Which Photoelectric Sensor Should I Be Using?

There are many variations within the category of photoelectric sensors, so how do you select the best sensor for your application? Below, I will discuss the benefits of different types of photoelectric sensors and sensing modes.

Through Beam

Through beam sensors consist of an emitter and a receiver. The emitter produces a beam of light, while the receiver identifies whether that light is present or not. So, when an object breaks the beam, an output is triggered by the receiver. Some of the advantages of using the simple through beam technology is that, unlike some of the other photoelectric sensors, it doesn’t matter the color, texture or transparency of your target.


What if you would like to have a through beam sensor, but don’t have enough room for two sensor heads in your application? Retroreflective sensors have an emitter and receiver within one housing and use a high-quality reflector to reflect the light beam back to the sensor head. This allows for easy connection of just one sensor head, but it doesn’t have the range of your typical through beam sensor. When using these types of sensors, you must factor in how small or reflective your target material is. If you are trying to sense a highly reflective material, then the light reflected back to the receiver could cause the sensor to think an object is present. If you are having these problems, but still want to use a retroreflective sensor, then you should consider versions with a polarizing lens. These lenses make the sensors insensitive to interference with shiny, reflective material.


Fork sensors include the transmitter and receiver in one housing, and they are already aligned. This saves time and energy during set up. Fork sensors are fantastic for small component and detail detection.


If you don’t have room for a sensor head on each side of your application or even a reflector, or you have had trouble with the alignment of a retroreflective sensor, a diffuse sensor may be a good choice. Diffuse sensors use technology to be able reflect light off the material and back to the sensor. This eliminates the need for a second device or reflector. This significantly reduces set up. You can simply place your target material in front of the sensor and teach it to that point. Once your object reaches that point, the light will be reflected back to the sensor, producing the output. While they are simpler to install, they also have a shorter range compared to through beam sensors and may be affected by your material’s color or the reflectivity or your background… Unless, you have a diffuse sensor with background suppression.

Background Suppression

Diffuse sensors have an emitter and receiver in one housing. In diffuse sensors with background suppression, the emitter and receiver are at a fixed angle so that they intersect at the position of your target material. This will help narrow the operating area (area in which your target material will be entering) and not let reflective material in the background have an influence in your detection.


Photoelectric sensors are simple to use when you need non-contact detection of a material’s presence, color, distance, size or shape, and with their various types, housing and sizes, you can find one that is ideal for your application.

Photoelectric Methods of Operation

Photoelectric sensors vary in their operating principles and can be used in a variety of ways, depending on the application. They can be used to detect whether an object is present, determine its position, measure level, and more. With so many types, it can be hard to narrow down the right sensor for your application while accounting for any environmental conditions. Below will give a brief overview of the different operating principles used in photoelectric sensors and where they can be best used.


Diffuse sensors are the most basic type of photoelectric sensor as they only require the sensor and the object being detected. The sensor has a built-in emitter and receiver, so as light is sent out from the emitter and reaches an object, the light will then bounce off the object and enter the receiver. This sends a discrete signal that an object is within the sensing range. Due to the reflectivity being target-dependent, diffuse sensors have the shortest range of the three main discrete operating principles. Background suppression sensors work under the same principle but can be taught to ignore objects in the background using triangulation to ensure any light beyond a certain angle does not trigger an output. While diffuse sensors can be affected by the color of the target object,  the use of a background suppression sensor can limit the effect color has on reliability. Foreground suppression sensors work in the same manner as background suppression but will ignore anything in the foreground of the taught distance.



Retro-reflective sensors also have the emitter and receiver in a single housing but require a reflector or reflective tape be mounted opposite the sensor for it to be triggered by the received light. As an object passes in front of the reflector, the sensor no longer receives the light back, thus triggering an output. Due to the nature of the reflector, these sensors can operate over much larger distances than a diffuse sensor. These sensors come with non-polarized or polarizing filters. The polarizing filter allows for the sensor to detect shiny objects and not see it as a reflector and prevents any stray ambient light from triggering the sensor.



