Presence-Sensing Devices, An Essential Guide

in Machine Safety Blog by

Estimated reading time: 19 minutes

Introduction – What are presence-sensing safety devices?

If you can’t use a permanent barrier for machine safety, or if the situation demands it, you might need a device that can tell when someone is there. These devices, called presence-sensing devices, use electronic sensors to notice if a person or object is in a certain area. Essentially, they can tell if something or someone is present or missing in a specific spot.

These devices may have been viewed as magical wizardry once but are now common and important in industrial safety. We will try to demystify presence-sensing devices for those unfamiliar with them and provide a foundational understanding to assist in making informed decisions regarding their selection.

Did you Know?

Often chosen because they allow for easier access, presence-sensing devices are not foolproof! We recommend using physical safeguarding whenever possible.

Understanding the Basics


Uses infrared light beams or lasers for detection. Examples include light curtains and garage door sensors.


Detects presence using a pressure-sensitive sensor. Examples include safety mats and bumpers.


Radiowaves are used to bounce off objects to detect movement. Examples include radar detectors.

One of the most common types of presence-sensing devices is the photoelectric sensor. These sensors use light beams (visible or infrared) to detect the presence of objects or people. They operate by emitting a beam of light; when the beam is interrupted by an object, the sensor triggers a response such as halting a machine.

Photoelectric beams can be stacked to create a “curtain” of infrared light across an area. These types of stacked photoelectric beams are called safety light curtains, which are a type of presence-sensing device.

Another widely used presence-sensing device is the pressure-sensitive safety mat. When a person steps on a mat, it signals to stop the machine or trigger an alarm. These mats are placed around hazardous areas and are particularly useful where personnel might inadvertently enter a dangerous zone.

These devices are connected to the machine’s control system, signaling to immediately stop any motors and movement. The basic principle is the same regardless of the chosen sensor type; an output is generated to stop a machine or sound an alarm.

The Role in Machine Guarding

Despite their differences, all these devices share a common goal: to enhance safety by detecting the presence of objects or individuals and initiating a response to mitigate potential hazards.

The selection of a presence-sensing device depends on several factors, including the specific safety requirements of the application, the type of machinery involved, the environmental conditions, and the characteristics of the objects or individuals to be detected.

Types of Presence-Sensing Devices

1. Safety light curtains

Imagine watching a movie where the main character tries to steal a valuable painting but must first cross visible laser beams. If anyone or anything tries to sneak past, the laser wall detects them and sets off an alarm. That’s pretty much what safety light curtains do in real life, but they protect people from getting hurt by machines, and unlike the movies, they are invisible!

A safety light curtain comprises a bunch of invisible light beams that create a kind of invisible wall of infrared light. These are set up around dangerous parts of machines, like where something is being cut or pressed. The machine automatically stops when anything interrupts the invisible light beams—like a hand or a tool. This happens super fast, so it can prevent accidents and keep people safe.

Light curtains are like invisible guardians that keep people safe around machines. They detect if someone or something is where it shouldn’t be. If the light curtain notices something wrong, it quickly tells the machine to stop, avoiding potential accidents.

2. Area Laser Scanners

Laser scanners are super intelligent tools that scan areas around them using laser light. They emit laser beams and then measure how long it takes for the light beam to bounce back after hitting an object. This process is called “time of flight” measurement.

The scanner can create a detailed map or 3D model of its surroundings by calculating the time it takes for the laser’s light to return. This is super useful for many things, like making accurate maps, helping robots navigate, or even designing video games.

In industries, these scanners are like the eyes of robots and automated systems, letting them “see” where they’re going or what they’re doing. For example, in a warehouse, laser scanners can help guide automated forklifts safely around obstacles, ensuring they don’t bump into shelves or people. In manufacturing, they can check if a part is made correctly by comparing it to a 3D model.

Laser scanners are also used in safety applications to detect the presence of people inside robotic cells, for example. These laser scanners can be programmed with a laptop to create a virtual “danger” zone, which is the area where the robot arm could hit a person. If a person enters this zone, the laser scanner signals the robot to stop its movement immediately, potentially saving the person from injury.

