Patent ID: 12247918

DETAILED DESCRIPTION

The present application provides a means for replicating and producing user-defined arbitrary spectra. Such means include producing a solar spectrum ranging from UV (wavelengths as low as 200 nm) to LWIR (wavelengths as high as 20 microns). The present application describes a solar spectrum generator for generating a precise spectrum closely replicating actual solar spectra. The spectra of the present application, which are generated by the solar spectrum generator, can be changed quickly from one specific spectrum to another specific spectrum. The solar spectrum generator of the present application can also be used for various applications such as sensor testing, solar panel testing, and producing customized UV, visible, and infrared spectra. The infrared spectra produced by the solar spectrum generator of the present application extend from shortwave infrared (SWIR) to LWIR.

The present application provides a means for producing spectrally precise spectra including an artificial solar spectrum ranging from UV to LWIR spectrum which closely simulates a natural solar spectrum. For the purpose of testing sensors, producing a precise artificial spectrum very similar to the spectrum of real sunlight is very useful and important.

In order to produce a spectrum that matches the spectrum of very hot objects (like the sun) without using an enormous power input, the present application utilizes UV, visible, and IR sources, each of which is individually controlled. In addition, the present application also uses: beam combiner optics, beam profiler optics, amplitude optimization software, an electrical output controller, an IR spectrometer, and a testing surface on which the object being tested can be placed. In a preferred embodiment, the testing surface is a 3-axis motion table.

The present application has the capability to modify, as required, a given spectrum extending from UV to LWIR by adjusting the outputs from each emitter source to match a variety of desired environmental conditions. This includes the capability to manipulate the spectrum to represent not only atmospheric conditions but even to produce arbitrary spectra that would not likely be found in a natural environment. The output of the system is a desired spectrum with sub-micron (below 1 micron) resolution. In a preferred embodiment, the spectrum has a resolution of at most approximately 0.1 micrometer increments. The spectrum is produced by a combination of light sources, including a blackbody radiator, a xenon/IR lamp, and lasers, all of which are combined to produce the desired output spectrum.

It has been observed that unexpected imageries are sometimes obtained when testing IR sensors in actual sunlight. These unexpected imageries are often due to internal reflections of a given sensor UUT in the sunlight environment. Such imagery is difficult to replicate with conventional IR sources due to several factors including: 1) the limited spectrum and power output of such conventional IR sources, 2) the complexity of the sensor optics and other components necessary to combine and convey the light from a source, and 3) the lack of a repeatable source. Limited testing was performed outdoors with real sunlight, but an indoor testing environment using the artificial solar spectrum of the present application provided much more flexibility and repeatability throughout the testing process. The present application provides a sensor testing environment that produces artificial spectra, including unexpected imageries due to internal reflections, that are very close in actual characteristics to actual solar spectra.

In an embodiment, the application creates beams that simulate solar energy from UV to LWIR spectra. The application can be used, for example, with a sensor UUT.

One of the problems overcome by the present application was the need to filter out undesirable IR noises created by the sun's IR radiation. The present application can produce spectra without such undesirable IR noise.

The sun produces spectral power equivalent to a blackbody radiator near 5800 K. Depending on the sensor's location in relation to actual solar rays, the spectral irradiance profiles vary, due to the sub-band absorption in the IR regions. The replication by a solar simulator of the total infrared (from NIR to LWIR) solar irradiance at the Earth's surface presents significant technological challenges. The system of the present application replicates the total solar spectral irradiance with a fine spectral resolution (with increments of at most approximately 0.1 micrometer). Other existing sources and optical systems that have been shown to accomplish this are not known by the applicant to exist at the current time.

The present application utilizes lower temperature blackbody radiator and xenon/IR lamp sources (between 1000-3000 Kelvin) to build the foundation spectrum. This includes visible light from black body radiation (FIG.1), mid LWIR light from black body radiation (FIG.2), and very LWIR light from black body radiation (FIG.3).

FIG.1plots wavelength (in hundreds of nanometers) vs. radiance (in microwatts/cm2/nanometer) at 1000 Kelvin. InFIG.1, the y scale is 100× magnified.

FIG.2plots wavelength (in micrometers) vs. radiance (in hundreds of microwatts/cm2/nanometer) at 1000 Kelvin.

FIG.3plots wavelength (in tens of micrometers) vs. radiance (in hundreds of microwatts/cm2/nanometer) at 1000 Kelvin.