Through-beam sensors have a separate body for the emitter and receiver and are placed opposite each other. The output is triggered once the beam has been broken. Due to the separate emitter and receiver, the sensor can operate at the longest range of the aforementioned types. At these long ranges and depending on the light type used, the emitter and receiver can be troublesome to set up compared to the diffuse and retro-reflective.



The previous three types of photoelectric sensors give discrete outputs stating whether an object is present or not. With photoelectric distance sensors, you can get a continuous readout on the position of the object being measured. There are two main ways the distance of the object is measured, time of flight, which calculates how long it takes the light to return to the receiver, and triangulation, which uses the angle of the incoming reflected light to determine distance. Triangulation is the more accurate option, but time of flight can be more cost-effective while still providing good accuracy.

Light type and environment

With each operating principle, there are three light types used in photoelectric sensors: red light, laser red light, and infrared. Depending on the environmental conditions and application, certain light types will fare better than others. Red light is the standard light type and can be used in most applications. Laser red light is used for more precise detection as it has a smaller light spot. Infrared is used in lower-visibility environments as it can pass through more dirt and dust than the other two types. Although infrared can work better in these dirtier environments, photoelectric sensors should mainly be used where build-up is less likely. Mounting should also be considered as these sensors are usually not as heavy duty as some proximity switches and break/fail more easily.

As you can see, photoelectric sensors have many different methods of operation and flexibility with light type to help in a wide range of applications. When considering using these sensors, it is important to account for the environmental conditions surrounding the sensor, as well as mounting restrictions/positioning, when choosing which is right for your application.

For more information on photoelectric sensors, visit this page for more information.

Top 5 Insights from 2019

With a new year comes new innovation and insights. Before we jump into new topics for 2020, let’s not forget some of the hottest topics from last year. Below are the five most popular blogs from our site in 2019.

1. How to Select the Best Lighting Techniques for Your Machine Vision Application

How to select the best vision_LI.jpgThe key to deploying a robust machine vision application in a factory automation setting is ensuring that you create the necessary environment for a stable image.  The three areas you must focus on to ensure image stability are: lighting, lensing and material handling.  For this blog, I will focus on the seven main lighting techniques that are used in machine vision applications.


2. M12 Connector Coding

blog 7.10_LI.jpgNew automation products hit the market every day and each device requires the correct cable to operate. Even in standard cables sizes, there are a variety of connector types that correspond with different applications.


3. When to use optical filtering in a machine vision application

blog 7.3_LI.jpgIndustrial image processing is essentially a requirement in modern manufacturing. Vision solutions can deliver visual quality control, identification and positioning. While vision systems have gotten easier to install and use, there isn’t a one-size-fits-all solution. Knowing how and when you should use optical filtering in a machine vision application is a vital part of making sure your system delivers everything you need.


4. The Difference Between Intrinsically Safe and Explosion Proof

5.14_LIThe difference between a product being ‘explosion proof’ and ‘intrinsically safe’ can be confusing but it is vital to select the proper one for your application. Both approvals are meant to prevent a potential electrical equipment malfunction from initiating an explosion or ignition through gases that may be present in the surrounding area. This is accomplished in both cases by keeping the potential energy level below what is necessary to start ignition process in an open atmosphere.


5. Smart choices deliver leaner processes in Packaging, Food and Beverage industry

Smart choices deliver leaner processes in PFB_LI.jpgIn all industries, there is a need for more flexible and individualized production as well as increased transparency and documentable processes. Overall equipment efficiency, zero downtime and the demand for shorter production runs have created the need for smart machines and ultimately the smart factory. Now more than ever, this is important in the Packaging, Food and Beverage (PFB) industry to ensure that the products and processes are clean, safe and efficient.


We appreciate your dedication to Automation Insights in 2019 and look forward to growth and innovation in 2020!



Mini Sensors Add Big Capabilities to Life Science Applications

1Miniaturization is one of the essential requirements for medical instruments and laboratory equipment used in the life science industry. As instruments get smaller and smaller, the sensor components must also become smaller, lighter and more flexible. The photoelectric sensors that were commonly used in general automation and applied in life science applications have met their limitations in size and performance.