Presence-sensing laser scanner

3. Safety Radar Detectors

Radar, an acronym for Radio Detection and Ranging, works by sending out radio waves and listening for echoes. When radio waves hit an object, they bounce back to the radar system, which calculates how long it takes for the waves to return to determine the object’s distance.

Radar can also detect the speed and direction of an object by analyzing changes in the frequency of the returned waves. This technology is widely used in various fields, including weather forecasting, air and sea navigation, and the military for detecting aircraft, ships, and vehicles.

Safety radar detectors are advanced devices designed to create invisible safety zones around machinery. They use radar technology to detect the presence of people or objects within these zones.

Electromagnetic waves are highly resistant to disturbances like dust, debris, smoke, fog, or darkness, and they travel at the speed of light, approximately 300,000 km/s. Unlike traditional safety systems, they can operate effectively under different conditions, including dust and debris, which makes them ideal for enhancing safety where traditional sensors might fail. They’re essentially a high-tech way to keep people safe around potentially dangerous equipment.

4. Pressure-sensitive mats and bumpers

Pressure-sensitive mats are like big, flat buttons you can step on. When you stand on them, they can tell you’re there because two conductors are pressed together, completing an electrical circuit. They’re used to keep people safe around machines or in certain areas by sending a signal to stop a machine or set off an alarm if someone steps on them. It’s like having a secret security guard on the floor watching over safe zones or dangerous spots.

Another type of pressure-sensitive device is a bumper. These can be placed along the edges of moving doors or objects to detect if a collision has occurred. For example, a bumper could be placed along the bottom edge of an automatic garage door.

Person walking on a pressure-sensitive safety mat

Key Features to Consider

When selecting presence-sensing devices for machine guarding, it’s important to consider the response time and sensitivity of the device, as these can significantly impact its effectiveness in preventing accidents. Additionally, evaluate the installation and maintenance requirements to ensure the device can be supported appropriately throughout its operational life.

We will use a standard light curtain for clarity in the following examples.


Sensitivity is how well the presence-sensing device can detect things. If it’s super sensitive, it can detect small objects, like a finger. But if the sensitivity is lower, it might only notice bigger stuff, like an arm or leg. Devices such as radar scanners can be so sensitive that they can detect the heartbeat of a human sitting motionless.

You will find that the resolution for a light curtain is the distance between light beams (in millimeters). We can categorize these resolutions into three categories of detection:

ResolutionProtectionDistance between beams
LowArm or Body50mm-90mm

You may be able to place a high-resolution light curtain closer to a hazard, provided that the response time of the light curtain, control system, and machine stop time is faster than the hand movement constant. (Click here to find out about the hand movement constant!)

A lower-resolution light curtain may not even detect a person’s hand reaching in, so it would need to be installed further away.

Response Time

Response time, in essence, refers to the latency between an input or stimulus and its corresponding output or reaction. In the case of light curtains, this stimulus is an object breaking the plane of the light beams, and the reaction is the system’s output to signal that breach.

Imagine you’re in a high-stakes environment like a factory with automated machinery. A light curtain is set up as a safety barrier around a dangerous machine. The light curtain detects this intrusion if something or someone crosses into the machine’s operational zone. The response time is the interval from when the light beam is interrupted to when the machinery is instructed to halt operations, ensuring safety.

A faster response time is analogous to having quicker reflexes. It’s crucial in preventing accidents and ensuring the machinery stops in time to avoid injury. In technical terms, a shorter response time means you can potentially place the light curtain closer to the dangerous area.

Therefore, in safety-critical applications such as with light curtains, the emphasis on minimal response time is akin to enhancing the reflexes of your safety measures. The goal is to ensure that protective actions are initiated instantaneously, minimize the risk of accidents, and improve overall safety protocols in environments where even a fraction of a second can make a significant difference.

Did you Know?

You must consider what is called the safety distance when using presence-sensing devices.

