lx remain. This graph answers in seconds "at what height do I install it and how many do I need?".

Case study · Real laboratory report

18 W LED floodlight measured in a goniophotometer

Throughout this guide we use the same laboratory report of an 18 W LED floodlight so you can see how each graph answers a different question of the project. These are the figures you get at a glance from its header:

1,173 lmTotal luminous flux

61.6 lm/WEfficacy of the whole unit

774 cdMaximum intensity

87,2°Beam angle at 50%

99.4% of the light is emitted into the lower hemisphere (CIE class "Direct"). It is an area luminaire for outdoor or warehouse use: as you will see in the glare section, its UGR rules it out of any office.

2. The 5 technical parameters for interpreting professional photometry

To read professional lighting photometry you must focus on five fundamental indicators that appear in any quality data sheet. This table summarises where to find them and which reference values to use:

Parameter	Where it appears in the report	Reference value	What to watch
Intensity distribution	Polar curve, isocandela diagram, .ldt / .ies file	cd/klm · beam angle	Symmetry, angle at 50% and real position of the maximum intensity.
Flux and illuminance	Header, cone diagram, isolux curves	lm · lx	Real lux at mounting height versus those required by EN 12464-1.
Colour rendering	Spectroradiometric report	Ra ≥ 80 / ≥ 90 · R9 ≥ 50	Do not stop at Ra: check R9 and the colour tolerance (SDCM).
Glare	UGR table (CIE 117 method), Söllner curves	UGR < 19 / < 22	Choose the row that matches the dimensions and reflectances of the room.
Flicker	Driver data sheet or flicker measurement	< 3% · index < 0.02	The figure must refer to the luminaire + driver as a whole, also when dimming.

← Swipe the table horizontally to see all columns →

2.1. Photometric distribution curves (polar diagram)

The polar graph is the heart of the photometry. It represents the luminous intensity, measured in candelas (usually per 1,000 lumens, cd/klm), at different angles.

Symmetrical luminairesSuch as classic downlights or smart LED panels compatible with Google Home and Alexa. They draw a globe- or uniform lobe-shaped curve: the light is distributed equally in all directions.

Asymmetrical luminairesTheir curve is deformed to one side. They are ideal for wall washers or street lighting, where the aim is to direct the light to a specific area without wasting it backwards.

Polar curve of a symmetrical downlight-type luminaire versus an asymmetrical wall-washer type — **Figure 4.** How to read a polar curve: 0° is the vertical pointing downwards and the radius is the intensity in candelas per 1,000 lm (cd/klm). The solid line is the C0-C180 plane and the dashed line the C90-C270 plane. A symmetrical luminaire draws a centred lobe; an asymmetrical one tilts it towards where it wants to throw the light.

Real polar curve of an 18 W LED floodlight with four C planes and an 87.2-degree beam angle — **Figure 5.** Real polar curve of the 18 W floodlight: the four curves (C0/180, C30/210, C60/240, C90/270) almost overlap, which means the distribution is symmetrical. The average beam angle at 50% is 87.2°: a very wide beam, typical of an area floodlight, not an accent spotlight.

Isocandela diagram of an LED floodlight with equal-intensity curves and a maximum of 774 candelas — **Figure 6.** Isocandela diagram: the same information seen "from the luminaire", like a topographic map of intensities. Each curve joins the points with the same intensity (95% = 736 cd, 50% = 387 cd, 5% = 39 cd). The maximum intensity, 774 cd, is not on the axis but at 27° (red cross): a detail you only see here and which explains the slight "ring" of light of many floodlights.

2.2. Luminous flux (lumens) and lighting level (lux)

Lumens versus lux: the difference that causes the most confusion

Luminous flux (lm): Total amount of light emitted by the complete luminaire. Quality data sheets state the flux of the luminaire, not just that of the LED chip, which is always higher because it does not deduct the losses of the optics and diffuser.
Illuminance (lx): Amount of light that actually reaches a surface (1 lx = 1 lm/m²). It depends on mounting height, angle and the reflectances of the room.
Healthcare example: A night-time circulation corridor may need barely 50 lx so as not to wake patients; a triage area needs between 750 and 1,000 lx.

