In modern logistics facilities, the metric for lighting success has shifted from simple floor brightness to task-specific visibility. For high-rack warehouses, the most critical performance indicator is often vertical illuminance ($E_v$). While horizontal lux ($E_h$) helps support safe movement on the floor, vertical lux is what allows operators to identify inventory, read small-print labels, and maintain high barcode scan rates.
Choosing between a circular high bay (often referred to as a "UFO" style) and a linear high bay is no longer a matter of aesthetic preference. It is a technical decision rooted in photometric distribution, mounting height, and the specific geometry of the racking aisles. This guide provides a spec-first analysis of how to optimize vertical light for racked environments, with an emphasis on alignment with energy codes and commonly referenced industrial lighting practices.
Editorial note on data and assumptions
Unless otherwise specified, the quantitative ranges in this article (e.g., typical lux targets, approximate percentage gains, suggested S/MH ratios, LLF ranges) are based on commonly cited industry practice and manufacturer-neutral design heuristics for enclosed warehouses with medium reflectance finishes (e.g., light-colored racks and walls) and aisle widths of roughly 8–12 ft at mounting heights of 25–40 ft. They are not a substitute for project-specific photometric calculations.
The Science of Vertical Illuminance ($E_v$) in Racked Aisles
Vertical illuminance refers to the amount of light falling on a vertical surface, such as the face of a storage rack or a shipping label. In a warehouse with 30-foot racks, the light must travel from the ceiling and penetrate deep into the narrow "canyon" of the aisle.
Barcode Legibility and Target Lux Levels
According to the ANSI/IES RP-7-21 Recommended Practice for Lighting Industrial Facilities, illuminance requirements vary significantly based on the task. For general warehouse aisles, a target of around 200 lux (approximately 20 foot-candles) is commonly referenced. However, for high-activity picking areas or where precision barcode scanning is required, practitioners often aim for approximately 150–300 lux specifically at the rack face, subject to scanner technology, print quality, and contrast requirements.
If vertical illuminance is not maintained within an appropriate range for the task, "shadow zones" can occur on the lower tiers of the racks. When vertical lux falls to roughly 100 lux or below under typical warehouse conditions (e.g., medium reflectance, standard 1D/2D label formats), barcode scanners may begin to struggle with contrast more frequently, which can contribute to higher error rates and slower picking cycles. Actual thresholds vary by device and environment, so field testing with the intended scanners is recommended.
Photometric Distribution: Symmetric vs. Asymmetric
The primary difference between standard high bays and aisle-specific fixtures is the beam geometry. Standard fixtures typically utilize a symmetric distribution (e.g., a 90° or 120° circular beam). While this can work well for open-floor manufacturing, it is often inefficient in narrow aisles because a significant portion of the luminous flux lands on the tops of the racks or the ceiling, rather than the vertical task surfaces.
In contrast, aisle optics utilize an asymmetric or rectangular distribution. These optics concentrate the light in a narrow "long and thin" pattern, directing lumens down the length of the aisle and onto the vertical rack faces.
Performance Analysis: UFO vs. Linear Optic Choice
The debate between circular and linear form factors often overlooks the fact that the internal optic—not the external shape—determines where the light goes. However, field experience and manufacturer-agnostic design simulations consistently show distinct performance gaps when comparing standard symmetric units to dedicated aisle-optic fixtures in typical racked-aisle geometries.
Approximate 20–30% Vertical Gain (Under Typical Assumptions)
Designers frequently report that switching from symmetric circular high bays to fixtures with aisle-focused optics can increase average vertical rack-face illuminance by roughly 20–30% at similar wattage and mounting height, when evaluated in representative layouts (e.g., 8–10 ft aisles, 30–40 ft mounting heights, medium reflectance). This uplift arises because aisle optics intentionally redirect more light toward the vertical rack faces instead of the tops of racks or the inter-aisle ceiling.
These figures should be treated as indicative, not universal. Actual gains depend on:
- Aisle width and mounting height
- Rack and wall reflectance
- Beam type (e.g., Type I/II vs. wide distribution)
- Fixture layout and S/MH ratio
For critical projects, run project-specific IES-based simulations (see “The Necessity of IES Simulations” below) rather than relying solely on rule-of-thumb percentages.
