Design, fabrication, and characterization of Fresnel ...

Design, fabrication and characterization of Fresnel lens array with spatial filtering for passive infrared motion sensors Giuseppe A. Cirino a, Robson Barcellos a, Spero P. Morato b, Allan Bereczki b , Luiz G. Neto c a

HoloPhotonics Equipamentos Ltda. R. Alfredo Lopes, 1717 - sala E-08 CEP 13560-460 – São Carlos – SP – Brazil Phone: (55)(16) 3362-6294, FAX: (55)(16) 3362-6261 [email protected] b

LaserTools Tecnologia Ltda R. Yosoji Tamaguti, 46 CEP 05379-130 - São Paulo – SP – Brazil Phone:(55)(11) 3768-0612, FAX (55)(11) 3714-1146 [email protected] c

EESC - University of São Paulo Av. Dr. Carlos Botelho, 1465 CEP 13560-970 - São Carlos - SP - Brazil Phone:(55)(16)3373-8135, FAX (55)(16)3373-8135 [email protected]

ABSTRACT A cubic-phase distribution is applied in the design, fabrication and characterization of inexpensive Fresnel lens arrays for passive infrared motion sensors. The resulting lens array produces a point spread function (PSF) capable of distinguish the presence of humans from pets by the employment of the so-called wavefront coding method. The cubic phase distribution used in the design can also reduce the optical aberrations present in the system. This aberration control allows a high tolerance in the fabrication of the lenses and in the alignment errors of the sensor. In order to proof the principle, a lens was manufactured on amorphous hydrogenated carbon thin film, by well-known micro fabrication process steps. The optical results demonstrates that the optical power falling onto the detector surface is attenuated for targets that present a mass that is horizontally distributed in space (e.g. pets) while the optical power is enhanced for targets that present a mass vertically distributed in space (e.g. humans). Then a mould on steel was fabricated by laser engraving, allowing large-scale production of the lens array in polymeric material. A polymeric lens was injected and its optical transmittance was characterized by Fourier Transform Infrared Spectrometry technique, which has shown an adequate optical transmittance in the 8-14 µm wavelength range. Finally the performance of the sensor was measured in a climate-controlled test laboratory constructed for this purpose. The results show that the sensor operates normally with a human target, with a 12 meter detection zone and within an angle of 100 degrees. On the other hand, when a small pet runs through a total of 22 different trajectories no sensor trips are observed. The novelty of this work is the fact that the so-called pet immunity function was implemented in a purely optical filtering. As a result, this approach allows the reduction of some hardware parts as well as decreasing the software complexity, once the information about the intruder is optically processed before it is transduced by the pyroelectric sensor. Keywords: Passive infrared sensor, pyroelectric detector, cubic phase distribution, wavefront coding, Diamondlike carbon (DLC) thin film, laser engraving mould.

Photonics North 2006, edited by Pierre Mathieu, Proc. of SPIE Vol. 6343, 634323, (2006) 0277-786X/06/$15 · doi: 10.1117/12.707928

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1. INTRODUCTION Passive infrared (PIR) motion sensors have been extensively used in domestic and corporative electronic security applications. Its principle of operation is based on the employment of a pyroelectric detector, which is responsible for transducing the incident IR radiation to an electric signal. Associated with the photo detector and conditioning circuitry, there is a multi zonal Fresnel lens, which monitors different spatial zones and collects the IR radiation from a body that moves inside the monitoring volume1. The lenses are designed to concentrate the incident IR radiation is on the pyroelectric detector surface, producing pulse-like electric signals at the pyroelectric detector output, when an intruder crosses the volumetric monitoring field of the sensor. The electrical waveform at the pyroelectric detector output depends on the distance between the detector and the target mass, on the amount of mass present in the target, and on the speed of the target with respect to the sensor which operates at the 8 - 14 µm wavelength range. Figure 1a shows the photography of a typical low cost passive infrared (PIR) pyroelectric motion sensor. When a human corporal mass (here called by target mass) moves through the volumetric monitoring field of the sensor, different Fresnel lenses of the array focus the IR radiation on the pyroelectric detector, generating an electrical signal at the pyroelectric detector output. Whenever electrical signal is present at the output of the pyroelectric detector, meaning that an intruder has been detected, the sensor will trip, as shown in figure 1b. The electrical current, I, in the pyroelectric detector is proportional to the rate of change in time of the temperature of the target, dT/dt, the pyroelectric coefficient of the detector, p(t), and detector electrode area, A:

