1. Field of the Invention
The present invention relates to narrowband imaging apparatus or systems, and in particular to compact, light weight imaging apparatus or systems suitable for detecting from a mobile platform optical emissions from combustion in the presence of daylight.
2. Brief Description of Prior Art
Threats to human life and property are often harkened by combustion. For example, small fires in the wilderness can become forest fires that spread to populated areas. Anti-aircraft missiles sent aloft by burning rocket fuel when launched by terrorists or enemy combatants often target civilian and military aircraft. At night, such combustion sources are readily detected by the light emitted. However, during daytime hours the detection presents a much greater challenge. This is because imaging sensors may be overwhelmed by daylight, including direct sunlight, sunlight scattered from the sky and clouds, and sunlight reflected from objects on the ground or in the air.
Daylight can also overwhelm imaging sensors used for viewing the health of crops, those used for LIDAR (Light Detection and Ranging), FLIR (Forward Looking InfraRed) systems, and laser communications receivers. Other relevant applications where extraneous light can overwhelm imaging sensors include welding observation, specialized cellular microscopy, and solar astronomy.
It is common practice in the art to use infrared (IR) image detectors to detect thermal emission from threat sources at electromagnetic wavelengths longer than those of visible light. The IR detectors used in such applications are expensive. Thus few private and commercial vehicles or aircraft are equipped with IR imaging threat detection systems. In addition, processing IR images to determine what sources are of interest is complex. It demands great processing power to reject clutter and various signal signatures that are not of interest. The cost of these powerful processors adds to the cost of a system based on an IR detector. The detectors and extensive processing lead to increased cost, size and weight of systems that rely on IR imagers.
Atomic Line Filters with passbands of <0.01 nm have been developed as an alternative method for improving the background light rejection of daylight image detectors. Faraday filters based on the Faraday effect are one type of atomic line filter used for this purpose. The Faraday effect refers to the rotation of polarized light when it passes through a medium in the direction of an applied magnetic field. The amount of rotation is proportional to the magnetic field strength and to the distance the light travels through the medium. Faraday filters are also known as Magneto-optical filters (MOF) for this reason. When the medium is an atomic vapor that effect occurs over a very narrow range of frequencies. This effect has been used to make narrow band filters for many years by directing a light beam along the axis of a cell containing an alkali metal vapor. Crossed polarizers are provided at opposite ends of the cell to prevent the passage of almost all light except light with frequencies near the atomic transitions. A magnetic field is applied along the axis of the light beam to rotate the polarization of the light at the transition wavelengths by 90° allowing it to pass through the second polarizer. Most useful for the rejection of background sunlight are Atomic Line Filters centered at the Sun's Fraunhofer Lines. The Fraunhofer lines are narrow minima in the spectrum of light from the sun, produced by absorption of light in the cooler regions of the sun's outer atmosphere at wavelengths corresponding to the atomic and molecular transitions of materials in these regions. Detection of optical emissions from combustion processes on Earth is facilitated at Fraunhofer line wavelengths because there is much less background light. In particular the light intensity within the Sun's Fraunhofer line is often only a few percent of the intensity outside the line. For example, Fraunhofer lines associated with a Solar Potassium absorption occur at 766.4 and 769.9 nm, have a width of about 0.02 nm, and have a central intensity about 80% lower than outside the lines.
On the other hand, combustion optical emissions contain intense narrow-band emissions that are formed by atomic and molecular optical transitions excited as part of the combustion process. They can occur throughout the optical spectrum. Terrestrial events of interest often include combustion of trace amounts of potassium that emit light at 766.4 nm and 769.9 nm. The combustion signal is therefore high and background sunlight signals are low, thus the contrast between combustion emission signal and ambient light is high at the Fraunhofer wavelengths. Combustion detection is therefore more favorable in the wavelengths band of Fraunhofer lines, and particularly in the band of 766.4 nm and 769.9 nm.
To take advantage of the detectability of this terrestrial combustion signal, a narrowband optical filter is needed that stops sunlight in other bands and passes light in a band about 0.01 nm wide that overlaps the potassium emissions at about 766.4 nm or 769.9 nm, or both.
Available MOF filters fall into two classes—cold cell and hot cell. The cold cell filters produce metal vapor by heating some central part of the cell and use a buffer gas to maintain a sufficient vapor population in the central part of the cell without allowing excessive diffusion of the vapor to the end windows. The hot cell filters heat the whole cell in an oven, using a cold finger to control vapor density. The cold-cell MOFs, have a limited field-of-view, are bulky and need continuous calibration to guarantee long-term stability. While prior implementations of the hot-cell MOF can have a moderate field-of-view, they are difficult to construct in a way that minimizes polarization-inducing stresses in the cell windows and oven enclosure windows. Furthermore, the hot cell filters require bulky ovens to maintain cell temperature and control vapor density. Thus, conventional MOF implementations have a limited usefulness for monitoring large sections of Earth or sky for fires, gunfire, missiles and other important combustion events. Especially on aircraft, the total volume and weight available for a combustion monitoring system, such as a missile warning system, is limited. A large array of narrow field of view cold cell MOFs, or hot-cell MOFs with bulky ovens are simply not feasible on an aircraft. Exemplary aircraft constraints for a combustion monitoring system are a volume no larger than about 10 centimeters (cm, 1 cm=10.sup.-2 meters) by 10 cm by 10 cm (i.e., a volume less than 1000 cubic cm) and a mass no greater than 1 kilogram (kg, 1 kg=1000 grams).
Prior art MOF systems capture signals using a camera or other detector external to the MOF and its container. An exit aperture window or windows are used to transmit the image to the external camera or other detector. Optical losses encountered at these windows and within the external camera's optics are a factor in the system detection limits. The mounts and support structure for the camera or other detector add weight and size to the combined MOF imaging system.
The highest performance prior art MOF imaging systems utilize state of the art cooled detectors and image intensifiers. These detectors produce extremely low electronic noise allowing near single photon counting performance. The housings for these detectors must incorporate thermal, environmental, and optical features that add weight, power consumption, size, and optical losses to the prior art MOF imaging system.
Where threats such as incoming missiles are captured by an MOF detector system it is crucial that the captured signal be recognized as a threat quickly, often within milliseconds, if effective counter-measures are to be deployed. Currently envisioned MOF systems utilize high-speed signal processing computers external to the MOF imaging camera for this recognition. The highest performance signal processing computer chips are often cooled to obtain lower noise. At the speeds required for threat detection even the signal delays caused by wire lead lengths between the detector and the signal processor can be significant.
United States Patent Application Publication No. 2007/0017281, published on Jan. 25, 2007 is perhaps the closest prior art to the present invention in that it also utilizes some of the same principles and components as the present invention. However, because of arrangement and positioning of the components and other differences, this prior art apparatus does not offer many of the advantages of the present invention.
In light of the foregoing, there is a clear need in the art for wide field of view and short length MOFs without bulky ovens. In particular, there is a need for a rugged, low cost, small size, low weight, and low power MOF that is coupled with a detector and signal processor having equivalent size, weight, and power features. The present invention fills this need.