In opposed-mode sensing, the sensor’s emitter and receiver are housed in two separate units. The emitter is placed opposite the receiver, so that the light beam goes directly from the emitter to the receiver. An object is detected when it “breaks” or interrupts the working part of the light beam, known as the effective beam. Depending on the application, opposed mode sensing provides the highest reliability whenever it can be implemented. This is because light passes directly from the emitter to the receiver. Then, when an object breaks the beam, the output will switch.

• Opposed-mode sensing offers the highest level of excess gain (sensing energy)
• Long sensing range
• Most robust for harsh environments
• Precise position sensing
• Small-part detection using lens apertures
• Impervious to surface reflectivity (the color or finish of the object)

A retroreflective sensor contains both the emitter and receiver elements in the same housing. It uses a reflector to bounce the emitted light back to the receiver. Similar to an opposed-mode sensor, it senses objects when they interrupt or “break” the effective beam. Because retroreflective sensing is a beam-break mode, it is generally not dependent upon the reflectivity of the object to be detected.

However, it can be tricked by shiny objects. For those targets, a polarized retroreflective sensor should be used to prevent proxing. Proxing is when an object with a shiny surface returns enough light to the sensor to mimic the photoelectric beam coming back from the reflector and causes the object to not be detected. In a polarized retroreflective sensor, the emitter sends light waves through a filter that aligns them on the same plane. These light waves bounce off the reflector, and return to a vertically polarized filter on the receiver. When this polarized light reaches a shiny target, the light is reflected back to the sensor on the same plane as it was emitted and is blocked by the filter, signaling a broken beam. When the polarized light hits the reflector, however, it is scattered into unpolarized light with light waves on both the horizontal and vertical planes. Some of this light will pass through the receiver’s filter and the sensor will detect the reflector and know the beam is unbroken.

A retroreflective-mode sensor offers a convenient alternative to opposed mode when space is limited, or if electrical connections are only possible one side of the installation. Retroreflective-mode sensors offer relatively long ranges.

• Second-highest excess gain mode
• Polarized model available to prevent the beam from proxing off shiny objects
• Coaxial optics available for clear objects and precision

Diffuse-mode sensors contain the emitter and receiver in the same housing but do not use a reflector. Instead, they detect an object when emitted light is reflected off a target and back to the sensor. With a diffuse-mode sensor, the object is detected when it “makes” the beam; that is, the object reflects the transmitted light energy back to the sensor. They are significantly affected by the reflectivity of the target objects, which can drastically shorten their range. These sensors should not be used in applications with very small parts that need to be detected, in parts-counting applications, or where a reflective background is close to the object to be sensed. Diffuse-mode sensors are very convenient and are often used when opposed or retroreflective-mode sensors aren’t practical.

• Low installation effort
• Does not require a reflector

Background-suppression (BGS) sensors are a diffuse-type sensor with a defined limit to their sensing range, ignoring any objects that lie beyond that range. There are two types of background-suppression sensors: fixed-field and adjustable-field. Both types use triangulation to determine the cutoff distance which allows the sensor to ignore anything beyond that point. The available excess gain inside the fixed sensing field is usually high, allowing sensing of less-reflective surfaces. A background-suppression sensor can often detect a dark target on a white background, as long as the background is past the sensor’s cutoff.

• Detects objects out to a set sensing distance
• Ignores background objects
• Very low color sensitivity

Excess gain is a measurement of the amount of light energy that the receiver element detects. A sensor needs an excess gain of one to cause the sensor’s output to switch “on” or “off.” However, contaminants in the sensing environment such as dirt, dust, smoke, and moisture can cause signal attenuation, so more excess gain will be required to receive a valid signal. Excess gain may be seen as the extra sensing energy available to overcome that attenuation.

An excess gain chart shows how much light energy is available at a given distance. The dirtier the environment, the more excess gain will be needed to overcome it. The graphs are logarithmic, which allows for a concise overview of data that varies by several orders of magnitude. Each minor tick increases by a factor of 1, and each major tick increases by a factor of 10. For example, starting at the origin and moving up the Y-axis, the graph’s ticks represent 1, 2, 3, etc. Once the tick gets to 10, the ticks represent 10, 20, 30, etc. When the tick gets to 100, then the ticks represent 100, 200, 300, and so on.

Visible LEDs help in the alignment and setup of a sensor, since the visible beam will provide a spot on the target. Red is the most common color for photoelectric sensors, because red diodes are inexpensive to make and the photodetectors in receivers are very sensitive to red light.

Materials will act differently to different wavelengths of light. A certain material may absorb one wavelength of light while reflecting another, or the contrast between two colors is low. In these cases, trying a different color LED, such as blue, can be a simple solution to the problem.

Infrared (IR) LEDs are invisible to the human eye but are very efficient at producing light. This efficiency can help IR sensors see farther than visible LEDs. However, because the beam is invisible, it can make alignment more difficult.

Typically, photoelectric sensors cannot see water because it is transparent to light in the visible spectrum. Fortunately, water efficiently absorbs the specific wavelength of 1450 nm, allowing for detection. Certain Banner sensors utilize long-wavelength infrared (LIR) LEDs operating at 1450 nm to detect liquids that contain water, while ignoring (burning through) clear or opaque containers.

Many Banner sensors use lasers for their emitted beams. Lasers use a small beam spot, delivering higher precision that is ideal for detecting very small objects or features. This beam remains very tight even over great distances, delivering precision detection at longer range.

The beam pattern represents the boundary within which the sensor will respond to a target. In opposed mode, the receiver can be anywhere within this pattern and will detect light from the emitter. In retroreflective mode, the beam pattern is dependent on the reflector being used. A smaller reflector will reflect less light, which results in a shorter range and a more narrow beam pattern. In diffuse mode, the target must be within the beam pattern to be detected. In diffuse mode, the beam pattern is created using a 90% white card, so different-colored targets will affect the beam pattern.

Sensors also have an effective beam, which is the “working” part of the light beam stretching from the emitter to the receiver. An object is detected when it breaks the effective beam. In opposed mode, the effective beam is established between the emitter and receiver. In retroreflective mode, because the emitter and receiver are housed in a single unit, the effective beam is established between the emitter, reflector, and receiver.

The latest Photoelectric Sensors Solutions brochure explores the many different types of Banner sensors and how to choose the right one for your application, whether it be object counting, quality control, object presence or absence, or other automation need.