What to Look for in Bi-Level Lighting
As discussed previously, bi-level lighting of infrequently used areas is a great way to reduced building operating costs while maintaining a safe environment. This blog entry is intended to help lighting professionals understand the differences between motion sensing technologies to help them choose the best option for their particular application.
Ultrasonic vs Passive Infrared vs High Frequency
The three leading technologies used in motion sensing include ultrasonic, passive infrared (PIR) and high frequency (HF). Ultrasonic and high frequency use sound while PIR uses heat, yet in each case, neither the sound nor heat sensed falls within the human-detectable range.
Ultrasonic sensors work on the Doppler principle. Similar to a ship’s sonar or a bat’s echolocation, a high frequency signal is continuously emitted, and signal disturbances are interpreted as motion. These signals are not line-of-sight limited, so motion can be detected around corners, such as when a person is walking up or down stairs. Ultrasonic sensors can typically pick up an entry door opening and motion from several floors above and below typical stairwells. The rush of air from a stairwell door opening, or a person walking in the stairwell is typically detected by ultrasonic sensors.
PIR sensors detect emitted infrared energy in the form of heat. A person (or animal) moving within a PIR sensor/lens assembly’s line-of-sight will trigger full illumination. Typically, Fresnel lenses are mounted on ceilings and detect motion in a 360o area. The higher the sensor/lens assembly is mounted, the wider the coverage are becomes, but with some shortcomings. A sensor lens detection pattern looks like long and short range ‘fingers,’ in that major motion is detected in alternating zones with more minor motion. As such, a sensor that works well on a warehouse ceiling may not offer the detection range and pattern needed for a stairwell; likewise, a detection pattern that works well on a ceiling be optimized for fixtures mounted on a wall, which is typical for a stairwell. Some broad coverage lenses are designed to have a range of up to six times the mounting height and when mounted on a wall, may offer reasonable coverage depending on the stairwell configuration.
HF, or microwave, sensors use very high frequency output and are similar to ultrasonic in that they use the Doppler principle. They can be mounted out of sight behind plastic fixture diffusers, and may detect movement through glass, wood, sheetrock and similar materials. They are not directly sensitive to the minor air movement that ultrasonic sensors may detect. The major drawback is that since they can detect motion through many materials, false detection is more likely; especially when there are windows, doors with windows, or sheetrock walls. Since they are less sensitive to air movement, they may not offer the detection benefits of ultrasonic in stairwells and corridors.
Dual-Tech sensors feature two technologies in one device. They may have PIR and ultrasonic, or HF. There is a common misconception that dual-technology sensors are a good choice for stairwells. This is not the case as these sensors are not using either technology to detect motion, but only will trigger when both technologies detect simultaneously. This is not desirable for stairwells as the benefits of ultrasonic or microwave will be negated and limited to the line of sight of the passive infrared portion of the sensor. Dual-Tech sensors are mostly used in security systems to minimize false detection.
Which Technology is the right choice?
From a strictly fundamental standpoint, ultrasonic is usually the better choice. Since it is not dependent on line-of-sight sensing, ultrasonic offers full illumination before a person actually steps into the area to be lighted, whereas with PIR, the person has to actually enter the area before full illumination is achieved. However, PIR sensing is less expensive, so PIR-equipped fixtures tend to cost less. HF sensors may be suitable for some applications especially if the aesthetics require that the sensors be concealed and not seen, but the building materials in the surrounding areas must be carefully evaluated to avoid nuisance tripping.
Choosing which technology to employ is somewhat application-specific—ultrasonic is typically the appropriate choice, but where budgets are an issue and/or where delayed full output lighting is not an impactful consideration, PIR can fit the bill. In stairwells or corridors in open areas or with strong airflow present, HF or PIR may be the better solution, but in the vast majority of commercial and residential high-rise buildings, ultrasonic offers superior detection.