Spacing-to-Mounting Height (S/MH) Ratios
The Spacing-to-Mounting Height ratio is one of the most informative metrics for predicting uniformity and minimum illuminance. For linear aisle fixtures, mounting spacing along the aisle often falls between about 0.8–1.5× the mounting height, depending on beam distribution and uniformity targets.
| Fixture Type | Ideal Application | S/MH Ratio (Aisle) | Vertical Lux Efficiency |
|---|---|---|---|
| Symmetric UFO | Open areas, staging, loading docks | 1.5 – 2.0 | Low (higher spill outside task area) |
| Narrow-Beam UFO | High-ceiling aisles (>35 ft) | 0.8 – 1.0 | Moderate |
| Linear Aisle Optic | Racked aisles (15–40 ft) | 1.0 – 1.5 | High (more targeted onto racks) |
Pro Tip: While conventional wisdom suggests that circular high bays are only for open areas, a well-designed circular fixture equipped with a Type I or Type II aisle optic can match or in some layouts exceed linear performance. The key is to review the IES (Illuminating Engineering Society) photometric file to confirm that the distribution is truly optimized for the actual aisle width and mounting height, rather than assuming performance based on form factor.
Debunking the "Form Factor" Myth
A common misconception is that linear high bays are inherently superior for aisles simply because they are long. In reality, the performance is dictated by the beam angle, intensity distribution, and the Spacing-to-Mounting Height ratio.
For example, a narrow-beam circular high bay at a 39-foot (12 m) mounting height can achieve around 200 lux on the floor and acceptable vertical illuminance on the rack faces with good uniformity if the spacing is tightened to an S/MH of approximately 0.9, assuming typical warehouse reflectance and standard pallet-rack geometry. This kind of result must be confirmed with a project-specific simulation rather than assumed.
Expert Warning: Relying solely on total lumens is a frequent specification error. A fixture might be rated at 30,000 lumens, but if it has a wide 120° beam in a narrow 8-foot aisle, a substantial fraction of those lumens may never reach the vertical task area. This can lead to higher energy use with relatively poor visibility on labels and rack faces.
Compliance, Controls, and Technical Artifacts
For professional procurement and facility management, a fixture must be more than bright; it should also be verifiable and aligned with national or regional energy and safety standards where applicable.
DLC Premium and Utility Rebates
To qualify for many utility rebates in North America, fixtures are typically required to be listed on the DesignLights Consortium (DLC) Qualified Products List (QPL). DLC Premium certification is viewed as a benchmark for higher-performance LED high bays, with requirements for higher efficacy (lumens per watt) and more stringent lumen maintenance criteria compared with standard DLC listings.
Safety and Durability Standards
Industrial fixtures are generally expected to carry UL 1598 certification, which is a core safety standard for luminaires. For the LED components specifically, UL 8750 addresses electrical and thermal safety for LED drivers and modules.
Advanced Controls Compatibility
Modern energy codes, such as California Title 24 and ASHRAE 90.1, often mandate the use of occupancy sensors and multi-level or continuous dimming in many warehouse applications.
- 0–10V Dimming: This is widely used as a commercial dimming standard. It allows for smooth transitions and is compatible with most centralized building management systems (BMS).
- Sensor Placement: Sensors should be positioned to minimize being "blinded" by the ends of the racks or obstructed by stored goods. In high-rack environments, integrated sensors on each fixture or carefully located remote sensors can provide more granular and effective control. Layouts should be validated in the field.
Installation Heuristics and Practical Implementation
To help ensure that a lighting design continues to perform close to its intended level over its rated life, designers must account for environmental factors and maintenance practices.
Accounting for Light Loss (LLF)
In dusty warehouse environments, initial lumen output will degrade over time due to dirt accumulation on the optics and gradual lumen depreciation of the LED packages. When sizing a system, professionals apply a Light Loss Factor (LLF).
For many enclosed warehouses, designers often assume an LLF in the range of approximately 0.8 to 0.85, depending on the local environment (dust levels, cleaning schedule) and maintenance program. This approach is intended to help ensure that, after years of operation, the delivered vertical illuminance remains within the desired target band (for example, 150–300 lux at the rack face in high-activity areas). Actual LLF values should be derived from manufacturer data, IES guidance, and site-specific conditions.
The Necessity of IES Simulations
Before fixtures are purchased in volume, a photometric simulation using IES LM-63-19 files (or equivalent format) is strongly recommended. Photometric software (e.g., commonly used industry tools) can plot vertical illuminance at various heights on the rack face and across the aisle.
