Visual Comfort: Comparing UFO & Linear High Bay Glare
Employee comfort in high-bay spaces is not just a “nice to have”. Visual fatigue from glare and flicker quietly erodes productivity, safety, and morale on every shift. This guide focuses on visual comfort in warehouses, shops, and industrial facilities, comparing UFO and linear high bay glare performance and outlining practical steps to design low‑fatigue lighting.
According to ANSI/IES RP‑7-17 Lighting Industrial Facilities guidance for industrial facilities, designers should target both appropriate illuminance and good glare control to maintain task performance and safety in high‑ceiling spaces.[^rp7] In practice, that means thinking beyond total lumens and focusing on how light is distributed in workers’ actual field of view.
Disclosure – product references
Any specific products or series mentioned in this article are examples used for illustration only and do not constitute third‑party endorsement or independent safety certification. Performance claims based on manufacturer data are indicated as such; readers should always review the original LM‑79 reports, LM‑80/TM‑21 data, and local code requirements for their own projects.
1. What “Glare” Really Means in High‑Bay Applications
1.1 Discomfort glare vs. disability glare
In high‑bay environments, two glare types matter:
- Discomfort glare – light sources that feel harsh or annoying but do not necessarily block vision. Symptoms include eye strain, headaches, and a desire to squint or look away.
- Disability glare – intense sources that wash out contrast on tasks (e.g., pallet labels, glossy packaging), making it objectively harder to see.
Most warehouse complaints fall under discomfort glare. Workers say “the lights are too sharp” even when measured foot-candles are within spec. That disconnect happens because horizontal illuminance alone does not describe how bright the luminaires appear in the field of view.
1.2 Why UGR and luminance matter
The Unified Glare Rating (UGR) is a standardized metric that estimates discomfort glare based on luminaire luminance, position, background luminance, and observer view. UGR is defined in CIE 117 and CIE 190 and is widely referenced for interior lighting design.[^cie117] For general industrial tasks, a commonly used practical target is UGR ≤ 22; for assembly or inspection work, UGR ≤ 19 tends to correlate with fewer complaints in practice.
However, as noted in comparisons of glare metrics in the technical literature, UGR was validated mainly for office‑type spaces with relatively uniform luminance patterns.[^cie190] In racked warehouses, it can underestimate localized discomfort from very bright point sources. That is why experienced designers pair UGR checks with:
- Luminance limits on the luminaires in key viewing angles (e.g., within 25–30° of line of sight).
- Vertical illuminance checks on rack faces or workstations, not just lux on the slab.
A common engineering approach is to maintain catalog or modeled UGR ≤ 22 while also keeping luminaire‑to‑background luminance ratios below about 1:10 in critical views. This combination generally tracks well with comfort feedback, but must always be validated with project‑specific modeling.
1.3 Common misconception: “More lumens = more productivity”
A persistent myth in high‑bay projects is that simply pushing more lumens automatically improves productivity. In real warehouses, this often backfires:
- One modeled project used 150 W UFOs at 10 m aiming for extremely high horizontal illuminance. The fixtures delivered >1,500 cd/m² apparent luminance within 25–30° of the line of sight in the lighting software, pushing calculated UGR values over 25 for some viewing directions.
- A comparable layout using linear low‑UGR fixtures with aisle optics spread the same lumens over ~1.7× the emitting area and reduced modeled peak luminance by ≈40–50%, bringing UGR into the 19–22 band for the same tasks.
These are design simulations based on manufacturer LM‑79 photometric files, not independent lab measurements. In field use, productivity improved with the linear layout even though average lux on the floor was similar, because pickers and drivers could keep their eyes in the task zone without constantly dodging bright sources.
2. UFO vs. Linear High Bays: How Form Factor Affects Glare
2.1 Beam patterns and emitting area
From a glare standpoint, the two dominant differences between UFO and linear high bays are:
- Emitting area – UFOs are compact, almost circular sources. Linears spread the LEDs over a longer housing.
- Beam pattern – UFOs commonly use round, symmetric beams (e.g., 90–120°). Linears can support rectangular or aisle optics like 60° × 120°.
In comparative uniformity modeling using LM‑79‑based IES files, a 150 W UFO (≈22,500 lm, 90° beam at 10 m) produced high luminance in a relatively small area. For workers, that can mean frequent exposure to bright discs in the upper visual field. The linear, by contrast, spreads the same flux over larger lenses, reducing the apparent source brightness at any given angle.