I = p (t ) A

dT dt

(1)

Figure 1c shows the photography of a Fresnel lens array typically employed in IR motion sensor applications. This array presents 24 detection zones, with 3 sets of lenses that monitor 3 different distance fields: a near field (typically 1-2 meters from the sensor), a medium distance field (typically 3-6 meters from the sensor), and a far field (typically 8-15 meters from the sensor).

.

Top Vie

high

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Ene

0=

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Movg

ALARM UI!

Vuliug.

V:•> (b)

(c)

50mm

Figure 1: (a) Typical low-cost passive infrared (PIR) motion sensor; (b) schematic view of the principle of a PIR sensor. Whenever a human corporal mass (target mass) moves through the monitoring volume field, an electrical signal at the pyroelectric detector output is generated, enabling the sensor to trip; (c) photography of a Fresnel lens array typically used in PIR sensors. Each lens is delimited by a rectangle.

Several companies have been employing various techniques in order to give the sensor the ability of distinguish human targets from pet targets. They are based on the use of microcontrollers, which make possible an electronic digital postprocessing of the electric signal generated by the pyroelectric transducer, i.e., the signal is collected and processed (filtered) afterwards 2. A low-cost solution, however, can be obtained by employing optical filtering. The phase

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distribution of the Fresnel lenses of the sensor can be modified in order to accomplish the distinction of the presence of humans from pets using only an optical processing. To solve the pet immunity problem in PIR motion sensors, an approach that consists of a hybrid optical pre-processing, also called wavefront coding 3,4,5, can be applied. Figure 2 shows the photography of the designed phase distribution, mapped in grayscale, of a Fresnel lens array typically employed in IR motion sensor applications. Figure 2a shows an image of the phase distribution of a conventional lens array, and figure 2b shows the phase distribution of the proposed Fresnel lens array with spatial filtering.

(a)

(b)

Figure 2: Photography of a multi-zonal, segmented Fresnel lens array typically used in PIR sensors. (a) grayscale image of the phase distribution of a conventional segmented Fresnel lens array, (b) grayscale image of the phase distribution of the proposed Fresnel lens array with spatial filtering.

Authors have reported the successfull employment of wavefront coding to correct aberrations present in incoherent image systems 6,7, making them invariant to misfocus within a predefined range of image plane distances as well as correcting the transversal chromatic aberration 8. Because of the aberration invariance, it is possible to use a low-cost and low-precision optics, to produce an imaging system with the performance of a high-cost, high precision system, or a near diffraction-limited optical system6. In this paper we verify the application of the cubic phase distribution to improve the immunity of PIR sensors to false alarms generated by domestic pets. More details about the design and application of wavefront coding can be found on previous works 9, 10. In order to fabricate such optical components in large-scale, replication techniques such as injection moulding, is widely used. Laser technology has attracted a great number of people interested in industrial applications, mainly in metallurgy and electronics. In a global market, a fierce competition induces the appearance of products that have the laser as one of their most important tools for value aggregation. Industrial application of lasers, mainly in sheet metal cutting, welding and marking are well-established techniques 11,12. Lasers are efficient marking and engraving tools and their application dominate sectors such as plastics, metals, alloys, silicon, ceramics and satisfy requirements of speed, quality, flexibility and price. Some of these qualities are not seen in traditional technologies. For all the reasons mentioned above, a mould for Fresnel lenses was fabricated by laser ablation. More details of the process is described in section 4

