Lighting Control Systems Explained: Dimming, Daylight Harvesting & Occupancy Automation

Lighting is the most visible and most controllable element of a building's energy profile. It represents approximately 30% of a commercial building's electricity consumption. More importantly, the technology for eliminating lighting waste is mature, proven, and cost-effective. A well-designed lighting control system can cut your lighting energy consumption by 40–60% — not through dimmer switches and manual discipline, but through automatic, sensor-driven, continuously optimised control.

This guide explains the three main dimming technologies, how occupancy and daylight sensors work, how to programme scenes for different spaces, and what to look for in LED fixtures to ensure dimming compatibility. Whether you are designing a new Grade A office, retrofitting a hospital, or specifying a university campus in Bangladesh, this guide gives you the technical foundation you need.

The Problem with Conventional Lighting

In a conventional office building, lighting works like this: someone walks in and flips a switch. The entire circuit — perhaps 20 fixtures across 200 square metres — turns on at full power. When the meeting is over, the lights may or may not be switched off. Cleaners flip everything on at 8 PM. The last person out forgets to switch off the floor. By morning, the lights have run for 14 hours, 8 of which served no one.

This pattern is not unusual — it is normal. Studies of conventional office buildings consistently show that lights are on and unoccupied for 40–60% of operating hours. Lighting control eliminates this waste automatically, without requiring any behaviour change from occupants.

DALI Protocol: The Digital Standard for Lighting Control

DALI — Digital Addressable Lighting Interface — is the international standard (IEC 62386) for digitally controlled lighting. It is the most capable and widely used lighting control protocol for commercial buildings.

In a DALI installation, each individual luminaire (or each fluorescent ballast / LED driver) has its own unique address on a 2-wire DALI bus. A DALI controller can send commands to individual luminaires, to groups of luminaires, or to broadcast all luminaires simultaneously. Commands include on, off, recall scene, direct dimming to a specific level, and query status.

The DALI bus runs alongside the power wiring but carries only low-voltage digital signals (15V DC, up to 250mA). Up to 64 devices can be addressed on a single DALI segment. Multiple DALI segments can be managed by a higher-level controller — a DALI gateway or a BMS interface — to create lighting zones across an entire floor or building.

DALI's key advantages for commercial lighting:

Individual luminaire feedback. Unlike simple relay switching, DALI reports back from each luminaire — confirming whether the fixture is on, its current dimming level, and whether a lamp fault has occurred. This enables predictive maintenance and remote fault reporting.

Group and scene flexibility. DALI groups are defined in software, not wiring. Reconfiguring a lighting zone from an open-plan layout to a cellular office layout requires a software change, not rewiring. This flexibility is extremely valuable for multi-tenant buildings where layouts change between tenancies.

Integration with BMS. DALI gateways bridge between the DALI bus and KNX, BACnet, or Modbus — enabling lighting to be managed as part of the overall building management system. Occupancy data from the BMS drives DALI dimming commands automatically.

Precise dimming. DALI provides 256 dimming steps from 0.1% to 100%, enabling smooth, flicker-free dimming across the full range. This is essential for daylight harvesting applications.

0-10V Dimming: The Analogue Alternative

Before DALI, the most common commercial dimming method was 0-10V analogue dimming. A separate 2-wire control signal (0–10 volts DC) is connected to each LED driver or fluorescent ballast. At 0V the driver dims to minimum (or off, depending on type). At 10V it runs at full output. The dimming controller varies this voltage to adjust light output.

0-10V is simpler and lower cost than DALI for basic applications. It is appropriate for simple zoned dimming where groups of fixtures are dimmed together. However, it has significant limitations compared to DALI:

0-10V is a one-way signal — the controller cannot receive feedback from fixtures. Fault detection requires separate monitoring systems. Groups are defined by wiring, not software. All fixtures on the same 0-10V circuit dim identically and simultaneously.

For daylight harvesting applications — where perimeter and interior luminaire rows need to dim independently in response to photocell feedback — DALI is significantly more effective. The individually addressable DALI approach allows the perimeter row to dim to 20% while the interior row maintains 80%, matching the natural light gradient across the space.

For new commercial projects in Bangladesh, DALI is the recommended choice. 0-10V remains appropriate for simple dimming in warehouses, car parks, and single-zone spaces where individual addressability is not required.

