Open-area beam smoke detection (OSID)

Open-area imaging beam detection protects large, tall spaces by watching light beams sent across the space from small emitters to an imager, which is a camera-like receiver. When smoke rises through a beam, it dims the light in a way the imager can recognise, and the system raises an alarm. It suits warehouses, atriums, arenas and halls where ceiling detectors would be too high, too many or too hard to maintain.

OSID is a trademark of its owner, Xtralis, now part of Honeywell, and is the best-known example of this approach. This guide describes the technology in general terms. Details such as range, number of emitters per imager and mounting rules depend on the specific product and its listing.

Why large open spaces are hard

Spot smoke detectors work well under ordinary ceilings. In a space with a very high roof, three things get in the way. Smoke from a small fire may cool and stop rising before it reaches the roof. A large roof needs many detectors, each of which has to be reached by a lift for testing. And in spaces such as warehouses and arenas, the roof may be crowded with structure, lighting and ductwork.

Beam detection solves part of this by watching a line across the space instead of a single point. Traditional projected beam detectors do this with one transmitter and one receiver, or a transmitter-receiver bouncing light off a reflector. They work, but they need careful alignment, and building movement from wind, temperature and snow load can knock them out of line over time.

How imaging beam detection works

An imaging system replaces the single receiver with an imager: a wide-angle optical sensor that sees a large part of the space at once. Emitters mounted around the space send short pulses of light toward it. The imager picks out each emitter's pulses within its field of view and measures how much light arrives.

Because the imager sees a wide area rather than a single point, alignment is much less critical. If the building moves slightly, the emitter simply appears in a slightly different spot in the image. And because one imager can watch several emitters, a single imager can cover a large area with beams fanning out in different directions.

Telling smoke from dust and objects

A beam detector that only measured how much light arrived could not tell smoke from a bird, a forklift mast, a hanging banner or a cloud of dust. Imaging systems deal with this by sending light at two wavelengths, ultraviolet and infrared, and comparing what arrives.

• Smoke particles are small and affect the shorter ultraviolet light more than the longer infrared light. The two signals drop by different amounts, which is the signature of smoke.
• Dust and steam are made of larger particles that dim both wavelengths about equally, so the system does not treat them as smoke.
• Solid objects block both wavelengths completely. The system reports a fault, prompting someone to look, rather than a fire alarm.

Beam height and stratification

In tall spaces, the warmest air collects under the roof. Smoke from a small or early fire may cool as it rises and stop at a level below the roof where its temperature matches the surrounding air. This is called stratification, and it means a beam placed right under the roof may never see the smoke.

Designers deal with this by placing beams at more than one height, or at a height chosen from the space's expected temperature profile, following the product's listing and CAN/ULC-S524. In atriums, beams across the open well at several levels are common.

Design considerations

• Stable mounting. Emitters and imagers go on solid structure such as walls and columns, not on light framing, ducts or anything that sways.
• Clear lines of sight. Check for overhead cranes, doors that roll up into the beam, signage, lighting, seasonal decorations and stacked goods that grow taller over time.
• Light conditions. Strong direct sunlight into the imager and reflective surfaces can cause problems; the layout should avoid pointing the imager at skylights or windows where possible.
• Power. Some emitters run on long-life batteries, which makes installation easier but adds battery replacement to maintenance. Others are wired.
• Zoning. Decide how beams map to fire alarm zones so responders can find the alarm location quickly.
• Access. Even though beams reduce the number of devices, the imager and emitters still need cleaning and testing, so plan how a technician will reach them.

A worked example

Consider a typical case: a distribution warehouse with a high roof, long rows of pallet racking and an existing fire alarm system that uses spot detectors under the roof deck. The owner reports that testing the roof detectors means closing aisles and bringing in a boom lift, and that several detectors sit above racking where they are almost impossible to reach.

A review for imaging beam detection would usually work through these steps:

1. Confirm the requirement. Check what the building and fire codes and the fire alarm design require in this space, and whether the insurer has its own expectations.
2. Survey the structure. Identify walls and columns stiff enough to carry imagers and emitters, and note cranes, roll-up doors, lighting and heaters that could cross the beams.
3. Choose beam heights. Consider the roof height, heating arrangement and racking height, and decide whether one level of beams is enough or two are needed.
4. Lay out coverage. Place imagers and emitters so the beams cover the floor area within the product's listed spacing, and map them to fire alarm zones.
5. Plan the integration. Confirm the fire alarm control unit has capacity, and plan verification of the changes to CAN/ULC-S537.

The outcome is fewer devices at roof level, testing that can be done with far less disruption, and detection better suited to how smoke actually behaves in the space.

Testing and maintenance

Beam detectors are part of the fire alarm system, so they are inspected and tested under CAN/ULC-S536 as your fire code requires, by qualified technicians, and verified to CAN/ULC-S537 after installation or changes. Testing usually uses test filters or the method the manufacturer specifies to confirm each beam can produce an alarm and a fault. Routine maintenance includes cleaning the optical windows, checking battery status where emitters are battery powered, and confirming nothing new has appeared in the beam paths.

How ehZee can help

Our VESDA and OSID smoke detection service lays out imagers and emitters for large spaces, checks lines of sight and beam heights, and integrates the system with your fire alarm. See also where very early smoke detection makes sense and fire alarm devices explained.

Common questions