Balanced Sensitivity Passive Infrared Lenses

Optical Engineering and Manufacturing

Abstract

 

The design of Fresnel lenses with balanced, or uniform sensitivity is a principal design goal. The weakest element of an optical system defines the performance of the whole system. Each element must provide enough signal to insure a sufficient signal for detection. Therefore the electronic gain must be set according to the weakest optical element. By balancing the optics and insuring there is no weakness in the lens. The electronics do not have to compensate for a weak area in the coverage of the lens, and the electronics designer can reduce electrical gain. In a balanced lens, there are no extremely sensitive areas either, therefore, with a balanced lens design and lower electronic gain, false alarms and false triggers are reduced.

 

Initial Design– The Detection Pattern

 

The design process begins by designing the coverage pattern for the optics. The coverage pattern is designed to cover a certain geometric pattern in front of the sensor. This geometric pattern is defined by maximum range, detection angle, and the sensor mounting height. Other specifications play an important role in the design of the lens. These include, the location of the pyroelectric detector in relation to the lens, the focal distance between the detector and the lens, and the curvature of the lens.

 

Displayed below is a preliminary pattern for a low profile lens. This lens is designed to cover a 90 degree angle to a distance of 20 meters. It is designed to work with a dual element pyroelectric device, positioned 23 mm behind the lens.

Plan view.                                                                  Side View

 

The zones are placed to optimize the performance of the sensor. In the lens above, there are three levels of zones, long range, intermediate range, and short range. The pattern should have no holes and should guarantee detection within two to three steps. The placement of inner and outer zones is designed to detect an intruder walking directly at the sensor as he crosses between levels of zones. There are fewer short range zones to compensate for the higher relative walk speeds of an intruder walking in close.

 

Infra-ótica lenses are designed to reduce common mode rejection. The design minimizes overlap between inner and outer zones. Excess overlap causes common mode rejection and much weaker signals from the dual element pyroelectric device. Most pyroelectric detectors have two elements. These elements have opposite polarity. When used with a Fresnel lens, the combination of detector and lens will produce a two-piece zone, with a positive and a negative part. The purpose of the opposing polarity is to reduce false detections. If the sensor is subjected to a sudden change in ambient temperature, each device will register the change equally, but will produce an opposing signal. The opposing signals cancel each other out and there is no false trigger.  Common mode rejection can occur when the overlap in zones is poorly designed or there is too much overlap.

 

Computer models to complete most of the detailed design work. The engineer loads the preliminary detection pattern into the model. The engineer used the model to adjust the positions and sizes of the lens elements to provide both maximum sensitivity and uniform sensitivity.

 

Second Design Step– Maximum Sensitivity

 

Once the pattern is established, the next step is to design the lens for maximum sensitivity. Our process is based on calculating and maximizing the energy transfer equation.

 

 

 

 

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In the sketch above, S1 is the target, usually a person. S2 is the lens. The target, S1 is radiating Energy E1 into space. The distance between the target and the lens is R12. N1 and N2 represent the normal vectors to the surfaces of the target and the lens. The target and the lens can have different sizes, positions, and orientations. For simplicity we model humans as a simple rectangular block. More detail would be desired, but the computer model would get overly complex. This sketch and equation are placed inside a computer model and with this model we are able to calculate the Energy E2 that each lens in the array receives from the target.

A second calculation is performed for the energy transfer from the lens to the detector. The layout and mathematics are similar, but the focusing power of the lens is considered in this model.

Our computer model allows us to consider lens surface area, range and angle to the target, transmittance of the lens, reflectance of the lens, target area, incidence angle on the pyroelectric detector, and amount of defocus of the lens element. We can quickly arrange the elements so we use elements from the center of the master lens. The center portion of the lens has the greatest sensitivity. We can also model and identify other factors such as detector or lens tilt that will improve overall sensitivity. Due to the physical layout of a sensor, it is impossible to place each element at its precise focal distance from the pyro. This model allows us to select the focal lengths that will minimize defocus and maximize the overall sensitivity of the lens.

Third Design Step– Balanced Sensitivity

The end goal of our design cycle is balanced sensitivity. Once we have identified the features that will provide maximum sensitivity, we begin optimizing the lens to provide balanced sensitivity. We can make adjustments to the lens to balance the sensitivities between elements. The easiest change is to vary the size of the elements. This lets us compensate for reduced sensitivity due to range or angle by increasing the surface area of the element. We can adjust the detector tilt to compensate for the weakest set of zones.

Considering all these factors allows us to balance the sensitivity of each element. The numbers below are for the lens element in the array. The graphs show the performance of each factor in the design. These factors can be traced to the radiation transfer equation discussed earlier.

The drawings below show a lens that is in production, the W-23. The long range elements have larger areas due to the decreased energy available from a distant target (high R12). The outer elements are larger to account for reflectance and transmission losses at extreme angles. The short range elements are larger to compensate for the thin detection zones that cover only a small portion of the target.

11 x Long Range Zones

6 x Mid Range Zones

3 x Short Range Zones

The chart below shows the relative sensitivity of each element.

Conclusion

Before the lens enters production Infra-ótica builds a prototype and subjects the lens to a battery of tests.

 

· Each element is tested for sensitivity.

· Walk test, to confirm detection in two to three steps, and no holes in the detection pattern.

· Compatibility with a variety of sensors available on the market.