Light the surfaces people need to see and understand
Horizontal illuminance describes light arriving on level surfaces such as desks and floors. Vertical illuminance describes light arriving on upright planes such as faces, walls, shelves, and signs. Good commercial lighting measures the planes that represent the real visual task.
Published · Last reviewed
Short answer
Use horizontal illuminance for workplanes, floors, counters, and other level tasks. Use vertical illuminance for faces, walls, displays, artwork, signs, shelving, and the direction people look. Many interiors require both. Define the position, height, orientation, extent, and maintained criterion of every calculation plane; a single average lux value on a generic floor or desk grid cannot describe the complete visual environment.
1. A plane changes what the lux value means
Illuminance is luminous flux incident on a surface per unit area, measured in lux. “Horizontal” and “vertical” identify the orientation of that receiving surface. The same point in a room can have different horizontal and vertical illuminance because light arrives from different directions. A downlight directly overhead may produce strong horizontal illuminance on a desk but relatively little light on a person's face.
A vertical plane is also incomplete unless its direction is stated. A wall facing north, a face looking toward a presentation, and a shelf facing an aisle receive different light. For people moving through a space, designers may review several vertical directions or use cylindrical or semi-cylindrical measures where the applicable method calls for them. For procurement, the essential lesson is simpler: always attach the requested value to a defined surface and orientation.
2. Match the calculation plane to what matters
| Visual need | Primary plane to study | Lighting implication |
| Reading, writing, assembly, checkout | Horizontal or inclined task plane at the actual working height. | Place controlled light on the task while checking hands, tools, and reflected glare. |
| Faces in meetings, reception, hospitality | Vertical planes at representative seated and standing face positions. | Use light with useful lateral or forward contribution, not only steep overhead beams. |
| Retail shelves and merchandise | Vertical planes aligned with display fronts, plus horizontal shelves where relevant. | Coordinate track or asymmetric distributions with customer viewing directions. |
| Walls, artwork, signs, teaching boards | Vertical grids on the actual target dimensions. | Review top-to-bottom and side-to-side uniformity, veiling reflections, and source visibility. |
| Corridors, lobbies, orientation | Floor for safe movement and vertical boundaries or signs for spatial understanding. | Combine circulation light with visible room surfaces and landmarks. |
| Video calls and presentations | Vertical face and background planes, plus horizontal table tasks. | Balance camera appearance, eye comfort, screen reflections, and scene controls. |
3. Horizontal illuminance supports work and movement
Horizontal measurements are central where detail lies on a desk, bench, counter, floor, or examination surface. Specify the height and boundaries of the task area instead of assuming every calculation should be at floor level or a standard desk height. Furniture layouts, machinery, shelves, and standing work can change both the correct plane and the shadowing conditions.
Average illuminance alone can hide dark points or excessive peaks. Review minimum values, uniformity, task surroundings, and the maintained condition required by the applicable project standard. Consider where the worker's head, body, hands, or equipment blocks light. A broad distribution or light from more than one direction may reduce hard shadows, while tightly focused light can be useful where contrast and containment are intentional.
4. Vertical illuminance shapes what occupants notice
People normally look outward, not down at a calculation grid. Light on faces supports communication and recognition. Light on walls and displays makes room boundaries, merchandise, graphics, and destinations legible. In circulation spaces, visible vertical surfaces can contribute to orientation and the impression of brightness even when floor illuminance is modest.
Vertical illuminance does not directly equal perceived brightness. Surface reflectance, color, texture, gloss, viewing direction, surrounding adaptation, and luminance contrast affect appearance. A dark wall can receive substantial illuminance and still look darker than a pale wall receiving less. For a deeper distinction between received light and apparent surface brightness, see the lumens, lux, candela, and luminance guide.
Faces also require qualitative review. Very steep overhead light can leave eye sockets dark; strong frontal light can flatten features; a bright source near the line of sight can cause discomfort. Use a mock-up with representative people, finishes, cameras, and viewing positions when facial modeling or hospitality atmosphere is important.
5. Distribution and placement decide the balance
Downlights and narrow direct luminaires commonly favor horizontal planes below them. Wide, batwing, asymmetric, wall-wash, pendant, and indirect distributions can send more useful light toward vertical surfaces or the room envelope. The fixture label is not enough: inspect intensity plots and calculate the exact IES or LDT file for the selected output, optic, trim, and accessory combination.
Placement matters as much as distribution. A wall washer too near or too far from its target can create poor top-to-bottom balance. A track light aimed from behind the viewer may reduce self-shadow on merchandise, while another position may produce glare or reflected images. Suspended direct-indirect lighting can serve desks and brighten ceilings, but partitions and ceiling finishes change the result. Compare these choices in the direct versus indirect lighting guide.
Calculation and review checklist
- Every task and visually important surface is identified on plans or elevations.
- Each calculation plane has a defined position, height, orientation, size, and grid.
- Criteria come from the applicable current standard and project brief.
- Average, minimum, uniformity, surrounding areas, and maintained conditions are reviewed as required.
- Exact luminaire photometry, output, optics, aiming, and accessories are modeled.
- Furniture, shelving, partitions, people, equipment, and daylight are represented realistically.
- Likely viewing directions, glare, reflections, shadows, and adaptation are assessed.
- Critical faces, materials, displays, and wall effects are verified in a sample or mock-up.
Mistakes and limitations
The most common mistake is treating a horizontal average as proof that the whole space is well lit. It says nothing by itself about faces, signs, walls, shelving, glare, or contrast. Another mistake is requesting “vertical lux” without stating the direction and height. A number without a defined plane cannot be reproduced or compared fairly.
Do not compare supplier calculations made with different room dimensions, reflectances, maintenance factors, grids, or photometric variants. Do not infer luminance from illuminance alone, especially on dark, colored, glossy, or textured materials. Calculation software predicts a model, not workmanship, color appearance, controls behavior, or occupant response. Emergency, energy, workplace, accessibility, and sector requirements also need separate qualified review.
What to include in an inquiry
Provide plans, elevations, sections, room and mounting dimensions, furniture or display layouts, task descriptions, user positions, viewing directions, surface finishes and reflectances, daylight conditions, target calculation planes, maintained criteria, uniformity requirements, controls, quantity, electrical supply, and destination. Mark important faces, walls, signs, shelves, or counters rather than sending only a room area.
Ask the supplier to identify the exact luminaire and photometric file used, aiming and mounting assumptions, maintenance factor, and calculation grid. Browse AOCEN's commercial lighting categories, consult the catalog library, or contact AOCEN with the calculation brief. Suitability, photometry, compliance evidence, controls, and commercial terms require project-specific written confirmation.
Source notes
The CIE International Lighting Vocabulary defines horizontal illuminance as illuminance on a horizontal plane and vertical illuminance as illuminance on a vertical plane. The Illuminating Engineering Society's Illuminance Selector FAQ explains that recommendations apply to identified task surfaces and are selected in an application context. Consult the current standard adopted for the project instead of transferring a generic lux target between unrelated spaces.