Key takeaway: At the same lumen package, a larger emitting area with directional optics usually yields lower apparent luminance and lower UGR for typical viewing positions, assuming similar surface reflectances and room geometry.
2.2 Vertical illuminance vs. “punch”
A common belief is that “UFOs are better for high‑ceiling point‑source tasks” because they are punchy and symmetric. Field experience and modeling tell a different story:
- Comfort and accuracy for pickers, drivers, and inspectors correlate strongly with vertical illuminance at head and eye height, not just horizontal lux on the slab. This is consistent with industrial lighting practices in ANSI/IES RP‑7.[^rp7]
- Linears with 60° × 120° or aisle optics can send more light to the faces of racks and workstations while keeping luminance peaks lower.
Modeling in typical warehouse volumes shows that when the same lumen package is used, the linear layout often delivers more uniform vertical illuminance on pallet faces with lower peak luminance in the operator’s view than a UFO layout, provided that spacing and alignment are controlled.
2.3 Spacing‑to‑height ratio and striping
Linears are not automatically comfortable. When the spacing-to-height (S/H) ratio drifts too high, they can create alternating bright and dark bands (striping):
- Practical experience shows that for narrow‑aisle optics, S/H > ~1.5 can drop the minimum‑to‑average illuminance ratio below 0.6, leading to visible stripe patterns on floors and pallet faces.
- Even a 10–15° misalignment of linear fixtures relative to the aisle can shift peaks and cast deep shadows on lower pallet levels.
At the same time, a well‑designed UFO layout with wide beams (e.g., 120°) and S/H ~1.0–1.2 can achieve uniformity ratios ≥0.7 and smooth vertical gradation. The message is simple: beam selection and spacing discipline matter more than form factor alone.
2.4 Practical optics rule of thumb
For most industrial layouts (subject to verification against applicable standards):
- Use linear high bays with 60° × 120° or aisle optics for racking aisles and inspection zones, where vertical illuminance and low glare are critical.
- Use UFOs with wide beams in open areas (staging, loading, sports) where wide spill is desirable and workers are less likely to look directly toward the luminaires.
A more detailed optics discussion and layout examples are explored in the dedicated guide on UFO vs linear high‑bay uniformity (manufacturer technical guidance, not an independent standard).
3. Diffusers, Reflectors, and UGR: Trading Efficacy for Comfort
3.1 How diffusers reduce glare
Diffusers and lens treatments are the primary levers for reducing high‑bay glare without changing the mounting height:
- Clear lenses maximize lumens but show individual LED chips as high‑luminance points.
- Micro‑prismatic lenses spread light slightly, blurring chip images and dropping peak luminance by a significant margin.
- Opal or frosted polycarbonate diffusers create a near‑uniform glowing surface with much lower peak luminance.
In practice, designers typically see:
- 5–8% efficacy loss with prismatic diffusers.
- 12–20% efficacy loss with opal/frosted lenses, depending on material and thickness.
These ranges are based on common manufacturer data for high‑bay optics; exact values must be confirmed from product‑specific LM‑79 and optical reports. The trade‑off is clear: a modest increase in fixture count often delivers a net win in visual comfort compared to overdriving harsh sources.
3.2 UGR improvement with low‑glare optics
Experience with low‑UGR high‑bay specifications shows a recurring pattern:
- Off‑the‑shelf UFOs with clear lenses often calculate at UGR 24–28 at 8–10 m mounting height in standard UGR rooms (e.g., 4H wide × 8H long), using catalog photometry.
- Purpose‑designed low‑UGR variants with larger emitting areas and micro‑prismatic diffusers are typically designed to target UGR ≤ 22, sometimes reaching the UGR 19–21 range for suitable layouts.
But these catalog numbers only apply to specific reference geometries. In real warehouses with tall racks, it is good practice to verify comfort with vertical illuminance and luminance modeling in aisle views. Without that check, it is common for “low‑glare” UFOs to show >2,000 cd/m² at 30° from nadir at eye level in calculations—correlating strongly with employee complaints.
3.3 Reflectors and uplight for softer contrast
Adding reflectors to UFO high bays can also improve visual comfort:
- Deep reflectors block high‑angle light from entering the eye directly, while shaping the beam.
- Reflectors with small uplight windows can send a portion of the flux upward, brightening the ceiling and reducing contrast between luminaires and background.