Check out the article here:


Think of a light curtain as an invisible safety net that uses light beams to detect objects crossing its path. Now, the “range” of this light curtain is essentially how far this net can stretch out to catch or detect things. It’s like the Wi-Fi signal in your house. Some routers have a strong signal that can reach the far corners of your home, allowing you to binge-watch your favorite shows from the comfort of your bedroom. Others might have a weaker signal, only covering the living room area, so you lose connection once you step too far away.

With light curtains, the range determines the maximum distance between the transmitter (which sends out the light beams) and the receiver (which catches those beams). If the light curtain has a long range, it can cover larger areas or distances, protecting a broader zone. A long-range light curtain is excellent for securing big machines or wide entry points. On the other hand, a shorter range means it’s more suited for smaller spaces or more precise detection needs, like safeguarding smaller machinery or specific parts of a process.

So, when we talk about the range of a light curtain, we’re basically talking about how big of an area it can effectively monitor and protect with its invisible light beams. The better the range, the larger the area it can keep safe, like how a good Wi-Fi router ensures you stay connected no matter where you are in the house.

Environmental Durability

Environmental durability is all about how well a device can withstand the different conditions in which it’s being used. Think about all the tough stuff in the environment: rain, dust, extreme heat or cold, and even the chance of getting splashed with water or chemicals. Environmental durability means it’s designed to keep working perfectly even when exposed to harsh conditions.

Think of environmental durability like a smartphone case. Some cases are super tough, designed for all sorts of accidents and conditions, protecting your phone no matter what. Similarly, environmental durability in devices like light curtains ensures they keep doing their job, protecting areas or machinery, without failing due to the environment they’re in. It’s about making them tough enough to handle whatever comes their way.

Watertight enclosure for safety light curtains

Dust and Dirt: In a place like a workshop, a lot of dust will fly around. All that dust may clog a light curtain. Therefore, a radar detector may be better than a light curtain in dusty environments because it can “see” through the dust.

Liquids: Whether it’s rain or spills, some devices are built to handle getting wet without short-circuiting or rusting. Some devices have add-on options for polycarbonate covers (see the adjacent image) to protect them from splashing liquids.

Temperature: If a sensor is used outdoors or in factories with extreme temperatures, it must keep working, whether freezing cold or blazing hot.

Chemicals: In industrial areas, harmful chemicals in the air or accidental spills might occur. A durable sensor can resist these chemicals without getting damaged. If the environment is very corrosive, look for plastic or stainless steel materials for the devices.

Selecting the Right Device

Assess Your Needs

Selecting a suitable presence-sensing device involves considering several factors to ensure safety, efficiency, and compatibility with your application. Let’s break down the steps and key considerations:

  1. First Step: Do a risk assessment on the machine you are trying to protect. Is a presence-sensing device necessary? Could personnel safety be achieved by placing physical barriers, guarding, or interlocking doors?
  2. Safety Requirements: Determine the level of safety needed. High-risk applications (like automatic robots) may require devices with higher safety ratings and redundancies, such as light curtains with self-checking features.
  3. Advanced Features: Do you need any type of advanced sensor features such as blanking, floating blanking, or muting? Also, look for nice-to-have features such as alignment indicators on light curtains.
  4. Evaluate the Sensing Area: Measure the area that needs monitoring. Devices like light curtains and laser scanners cover different sizes and shapes of areas. For large or irregular areas, laser scanners might be more adaptable. Identify any potential obstacles that could interfere with the device’s operation and consider how people or machinery will access the area.
  5. Response Time: How quickly does the system need to react? Faster response times are crucial for high-speed machinery. Do a stop-time analysis to figure out how fast the machine can stop. A light curtain is useless if it takes the machine 30 seconds to halt because the hazard is still there long after the signal to stop has been sent.
  6. Resolution: What are you trying to detect? Is it a whole person trying to enter a large area or someone’s hand reaching into a running machine? How close to the hazard do you need to detect presence?
  7. Assess Environmental Durability Requirements: As discussed earlier, the device must withstand its operating environment, whether exposed to dust, water, extreme temperatures, or chemicals. Consider the environment where you will install the device. Is it indoors or outdoors? Are there extreme temperatures, moisture, dust, or chemicals?
  8. Check Compatibility and Integration: Check to make sure that you can easily connect the device to your existing electrical system, including checking for compatible outputs and connectivity options that match your control system requirements.
  9. Budget and Maintenance: Consider the initial cost and the ongoing maintenance expenses. Some devices might be cheaper upfront but require more maintenance or shorter lifespans.
  10. Research and Consult: Each type of device, be it light curtains, pressure mats, laser scanners, or radar detectors, has its strengths and applications where it performs best. Consult with safety engineers or product specialists. They can provide valuable insights based on your specific needs and might suggest solutions you hadn’t considered.

