FAQ on optical proximity sensing: part 1

A light source and photosensor combine for effective, reliable, non-contact proximity detection of nearby objects.

Non-contact proximity sensors detect the presence or absence of nearby objects (including animals and people) using basic physics phenomena such as capacitive and inductive electromagnetic fields, light, or sound. Many techniques are used for this sensing, each offering different operational attributes in specific applications and environments.

This proximity sensing is a ubiquitous yet often invisible function in our society. Automation is impossible in a modern manufacturing process without sensors, but it goes far beyond that. The sensing requirement may be for a random event, such as a person entering/leaving a room (legitimate or an intruder) or to indicate to an elevator controller that someone is in the path of a door closing. Alternatively, it is needed for a more deterministic and predictable role, such as checking for products moving along a high-speed production line, as seen in Figure 1.

proximity detection — Figure 1. Sensors must be sited close to their target, be rugged and reliable, and use minimal wiring, especially in industrial environments. (Image: Dynamic Measurement & Control Solutions LLC)

In a basic arrangement, these sensors are almost always housed in a relatively compact enclosure and mounted using a simple yet rugged bracket. The associated cable is also rugged, usually with just three wires for power, ground, and output signal; sometimes, a fourth wire is used for signal ground.

Keep in mind that proximity sensing is a non-contact arrangement. Other sensing techniques, such as using a switch, rely on physical contact, and these have their role in some situations. However, in most industrial, commercial, and even residential applications, requiring such physical contact would be unsuitable, awkward, unreliable, or have other drawbacks.

Unsurprisingly, no proximity-sensing technology best fits all target types, sizes, distances, operating environments, and installation specifics. As a result, many different technologies incorporating various physics phenomena are used for non-contact proximity sensing,

The most common proximity sensing technology is also the oldest. Called optical sensing, the concept is simple enough: visible or infrared (IR) light is projected at the target and then is reflected by the target or, in the absence of the target, passes through the target zone. Optical proximity sensing is also often called “photoelectric sensing.” Compared to the many other non-contact presence/absence proximity-sensing technologies in use that are based on different physics principles, optical is the most intuitive as it involves just interruption and reflection of a beam of light.

This FAQ looks at the basics of photoelectric sensing, its variations, and its enhancements. While this form of proximity sensing is fairly straightforward in principle, as with all technologies, its subtleties affect effective use and installation.

Q: What is the basic principle of optical proximity sensing (photoelectric sensing)?
A: It’s extremely simple, direct, and intuitive — among its many virtues. It starts with a beam of light directed towards the target area of interest. Depending on the installation arrangement, a light sensor then looks for a reflection from the target; alternatively, if the senior is on the opposite side of the light source, the sensor looks for the presence or absence of light passing through the target area.

Q: Sounds simple enough, so what are some complicating factors?
A: Many factors affect installation and performance, including target distance (a few inches to several feet or more), target size, target optical characteristics (reflective, diffuse, or transparent) and color, target position, consistency, and orientation, operating atmosphere (clean to dusty), and available space.

Q: What are the three basic arrangements for optical proximity sensing? What are their key attributes?
A: There are three types; through-beam, retroreflective, and diffused. A combination of the above considerations and other factors determines the right choice in a given application setting.

Q: What is “through-beam?”
A: This is the most direct and obvious approach, with separate transmitter and receiver nodes on either side of the target zone, shown in Figure 2. The transmitter light source and receiver photosensor must be pointing directly at each other, with the target zone between them.

Q: What are the attributes of this simple and obvious arrangement?
A: The through-beam scheme offers a relatively longer range, reliability (meaning consistent performance), and higher accuracy. Since the light only travels in one direction, it is well suited to situations with wide door openings, such as garage doors. However, the cost is higher due to the need for two components, including their housing and wiring; challenges in detection through thin, clear objects due to light refraction; setup and alignment of two separated units; and issues with mounting-space requirements and cable management.

Q: How does retroreflective arrangement work?
A: In the retroreflective arrangement, as shown in Figure 3, the sensing system contains both the transmitter and receiver in the same housing. The light source projects the beam to the target, and beyond the target is a retroreflector, which is aligned to reflect the beam back into the photo-sensing receiver in that housing.

When the target object is absent, the beam is reflected back to the sensor; when it is present, some or all of the beam is blocked, and only a small part (if any) gets back to the sensor.

Note that a retroreflector is not a simple mirror. It would reflect impinging light at an angle and, therefore, away from the source unless they were in perfect alignment; basic physics states that the angle of incidence equals the angle of reflection. Instead, the retroreflector is a relatively flat but three-dimensional, inexpensive passive component with a surface pattern of tiny “inside cubes” that reflect light back to the source regardless of the angle of incidence. Rugged versions were placed on the moon by astronauts and then used to measure the distance from Earth to the moon via round-trip laser-beam transit time.

Q: Retroreflective seems simpler to set up and is less costly than through-beam. What are the actual pros and cons?
A: Certainly, the hardware cost is lower, and setup is easier, as there is only one electromechanical module and less cabling, so proving power and doing alignment is easier (the far-end reflector is relatively easy to install and align). However, the maximum sensing distance is shorter as it depends on the amount of light related back to the sensor from the retroreflector. Keep in mind that only a small amount of reflected light is available even under the best circumstances, and it is a point-like source that spreads with the returned intensity decreasing in proportion to the square of the distance.

Shiny or reflective objects like mirrors, cans, and plastic-wrapped juice boxes create a challenge for retro-reflective photoelectric sensors. These targets sometimes reflect enough light to trick the receiver into thinking the beam was not interrupted, causing erroneous outputs. Applications for retroreflective include baggage conveyors at airports, vehicle detection at toll gates, and some material-handling applications. Due to its simplicity in setup, the retroreflective mode is more widely used than the more obvious through-beam scheme.

Q: What is the diffused approach?
A: The diffused approach (also called the diffuse-reflective or simply reflective approach) is direct and intuitive but has operational issues. As with the retroreflective arrangement, the sensor unit contains both a transmitter and a receiver. However, instead of needing a discrete reflector to return the beam to the receiver, the sensor is directed at the target object itself, which must reflect some light back to the receiver, as shown in Figure 4.

Q: The diffused approach seems simple enough: just aim and look for the reflected light. What are real-world considerations?
A: A diffused photoelectric sensor is the cheapest as only one point of installation is required, and no far-end sensor or retroreflector is needed. Rather than relying on a reflector to bounce back the beam, the sensor relies on the backscattering of target objects passing in front of the beam to reflect some light back. However, the detection distance is much shorter since only a very small fraction of the incident light is reflected back to the sensor. As a result, the sensor may have difficulties detecting the object depending on its material, reflectivity, orientation, color, and flatness.

Part 2 of this article goes beyond the principles and looks at some physical implementations of the sensing system, input/output specifics, and other considerations.