This ceiling‑brightening effect helps keep luminance ratios in check and can be especially useful in gyms, aircraft hangars, and tall workshops where occupants frequently look upward. The exact uplight percentage depends on the specific reflector design and should be confirmed from manufacturer photometric data.
3.4 Pro Tip: Budget for diffuser losses upfront
Pro Tip: When converting from bare‑lamp HID or clear‑lens UFOs to low‑UGR optics, do not try to match illuminance with the same installed wattage. Plan for diffuser losses from the start:
- Estimate the diffuser penalty from product data (e.g., ≈5–8% for a prismatic shield, ≈12–20% for an opal lens).
- Size the new layout based on target illuminance and uniformity, not on one‑for‑one wattage swaps.
- Increase fixture count or wattage slightly to offset losses, while keeping luminance caps and UGR targets.
This approach yields layouts where the measured lux stays on target, but perceived brightness is softer, with fewer complaints of “too harsh” or “too bright”.
4. Flicker and Drivers: The Hidden Comfort Variable
4.1 Why flicker matters in high‑bay spaces
Glare is not the only comfort issue. Driver flicker can quietly undermine visual performance even when luminance and UGR look acceptable on paper.
IEEE 1789‑2015 Recommended Practices for Modulating Current in High‑Brightness LEDs for Mitigating Health Risks to Viewers provides guidance on flicker levels and frequencies.[^ieee1789] Commentaries on IEEE 1789 note that, around the 100–120 Hz range common in LED driver ripple, a percent flicker on the order of 8% or less is generally considered “low risk”, and values of ≈3% or less are often associated with little to no observable effect for most people.[^flicker-commentary]
Field measurements on budget high bays frequently show 20–40% flicker at these frequencies, especially under certain dimmed conditions. At those levels, users may experience:
- Eye strain and headaches over long shifts.
- Difficulty tracking moving objects, such as forklifts or cranes, particularly in peripheral vision.
- A vague sense of visual fatigue even when illuminance meets standards.
4.2 How to specify low‑flicker high bays
To control flicker, specifiers should require:
- Driver type and control method: Prefer constant‑current drivers with high‑frequency pulse‑width modulation (PWM) in the kHz range or, where available, analog dimming schemes with minimal ripple. Always verify with manufacturer data, as implementation details vary.
- Quantified flicker metrics: Request percent flicker and flicker index data at full output and at key dimming levels from an LM‑79 or equivalent photometric/electrical test report.
- Compliance references: Ask vendors to state how their products align with IEEE 1789 recommendations or to provide independent lab reports.
The incremental cost of higher‑quality drivers is generally modest compared with the long‑term benefits: reduced complaints and better performance in visually demanding tasks.
4.3 Expert warning: Dimming can re‑introduce flicker
Expert Warning: A common surprise in retrofits is that fixtures test as low‑flicker at full output but exhibit severe flicker when dimmed.
- Some 0–10 V drivers achieve flicker control at 100% but rely on low‑frequency PWM to dim, pushing percent flicker into the 20–40% range at typical setpoints.
- In warehouses using occupancy or daylight sensors, luminaires may operate dimmed for most of the day, exposing workers to the worst flicker regime for hours.
When commissioning sensorized high bays, always verify flicker performance under the expected dimmed states, not just at full brightness.
5. Case Studies: UFO vs. Linear for Visual Comfort
The following case studies are based on project‑style simulations and field feedback using LM‑79 photometric files in common lighting software. They are illustrative examples, not peer‑reviewed clinical trials.
5.1 20 ft shop: balancing comfort and punch
A metalworking shop with a 20 ft (≈6 m) ceiling replaced aging metal halide fixtures. Two options were modeled:
- Option A: UFO high bays at 150 W each, 90° beam, S/H ≈ 1.2.
- Option B: Linear high bays at 130 W each with 110° beam, S/H ≈ 1.4.
Results from the layout and field feedback:
- Both achieved ~40–45 foot‑candles (430–480 lx) on the workplane, in line with typical industrial recommendations such as ANSI/IES RP‑7 for medium‑duty tasks (exact target should be verified per task table).[^rp7]
- The UFO layout produced calculated UGR values in the 24–26 range from common viewpoints and strong highlights on glossy machinery.
- The linear layout, with a larger emitting area, reduced modeled UGR to ≈21–22 and smoothed reflections on machine surfaces.
Shop staff reported less squinting and better comfort during long fabrication shifts under the linear layout, even though the average illumination was similar.