This process allows designers to:
- Validate that the chosen fixture and spacing meet applicable IES recommendations for the task
- Check minimum, average, and max/min ratios for both horizontal and vertical illuminance
- Compare alternative optics (e.g., symmetric vs. aisle-specific) on a like-for-like basis
Where OSHA or local safety regulations reference specific lighting requirements, those should be reviewed in parallel with the simulation results to help ensure compliance.
Checklist for Warehouse Lighting Specification
- Define Target $E_v$: For racked picking aisles, many practitioners aim for approximately 150–300 lux on the rack face, adjusted for task difficulty and scanner requirements. Confirm targets against current standards and internal safety policies.
- Verify DLC Listing: Confirm that the model appears on the DLC QPL (Standard or Premium) if utility rebates or specific program requirements apply.
- Request LM-79 and LM-80 Reports: LM-79 provides measured photometric performance for the complete luminaire, while LM-80 supports lumen maintenance projections for the LED packages, both from qualified testing laboratories.
- Confirm Control Compatibility: Specify 0–10V dimming (or another suitable protocol) and ensure fixtures are sensor-ready where code or operational strategy calls for advanced controls.
- Run Photometrics: Use manufacturer-supplied IES files to simulate the exact aisle width, mounting height, and reflectance conditions. Review vertical illuminance at multiple shelf heights, not only on the floor.
Key Takeaways for High-Rack Lighting
Optimizing a warehouse for vertical light requires moving beyond "one-size-fits-all" solutions. By prioritizing aisle-appropriate optics and verifying performance through technical documentation, facility managers can improve operational visibility and consistency.
- Vertical Lux Matters: For picking and scanning tasks, visibility at the rack face is typically more critical than floor brightness alone.
- Optics Over Form: Whether circular or linear, the beam distribution and S/MH ratio must correspond to the aisle geometry and task requirements.
- Targeted Efficiency Gains: In many typical racked-aisle layouts, aisle-focused optics can deliver on the order of 20–30% higher average vertical illuminance than wide symmetric beams at comparable power, but the actual gain should be confirmed via simulation.
- Compliance Is Foundational: DLC listing, UL certification, and adherence to local codes (e.g., Title 24, ASHRAE 90.1 where applicable) support both safety and, in many regions, rebate eligibility.
- Plan for Depreciation: Applying an LLF in the 0.8–0.85 range (or a project-specific value) helps account for lumen depreciation and dirt accumulation so that minimum vertical illuminance levels are maintained over time.
Frequently Asked Questions (FAQ)
What is the difference between 0–10V and 1–10V dimming?
While both use a low-voltage control signal, 0–10V dimming typically allows the light to dim down to very low levels and, in many implementations, effectively to "off" with suitable drivers and control gear. 1–10V dimming usually dims to a minimum of about 10% brightness rather than to zero. For warehouse energy savings and code compliance, 0–10V is often preferred because it supports deeper dimming and, with the right controls, can allow lights in unoccupied aisles to turn off or go to a very low setpoint.
Can I use circular high bays in a warehouse with 40-foot ceilings?
Yes. Circular high bays can be suitable at 40-foot mounting heights if you select a fixture with an appropriately narrow beam angle (e.g., 60° or 90°), so that sufficient light reaches the floor and vertical rack faces. Wide 120° beams at that height tend to produce more scatter and reduced vertical illuminance in narrow aisles. Always verify performance with a project-specific photometric analysis.
Why is an LM-79 report important?
An LM-79 report is effectively a performance summary for an LED luminaire from a qualified testing laboratory. It provides measured data on total lumens, intensity distribution, efficacy (lm/W), and color characteristics under defined test conditions, helping specifiers validate that catalog claims are supported by independent measurements.
YMYL Disclaimer: This article is for informational purposes only and does not constitute professional engineering, safety, or electrical design advice. Warehouse lighting designs should be reviewed and, where required, stamped by a qualified professional to ensure compliance with local building codes, OSHA or other applicable safety regulations, and the National Electrical Code (NEC) or local equivalent. Individuals with photosensitive or vision-related conditions should consult a healthcare or occupational-safety specialist before working under or specifying high-intensity industrial lighting.
About this guide
This content has been compiled and technically edited based on commonly referenced IES standards, UL/DLC program criteria, and field feedback from warehouse and industrial lighting projects. It is intended as a neutral, manufacturer-agnostic resource and should be used together with project-specific photometric studies and the documentation supplied by fixture manufacturers and accredited testing laboratories.