How Hot is Hot? – The Basics of Infrared Temperature Sensors

Detecting hot objects in industrial applications can be quite challenging. There are a number of technologies available for these applications depending on the temperatures involved and the accuracy required. In this blog we are going to focus on infrared temperature sensors.

Every object with a temperature above absolute zero (-273.15°C or -459.8°F) emits infrared light in proportion to its temperature. The amount and type of radiation enables the temperature of the object to be determined.

In an infrared temperature sensor a lens focuses the thermal radiation emitted by the object on to an infrared detector. The rays are restricted in the IR temperature sensor by a diaphragm, to create a precise measuring spot on the object. Any false radiation is blocked at the lens by a spectral filter. The infrared detector converts radiation into an electrical signal. This is also proportional to the temperature of the target object and is used for signal processing in a digital processor. This electrical signal is the basis for all functions of the temperature sensor.

There are a number of factors that need to be taken into account when selecting an infrared temperature sensor.

  • What is the temperature range of the application?
    • The temperature range can vary. Balluff’s BTS infrared sensor, for example, has a range of 250°C to 1,250°C or for those Fahrenheit fans 482°F to 2,282° This temperature range covers a majority of heat treating, steel processing, and other industrial applications.
  • What is the size of the object or target?
    • The target must completely fill the light spot or viewing area of the sensor completely to ensure an accurate reading. The resolution of the optics is a relationship to the distance and the diameter of the spot.

  • Is the target moving?
    • One of the major advantages of an infrared temperature sensor is its ability to detect high temperatures of moving objects with fast response times without contact and from safe distances.
  • What type of output is required?
    • Infrared temperature sensors can have both an analog output of 4-20mA to correspond to the temperature and is robust enough to survive industrial applications and longer run lengths. In addition, some sensors also have a programmable digital output for alarms or go no go signals.
    • Smart infrared temperature sensors also have the ability to communicate on networks such as IO-Link. This network enables full parameterization while providing diagnostics and other valuable process information.

Infrared temperature sensors allow you to monitor temperature ranges without contact and with no feedback effect, detect hot objects, and measure temperatures. A variety of setting options and special processing functions enable use in a wide range of applications. The IO-Link interface allows parameterizing of the sensor remotely, e.g. by the host controller.

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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

Timing is everything – Which light is the right light?

Shortly after posting my last blog, Which light is the right light, I had a customer call with a problem in a machining cell.  They are using a self-contained through-beam sensor, in the form of a fork sensor, with a red light source. They required a small light spot to detect a tool.  As in most machining centers, there is a lot of coolant flying around in the cell and a fine mist in the air.  When water based coolants dry, they separate and leave a white film on surfaces, including photoelectric lenses.  This customer had to shut down their cell and clean off the lens at least once per shift, which was costing them production, time, and money because of false signals.

As we spoke on the phone, I suggested that they use the infrared version because we can burn through the contamination in the environment, in this case the film left behind from the coolant.  The customer wanted some sort of idea of how much residue we could burn through so I did some simple testing and sent him the following pictures.  Picture 1 is a heavy dusting, of all things, coffee creamer.  Picture 2 is a nice dollop of grease from a grease gun and picture 3 is a film of hand cream.

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Back to the Basics – Which light is the right light?

The emitters in photoelectric sensors give off a light that is received by a separate receiver, reflected back to a receiver by a reflector, or reflected back by the object itself. Back in the good ‘ol days, the light source was incandescent, however they ran hot and tended to have a short life. Now solid state devices, LED’s, are used because they use less energy, they can be pulsed very rapidly and you can use different colors for special applications.

Typically we refer to light sources in photoelectrics as red light, infrared, and laser. All have their advantages and disadvantages, and picking the wrong light source, can either make your application successful, or let’s say less desirable than you had hoped.

Red light photos are probably the most favored because they are easy to set-up, and confirmation that the sensor is working properly is easy since you have a bright light that you can focus on your target. However, it is important that you aim the sensor correctly if you have the sensor installed near an operator as the light can be rather annoying if it is in their eyes.

Continue reading “Back to the Basics – Which light is the right light?”