5.2 Narrow warehouse aisles: vertical lux vs. striping
A racked warehouse with 30 ft (≈9 m) mounting height evaluated two concepts for pallet‑picking aisles:
- Concept 1: UFOs on a grid above aisles.
- Concept 2: Linear aisle luminaires with 60° × 120° optics aligned to each aisle.
Key observations:
- The UFO layout gave uniform floor lux but left uneven vertical illuminance on pallet faces, with pronounced bright circles in the upper field of view.
- The initial linear design used S/H ≈ 1.8, which created noticeable striping and contrast bands on racks.
- After adjusting to S/H ≤ 1.5 and tightening alignment, the linear layout produced vertical illuminance ≥150–200 lx on pallet faces with smoother gradation and lower apparent glare. These ranges are consistent with the vertical illuminance levels commonly used for warehouse picking in practice, but specifiers should always confirm against the latest RP‑7 tables.[^rp7]
Forklift operators reported less visual fatigue, especially when scanning upper rack levels.
5.3 Inspection area: low‑UGR linears vs. shielded UFOs
In an electronics inspection area adjacent to a warehouse, glare and reflections on glossy PCBs were causing quality issues. Two strategies were considered:
- Low‑UGR linear fixtures with opal diffusers.
- UFO fixtures fitted with deep reflectors and prismatic shields.
Both designs targeted UGR ≤ 19 and 500–750 lx on horizontal and vertical planes, in line with typical illuminance for detailed assembly/inspection tasks in many industrial guidelines.[^rp7]
- The linear solution required ~10–15% higher installed lumens to offset diffuser losses but yielded very uniform luminance and almost no visible hotspots.
- The shielded UFO solution used slightly fewer fixtures but demanded precise aiming and reflector choice to keep high‑angle luminance under control.
Ultimately, the facility chose low‑UGR linears because they provided more forgiving, uniform lighting with less dependence on exact aiming.
6. Applying This to Real Products and Specifications
6.1 UFO high bay: when and how to use it comfortably
A high‑efficacy UFO with a forged aluminum housing and IP65 rating is well‑suited for harsh industrial environments. When using UFOs, consider the following for visual comfort:
- Add reflectors in visually sensitive areas to shield high‑angle light and optionally introduce uplight for softer contrast.
- Respect S/H ≤ 1.2 in typical open‑area layouts to avoid over‑bright singular sources and keep uniformity strong.
- Pair with controls that dim smoothly without inducing flicker, particularly when occupancy sensors will hold fixtures at partial output for most of the day.
As an example, the Hyperlite LED High Bay Light – Black Hero Series, 29000 lumens combines up to 140 lm/W efficacy, selectable wattage and CCT, 0–10 V dimming capability, and an optional reflector. Performance values in this paragraph are based on manufacturer‑published data, not independent third‑party certification.
6.2 Linear high bay: low‑UGR workhorse for aisles and task zones
For racked aisles and precision tasks, a linear high bay with selectable wattage and CCT, 1–10 V dimming, and DLC 5.1 Premium certification offers a strong balance of performance and comfort. A representative linear high bay with a 110° beam, rugged steel housing, and 150 lm/W efficacy can be configured via selectable wattage to match target illuminance while maintaining good uniformity.
The Linear High Bay LED Lights – HPLH01 Series, 18200 lumens illustrate this approach: selectable wattage (down to 40% of maximum), selectable CCT, and compatibility with motion sensors and emergency options give specifiers the flexibility to tune both light levels and controls without compromising comfort. DLC Premium certification, where applicable, is based on LM‑79 performance data and LM‑80/TM‑21 lifetime projections reviewed by the DLC program; designers should review the corresponding DLC listing for the specific SKU they plan to use.
6.3 Recommended UGR and layout targets
Drawing on ANSI/IES RP‑7, CIE glare guidance, and industrial practice, a practical set of starting targets is:
- General warehouse and circulation: UGR ≤ 22, average horizontal illuminance ≈200–300 lx, vertical illuminance on rack faces ≥150 lx.
- Assembly and inspection: UGR ≤ 19, average horizontal illuminance ≈500–750 lx, strong vertical illuminance at eye level with limited reflections.
- High‑bay sports or multi‑use halls: UGR in the low 20s or better, with careful control of high‑angle luminance in spectator views.
Exact illuminance and UGR targets should always be confirmed from the latest version of RP‑7 and any local regulations or client standards.