2.jpgSensors used in these complex applications require numerous special characteristics such as high-quality optics, unique housing designs, precise LEDs with the best suited wavelength and the ability to be extremely flexible to fit in the extremely small space available. Sensors have been developed to meet the smallest possible installation footprint with the highest optical precision and enough flexibility to be installed where they are needed. These use integrated micro-precision optics that shape and focus the light beam exactly on the object without any undesirable side-effects to achieve the reliability demanded in today’s applications.

Previously many life science applications used conventional plastic fiber optic cables that were often too large and not flexible enough to be routed through the instruments. An alternative to the classic fiber cables is a “wired” fiber with precision micro-optics and extremely flexible cables with essentially no minimum bending radius and no significant coupling losses. Similar to a conventional fiber optic sensor, an external amplifier is required to provide a wide variety of functionalities to solve the demanding applications.

These sensors can be used in applications such as:

  • Precise detection of liquid levels using either attenuation or refraction with a small footprint
  • Reliable detection of transparent objects such as microscope slides or coverslips having various edge shapes
  • Detection of transparent liquids in micro-channels or capillaries
  • Reliable detection of individual droplets
  • Recognition of free-floating micro-bubbles in a tube that are smaller than the tube diameter and that cannot be seen by the human eye
  • Recognition of macro-bubbles that are the diameter of small tubes

For more information on photoelectric sensors that have the capability to meet the demands of today’s life science applications visit www.balluff.com.

Make Clear Water Visible to Your Sensors

In some industries such as life sciences it is necessary to detect clear water or clear liquids in a container or tube. This is even more challenging when the diameters of the tube are small, and the tube thickness is nearly as large as the stream of liquid.

The attenuation or gradual reduction of the intensity of the light beam in water and air can be directly compared. The attenuation of light in water can be attributed to light entering water at any angle other than at a right angle and can be refracted. The measurement of light through a tube is different because not only is the light attenuated by the liquid, but depending on where the light passes through the tube it can be refracted, diverted and or focused. As a result, the signal differences can be low.

Attenuation is typically the first choice if the liquids are opaque or colored. The requirements of the shape of the light beam and the alignment of the sensor add more complication to the application. The attenuation effect appears weaker in clear liquids. The principle does not work with reflective sensors since reflection is a surface effect and the light must pass through the liquid.OPTO_appl_08_sw-water

From spectral analysis it’s has been determined that the attenuation characteristics of water are heavily dependent on the wavelength of the light that is conducted through it. Sensors were developed for such applications. Typically, these sensors utilize LED’s in the upper infrared range of 1,450 nm. At this wavelength water literally absorbs the light and becomes opaque making detection more simplified and reliable.

This principle even works for fine capillaries and microchannels. Liquid detection can be very precise depending on the sensor size and the effective light beam. Light beams as small as 0.4mm can provide high resolution for small thin tubes typically found in microfluidics applications.

Versions of these sensors exist for applications that involve less transparent or semi-transparent vessels. Light at the 1450nm wavelength can pass through these containers or tubes and can be attenuated by the water. The main factor is that enough light makes its way through the walls of the container.

Through-beam sensors were developed for applications such as detecting clear liquids. These sensors are also available in extremely small dimensions and usually require an amplifier, or they can be supplied in a rugged fork sensor housing. The required sensor dimensions conform to the geometry of the vessel or container.

For more information on sensors for these types of applications contact your local Balluff representative or contact us at www.balluff.com.

Using Photoelectric Sensors in High Ambient Temperatures

Photoelectric sensors with laser and red-light are widely used in all areas of industrial automation. A clean, dust-free and dry environment is usually essential for the proper operation of photoeyes, however, they can be the best choice in many dirty and harsh applications. Examples of this are raw steel production in steel mills and further metallurgical processes down to casting and hot-rolling.

Cutting of billets at casting – Photo: M.Münzl
Cutting of billets at casting – Photo: M.Münzl

Photoelectric sensors are especially useful in these environments thanks to their long sensing distance and their ability to detect objects independent of their material.

Most photoelectric sensors are approved to work in ambient temperatures of 55 to 60 °C. The maximum temperature range of these sensors is most often limited by the specifications of the optical components of the sensor, like the laser-diodes, but by taking certain precautions photoelectric sensors can provide optimal use in much hotter applications.

