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Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
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Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources Abstract
We describe a new class of eye safe near infrared (~810 nm) illuminators based on diode laser sources. These illuminators are the most efficient available by a large margin. We show examples of how the high output allowed by lasers can be used to illuminate very large areas and improve images in difficult lighting situations. We also show examples of how the narrow spectral band of laser light can be used with filtered cameras to achieve unique results in applications such as facial recognition in open daylight. Application data on effective ranges and other factors are presented. Comparisons are made with other lighting systems such as arc lamps, tungsten lamps and light emitting diodes. We also compare results of systems that operate without supplemental light such as intensified cameras and thermal infrared cameras.
Introduction
Recently a revolutionary new technology has become available for applications in security and surveillance systems. The technology is derived from diode laser-based near infrared (NIR) illuminators developed for automotive night vision systems—primarily at Ford Motor Company. These systems make use of proprietary optical concepts that allow high power (20 watts or more) diode lasers to be used with complete safety in illumination devices. The characteristics of diode lasers allow performance features achievable in no other way, as will be shown in this paper. We describe the basic concepts of these illuminators, their unique features and illustrations of their applications.
Technical Overview
Diode lasers are attractive light sources for security applications because of several unique features. Diode lasers are the most efficient light sources known. Efficiencies of 60% or more have been demonstrated and more gains are coming. The compact size of the source allows very efficient optics. High efficiency translates into lower heat loads and thus allows more power to be delivered in compact units. Of course high efficiency results in less power consumption, which is important self powered systems. Typical laser power used in our units is in the range of five to forty watts. Diode lasers emit light in a very narrow wavelength band—typically about one or two nanometers wide. This is very useful when one wants to reject light from high ambient lighting or bright sources in the field of view. Our systems can use light over a wide range of wavelengths. Typically we use light in the 800–900 nanometer
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©2007 Electrophysics Corp. All rights reserved.
Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
Upper Atmospheric Targets
range. These wavelengths are compatible with commonly available CCD and CMOS cameras. Diode lasers can be modulated extremely rapidly to facilitate strobe effects. This can be useful if rapidly moving objects are being observed. A typical illuminator system consists of one or more diode lasers, a power supply, an optical system that provides an eye safe beam and a tamper proof enclosure. The optical system in the types we are discussing makes use of a class of optics termed sheet optics.
FIGURE 1: A typical sheet optic configuration
The term sheet optic refers to a class of optics developed and patented by the Ford Motor Company in cooperation with the Technology Integration Group, Inc. to allow eye safe use of high power diode lasers as illumination sources. There are various configurations of sheet optics but they all accomplish their function by allowing the laser beam to spread over an array of optical elements which form a beam of large cross sectional area—large enough so that power densities are below eye safety limits. Reflective, refractive and diffractive optical elements are used in various combinations. The name sheet optic comes from the fact that these elements are usually incorporated in a relatively thin planar optic such as shown in Figure 1. In this configuration, an expanding beam (from a fiber for example) is collimated by the molded lens at the lower corner. The collimated beam is reflected by total internal reflection from a row of surfaces on the lower edge to a second set of reflecting surfaces on the back face. The beams reflected from the back surfaces pass through the front surface forming a broad area beam. Typically, beam expansions of the order of 50,000 to 1 are obtained with efficiencies of 70% or better in an optic a few millimeters thick.
©2007 Electrophysics Corp. All rights reserved.
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Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
Often it is useful to spread the relatively highly collimated beam from a sheet optic into a broader pattern. One method of doing this is to use holographically generated diffusion screens. These can form beams with a wide variety of power distributions with efficiencies above 90%. A typical pattern used for area lighting is 20 degrees wide and 10 degrees high (FWHM).
Types of Laserbased Near Infrared Illuminators
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Laser-based illuminator types fall into two basic categories. The simpler of the two, and the most common type, is the self-contained unit in which the laser, optics and power supply are enclosed in a single tamper proof enclosure. For specialized applications it is sometimes desirable to mount the laser and power supply in an enclosure of their own and send the light via optical fiber to a separate enclosure containing only the optics. Fiber coupled illuminators are vulnerable to fiber breakage or disconnects which could cause dangerous levels of radiation to be emitted. There are various types of interlocks to prevent this. Some examples of these types are shown in Figures 2-5.
Figure 2. A 5-watt, self-contained laser-based illuminator
Figure 3. A 20-watt, self-contained laser-based illuminator
Figure 4. A 40-watt, self-contained laser-based illuminator
Figure 5. The optical head of a 30-watt, fiber coupled, laser-based illuminator
©2007 Electrophysics Corp. All rights reserved.