A dedicated guide on low‑UGR high bay lighting provides more detailed examples of how these targets translate into luminaire selection and aiming (again, a manufacturer resource rather than an independent standard).
7. Step‑by‑Step Checklist: Designing for Low Glare and High Comfort
Use this checklist when choosing between UFO and linear high bays for a project focused on employee comfort and productivity.
7.1 Define tasks and comfort targets
- Map visual tasks: Identify zones for pallet picking, driving, inspection, assembly, and occasional upward viewing (e.g., gyms or arenas).
- Set UGR goals: Use UGR ≤ 22 for general tasks and aim for ≤ 19 in critical visual zones, checking against CIE guidance and project requirements.
- Set illuminance targets: Use ANSI/IES RP‑7 and local standards as a baseline. For typical warehouses, start with ≈200–300 lx general, ≈150–200 lx vertical on racks, then refine based on the specific task category in RP‑7.
7.2 Choose form factor by zone
- Racking aisles: Prefer linear high bays with aisle optics (e.g., 60° × 120°) for better vertical illuminance and lower apparent glare.
- Open floor and staging: Use UFO high bays with wide beams where wide spill is helpful and workers seldom look directly toward fixtures.
- Inspection or precision tasks: Use low‑UGR linear fixtures with prismatic or opal diffusers, or shielded UFOs with deep reflectors.
7.3 Control luminance and uniformity
- Limit S/H ratios: Keep spacing ≤ 1.5× mounting height as a practical upper bound, and closer to 1.0–1.2 in critical aisles.
- Specify diffusers: Accept a 5–20% lumen penalty (per product data) to gain significantly lower luminance and UGR.
- Check luminance ratios: Aim for luminaire‑to‑background ratios below ≈1:10 in key views to avoid harsh contrast, adjusting based on modeling results.
7.4 Control flicker and controls
- Require low‑flicker drivers: Ask for percent flicker data and driver modulation frequency, targeting alignment with IEEE 1789 “low‑risk” guidance.[^ieee1789]
- Test dimmed operation: Verify flicker at typical sensor‑driven setpoints, not just full brightness.
- Zone controls thoughtfully: Use separate zones for fast‑moving equipment vs. static storage to avoid unnecessary dimming and brighten/dim cycles in operators’ field of view.
7.5 Verify with photometric files and standards
- Obtain LM‑79 reports and IES (.ies) files: These define luminaire output and distribution and are required for accurate modeling. The IES LM‑63 format is the widely used standard for electronic transfer of photometric data.[^lm63]
- Model vertical illuminance: Use software such as AGi32 or similar, importing LM‑63‑compliant IES files, to verify vertical illuminance and UGR.
- Cross‑check standards: Confirm that your layout aligns with ANSI/IES RP‑7 for industrial tasks and, where applicable, local energy codes (e.g., ASHRAE 90.1 or IECC) for power density and controls.
8. UFO vs. Linear High Bay Glare: Summary Comparison
The table below summarizes how UFO and linear high bays differ from a visual comfort perspective.
| Feature / Criterion | UFO High Bays | Linear High Bays |
|---|---|---|
| Typical beam pattern | Symmetric circular (90–120°) | Rectangular or aisle (e.g., 60° × 120°) |
| Emitting area | Compact; higher luminance per unit area | Larger; lower luminance per unit area at same lumens |
| Best applications | Open floor, staging, sports, general large spaces | Racking aisles, inspection, assembly, task‑oriented zones |
| Typical UGR behavior | UGR 24–28 with clear lenses at 8–10 m mount; lower with reflectors/diffusers (per catalog photometry) | UGR 19–22 achievable with low‑glare lenses and correct spacing (per catalog photometry) |
| Vertical illuminance on racks | Highly layout‑dependent; can under‑light lower levels | Generally better vertical coverage with aisle optics |
| Risk of striping | Low if S/H ≤ 1.2 and beams wide | Elevated if S/H > 1.5 or fixtures misaligned |
| Glare mitigation options | Deep reflectors, uplight windows, prismatic shields | Prismatic/opal lenses, larger emitting area, aisle optics |
| Integration with sensors | Good, but watch dimming‑induced flicker | Good; often paired with 1–10 V dimming and aisle zoning |
For deeper discussion of UGR, luminance control, and layout examples, see the guide on designing a high‑bay layout for warehouse safety and the warehouse lumens guide for UFO high bays. These are manufacturer resources intended to complement, not replace, formal standards.