Maximize the distance
In steel production many parts of the process are accompanied by high ambient temperatures. Liquid steel and iron have temperatures from 1400 to 1536 °C. Material temperature during continuous casting and hot-rolling are lower but still between 650 and 1250°C.

The impact of heat emission on the sensors can be reduced significantly by placing the sensor as far from the target object as possible, something you can’t do with inductive sensors which have a short range. Very often the remote mounting will allow the sensor to operate at room temperature.

If you intend to detect quite small objects with high precision, the maximum distance for the installation might be limited. For this purpose chemical resistant glass fibers are suitable and can handle temperatures up to 250 °C. These pre-fabricated fiber optic assemblies can be easily attached to the sensor. The sensor itself can be mounted on a cooler and protected place.

Detect Glowing Metals
If you want to reliably detect red-hot or white glowing steel parts with temperatures beyond 700 °C, you won’t be able to use standard laser or red-light sensors. Red-hot steel emits light at the same wavelength that it is used by photoelectric sensors. This can interfere with the function of the sensor. In such applications you need to use sensors which operate based on infrared light.

Add Protection

Sensor enclosure and protective cable sleeve
Sensor enclosure and protective cable sleeve

At many locations in the steel production process, the extensive heat is only temporary. In a hot rolling mill, a slab runs through a rougher mill multiple times before it continues to a multi-stage finishing mill stand to be rolled to the final thickness. After that the metal strip runs into the coiler to be winded up.
This process runs in sequence, and the glowing material is only present at each stage of production for a short time. Until a new slab runs out of the reheating furnace, temperatures normalize.

Standard sensors can work in these conditions, but you do run the risk of even temporary temperature hikes causing sensor failure and then dreaded downtime. To protect photoelectric sensors against temporary overheating, you can use a protective enclosure. These can provide mechanical and thermal protection to the sensors which often have plastic bodies. Additional protection can be achieved when a heat resistant sleeve is used around the cable.

Photoelectric sensors do have their limits and are not suitable for all applications, even when precautions are taken. Ask yourself these questions when deciding if they can be the right solution for your high temperature applications.

  • Which distance between the hot object and sensor can be realized?
  • What is the maximum temperature at this location?
  • How long will the sensor be exposed to the highest heat levels during normal operation and at breakdown?

Polarized Retroreflective Sensors: A Solution for Detecting Highly Reflective Objects

The complexity of factory automation creates constant challenges which drive innovation in the industry. One of these challenges involves the ability to accurately detect the presence of shiny or highly reflective objects. This is a common challenge faced in a variety of applications, from sensing wheels in an automotive facility to detecting an aluminum can for filling purposes at a beverage plant. However, thanks to advancements in photoelectric sensing technologies, there is a reliable solution for those type of applications.

Why are highly reflective objects a challenge?

Light reflects from these types of objects in different directions, and with minimum energy loss. This can cause the receiver of a photoelectric sensor to be unable to differentiate between a signal received from the emitter or a signal received from a shiny object. In the case of a diffuse sensor, there is also the possibility that when trying to detect a shiny object, the light will reflect away from the receiver causing the sensor to ignore the target.

So how do we control the direction of the light going back to the receiver, and avoid false triggering from other light sources? The answer is in polarized retroreflective sensors.

Retroreflective sensors require a reflector which reflects the light back to the sensor allowing it to be captured by the receiver. This is achieved by incorporating sets of three mirrors oriented at right angles from each other (referred to as corner cubes). A light beam entering this system is reflected by all three surfaces and exits parallel to the incident beam. Additionally, corner cubes are said to be optically active as they rotate the plane of oscillation of the light by 90 degrees. This concept, along with polarization, allow this type of sensor to accurately detect shiny objects.


Light emitted by a regular light source oscillates in planes on dispersal axes. If the light meets a polarizing filter (fine line grid), only the light oscillating parallel to the grid is let through (see figure 1 below).