Electrophysics Resource Center: Night Vision
Applications of Laser-Based Near Infrared Illuminators
Figure 6. View of part of Long Island Sound in ambient light
Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
For many infrared illumination applications devices making use of LED’s or incandescent light sources are adequate. However, for some application areas laser-based illuminators are unsurpassed and may be the most economical or the only way to accomplish the task. The characteristics of laser-based illuminators that we want to emphasize in this paper are the very high output and the very narrow wavelength band of the output. High output obviously allows illuminators to be effective at long distances and over broad areas. An example is a water scene shown in Figures 6 and 7, which compare the scene as seen in ambient light and the scene as illuminated by a 40-watt illuminator. In these pictures the people standing on the breakwater beyond the anchored boat are clearly visible at a distance of 1300 feet and there is discernable illumination on the brightly lit shore over 2000 feet away. Another illustration is shown in Figure 8 in which a 15+ acre dog racing facility is illuminated by a single 40-watt unit. All pictures were obtained with standard B&W CCD cameras. Figure 7. The same view as Figure 6 but with infrared illumination. People on breakwater are about 1300 feet from illuminator.
Figure 8. Section of 15-acre dog racing facility illuminated by 40 watt NIR illuminator. Man is about 1200 feet from the illuminator.
Often it is important to be able to see into areas near bright lights, which effectively blind the camera if no auxiliary lighting is used. A powerful illuminator can accomplish this under some circumstances with a standard camera. An example is shown in Figures 9 and 10 where a pier with bright lights on it is illuminated. The end of the pier is about 400 feet from the illuminator. Notice how the under side and dark areas become very visible when the illuminator is on. Under extreme conditions it is desirable to reject as much ambient light as possible. Because of its very narrow spectral band width the light from a laser-based illuminator allows exceptional rejection through the use of narrow bandwidth interference filters. All other light sources have much broader spectral bandwidth than that of a laser (1-2 nm) making it the light source of choice for this type of application. An example is the use of laser illuminators and narrow band filters to see past the headlights of oncoming vehicles. Figures 11 and 12 illustrate this.
©2007 Electrophysics Corp. All rights reserved.
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Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
Figure 9. A section of a pier illuminated by working lights
Figure 10. A section of the same pier illuminated by a 40-watt illuminator
Figure 11. Camera without filter facing oncoming headlights
Figure 12. A scene similar to that of Figure 11 but with laser illumination and a filtered camera
A specialized application of these effects is in the area of facial recognition. Uncontrolled variations in lighting make facial recognition difficult. A recent study by DARPA and NIST has shown that while the best systems can achieve better than 90% accuracy in controlled lighting conditions, uncontrolled lighting such as found outdoors reduces accuracy to an unacceptable 50% range1. Some examples of the use of NIR laser illumination with a filtered camera are shown in Figures 13-16. Figure 13. Image of subject in bright sun only
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Figure 14. The same subject in sun but with added NIR laser illumination
Face Recognition Vendor Test 2002: Overview and Summary P. J. Phillips, P. Grother, Ross Micheals, D. M. Blackburn, E. Tabassi, J. M. Bone
©2007 Electrophysics Corp. All rights reserved.
Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
Figure 15. Subject illuminated by overhead lights
Figure 16. The subject in overhead light but with NIR laser illumination
Under some circumstances the ambient conditions are so difficult that both high laser power and narrow band filtering are necessary to obtain a satisfactory image. An example is facial recognition through a car windshield in bright sunlight. If white light were used to fill in the shadows and overcome glare, it would require many thousands of watts of power. The light would be extremely annoying or even a safety hazard to say nothing of revealing the fact that facial recognition is taking place. However, good images can be obtained with a laser and filtered camera as can be seen in Figures 17 and 18. A 40-watt illuminator and a 15 nm wide filter centered at 810 nm were used. The subjects are unaware of the illumination. Figure 17. View through windshield in bright sun
System Performance
Figure 18. View through windshield in bright sun with NIR laser illumination and filtered camera
The performance of a surveillance system is a complex function of many factors. Among the more important are the following. • Illumination pattern of the source • Power of the illumination source • Sensitivity of the camera • Resolution of the camera • Noise in the camera and channels • Characteristics of the display • Ambient lighting conditions at site • Weather conditions at site • Wavelengths at which the system operates
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Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
Performance questions also depend on what is necessary to be observed. For example the ability to read painted features such as identification numbers may or may not be important. It is impossible to give a simple analysis of all these factors however a commonly sought performance parameter is the area that can be observed with a system. As an example we have calculated the maximum range that can be observed with typical sensitive standard CCD cameras when illuminators of varying power and beam patterns are used. These calculations are based on analysis of field results using Watec LCL 902K and Sentech STC-H160BC cameras with f-1.4 lenses. Illuminators with nominal laser powers of 5, 20 and 40 watts were used. Ambient conditions are assumed to be those found where there is no significant light from other sources. The results of these calculations are summarized in Figure 19. The maximum effective range is given as a function of the product of the nominal horizontal and vertical angles for a range of laser powers. The angles are the total spread angle of the beam at the half power level. The useful beam spread is usually wider than the nominal spread. Figure 19: Maximum effective range of illuminators of varying powers and beam spreads
These results can be calculated from Equation 1. R=53666*P½/(ΘH*ΘV )
1)
Where R is the effective range in feet, P is the nominal power of the internal laser, ΘH is the horizontal spread angle of the beam in degrees (FWHM) and ΘV is the vertical spread angle of the beam in degrees (FWHM).