Smart Relay: On/Off Control with Scheduling

The third category — smart relay — does not dim at all. Smart relays are electronically controlled circuit breakers that switch lighting circuits on or off under BMS or time-clock control. They add scheduling, occupancy response, and remote monitoring to conventional on/off lighting without the cost of dimming infrastructure.

Smart relays are appropriate for spaces where dimming is not required: loading bays, stairwells, equipment rooms, external car parks, and other utility areas. They are also used alongside DALI — DALI controls the dimming within zones, while smart relays handle emergency lighting testing, after-hours circuit switching, and holiday mode control.

Occupancy vs. Vacancy Sensors: Choosing the Right Detection Mode

The terms "occupancy sensor" and "vacancy sensor" describe the same hardware operated in different modes. The choice of mode has significant implications for both energy savings and occupant experience.

Occupancy Mode (Auto-On)

In occupancy mode, the sensor automatically turns lights on when motion is detected and automatically turns them off after a set timeout period (typically 5–20 minutes of inactivity). The occupant never touches a switch — entering the room is enough to trigger lighting.

Occupancy mode maximises energy savings because lights can never be left on accidentally — they always turn off after vacancy. However, some occupants find automatic switching disruptive, particularly in private offices where sedentary work (reading, thinking, working at a desk) produces little movement, causing lights to switch off unexpectedly.

Vacancy Mode (Auto-Off)

In vacancy mode, the occupant must manually switch on the lights when entering, but the sensor automatically switches them off after detecting vacancy. This gives occupants control over when lights come on — eliminating the surprise of auto-on in sensitive environments — while still ensuring automatic switch-off when rooms are vacated.

Vacancy mode is recommended by ASHRAE 90.1 for private offices, conference rooms, and other spaces where auto-on can be disruptive. Studies show that vacancy mode still captures 75–85% of the energy savings achievable with occupancy mode, while producing higher occupant satisfaction.

Sensor Technology

PIR (Passive Infrared) sensors detect motion through body heat radiation. They are effective for detecting movement across their field of view but can miss sedentary occupants in private offices. Ultrasonic sensors emit sound waves and detect disturbance in the reflected pattern — more sensitive to minor movement, better for sedentary detection. Dual-technology sensors combine PIR and ultrasonic for high reliability in demanding environments like meeting rooms and healthcare spaces.

For open-plan offices, PIR sensors in a grid pattern provide adequate detection at reasonable cost. For private offices and meeting rooms, dual-technology sensors prevent the annoying mid-meeting switch-off that undermines occupant confidence in the system.

Daylight Harvesting: The Photocell and Dimming Feedback Loop

Daylight harvesting is the most impactful lighting automation feature for perimeter office zones. The principle is simple: measure the natural light available at the work plane, and automatically reduce artificial lighting to maintain the target illuminance level using the minimum artificial contribution.

A photocell (light sensor) is mounted either at ceiling level — measuring daylight reaching the sensor from the window — or at desk level, measuring actual illuminance at the work surface. The photocell continuously reports measured lux to the DALI controller. The controller compares this to the target setpoint — typically 300–500 lux for office work, as specified in CIBSE LG7 guidance — and adjusts the DALI dimming command accordingly.

When sunlight is bright and generous amounts of daylight reach the perimeter desks, artificial lighting dims to near zero. As a cloud passes or daylight fades in the afternoon, artificial lighting automatically increases to maintain the target illuminance. The transition is smooth and imperceptible — occupants experience consistent light quality without any manual intervention.

Closed-loop vs. open-loop control: Closed-loop daylight harvesting (photocell at desk level) provides the most accurate illuminance control but requires careful sensor placement and calibration. Open-loop control (photocell near the window measuring incident daylight) is simpler to install but requires calibration tables to translate window illuminance to desk illuminance. For high-quality commercial fit-outs, closed-loop control is recommended.

Energy savings from daylight harvesting: Building research consistently documents 20–40% reductions in perimeter zone lighting energy. Over a typical office's 250-day operating year, with perimeter zones receiving adequate daylight for 4 hours per day on average, the savings are meaningful. For a 500 m² perimeter zone with 20W LED luminaires at 2.5m spacing, saving 30% on lighting for 4 hours per day represents approximately 7,500 kWh per year — at BDT 11 per kWh, that is BDT 82,500 annually from this single zone.

Scene Programming for Meeting Rooms

Meeting rooms benefit significantly from scene programming — the ability to create predefined lighting states (combinations of zone levels and colour temperatures) that can be recalled instantly with a single button press or automatically triggered by an AV system.