Figure 1_AR
Figure 1

In polarized retroreflective sensors, a horizontal polarized filter is placed in front of the emitter and a vertical one in front of the receiver. By doing this, the transmitted light oscillates horizontally until it hits the reflector. The corner cubes of the reflector would then rotate the polarization direction by 90 degrees and reflect the light back to the sensor. This way, the returning light can pass through the vertical polarized filter on the receiver as shown below.

Figure 2_AR
Figure 2

With the use of polarization and corner cubed reflectors, retroreflective sensors can create a closed light circuit which ensures that light detected by the receiver was sourced exclusively by the emitter. This creates a great solution for applications where highly reflective targets are influencing the accuracy of sensors or causing them to malfunction. By ensuring proper operation of photoelectric sensors, unplanned downtime can be avoided, and overall process efficiency can be improved.

Measuring Distance: Should I Use Light or Sound?

Clear or transparent sensing targets can be a challenge but not an insurmountable one. Applications can detect or measure the amount of clear or transparent film on a roll or the level of a clear or transparent media, either liquid or solid.  The question for these applications becomes, do I use light or sound as a solution?

An application that measures the diameter of a roll of clear labels.

In an application that requires the measurement of the diameter of a roll of clear labels, there are a number of factors that need to be considered.  Are the labels and the backing clear?  Will the label transparency and the background transparency change?  Will the labels have printing on them?  All of these possibilities will affect which sensor should be used. Users should also ask how accurate or how much resolution is required.

Faced with this application, using ultrasonic sensors may be a better choice because of their ability to see targets regardless of color, possible printing on the label, transparency and surface texture or sheen.  Some or all of these variables could affect the performance of a photoelectric sensor.

Ultrasonic sensors emit a burst of short high frequency sound waves that propagate in a cone shape towards the target.  When the sound waves strike the target, they are bounced back to the sensor. The sensor then calculates the distance based on the time span from when the sound was emitted until the sound was received.

In some instances, and depending on the resolution required, a time of flight sensor may solve the above application. Time of Flight (TOF) sensors emit a pulsed light toward the target object. The light is then reflected back to the receiver. The elapsed time it takes for the light to return to the receiver is measured, thus determining the distance to the target. In this case, the surface finish and transparency may not be an issue.

Imagine trying to detect a clear piece of plastic going over a roll.  The photoelectric sensor could detect it either in a diffuse mode or with a retroreflective sensor designed for clear glass detection.  But what if the plastic characteristics can change frequently or if the surface flutters.  Again, the ultrasonic sensor may be a better choice and also may not require set up any time the material changes.

So what’s the best solution?  In the end, test the application with the worst case scenario.  A wide variety of sensors are available to solve these difficult applications, including photoelectric or ultrasonic. Both sensors have continuous analog and discrete outs.  For more information visit www.balluff.com.


Technological Alternative to Fiber Optics

Photoelectric applications with space restrictions, small part detection, high temperatures, or aggressive harsh environments may be solved using fiber optic sensors. These sensors allow the electronics to be mounted out of harm’s way while at the same time focusing the light beam on a small target. The sensing tips can be manufactured in a wide variety of housings for unique mounting requirements.

Fiber optic sensors require two components: a remote mounted amplifier, and the fiber optic cable(s). The amplifiers can be basic, with few features, or advanced with many configurable options and digital displays. The fiber optic cables are made of either plastic or glass fibers, each with advantages and application specific solutions.

Many applications, primarily those in the medical Technological Alternative to Fiber Optics 1sciences and semiconductor industries, cannot be solved with fiber optic or miniature photoelectric sensors because they are physically too large to fit in the instruments. Additionally, the cables are typically not flexible enough to be routed through the instruments.  Today, highly flexible and miniature sensors are are being incorporated in other industries due to today’s demands of smaller machines and tools.

MICROmote® sensors are miniaturized photoelectric Technological Alternative to Fiber Optics 2sensors with separate amplifiers that are also available with a variety of functionalities. Their highly flexible, electric sensor cables make them a genuine technical alternative to conventional fiber optics. The photoelectric sensor heads have extraordinarily small dimensions, excellent technical characteristics, and outstanding flexibility for application-specific solutions.

