A proximity sensor is a device which detects objects when they get within a certain distance from the sensor. The proximity sensor will then usually send a signal to an electronic circuit (proximity switch) to perform a simple action, such as opening a door, turning on a light, sounding an alarm for industrial safety, or deactivate embedded programs. Many proximity circuits detect anything that comes too close. Others are designed to pick up on a particular type of object and ignore everything else. There are four basic types of proximity sensors: infrared, acoustic, capacitive, and inductive.
Infrared (IR) proximity sensor work by sending out beams of invisible IR light. A photodetector on the proximity switch detects any reflections of this light fall on it. These reflections allow IR proximity switches to determine whether there is an object nearby. As proximity sensors with just a light source and photodiode, they are susceptible to false readings due to background light. The complexity of these switches can vary to compensate for this stray light. There are a broad range of IR proximity sensors applications including extended range (>100cm), one-dimensional linear position sensing, and cell phone and/or hand-held appliances at <5cm.
To calibrate and IR proximity sensor a reference target of diffuse reflectance is needed. The IR sources are typically an IR LASER or LED. The application of the proximity sensor determines the amount of irradiance or flux falling on the photodiode sensor. The intended light falling on the photodiode is the light from the IR source after reflecting off a near Lambertian target of known reflectance. If there is any transmissive medium between the IR source and target or the photodiode and the target, it needs to be compensated for as well. The reference target’s reflectance imitates the application object and is used to calibrated the proximity sensors IR source to achieve the light level at the photodiode that will active the devices proximity switch.
Labsphere’s Spectralon® is a high diffuse reflectance reference material. These durable, chemically inert materials are offered in reflectance values ranging from 2% to 99%. The materials and Labsphere’s unique in house processes are ideal for tailoring the reflectance levels (i.e. 10%, 12%, 15%, 18%, 20%, 30% and 48%) and form factor to the sensor application. A good example of the applied target material is an industrial safety application by SICK. To learn more download the Case Study below.