2. ABERRATION INVARIANCE APPLIED IN THE DESIGN OF CUBIC PHASE LENS An incoherent imaging system is linear in intensity and its point spread function (PSF), is the squared magnitude of the amplitude impulse response h(xi,yi) 13. The variables xi and yi are the coordinates in the image plane. The optical transfer function (OTF) H(u,v) of the system can be defined from h(xi,yi) by +

  h(x , y ) i

H(u,v) =

i

2

exp[ j2 ( x i u + y iv )] dxdy

 +

  h(x , y ) i

i

2

(2)

dxdy



The complex function P(xp,yp) may be referred as the generalized pupil function. The PSF of an aberrated incoherent system is simply the squared magnitude of the Fraunhofer diffraction pattern of an aperture with amplitude transmittance

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P(xp,yp). The most common approach to obtain a focus-invariant incoherent optical imaging system is to stop down the pupil aperture or to employ an optical power absorbing apodizer, with possible phase variations. The aperture can be viewed as an absorptive mask in the pupil plane of an optical imaging system. Although these methods increase the amount of depth of field, there is a loss in optical power at the image plane and the reduction of the diffraction-limited image resolution 2,5,13. A non-absorptive separable cubic phase mask can be included in the exit pupil to produce an optical PSF that is highly invariant to misfocus 2,5. Considering the cubic phase distribution, the PSF is not directly comparable to that produced from a diffraction-limited system PSF. However, because the new OTF has no regions of zeros, digital processing can be used to restore the sampled intermediate image. If the PSF is insensitive to misfocus, all values of misfocus can be restored through a single, fixed, digital filter. This combined optical/digital system produces a PSF that is comparable to that of the diffraction limited system PSF, but over a far larger region of focus. The cubic phase distribution necessary to produce this focus-invariance is described by equation 4:

[

(

3

P(x p , y p ) = exp j20
x p + y p

3

)]

(3)

When a focusing error is present, the image of an object is formed before or after the image plane. This plane is located at a distance zi from the exit pupil 13. The misfocus in the generalized pupil function P(xp,yp), for an image located at an arbitrary distance za from the exit pupil, results in the equation 5:

1 1 1 2

2 P(x p , y p ) = P(x p , y p )exp jk x p + y p

2 za zi

(

)

(4)

The amplitude impulse response h(xi,yi) of this new lens is calculated by the equation 5: +

h(x i , y i ) =

  exp[ j20 ( x



3 p

+ yp

3







)] exp jk 12  z1  z1  ( x a

i

2 p

 2 + y p  exp  j2 ( x p x i + y p y i ) dx i dy i 

)

[

]

(5)

Figure 3 compares the image of the PSF for zi=za=50 mm and for zi=50 mm and za=70 mm. The cubic phase distribution generates a constant PSF over a far larger region of focus, compared to conventional image system. The intensity distribution of the intermediate images of the PSF remains constant for different image planes.

(a)

(b)

Figure 3: (a) Image of the PSF for zi=za=50 mm; (b) Image for zi=50 mm and za=70 mm. The cubic phase distribution generates a constant PSF over a relatively large region of focus. These results are calculated using the relation:

G i ( u , v ) = H ( u , v )G g ( u , v )

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(6)

The OTF H(u,v) of the system is calculated using equation 2. The frequency spectra of Ig, the input image, and Ii, the output image, are defined by: +

  I (x , y g

Gg (u,v) =

g

g

[

]

)exp  j2 ( x g u + y g v ) dx g dy g



(7)

+

  I (x , y g

g

g

)dx g dy g



+

  I (x , y )exp[ j2 ( x u + y v )]dx dy i

Gi (u,v) =

i

i

i

i

i

i

 +

  I (x , y )dx dy i

i

i

i

(8)

i



As shown in figure 2, cubic phase optical systems are insensitive to misfocus. Because the new OTF has no regions of zeros, the original images can be recovered from the focus invariant images after simple digital filtering using equation 9, as desired 2.