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©2007 Electrophysics Corp. All rights reserved.
Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
Comparisons to Other Systems
There are other surveillance systems that rely on completely different concepts. Most notably, thermal infrared and supersensitive cameras do not depend on separate illumination. Thermal cameras detect heat radiating from objects of interest and super-sensitive cameras rely on low levels of ambient illumination. Each has its strengths and weaknesses. Thermal cameras have effectively infinite range because the light source is the object itself. They also highlight heat-emitting objects. However, their range is effectively limited by the resolution of the detector. The visibility of a scene depends on variations of temperature and emissivity, which cause details such as paint markings to be lost. They also suffer from degraded performance under certain ambient temperature conditions. A comparison of a thermal camera image and a laser-illuminated image is shown in Figures 20 and 21.
Figure 20. A view of a road with a thermal camera
Figure 21. A view of a road with NIR laser illumination and a CCD camera
Note that compared to the laser/CCD system the thermal camera shows lower contrast, poorer resolution, a narrower field of view and does not show painted lines and signs at all. Supersensitive cameras operating unaided can often see quite well if the ambient illumination is suitable. Many of these cameras can also work well with supplemental NIR illumination. The unaided cameras run into difficulties when it is extremely dark or when there are regions of bright illumination and dark areas in the same scene. Under these conditions it is necessary to provide enough supplemental light to overcome the ambient light. For applications such as facial recognition or seeing into vehicles neither thermal nor supersensitive cameras provide any advantage. Thermal cameras record the temperature distributions not facial features. The wavelengths used by thermal cameras generally do not penetrate glass. Increased sensitivity of supersensitive cameras does nothing to rectify interference from sunlight or other awkward sources.
©2007 Electrophysics Corp. All rights reserved.
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Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
Electrophysics Resource Center: Night Vision
NIR illuminators based on incandescent lamps with filters or light emitting diodes (LED’s) are widely used for surveillance. Compared to laser-based illuminators both are less efficient and lower powered. Both have much broader wavelength bands than lasers. They are, however, less expensive and may be perfectly suited to certain applications. Some applications where lasers excel are • Illuminating very large areas. Forty watts of laser-based NIR illumination with a total power draw of about 100 watts has been shown to effectively illuminate 15-20 acres. It costs between $2,000 and $3,000 to install a lighting pole. If a pole mounted conventional NIR fixture can illuminate about an acre, it would cost in the vicinity of $50,000+ to do the same job. • Illuminating areas at distances beyond the reach of other illuminators. Examples are large dams, bridges scenes over water and airfields. • Situations where extremes of object brightness are found. Examples are facial recognition outdoors and in vehicles. For comparison visible arc lamps for daylight fill are blindingly bright, hot and cost well over $10,000. • Scenes with bright light sources such as streetlights or headlights are in the field of view. • Portable or self contained systems. Laser-based illuminators are by far the most efficient light sources available which translates into lighter, more compact and long-lived power supplies.
Eye Safety Considerations
The commercially available laser-based illuminators that we describe in this paper are classified as Class 1 devices according to the standards of the Center for Devices and Radiological Health (CDRH) of the United States Food and Drug Administration. They also comply with the requirements of the International Electrotechnical Commission (IEC) for Class 1 devices. Class 1 devices are considered to present no danger of eye damage in any configuration or mode of use. The reason it is possible for these illuminators to be a Class 1 device even though it contains a powerful laser is that the internal optics expand the beam to a large enough cross section so that the power density is within safe limits. Of course this requires that users not open the device and the enclosures are so labeled and protected by other means.
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©2007 Electrophysics Corp. All rights reserved.
Electrophysics Resource Center: Night Vision
Conclusions
©2007 Electrophysics Corp. All rights reserved.
Application Analysis of Near-Infrared Illuminators Using Diode Laser Light Sources
We have described a revolutionary new class of near infrared illuminators based on diode laser light sources. Through the use of proprietary optics these illuminators can use tens of watts of laser power with complete eye safety. The high efficiency and narrow spectral band width of laser light make it ideal for a variety of surveillance and night vision applications. We have shown applications to large area lighting and fill lighting in night and daylight scenes that could be accomplished in no other way.
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