Diagram of lumens versus lux and indicative illuminance levels by type of space — **Figure 7.** Lumens leave the luminaire; lux reach the surface (1 lx = 1 lm/m²). The same luminaire gives more or fewer lux depending on height, angle and the reflectances of the room. On the right, indicative illuminance levels according to the use of the space.

Isolux curves in plan of an LED floodlight mounted at a height of 10 metres — **Figure 8.** Isolux curves in plan of the floodlight mounted at a height of 10 m: the central area receives about 3 lx (50% curve) and the outermost curve (0.18 lx, 3%) reaches about 12 m from the axis. With 1,173 lm at 10 m the illuminance is low; the report makes this clear and avoids surprises on site.

Floor illuminance section of an LED floodlight at 10 metres in the C0 and C90 planes — **Figure 9.** Floor illuminance section at a height of 10 m, in the two main planes: maximum of ~6.2 lx under the luminaire and falling to zero at ±12 m. If your project requires 50 lx on an open area, this graph tells you to lower the height, multiply the units or switch to a higher-flux floodlight.

2.3. Colour rendering index (CRI/Ra) and the critical R9 value

The CRI (or Ra) measures the ability of the light to show the true colours of objects compared with a reference source. Ra ≥ 80 is the standard for offices, but in critical projects you need more:

For visual diagnosis environments, pharmacies or hospitals, it is non-negotiable to have Ra ≥ 90.
The R9 index is vital. Standard Ra averages eight pastel colours (R1–R8), but R9 measures saturated red. In healthcare spaces, a low R9 visually dulls blood or tissue, which is why the most demanding healthcare specifications call for luminaires with Ra ≥ 90 and R9 ≥ 50.
In retail, a high R9 makes the difference in meat, fruit, fashion or jewellery. We explain it in detail in our guide to LED strip lighting for jewellers.

Spectroradiometric report of a 4000 K LED with Ra 89.6, R9 57 and a 1.9 SDCM MacAdam ellipse — **Figure 10.** Real spectroradiometric report of the 4,000 K channel: Ra = 89.6 and R9 = 57. On the left, the spectrum with the blue peak of the chip at 449 nm and the drop in deep red; on the right, the MacAdam ellipse with 1.9 SDCM (very good colour consistency). It comfortably meets R9 ≥ 50 and comes close to Ra ≥ 90: ask the manufacturer for the certified value if the specification requires a strict 90.

Spectral report of the 3500 K intermediate mode with Ra 89.4, R9 46 and 7.5 SDCM — **Figure 11.** Same product, intermediate channel of ~3,500 K (3,340 K measured, a mix of the two chips): Ra = 89.4, practically the same… but R9 = 46. With an almost identical Ra, this mode would NOT meet the healthcare requirement of R9 ≥ 50. It is the perfect example of why you have to go down to the R9 line and not stop at Ra. Note also the 7.5 SDCM: colour tolerance shoots up when channels are mixed.

Golden rule: do not stop at Ra, go down to the R9 line

Two modes of the same product can have a practically identical Ra (89.6 and 89.4) and yet one meets the healthcare requirement of R9 ≥ 50 and the other does not (57 versus 46). Also check the SDCM: below 3 MacAdam steps the colour consistency between luminaires is very good; above 5 you will start to notice differences in tone on the ceiling.

R1 to R9 bar chart comparing a standard LED with an R9 of 5 and a high colour rendering LED with an R9 of 62 — **Figure 12.** Typical comparison between a standard LED and a high colour rendering LED, sample by sample (R1-R9): both have an "acceptable" Ra, but R9 goes from 5 to 62. The colours under each bar are the reference samples that are evaluated.