G g (u , v) = Gi (u , v) / H (u , v)

(9)

1. APPLICATION OF CUBIC PHASE DISTRIBUTION IN PASSIVE INFRARED MOTION SENSORS As stated above, in PIR sensors the lens array is used to concentrate the infrared radiation over the detector and not to generate an image. Regular lenses do not distinguish the presence of humans from pets. In order to assure this distinction, a cubic phase distribution is added to a regular (quadratic phase) lens, introducing the wavefront coding in the system. In our application, the new lenses are used as a spatial filter rather than an imaging system. When a target mass is moving through the volumetric monitoring field of the sensor, i.e., through the field of view of different segmented Fresnel lenses, the generated distribution of energy also moves over the detector surface. The goal is to introduce a spatial filtering that helps in the distinction between the patterns generated by a human and a pet body. This is done by using the PV(xp,yp) phase distribution 2-7:

[

PV (x p , y p ) = exp j20
.y p

3

]

(10)

Figure 4a shows the input test image used in the computer simulation. Figure 4b shows the phase distributions of equation 10 multiplied by the phase distribution of a conventional lens. Figure 4c shows the corresponding output image. One can note from figure 4c that images of the human bodies have higher brightness when compared to the image of the small pet. In order to show how the lens can be used in the distinction of human targets from pet targets, we designed and manufactured a lens set on amorphous hydrogenated carbon thin film, by employing, micro fabrication process techniques, such as microlithography and plasma etching. Although the sensor operates in the 8-14 µm wavelength range, a diffractive Fresnel lens was fabricated, which operates in the visible portion of the spectrum, with a central wavelength of 500 nm. This is suitable to show that the principle works, being directly applied in the desired wavelength range (mid infra-red). For comparative purposes, besides each cubic-phase distribution Fresnel lens, we fabricated also a conventional quadratic-phase distribution Fresnel lens. Both lenses were submitted to the same steps during the whole fabrication process.

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Cf

(a)

(b)

(c)

Figure 4: (a) Input test image used in the computer simulation; (b) resulting Fresnel lens with spatial filtering, from equation 11; (c) the corresponding output image. One can note that the image of the human body has higher brightness when compared to the image of a small pet. Figures 5a and 5b show the resulting images from the conventional (quadratic phase profile) and the proposed lens (cubic phase profile), respectively. The object plane is illuminated by a diffused incoherent white light source. The lens images the object directly on the plane of the CCD camera. From figure 5 one can see that, because the cubic phase profile spreads out the optical energy with respect to the collecting area of the pyroelectric sensor, the optical power falling onto the detector surface is attenuated for targets that present a mass horizontally distributed in space (e.g. pets) while the optical power is enhanced for targets that present a mass vertically distributed in space (e.g. humans). Therefore we have shown that the wavefront coding approach can be used to allow some blurring of the PSF of the optical system in a way to allow discrimination between humans and pets for passive infra-red motion sensor applications.

(a)

(b)

Figure 5 - Intensity images generated from the Fresnel lenses used as preliminary experiments in order to prove the proposed principle in the visible portion of the spectrum, with a central wavelength of 500 nm. (a) image from the conventional (quadratic phase profile) lens; (b) image from the proposed lens (cubic phase profile). It is notable the enhancement of the human body intensity distribution.

