Shedding Light on Different Types of Photoelectric Sensors

Photoelectric sensors have been around for more than 50 years and are used in everyday things – from garage door openers to highly automated assembly lines that produce the food we eat and the cars we drive.

The correct use of photoelectric sensors in a manufacturing process is important to ensure machines can perform their required actions. Over the years they have evolved into many different forms.

But, how do you know which is the right sensor for your application?  Let’s take a quick look at the different types and why you would choose one over another for your needs.

Diffuse sensors

    • Ideal for detecting contrast differences, depending on the surface, color, and material
    • Detects in Light-On or Dark-On mode, depending on the target
    • Economical and easy to mount and align, thanks to visible light beams
    • Shorter ranges as compared to retroreflective and through-beam sensors
    • IR (Infrared) light beams available for better detection in harsh environments
    • Laser light versions are available for more precise detection when needed
    • Mounting includes only one electrical device

Diffuse sensor with background suppression

    • Reliable object detection with various operating ranges, and independent of surface, color, and material
    • Detects objects against very similar backgrounds – even if they are very dark against a bright background
    • Almost constant scanning range even with different reflectance
    • Only one electrical device without reflectors or separate receivers
    • Good option if you cannot use a through-beam or retroreflective sensor
    • With red light or the laser red light that is ideally suited for detecting small parts

Retroreflective sensors

    • Simple alignment thanks to generous mounting tolerances
    • Large reflectors for longer ranges
    • Reliable detection, regardless of surface, color, and material
    • Polarized light filters are available to assist with detecting shiny objects
    • Mounting includes only one electrical device, plus a reflector
    • Most repeatable sensor for clear object detection; light passes through clear target 2X’s giving a greater change in light received by the sensor

Through-beam sensors

    • Ideal for positioning tasks, thanks to excellent reproducibility
    • Most reliable detection method for objects, especially on conveyor applications
    • Extremely resistant to contamination and suitable for harsh environments
    • Ideally suited for large operating ranges
    • Transmitter and receiver in separate housings

Fork sensors

    • Different light types (red light, infrared, laser)
    • Robust metal housing
    • Simple alignment to the object
    • High optical resolution and reproducibility
    • Fork widths in different sizes with standardized mounting holes
    • Identical mechanical and optical axes
    • The transmitter and receiver are firmly aligned to each other, yielding high process reliability

The next time you need to choose a photoelectric sensor for your manufacturing process, consider these features of each type to ensure the sensor is performing optimally in your application.

The Foundation of Photoelectric Sensors

PhotoelectricsThe foundation of a photoelectric sensor is light!  Without the light you have a housing with some electronics in it that makes an interesting object to leave on your desk as a conversation starter.  Is all light the same?  Does the light source really matter?  When do you select one over the other?

Red light or red LED light sources are the most favored as they are easy to set up and confirm that the sensor is working properly since you have a bright light that you can focus on your target.  Depending on the lensing the light spot size can vary from a pin point to a spot that can be several centimeters square or round.  It is important that you aim the sensor correctly if you have the sensor installed near an operator so as the light is not shining in their eyes as it can be rather irritating.

There are several misconceptions with the laser light.  Many think that lasers are the most powerful light and can penetrate anything.  Also there is the concern that lasers will cause damage to the human eye.  Lasers in photoelectric sensors are typically available as either a Class 1 or Class 2.  Class 1 lasers are safe under normal use conditions and are considered to be incapable of damage.  Class 2 lasers are more powerful, however it is the normal response of human eye to blink which will limit the exposure time and avoid damage.  Class 2 lasers can be hazardous if looked at for extended periods of time.  In either case viewing a laser light with a magnifying optic could cause damage.

Lasers provide a consistent light with a small beam diameter (light spot) that provides a perfect solution for small part detection.  Although the light beam is small and concentrated, it can be easily interrupted by airborne particles.  If there is dust or mist in the environment the light will be scattered making the application less successful than desired.  In some cases the sensing distance will be greater with a laser light than with a red light.

Infrared LED’s will produce an invisible, to the human eye, light while being more efficient and generating the most light with the least amount of heat.  Infrared light sources are perfect for harsh and contaminated environments where there is oil or dust.  Also infrared through-beam sensors are sometimes capable of “seeing through” a package or object which is sometimes preferred to solve an application.  The ability to see though an object or dirt makes this light source perfect in very contaminated environments when the contamination builds up on the lens or reflector.

In all cases LED’s are modulated or turned on and off very rapidly.  This modulation determines the amount of light a photoelectric sensor can create and prolongs the life of the LED.  In addition, the sensor receiver is designed to look for the modulated light at the same frequency to help eliminate ambient light causing the sensors output to false trigger.

We have determined that all light sources are not the same each with their benefits and drawbacks.  Selection of the light source really depends on the application as often red lights have been installed in very contaminated applications that required the power of the infrared.

If you are interested in learning more about the basics of photoelectrics request the Photoelectric Handbook or visit

Back to the Basics – Photoelectric Light Source

Welcome to the first in a series of getting back to the basic blogs about photoelectric sensors.

LightTypeAll photoelectric sensors require a light source to operate. The light source is integral to the sensor and is referred to as the emitter. Some light sources can be seen and may be of different colors or wavelengths for instance red, blue, green, white light or laser or one you cannot see, infrared. Many years ago photoelectric sensors used incandescent lights which were easily damaged by vibration and shock. The sensors that used incandescence were susceptible to ambient light which limited the sensing range and how they were installed.

Today light sources use light emitting diodes (LED’s). LED’s cannot generate the light that the incandescent bulbs could. However since the LED is solid state, it will last for years, is not easily damaged, is sealed, smaller than the incandescent light and can survive a wide temperature range. LED’s are available in three basic versions visible, laser and infrared with each having their advantages.

Visible LED’s which are typically red, aid in the alignment and set up of the sensor since it will provide a visible beam or spot on the target. Visible red LED’s can be bright and should be aimed so that the light will not shine in an operator’s eyes. The other color visible LED’s are used for specific applications such as contrast, luminescence, and color sensors as well as sensor function indication.

Laser LED’s will provide a consistent light color or wavelength, small beam diameter and longer range however these are generally more costly. Lasers are often used for small part detection and precision measuring. Although the light beam is small and concentrated, it can be easily interrupted by airborne particles. If there is dust or mist in the environment the light will be scattered making the application less successful than desired. When a laser is being used for measuring make sure the light beam is larger than any holes or crevasses in the part to ensure the measurement is as accurate as possible. Also it is important to ensure that the laser is installed so that it is not aimed into an operator or passerby’s eyes.

Lastly, the infrared LED will produce an invisible, to the human eye, light while being more efficient and generating the most light with the least amount of heat. Infrared light sources are perfect for harsh and contaminated environments where there is oil or dust. However, with the good comes the bad. Since the light source is infrared and not visible setup and alignment can be challenging.

LED’s have proved to be robust and reliable in photoelectric sensors. In the next installment we will review LED modulation.

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