FUJI & CO.(Piezo Science)

DESIGN PRINCIPLES OF INTRUSION SENSOR OPTICAL SYSTEMS

The primary function of the optical system is to concentrate the heat energy from the target onto the detector.  The surface area of the mirror or lens is larger than the that of the detector element thereby providing gain by intercepting more radiation than would the detector alone. From a system perspective, optical and electronic gain are somewhat interchangeable, but optical gain has the advantage of not degrading the signal to noise ratio. A bigger detector could be constructed which would yield a similar benefit but diffuse energy collected directly by a detector element has little spatial information and cannot readily supply the chopping action previously described as being essential to efficient detection. Optics provide other benefits, providing the number and separation of the fields-of-view, determination of range, target size discrimination and help on the task of making the background invisible.

Because a PIR sensor is not very demanding of image quality, objectives used to focus energy on the detector can be relatively unsophisticated reflective or refractive optics. Refractive optics and usually found in inexpensive sensors which use polyethylene fresnel lens configurations. Polyethylene is the only inexpensive material that will transmit far infrared wavelengths but it has the disadvantage of having a fairly high transmission loss. The fresnel lens configuration provides a thinner lens than a conventional one keeping the loss to a manageable level. Such a lens is cheap and easy to use. Other materials capable of transmitting far IR, germanium, silicon, sapphire and various exotic salts and minerals, are too expensive or impractical for industrial intrusion sensors but can sometimes be found in military systems.

Mirror optics are more efficient than refractive optics even when the loss of the necessary protecting window is included. Typically transmission loss is about 70-85% compared with 30-50% in the typical polyethylene fresnel lens. Mirrors also allow greater design flexibility allowing such innovations as curtain shaped fields and multiple focal lengths which would be difficult using fresnel lens technology.

Conventional "Beam Forming" Optics

Sensitivity/Range Profile

Conventional fields-of-view, employing either lens or mirror, are commonly called beams or fingers. The term is obviously a misnomer since it implies something active. Anyhow, if an intruder enters a FOV an image of him is formed in the plane of the detector. When the image of the intruder occupies the entire area of the detector elements then all of the available energy will be converted into an electrical signal. The range at which the image size is matched to the detector size is called the maximum design range and is determined by the focal length of the lens and the element size. The sensor is adjusted to yield its design sensitivity at this range. The variation of sensitivity with distance from the sensor is termed sensitivity range profile and for this type of optical system functions as follows.

In the diagram:

If the intruder moves away from the sensor from maximum design range, his image will no longer fill the element area and the irradiance will reduce in accordance with the inverse square law. Correspondingly the sensitivity of the sensor will fall off as the intruder moves further away until at some range he will no longer be detectable. The exact range at which this happens is variable and depends on his contrast to the background which in turn depends on the room temperature and how he is clothed.

If the intruder moves closer to the sensor from the maximum design range, his irradiance will increase in accordance with the inverse square law. However, in this case his image size grows larger as he approaches close and exceeds the the area of the detector element. The part of the image not on the detector is not converted to signal. The increase due to the inverse square law and the loss of area outside of the detector are self compensating and the sensitivity of the sensor is essentially constant independent of range at ranges less than the maximum design range.

In practice this is not quite true. First the intruder does not have uniform temperature distribution over his whole body so the irradiance of the part of the image that illuminates the detector is variable. Secondly, the detector itself is not perfect and heat from the enlarged image is absorbed by the substrate and may be conducted into the adjacent element causing sensitivity to increase. Eventually at even closer distance the image fills the whole substrate and the effect is stabilized. Because of these factors, while the change in sensitivity inside maximum design range is less than outside, sensitivity does vary.

Characteristics of Conventional Beam-Forming Optics

Most sensors employing "beam forming" optics use the same focal length for all fields-of-view. In a fresnel lens this is almost a necessity to achieve an acceptable external appearance. These systems have the disadvantage of becoming very sensitive to small targets such as pets or rodents at close range.

The fields-of-view under the main fields, which provide under crawl protection can intersect with the floor quite close to the sensor. Small animals will fill these fields-of-view and even drafts blowing across a carpet can cause false alarms when so magnified. The increased and undesirable sensitivity has been referred to as claustrophobia by some designers. The figure below shows this effect. For stability it is necessary that the background area is maximized.

Because of these factors, it is important that the sensor optics be designed with the desired area of coverage in mind. It is a risk to install a sensor designed for 50 feet in a 12 foot office. Because of this a universal PIR sensor suitable for any location and range is probably not possible.

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Last modified: October 25, 2009