The 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 www.balluff.us/photoelectric.