2.4. Glare control (UGR)

The Unified Glare Rating (UGR) calculates the visual discomfort produced by a luminaire as a function of its luminance, its position and the surroundings. If you are reading a photometry for a continuous workspace, this value is your main legal and health limit. The lower, the more comfort:

≤ 16Technical drawing and very high visual precision tasks

< 19Offices, medical consulting rooms, laboratories, inpatient wards

< 22Waiting rooms, reception, areas with lower visual demand

< 25Warehouses, corridors, circulation areas

< 28Industrial passageways

UGR scale with the usual thresholds of EN 12464-1:2021 (from most comfort to most glare)

UGR scale of EN 12464-1 with thresholds 16, 19, 22, 25 and 28 and the spaces to which they apply — **Figure 13.** UGR scale with the usual thresholds of EN 12464-1 and the spaces to which they apply. The lower, the more visual comfort. UGR < 19 is the limit for offices, consulting rooms and laboratories.

UGR table by CIE 117 method of an LED floodlight with values between 23.8 and 26.9 — **Figure 14.** Real UGR table (CIE 117 method) of the 18 W floodlight: values between 23.8 and 26.9 depending on the room dimensions (x and y in multiples of the height H), the reflectances of ceiling/walls/working plane and whether the luminaire is viewed crosswise or endwise. Practical reading: it is an outdoor or industrial warehouse luminaire; in an office requiring UGR < 19 it would not be admissible. The designer chooses the row that matches their room.

Luminance limitation curves by the Söllner method with a table of luminances by angle — **Figure 15.** Luminance limitation curves (Söllner/CIBSE method, which predates UGR but is still present in many reports). The table on the right is very revealing: at 45° the luminance is 75,600 cd/m² and at 70° it is zero. In other words, the luminaire glares a lot if you see it at low angles, but is fully shielded above 65°.

2.5. Flicker and drivers

Although flicker is not drawn on the polar curve, it is an essential photometric and electrical figure. Invisible flicker at 100 Hz can cause migraines, headaches and eye strain over long working days.

How flicker is calculated Flicker Percent (%) = (L_max − L_min) / (L_max + L_min) × 100

A professional data sheet, especially for demanding environments or operating theatres, must specify a Flicker Percent < 3 % and a Flicker Index < 0.02. To achieve this it is key to combine the luminaire with highly reliable power supplies, such as GXtronic drivers, which stabilise the output current.

Flicker comparison: basic driver with 30 percent flicker versus a stabilised driver below 3 percent — **Figure 16.** Flicker cannot be seen on the polar curve, but it can be measured: Flicker Percent = (max − min)/(max + min) × 100. Above, a basic driver with 30% modulation at 100 Hz (invisible, but it causes fatigue); below, a stabilised driver under the 3% required in healthcare environments and offices.

Need technical advice for your next project? Browse our catalogue of luminaires with laboratory-tested photometry and .ldt / .ies files ready for Dialux.

See catalogue

3. How to read a photometry step by step

This is the reading order designers use to validate a luminaire in a few minutes, before opening the calculation software:

Check the report headerTotal flux (lm), power (W), efficacy (lm/W), maximum intensity (cd) and CIE class. Check that the laboratory and the model match the product you are going to buy.
Interpret the polar curveIdentify whether the distribution is symmetrical or asymmetrical and note the beam angle at 50%: decide whether the luminaire is suitable for accent or general lighting.
Consult the isocandela diagramLocate where the maximum intensity really is. If it is not on the axis, a ring of light may appear on walls or floor.
Calculate the lux at your mounting heightUsing the cone diagram or the isolux curves, check the average illuminance and the illuminated diameter against what EN 12464-1 requires for that use.
Verify Ra, R9 and SDCMRead the spectral report for each colour temperature or channel, not only the most favourable one. In selectable-CCT luminaires, the intermediate modes usually have worse R9 and more SDCM.
Find your row in the UGR tableChoose the combination of dimensions (x, y in multiples of H), reflectances and viewing direction that matches your room, and compare it with the limit for the space.
Demand the flicker figureAsk for the Flicker Percent and Flicker Index of the luminaire + driver as a whole, also at minimum dimming if you are going to use DALI or Tunable White.
Pre-size and validate in softwareUse the utilisation factor table to estimate the number of luminaires and confirm the result by importing the .ldt or .ies file into Dialux or Relux.

