Optical vs Heat vs Multi-Sensor Detectors: What’s the Difference?

Choosing the right fire detector is an important part of designing an effective fire detection system. But with different technologies available, understanding which type of detector is suited to a particular environment is not always straightforward. 

Optical smoke detectors, heat detectors and multi-sensor detectors all identify signs of fire differently. An optical detector responds to smoke particles, a heat detector responds to changes in temperature, while a multi-sensor detector can analyse information from more than one sensing technology. 

Each has advantages in the right application. Understanding how they work, and the differences between them, can help system designers and installers select detection appropriate to the fire risk and surrounding environment.

What is an Optical Smoke Detector? 

An optical smoke detector, sometimes referred to as a photoelectric smoke detector, is designed to detect smoke particles in the air. 

The detector uses an optical sensing system to identify changes caused by smoke entering the sensing area. Apollo optical detection products, for example, use the principle of photoelectric light scattering. 

Because an optical detector is looking for the presence of smoke rather than an increase in temperature, it can identify developing fires before there has necessarily been a significant rise in ambient temperature. 

In simple terms: an optical detector is primarily looking for smoke. 

What is a Heat Detector? 

Unlike an optical smoke detector, a heat detector responds to temperature rather than smoke. 

Depending on the detector and its classification, heat detection can respond when a defined temperature threshold is reached or according to the way temperature rises over time. 

This distinction can make heat detectors suitable for environments where smoke detection could be affected by conditions that occur as part of normal activity, such as dust or steam. 

A kitchen is an easy way to understand the principle. Cooking can routinely produce steam, fumes or airborne particles that might make conventional smoke detection difficult in some circumstances. A correctly selected heat detector responds to temperature instead, although the appropriate solution will always depend on the specific application and applicable fire system design requirements. 

There is an important trade-off to understand, however. Because the detector needs to identify a sufficient change in temperature, heat detection and smoke detection should not be considered interchangeable technologies. 

The detector should be chosen according to the anticipated fire risk, environmental conditions and relevant standards or system requirements. 

In simple terms: a heat detector is primarily looking for heat rather than smoke. 

What is a Multi-Sensor Detector? 

A multi-sensor fire detector combines two or more sensing technologies within a single device. 

An optical/heat multi-sensor, for example, monitors both smoke and temperature. The detector can use information from these sensors to make a more informed assessment of changing conditions. 

Apollo’s Soteria Multi-Sensor Detector (Optical/Heat), for example, combines advanced PureLight optical sensing technology with two thermistors for heat detection. It offers five operating modes designed to support fire detection and false alarm management across different applications. 

Likewise, the Discovery Multi-Sensor Detector (Optical/Heat) combines optical smoke and thermistor temperature sensors, with the signals used together to support improved false alarm management.  

This ability to consider more than one characteristic of a developing fire is one of the key differences between a multi-sensor detector and a detector relying on a single sensing principle. 

Multi-sensor technology can go beyond optical and heat sensing too. For example, the Soteria Tri-Sensor Detector (Dual Optical/Heat/CO) incorporates dual optical sensing, heat detection and a carbon monoxide sensor, providing three methods of detection within the device. 

In simple terms: a multi-sensor detector can look at multiple indicators of fire to make its detection decision. 

Is a Multi-Sensor Detector Better than a Smoke Detector? 

Not necessarily. A multi-sensor detector provides additional information by monitoring more than one fire phenomenon, but detector selection should be based on the needs of the application rather than the number of sensors within a device. 

Where false alarm stimuli are a concern, sophisticated multi-sensor detection can offer a number of advantages, including: 

• Improved false alarm management by evaluating multiple indicators of fire. 

• Better discrimination between genuine fire conditions and transient environmental stimuli. 

• Greater flexibility through different operating modes or sensitivity settings. 

• Reliable detection across a wider range of fire types by combining different sensing technologies. 

In other applications, a dedicated optical or heat detector may be more appropriate. 

The question therefore shouldn’t simply be “Which detector is best?” but “Which detection technology is best suited to this particular risk and environment?” 

When Should You Choose an Optical Detector Instead of a Heat Detector? 

The decision starts with understanding what you are trying to detect and what conditions exist within the space. 

An optical detector may be appropriate where smoke is expected to provide an important early indication of fire and normal environmental conditions allow reliable smoke detection. 

A heat detector may be considered where normal airborne conditions could make smoke detection unsuitable, provided heat detection is appropriate for the fire risk and compliant with the system design requirements. 

Multi-sensor detection can provide another option where analysing more than one fire characteristic can improve the detection decision. 

There are several questions worth considering: 

• What potential fire types exist in the area? 

• Is smoke likely to develop before significant heat? 

• Are steam, dust or other airborne particles present during normal operation? 

• Could normal environmental conditions contribute to unwanted alarms? 

• How quickly does a developing fire need to be identified? 

• What do the applicable fire detection standards and fire risk assessment require? 

These considerations demonstrate why detector selection should form part of the wider fire system design rather than being considered in isolation. 

Which Type of Fire Detector Do I Need? 

There is no single answer that applies to every building. 

Optical smoke detectors detect smoke and can provide an effective solution across a wide range of applications. 

Heat detectors respond to temperature and can be useful where the normal environment makes smoke detection inappropriate. 

Multi-sensor detectors combine sensing technologies, such as optical and heat detection, allowing the device to evaluate multiple characteristics associated with fire. 

Ultimately, detector selection should be based on the fire risk, application, environment, system design and applicable standards. 

For professional fire detection systems, the system should be designed, installed and maintained by appropriately competent professionals in accordance with the relevant requirements. 

Find the right fire detection solution 

From optical and heat detection to advanced multi-sensor technology, Apollo Fire Detectors develops fire detection solutions for a wide variety of applications. 

Explore our products, learn more on our training Hub, or find an Apollo partner to discuss the requirements of your next fire detection system. Apollo Fire Detectors has specialised in manufacturing fire detection and alarm devices since 1980.