Key Takeaways for Specifiers and Facility Managers
- Visual comfort is a productivity variable. Workers in high‑bay environments often tolerate harsh lighting until fatigue or errors force change. Proactively controlling glare and flicker reduces those hidden costs.
- Form factor alone does not define glare. UFOs and linears can both be comfortable or uncomfortable depending on optics, spacing, and controls. The critical variables are emitting area, beam shape, S/H, and luminance ratios.
- Diffusers and reflectors are strategic tools, not afterthoughts. Accepting a moderate efficacy penalty for low‑UGR optics usually pays back in fewer complaints and better task performance.
- Driver quality and dimming strategy matter. Ensuring low flicker at operating dimmed levels is just as important as choosing wattage and CCT.
- Model vertical illuminance and UGR, not just horizontal lux. Pair LM‑79 data, IES files, and software like AGi32 with ANSI/IES RP‑7 targets to create layouts that feel as good as they look on paper.
When selecting between UFO and linear high bays, put employee comfort, vertical visibility, and glare control on equal footing with efficiency and cost. In most facilities, that balance is what actually drives long‑term productivity.
Frequently Asked Questions
Q1: Are UFO high bays always more glaring than linear high bays?
Not always. UFOs have smaller emitting areas and often higher luminance, which can increase UGR, especially with clear lenses. However, with deep reflectors, diffusers, and careful spacing, UFOs can meet UGR targets. Linears generally have an easier path to low glare in aisles and task zones due to their larger emitting area and directional optics.
Q2: How do I know if my existing high‑bay lighting has a flicker problem?
A quick field check is to use a smartphone camera in slow‑motion mode and observe the luminaires while dimmed. Severe banding often indicates high flicker. For critical facilities, have an electrical or lighting professional measure percent flicker and flicker index with a meter and compare the results with IEEE 1789‑based guidance or local criteria.
Q3: What is a reasonable starting point for UFO spacing in a 25 ft warehouse?
A common starting point based on experience is keeping spacing at or below ≈1.2× the mounting height for wide‑beam UFOs. For a 25 ft mounting height, that suggests ≈25–30 ft spacing, adjusted based on the required illuminance, reflectance of surfaces, and whether the area is open or racked. Final spacing should be validated with a layout using product IES files.
Q4: Do diffusers always reduce efficiency too much to be worthwhile?
No. While diffusers typically reduce efficacy by ≈5–20%, the trade for significantly lower luminance and better UGR is usually favorable in people‑intensive spaces. Designers can compensate by slightly increasing fixture count or wattage to maintain target illuminance while still delivering a net gain in visual comfort.
Q5: What documentation should I request from manufacturers to evaluate glare and comfort?
At minimum, request LM‑79 reports, IES (.ies) files, LM‑80/TM‑21 lifetime data, and any available UGR tables or glare classifications. For sensorized or dimmed systems, ask for flicker metrics at full and dimmed output. These documents allow a lighting designer or engineer to model UGR, vertical illuminance, and flicker risk accurately.
Disclaimer: This article is for informational purposes only and does not replace professional engineering, electrical, or safety advice. Lighting designs for commercial or industrial facilities should be reviewed and stamped by qualified professionals, and all installations must comply with applicable codes and standards such as NFPA 70 (National Electrical Code), ANSI/IES RP‑7, and local building regulations.
[^rp7]: ANSI/IES RP‑7‑17, Lighting Industrial Facilities. Illuminating Engineering Society. (Access via IES or ANSI webstore.) [^cie117]: CIE 117:1995, Discomfort Glare in Interior Lighting. International Commission on Illumination (CIE). [^cie190]: CIE 190:2010, Recommended System for Mesopic Photometry Based on Visual Performance and related CIE publications on UGR. [^ieee1789]: IEEE 1789‑2015, IEEE Recommended Practices for Modulating Current in High‑Brightness LEDs for Mitigating Health Risks to Viewers. IEEE Standards Association. [^flicker-commentary]: For example, engineering summaries of IEEE 1789 (e.g., manufacturer and research briefs) commonly interpret the standard as suggesting percent flicker ≤≈8% at ~100–120 Hz as a “low‑risk” region and ≈3% or less as typically not visually perceptible for most observers. [^lm63]: IES LM‑63‑17, Approved Method: IES Standard File Format for the Electronic Transfer of Photometric Data and Related Information. Illuminating Engineering Society.