4. Current standards in photometry and professional lighting (2026)

Knowing the technique is of little use if the project does not comply with current legislation. When reading professional lighting photometry, cross-check the manufacturer's data with the following references:

Standard / guide	Scope	What it sets for your project
EN 12464-1:2021 (UNE-EN 12464-1)	Indoor workplaces	Main European guideline: maintained illuminance, UGR limit, uniformity and minimum Ra for offices, hospitals, laboratories and administration.
HTM 08-03	Healthcare environments (British technical guide)	Very demanding reference requirements for ICUs, operating theatres and clinical areas; often used as a quality standard in European hospital projects.
UNE-EN 60598-2-25	Luminaires for clinical areas	Particular safety requirements for luminaires used in hospitals and healthcare centres.
DALI-2 (IEC 62386)	Lighting control	Precise dimming, real-time fault reporting and dynamic scenes adjusted to the biological cycle: photometry stops being static.
EN 13032-2	Presentation of photometric data	Format of luminaire data for indoor calculation, including utilisation factor tables.

← Swipe the table horizontally to see all columns →

You can consult and purchase the official text of the UNE-EN 12464-1 standard on the portal of UNE, the Spanish Association for Standardisation.

5. Circadian cycles (Human Centric Lighting) through photometry

The big trend of 2026 is people-oriented design (Human Centric Lighting or HCL). Modern photometric data include parameters of Tunable White technology (dynamic white): the luminaire changes colour temperature and intensity throughout the day.

2,700 KNight50 lx · warm light that respects rest

3,500 KTransitionDawn and dusk · smooth change, no jumps

4,000 KDay500 lx · neutral light that activates and maintains alertness

Typical Tunable White sequence in an Intensive Care Unit

In Intensive Care Units (ICU), a cycle that provides 4,000 K at 500 lx during the day and transitions smoothly to 2,700 K at 50 lx at night helps preserve patients' circadian rhythm. Some clinical studies have associated this type of lighting intervention with reductions in the incidence of delirium of up to 35%.

Tunable White circadian cycle in an ICU: 4000 K and 500 lux by day, 2700 K and 50 lux at night — **Figure 17.** Example of a Tunable White circadian cycle in an ICU: 4,000 K and 500 lx during the day, a smooth transition at dusk and 2,700 K with 50 lx at night. The drivers must travel the curve without colour jumps or an increase in flicker.

To implement these dynamic installations you need luminaires with drivers that offer a flawless colour transition without altering the flicker level at any point on the curve.

6. Advantages and disadvantages: why you must analyse the luminaire correctly

Skipping the photometry and installing "by eye" carries serious risks in the field of contract and architecture.

Advantages of a correct photometric analysis

Maximum energy efficiencyYou install the right number of luminaires, without over-lighting (and wasting energy) or falling short.

Guaranteed regulatory complianceBy verifying parameters such as UGR < 19, you protect your client against failed workplace inspections.

User comfortYou create clear, pleasant spaces that avoid visual rejection and improve productivity.

Aesthetic integrationIt allows you to hide the light sources and make the architecture itself shine, ideal with profiles and high-density COB LED strips.

Risks of ignoring the photometry

Complaints and returns: in commercial installations, an error in the beam angle creates unacceptable dark areas and forces the replacement of the whole batch of floodlights or GU10 spotlights.
Adverse health effects: installing downlights with high direct glare over bed-bound patients or workers causes discomfort and eye strain.
Poor colour rendering: buying without looking at R9 ruins the presentation of products in fashion stores and can lead to errors in clinical assessment.
Chip data instead of luminaire data: calculating with the flux of the bare LED overestimates the real lux. Always demand the report of the complete luminaire.

7. Real cases: from the polar curve to the installation

Case 1: high-performance offices

An architecture studio had to light a coworkingspace. When downloading the LDT files and processing them in Dialux, they found that the standard LED panels planned generated excessive glare on the screens.

The solution: reading the photometry made it possible to switch to smart LED panels with a microprismatic diffuser, achieving a UGR below 19 and adding voice control (Google Home / Alexa) to adjust meeting scenes.