4. FABRICATION OF THE INJECTION MOULD BY LASER ABLATION Lasers are now substituting traditional engraving methods in the mould making industry. Amongst the laser systems that are currently used in industry for marking and engraving, the most popular is the continuously pumped and Q-switched Nd:YAG laser. These systems are the ones with highest peak power outputs specially required for marking and engraving 14. This high peak power characteristic is especially interesting when engraving metals and alloys with high hardness numbers and when one needs high graphic resolution of engraving. High peak power characteristics are also

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important since they allow that a large number of metals, their alloys and a wide range of painted or pigmented plastics to be marked. The combination of Nd:YAG lasers with Q-switching devices and scanning heads that divert the laser beam in a x-y plane 11, have the ability of high speed marking any kind of material in a computer controlled way, duplicating any vectorial graphic image, including lines with variable width with precision down to 25 µm. Besides, any graphic element (ideogram, logo, etc.) can be immediately altered by the computer before the next marking job takes place. The image that is going to be marked on a part is generated by a graphic software that is exported to a dedicated software that controls the laser and other components of the marking equipment like shutters, Q-switchers and scanners. The job duration is determined basically by the length of text and the image complexity. Marking systems include, besides the Nd:YAG laser cavity, the lamp power supply, primary cooling circuit, RF power supply (Q-switcher), scanning head, a xyz milling base with optical rules for increased precision in positioning, micro computer and a CAM type software. Graphic data to be engraved on a part are created, edited and manipulated utilizing CAD software. It is important to understand the system operational basis, its parameterization and the requirements for the creation and edition of vector files that will be used in the laser processing. To obtain an efficient marking process, knot points, fillings and contours must be carefully manipulated in the file that contains the vectorial graphic. Figure 6 shows a schematic diagram of the laser cavity coupled to a beam delivering system composed by a scanning head which scans the x-y plane. The beam used by this head is focused through a f-theta flat field lens. X-AXIS ALVAN OMETE R

V-AXIS

LASER

GA LVA NOM ETER

CAVITY

FLAT-Fl E LU

FOCUS LENS

x

Figure 6 - Schematic view of a laser marking and ablation system. The laser is deflected by the X-Y scanning heads and focused at the surface mould.

The method we built is based on that described by Alessandro in reference 15, but a bitmap input file is used, instead of a stl file because for the resolution required, the stl file had a huge size, turning this method impracticable. For processing the files, some graphic software was used like CorelDRAW Graphics Suite, Adobe Photoshop, AutoCAD, and Fobagraf that is the laser controlling CAM. For the production of “slices” the desired regions of the original bitmap was selected. Then, these regions were vectorized and a filling (laser processing area) was applied to each slice. Figure 7 illustrates the described process. The so obtained slices are then grouped in a way that the distance between two slices was not bigger than the depth of focus within which the material removal rate was considered to be constant. This value was determined in previous experiments as being smaller than 400
m for the conditions used in the experiment. Additionally laser parameters were studied taking into account the material removal rate, level of detail and surface finishing. It was also taken into consideration the kind of steel to be micro machined and the previous thermal treatment used to obtain a hard surface. The engraving was done by adjusting the focal distance for each set of slices. At

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the end of the process the surface was scanned with the laser in CW mode to do a controlled melting of the generated surface. This surface was then acoustically polished and a specular 3D Fresnel set of figures was obtained.

Figure 7 - Steps of image treatment to engrave a surface from a gray scale bitmap. (a) original bitmap; (b) Bitmap with white selected colors and remaining black; (c) vectorization that resulted in a curve that can be filled; (d) filled figure. This process is repeated for each layer.

This polishing is executed manually and greatly depends on the ability and skills of the operator like in any other manual optical polishing procedure. The peaks were removed with the tool tip without an excessive polishing that could risk the surface geometry well established by the laser micromachining. The polishing applied to the laser micro machined surface allowed the production of a mould for Fresnel lens operating in the infrared region with specular reflection, despite of the surface finishing left by the laser ablation.

0 0

M 0

0 0

Ci

0 o 0 o 0

Cii

0 0 0

Ni

h

CD

w

0

Removed Material [nm]

M Cii

0 0

CO

0 0 0

Figure 8 shows the relationship between the incident energy and depth of the generated surface by removal of the material. Both laser energy and depth were normalized for each laser pulse. A linear behavior of the ablation process is observed.