Schedule a call with us today.

  • Discuss your application to figure out if you need a presence-sensing device.
  • Which presence-sensing device will fit your needs?
  • Do an on-site visit and assessment of your machinery.

    Cost Considerations

    Choosing the right presence-sensing device, like a light curtain, pressure mat, laser scanner, or radar detector, involves weighing various cost considerations to ensure you get the best value and effectiveness for your needs. Let’s break it down:

    1. Initial Purchase Price: This is the first and most obvious cost. Different types of devices come with different price tags. Generally, sophisticated technologies like laser scanners or radar detectors might cost more upfront than simpler ones like pressure mats. It’s important to look beyond just this initial cost, though.
    2. Installation Costs: Some devices are plug-and-play, while others may require professional installation, especially if you need to integrate into existing systems. For instance, installing a light curtain around machinery might be more complex and costly than laying down a pressure mat. Environmental installation costs, such as the distance between the electrical panel and sensors, will also increase costs due to the length of the cable and conduit.
    3. Maintenance and Repair Costs: Every device needs maintenance over time to keep it running smoothly. However, due to their complexity or the environment they’re used in, some devices may require more frequent maintenance or have higher repair costs. For example, dust and debris might affect a laser scanner’s accuracy more easily, necessitating regular cleaning and occasional recalibration. Use bollards to prevent accidental impact with forklifts or pallet jacks.
    4. Operational Efficiency: Consider how the device will affect your operation’s efficiency. A device that frequently malfunctions or needs cleaning could lead to downtime or reduced productivity, indirectly increasing costs.
    5. Compatibility and Integration: If the new device needs to work with other systems or machinery, you may need additional equipment or software to integrate with it properly. Presence-sensing devices may often need safety relays or safety PLCs to work correctly.
    6. Safety and Compliance: Incorrectly wired or installed devices can make machines more dangerous. They can cause a false sense of security or allow someone to bypass the device easily. For example, don’t install a light curtain too high. It will allow operators to crawl underneath, negating the light curtain’s purpose. Have the final system checked over by a safety expert to make sure that everything has been installed correctly and according to specifications.

    Safety Standards

    We use these devices to keep people safe. They’re important for safety, so set them up and use them properly by following the appropriate safety standards. Make sure to follow safety rules because they exist for a good reason.

    Look for certifications or compliance with international safety standards such as ISO (International Organization for Standardization) and IEC (International Electrotechnical Commission) or regional standards like OSHA (Occupational Safety and Health Administration) in the United States, CSA in Canada, or CE marking in Europe. These standards ensure the device meets specific safety requirements for design, installation, operation, and maintenance.

    Here are some key standards and considerations:

    1. International Standards for Safety of Machinery

    • ISO 13855: Establishes the positioning of safeguards for the approach speeds of parts of the human body.
    • ISO 13857: Provides safety distances to prevent hazard zones from being reached by the upper and lower limbs.
    • IEC 61496: Specifies general requirements and tests for electro-sensitive protective equipment, specifically light curtains and light grids, regarding functional safety and design.

    2. European Standards (EN)

    • EN 61496: This standard mirrors the IEC 61496 standards but is specifically tailored for the European market. Compliance with these standards is often necessary for obtaining CE marking, which indicates that a product meets EU safety, health, and environmental protection requirements.
    • EN ISO 13855: Provides guidelines similar to ISO 13855 but is adopted explicitly for European conformity.