Utilisation factor table according to EN 13032-2 by room index and reflectances — **Figure 18.** Utilisation factor table (EN 13032-2) from the report: this is what Dialux uses "behind the scenes". Example: in a room with room index k = 2 and reflectances 0.7 / 0.5 / 0.2 the UF is 1.01 (it can exceed 1 due to inter-reflections in very bright rooms), so the number of luminaires comes from N = E × A / (Φ × UF × FM). With these tables you can do a manual pre-sizing before opening the software.

Quick pre-sizing with the utilisation factor N = (E × A) / (Φ × UF × FM)

N: number of luminaires · E: required illuminance (lx) · A: area (m²) · Φ: flux of each luminaire (lm) · UF: utilisation factor from the table · FM: maintenance factor (typically 0.8). Example: a 100 m² office that needs 500 lx, with 3,600 lm luminaires, UF = 0.85 and FM = 0.8, requires N = 50,000 / 2,448 ≈ 20.4 → 21 luminaires.

Case 2: cleaning and instrument areas

In a laboratory, high-ingress-protection luminaires were specified because of aggressive cleaning with water jets.

The solution: the technical analysis made it necessary to look for IP65 or IP67 waterproof luminaires that maintained Ra ≥ 80 and guaranteed more than 500 lx on the working plane, without interfering with the laminar flow hoods.

Frequently asked questions about lighting photometry

What is a .ies or .ldt photometric file?

It is the digital file that describes the luminous intensity distribution of a luminaire measured in a goniophotometer. The .ies format (IESNA LM-63) is the North American standard and .ldt (EULUMDAT) the European one; both are imported into Dialux or Relux to calculate the lux, uniformity and UGR of the project.

What is the difference between lumens and lux?

Lumens (lm) measure the total light the luminaire emits; lux (lx) measure the light that reaches a surface: 1 lx equals 1 lm spread over 1 m². The same luminaire gives more or fewer lux depending on mounting height, beam angle and the reflectances of the room.

How do you read the polar curve of a luminaire?

0° is the vertical pointing downwards and the radius indicates the intensity in candelas per 1,000 lm (cd/klm). The solid line usually represents the C0-C180 plane and the dashed line the C90-C270 plane. If both overlap, the distribution is symmetrical; if the lobe tilts, it is asymmetrical.

Why does R9 matter if I already have Ra ≥ 90?

Ra averages only eight pastel colours (R1–R8) and does not include saturated red. Two luminaires with the same Ra can have very different R9 values: in a real example, Ra 89.6 with R9 = 57 versus Ra 89.4 with R9 = 46. In healthcare, food retail or fashion it is advisable to require R9 ≥ 50.

What UGR does an office or a medical consulting room need?

According to EN 12464-1:2021, offices, consulting rooms, laboratories and inpatient wards require UGR < 19. Waiting rooms and areas with lower visual demand allow up to UGR < 22, and corridors or warehouses up to UGR < 25.

What flicker values are acceptable in LED lighting?

In healthcare environments and demanding workplaces a Flicker Percent below 3% and a Flicker Index below 0.02 are recommended. It depends above all on the driver, so the figure must refer to the luminaire + driver as a whole.

What standards govern workplace lighting in 2026?

The main reference in Europe is EN 12464-1:2021 (UNE-EN 12464-1 in Spain). In healthcare environments it is complemented by EN 60598-2-25 for luminaires in clinical areas and by technical guides such as HTM 08-03; dynamic control is based on the DALI-2 protocol (IEC 62386).

Conclusion: photometry separates an amateur installation from a professional project

Learning how to read professional lighting photometry turns you into an expert capable of anticipating problems and offering high-value technical solutions. Understanding the difference between luminous flux and lux, controlling glare (UGR) and verifying the crucial red rendering index (R9) makes the difference between an amateur installation and a professional project backed by the 2026 standards.

Light is a clinical tool, a working environment and a temporary home. Apply empathy in your designs by thinking about who will inhabit that space.

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Article written by the Factorled technical team · factorled.com