Figure 8 - Characteristic curve for the STAVAX ESRTM steel from Uddeholm Inc., 51 HRC hardness, when ablated by a Nd:YAG laser writing system. One can see the linear relationship between the laser pulse energy and the amount of removed material

per laser pulse (spot laser of 50
m). Figure 9a shows the mould after the laser ablation and ultra sound polishing; figure 9b shows a sample of a lens array fabricated in polyethylene using the mould of figure 9a, by injection moulding process. Utilizing this mould for the injection of plastic lenses resulted in efficient movement detection systems, thus validating this method, as described by the results show in the next section.

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(a)

(b)

Figure 9 – Photographs of (a) the mould fabricated by laser ablation on hardened steel; (b) sample of a lens array fabricated in polyethylene using the mould of figure (a), by injection moulding process.

1. PERFORMANCE RESULTS OF THE SENSOR The first task to be accomplished for a good performance of the sensor is to make an optical characterization of the polymeric material of the lens. The polyethylene is widely used for this end 16. Figure 10 shows a transmittance curve of this material (1 mm thickness plate) obtained by Fourier Transform InfRared Spectroscopy (FTIR). This curve shows a window with acceptable transmittance in the range of 8-15 µm wavelength, as desired. 70 60

Sample #2

1mm thick

50

w

(j40

z

1-30

I-

z I-

20 10

WAVELENGTH [urn]

Figure 10 - Transmittance curve of polyethylene material (1 mm thickness plate) obtained by Fourier Transform InfRared Spectroscopy (FTIR). This curve shows a window with acceptable transmittance in the range of 8-14 µm wavelength: the sensor operates at 10 µm.

In order to validate the functional performance of the sensor, a climate-controlled test laboratory facility (10 m by 14 m by 3.5 m size) was constructed, following a standard procedure published by the Interagency Advisory Committee on Security Equipment – Intrusion Detection Subcommittee (IACSE protocol), from an effort of UL (Underwriters Laboratories), Sandhia National Laboratories e SIA (Security Industry Association) 17. Figure 11 shows the test facility used to evaluate the sensor’s performance. Figure 11a shows a schematic view of the pattern used for mapping the volumetric detection of the sensor, extracted from the IACSE standard; figure 11b is a photography of the a climatecontrolled test laboratory facility used in this experiments; figure 11c is a schematic diagram, extracted from the IACSE standard, showing how the test has to be performed; figure 11d is a photography of the test to determine the detection zones of the sensor being performed. Beginning outside the detection area, along the arc nearest the sensor (starting point), the person (test target) shall move forward along the arc at a rate of 0.15 m/s until an alarm is generated. At this

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point the test target shall stop and mark the location. The test target shall continue along the same arc making sure that the sensor remains in constant alarm until the test target moves out of the detection zone.

(a)

(b)

(d)

(c)

Figure 11 - (a) Schematic view of the pattern used for mapping the volumetric detection of the sensor, extracted from the IACSE standard; (b) photography of the a climate-controlled test laboratory facility used in this experiments; (c) schematic diagram, extracted from the IACSE standard, showing how the test has to be performed; (d) a photography of the test to determine the detection zones of the sensor being performed.

Figure 12 shows the map obtained for a detection zone of the sensor, with a human test target and with a small pet (14 Kg) test target. One can note that the sensor operates normally with a human target, with a detection zone reaching 12 meters with an angle of 100 degrees. On the other hand, when a small pet runs through a total of 22 different trajectories no sensor trips are observed. The small circles correspond to the position where the sensor trips, while the straight and curved lines correspond to pet trajectories.