Typical meeting room scenes include:

"Presentation" scene: Dimmed front lighting (near the screen), reduced side lighting, brighter rear lighting for note-taking. Triggered automatically when the projector or display is switched on.

"Video Conference" scene: Even, glare-free illumination across all occupant positions. Avoids hotspots or shadows that degrade camera image quality. Warm-white (3000K) colour temperature for flattering skin tones.

"Collaboration" scene: Full illumination at 500 lux, neutral-white (4000K). All luminaires at consistent brightness for face-to-face meetings and document review.

"Cleaning" scene: Full illumination at 100%, all zones. Triggered by the cleaning schedule or a dedicated switch, ensuring cleaners have adequate light without needing to operate the main control panel.

Scene programming in DALI is stored in the DALI controller, not in each individual luminaire. Scenes can be recalled by physical button, BMS command, calendar schedule, or AV system integration. For Bangladesh commercial fit-outs, integrating lighting scenes with AV control systems (via Crestron, AMX, or similar platforms) creates a seamless experience that tenants and their clients appreciate.

LED Fixture Considerations for Dimming Compatibility

Not all LED luminaires dim equally well. Before specifying lighting control, verify the dimming compatibility of every LED fixture in the project. Incompatible fixtures will cause problems — flickering at low levels, minimum dimming levels that are too high, audible buzzing from drivers, or control system instability.

Key specifications to verify:

Driver compatibility: LED drivers (power supplies) must be specified to match the dimming protocol. For DALI dimming, the driver must have a DALI interface. For 0-10V, the driver must have 0-10V input terminals. Not all LED drivers support both. Verify the exact driver model — not just the fixture model — against the control system specification.

Minimum dimming level: Many LED drivers claim "dimmable to 1%" but in practice flicker becomes visible below 5–10%. Specify the minimum dimming level required (typically 5% for meeting rooms, 1% for theatrical and hospitality applications) and verify that the selected driver achieves this without visible flicker under IEC 61547 measurement criteria.

Colour temperature consistency at dim levels: Some LED sources shift colour temperature as they dim — appearing warmer (lower Kelvin) at lower levels. For office environments where colour rendering consistency is important, specify drivers with constant colour temperature across the dimming range, or use tunable-white (dual-channel) LED sources.

Inrush current: Large LED installations with many dimmer-controlled fixtures can produce significant inrush current at switch-on. Verify that the relay switches and circuit breakers are rated for LED inrush, which can be 10–20 times the steady-state running current.

For Bangladesh projects, ensure that selected LED luminaires carry CE or UL certification and that drivers are sourced from reputable manufacturers — Tridonic, Osram, Inventronics, or equivalent. Uncertified LED drivers are common in the Bangladesh market and may fail prematurely or perform inconsistently with control systems.

Energy Savings Summary: 40-60% of Lighting Energy

Combining occupancy-based switching (auto-off), daylight harvesting (perimeter dimming), and scene control (demand-appropriate levels), a well-designed lighting control system consistently achieves 40–60% reductions in lighting energy consumption:

• Occupancy sensing alone: 20–30% saving (eliminating unused-space waste)
• Daylight harvesting alone: 20–35% saving in perimeter zones
• Scene control alone: 10–20% saving (demand-appropriate levels vs. full-on defaults)
• Combined: 40–60% total lighting energy reduction

For a 5,000 m² commercial office with 15W/m² installed lighting power (typical LED specification), annual lighting consumption without control: approximately 150,000 kWh. With a 50% saving from full lighting control: 75,000 kWh saved per year. At BDT 11 per kWh: BDT 825,000 annual saving. Lighting control system investment for 5,000 m²: approximately BDT 25–40 lakh. Payback: 3–4.8 years.

Designing Your Lighting Control System

A successful lighting control project starts with a zone plan — defining which luminaires are controlled together, what the target illuminance is for each zone, what sensors and control interfaces are needed, and how the system integrates with the BMS. This design work requires a qualified lighting control engineer with experience in DALI programming and sensor placement.

SSAL's building automation team designs and commissions DALI lighting control systems for offices, hospitals, educational institutions, and industrial facilities across Bangladesh. We work with leading DALI controllers and integrate with BMS platforms from Schneider Electric, Siemens, and Honeywell.

Get Free Consultation — Contact our lighting control specialists to discuss your project. We will review your floor plans, model the energy savings, and propose a lighting control design that delivers measurable results from day one.