Similar to fiber optic sensors, these micro-optic photoelectric sensors function as either a through-beam or diffuse type sensor with comparable sensing ranges. Unlike fibers, the wired sensing heads are inherently bifurcated type cables so that there is only one connection to the amplifier.

Unlike conventional fiber optic cables,Technological Alternative to Fiber Optics 3 there are no significant coupling losses, minimum bending radius and cyclic bending stresses.  The patented precision elements produce extremely small beam angles with sharply defined light spots unlike standard fiber optics where the beam angle is a function of the fiber geometry.  Additional lenses must be used if the light beam of a fiber optic cable must be focused which adds to the costs.

MICROmote® photoelectric sensors for water detection use a specific wavelength at which water absorbs more light. This significantly simplifies the detection of liquids with high water content using optical sensors. The combination of an ultra-compact design and powerful micro-optics allows for reliable use in capillary tubes where other sensing devices are stretched to their limits.

These sensors can also be used as precision tube Technological Alternative to Fiber Optics 4sensors for detecting bubbles through use of either light refraction or attenuation through the air, or liquid column within the tube. They provide excellent detection for even the smallest air-to-liquid transitions and are reliable for all liquid types, even clear liquids.

In addition, these sensorsTechnological Alternative to Fiber Optics 5 are designed to detect free-floating microbubbles in transparent liquids. Microbubbles refer to little gas bubbles with dimensions smaller than the inside diameter of the tube. Uniform lighting is achieved in the liquid column by using a concentrated arrangement of multiple light beams with very uniform intensity distribution. Gas bubbles that move through this field induce a signal jump in the built-in photoelectric receiver elements

For more information on this technological alternative to fiber optics visit www.balluff.com.

Back to the Basics: Object Detection

In the last post about the Basics of Automation, we discussed how humans act as a paradigm for automation. Now, let’s take a closer look at how objects can be detected, collected and positioned with the help of sensors.

Sensors can detect various materials such as metals, non-metals, solids and liquids, all completely without contact. You can use magnetic fields, light and sound to do this. The type of material you are trying to detect will determine the type of sensor technology that you will use.

Object Detection 1

Types of Sensors

  • Inductive sensors for detecting any metallic object at close range
  • Capacitive sensors for detecting the presence of level of almost any material and liquid at close range
  • Photoelectric sensors such as diffuse, retro-reflective or through-beam detect virtually any object over greater distances
  • Ultrasonic sensors for detecting virtually any object over greater distances

Different Sensors for Different Applications

The different types of sensors used will depend on the type of application. For example, you will use different sensors for metal detection, non-metal detection, magnet detection, and level detection.

Detecting Metals

If a workpiece or similar metallic objects Object Detection 2should be detected, then an inductive sensor is the best solution. Inductive sensors easily detect workpiece carriers at close range. If a workpiece is missing it will be reliably detected. Photoelectric sensors detect small objects, for example, steel springs as they are brought in for processing. Thus ensures a correct installation and assists in process continuity. These sensors also stand out with their long ranges.

Detecting Non-Metals

If you are trying to detect non-metal objects, for example, the height of paper stacks, Object Detection 3then capacitive sensors are the right choice. They will ensure that the printing process runs smoothly and they prevent transport backups. If you are checking the presence of photovoltaic cells or similar objects as they are brought in for processing, then photoelectic sensors would be the correct choice for the application.

Detecting Magnets

Object Detection 4

To make sure that blister packs are exactly positioned in boxes or that improperly packaged matches are sorted out, a magnetic field sensor is needed which is integrated into the slot. It detects the opening condition of a gripper, or the position of a pneumatic ejector.


Level Detection

What if you need to detect the level of granulate in containers? Then the solution is to use capacitive sensors. To accomplish this, two sensors are attached in the containers, offset from each other. A signal is generated when the minimum or maximum level is exceeded. This prevents over-filling or the level falling below a set amount. However, if you would like to detect the precise fill height of a tank without contact, then the solution would be to use an ultrasonic sensor.

Stay tuned for future posts that will cover the essentials of automation. To learn more about the Basics of Automation in the meantime, visit www.balluff.com.