6. CONCLUSIONS In this work we described the wavefront coding method applied to passive infrared motion sensors. The new proposed lens array has the ability to distinguish the presence of humans from pets. The lenses are designed considering a cubic phase distribution placed in the exit pupil plane. We verified the behavior of the cubic phase mask in one-dimensional systems, testing this distribution in the vertical direction. We simulated several qualitative human bodies in different conditions, as well as other features, such as small pets. Through the fabrication of lens with conventional quadratic phase profile as well as cubic phase profile, we demonstrate that the proposed system is able to enhance the information of the vertical features, making possible, in this way, the distinction of the presence of humans from pets. The novelty of this work is the fact that the so-called pet immunity function was implemented in a purely optical filtering. As a result, this approach allows reducing some hardware parts as well as decreasing the software complexity once the information about the intruder transduced by the pyroelectric sensor has already been optically processed. The authors would like thanks to FAPESP for financial support.

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(a)

(b)

Figure 12 - Resulting map for a detection zone of the sensor, (a) obtained with a human test target and (b) for a small pet (14 Kg) test target. Its detection zone reaches 12 meters with an angle of 100 degrees. When a small pet runs through a total of 22 different trajectories no sensor trips are observed The small circles correspond to the position where the sensor trips, while the straight and curved lines correspond to pet trajectories.

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2.

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6.

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7.

Cathey, W. T.; Dowski, E. R.; and Alan R. FitzGerrell, A. R. "Optical/Digital Aberration Control in Incoherent Optical Systems" Publication of the Imaging System Laboratory, Department of Electrical Engineering, University of Colorado, Boulder, available (march 29/2006) a t: http://www.colorado.edu/isl/papers/aberration/paper.html

8.

Wach, H.B., Cathey, W. T.and Dowski, E. R “Control of Chromatic Focal Shift Through Wavefront Coding” Publication of the Imaging System Laboratory Department of Electrical Engineering, University of Colorado, Boulder, available (march 29/2006) at: http://www.colorado.edu/isl/papers/chromaberr.pdf

9.

Cirino, G.A.; Neto, L.G. “Design of cubic distribution lenses for passive infrared motion sensors”, Thermosense XXV, Orlando, editors: K.E. Cramer & X.P. Maldague, Proceedings of the SPIE vol. 5073, pp.476-484, SPIE, Orlando-FL (2003).

10. Cirino, G.A.; Neto, L.G. “Optical implementation of cubic-phase distribution lenses for passive infrared motion sensors”. In: SPIE's Defense And Security - Thermosense XXVI, 2004, Orlando, FL, USA. Proceedings of SPIE. Bellingham, Washington, USA : Editor(s): Douglas D. Burleigh, K. Elliott Cramer , G. Raymond Peacock,. v. 5405. p. 189-198, (2004) 11. Wetter, N.U., Rossi, W. de, Grassi, F. and Steen, W.M., ICS Lectures on Industrial Applications of Lasers, S. P. Morato, editor, UNIDO / ICS, Vienna (2000). 12. Steen, W.M., Laser Material Processing, Springer-Verlag, London (1991). 13. Joseph W. Goodman. “Introduction to Fourier Optics”, pp.137, McGraw-Hill Publishing Company, Second Edition, New York (1996). 14. Wang, W; Wang, L.; Guo, N.; The research and development of a Nd:YAG laser engraving system, SPIE vol. 2888, p. 207-212 (1996). 15. Alessandro Melo de Ana, Edison Puig Maldonado and Spero Penha Morato. “Tridimensional laser engraving of industrial injection moulds”. Annals of Optics, XXV National Meeting of Physics of Condensed Matter – Brazilian Physics Society, Brazil, pp 212-214, (2002). 16.

PolyIR Brochure Fresnel Technologies Inc. (march 29/2006) http://www.fresneltech.com/pdf/POLYIR.pdf

17.

“Sandia National Laboratories and Underwriters Laboratories Poised for PIR Performance Testing” available at (march 29/2006) h t t p : / / w w w . s i a o n l i n e . o r g / S I A _ n e w s _ v i e w . a s p ? i d = 6 8 7 6 2 2 5 5 see also(march 29/2006) http://www.siaonline.org/data/standards/sandia/pir_test_procedure.pdf

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