    3. North American Standards

    • ANSI B11.19: Covers performance criteria for the design, construction, operation, and maintenance of the protective equipment used to safeguard personnel around machinery, including light curtains.
    • ANSI/RIA R15.06: Safety Requirements for Industrial Robots
    • OSHA 29 CFR 1910.212: General requirements for all machines specifying that point of operation guarding must be provided to protect the operator and others from hazards.
    • CSA Z432 is a Canadian standard titled “Safeguarding of Machinery.” It provides guidelines and requirements for the design, manufacture, installation, maintenance, and operation of machinery to protect people from the hazards of interacting with machinery. The primary focus is preventing injuries by ensuring that machinery is safely guarded and adequate safety measures are in place.
    • CSA Z434 Is a Canadian standard titled “Industrial Robots and Robot Systems.” This standard provides guidelines and requirements for designing, integrating, implementing, operating, and maintaining industrial robots and robot systems. Its main goal is to ensure the safety of workers and the general environment around robotic systems in industrial settings.

    4. Product Certifications

    • CE Marking: For Europe, indicating that a product meets EU safety, health, and environmental requirements.
    • TÜV Certification: A globally recognized certification indicating that a product has been tested for safety and meets the requirements of relevant European Directives or Regulations.
    • UL or CSA Listing: In the United States and Canada, a UL or CSA mark signifies that the product meets specific safety standards.

    Ensuring that presence-sensing devices are selected, installed, and maintained according to these standards and considerations is important for the safety of machine operators and other personnel. It not only helps prevent accidents but also ensures compliance with legal and regulatory requirements, thereby protecting organizations from liability and ensuring the safety and well-being of their employees.

    Installation of Presence-Sensing Devices

    1. Ensure you have performed a risk analysis by a professional or engineer and figured out the required performance level (PL). For high-risk applications, you may need to design the safety system to have safety monitoring, fail-safe components, and redundancy.
    2. Make sure you install the sensor following this guide on safety distances:
    3. Get yourself a laser alignment tool for light curtains! They will immensely help you align the sender and receiver, especially if you use mirrors to go around corners.
    4. Ensure you have sufficient cable to run from the control panel to your sensor. These cable runs are not always straight and may need to follow up walls or conduit.
    5. Install junction boxes near the sensor to simplify future sensor and cable replacements.
    6. Ensure that you have everything you need, such as a DC power supply, safety relays, contactors, mounting brackets, cables, conduits, connectors, etc.


    Q: Can presence-sensing devices be used on manufacturing equipment?

    Yes, presence-sensing devices are ideal for safeguarding manufacturing equipment such as press machines, injection molding machines, and robotic assembly lines. They help prevent injuries by detecting when a person or object is too close to dangerous parts of the machinery.

    Q: Are presence-sensing devices suitable for packaging machinery?

    Absolutely. Packaging machinery, including fillers, cappers, and labelers, can be safeguarded with presence-sensing devices. They ensure that the machinery stops or does not start if someone is in a hazardous area.

    Q: Can metalworking machinery be protected using presence-sensing devices?

    No, you should safeguard metalworking machinery like lathes, milling machines, and grinders with physical safeguards to prevent contact with moving parts, flying debris, and coolants.

    Q: I am new to these types of machine-safeguarding devices. Where can I get help?

    We are always here to help! Ferndale Safety can offer you free direct consultation and provide product information, installation guidance, and advice on selecting the right device for your needs. From on-site guarding consultation to installation, we want to ensure your long-term success. Contact us today!

    Further information

    For a more in-depth and technical reference, check out the ReeR Safety Guide here.


    In conclusion, understanding the variety of presence-sensing devices available will help you select the most suitable option for machine guarding. Each type of sensor has its unique advantages and limitations, making it important to carefully assess the specific needs of your operation before deciding. By prioritizing workers’ safety and considering your machinery’s operational requirements, you can choose a presence-sensing device that effectively minimizes risks and enhances safety in the workplace. This foundational knowledge is just the beginning, and further exploration and consultation with safety experts can provide deeper insights into the best practices for selecting and implementing these critical safety measures.