The present application also utilizes more recent technology development items such as laser diodes and QCL laser sources to fill the spectrum.FIG.4shows a NIR laser diode spectrum (1.06 micron with 3 nm bandwidth) plotted in wavelength (in nanometers)′vs. normalized amplitude. Specifically, the NIR laser diode spectrum inFIG.4is a spectral profile of laser radar (LADAR) projector laser.FIG.5shows a sample tunable QCL spectrum (7.9-8.5 micron QCL with 1 nm bandwidth) plotted in wavelength (in micrometers) vs. normalized amplitude. Overall, at upper atmosphere, the solar irradiance is approximately 0.2 watts per square centimeter per micron at the 0.5 micron spectral peak. The irradiance rapidly decreases to less than 0.05 watts per square centimeter per micron below 0.4 micron and above 1 micron. Beyond 2 microns, the irradiance level is well below 0.01 watts per square centimeter per micron.

It was found that for the case of 5% power being coupled into the sensor unit with a 20 centimeter aperture, the area that needs to be illuminated is 314 square centimeters. And, for a midwave case (having a wavelength around 4 microns), the power needed is less than 2 W per micron. Since, several lasers will be used per micron for the midwave, the laser output power required will be less than 1 W. Thus, the use of low power lasers of various wavelengths works well to allow the present application to simulate various atmospheric and weather conditions (including altitudes).

By efficiently superposing several light sources one upon another, the combined light sources together produce the desired spectrum. For the purposes of this application, to “superpose” is defined as “to lay or place on, over, or above something else”. Thus, several light sources are beamed to a given target so that the light beams are superposed one upon another at a certain designated area. This is in contrast to conventional solar simulator systems which start with a very broad and uncontrollable light that has to be filtered out and have light subtracted from it to produce anything close to the desired spectrum. Such a conventional approach does not produce precise spectra and does not allow spectra to be changed rapidly. In contrast, the present application does produce precise spectra and allows for various conditions of atmosphere and material characteristics to be simulated quickly without changing sources, and does not require using filters to subtract portions of the combined spectrum. In some cases, embodiments of the present application may however add filters when needed in order to simulate specific atmospheric conditions or to modify amplitude to match a desired spectrum.

FIG.6shows a block diagram illustrating the interrelationships of the functional components of an embodiment of the present application. The parts include: spectrum amplitude optimizer software110, a spectrum controller112, an emitter controller114, a xenon/IR lamp116, a blackbody radiator118, a UV laser120, a visible laser122, an NIR laser124, an SWIR laser126, an MWIR laser128, an LWIR laser130, a QCL laser source154, an integrating sphere132, a beam profiler134, a spectrometer136, and a sensor UUT138. The spectrometer136in turn feeds back to the spectrum controller112.

FIG.7shows a schematic diagram of an embodiment of the present application. In it, an electrical source144is connected by electrically conductive cables174to several laser emitters148, on the lower left hand side and lower right hand side of the schematic, respectively. On the lower left of the schematic, two of the laser emitters148beam light up to two collimating mirrors142, respectively. The two collimating mirrors142bend and reflect light so that the light beams into an integrating sphere132after passing through a diffraction grating170. The integrating sphere132works in combination with at least one diffraction grating170. The diffraction grating170is incorporated into the integrating sphere132so that several different laser sources can be combined into a single input port172of the integrating sphere132. The number of ports and port sizes, including the size of both the input port172and the output port (not shown) are limited by the diameter of the integrating sphere132and other design constraints. In general, fewer ports are more desirable. A simple theoretical analysis indicates that an up to 2 micron separation is necessary to combine two to three laser sources into a single input port172. This can be accommodated by a custom built diffraction grating170. The diffraction grating170functions to minimize the number of input ports. On the lower right of the schematic ofFIG.7, two of the laser emitters148are shown beaming light directly to the integration sphere132through optical/hollow fibers146rather than by means of collimating mirrors142. The integrating sphere132is used to uniformly scatter light rays. In the present application, for experiments with the integrating sphere132in the IR band, a gold coated, reflective, integrating sphere132has been utilized. However, an aluminum integrating sphere132, which is more difficult to maintain due to oxidation concerns, is preferred for visible spectrum applications because it minimizes the presence of gold colored tint in the visible spectrum. There are several ways to minimize the oxidation concerns arising from the aluminum integrating sphere132. One way uses an environmental chamber (not shown) to establish conditions in which the optimal materials (such as aluminum) can be used in the integrating sphere132. Another way uses coatings such as magnesium fluoride or lithium fluoride to decrease oxidation in an aluminum integrating sphere132. Coatings can provide good spectral reflectance down to the deep UV region, but the directional reflectance can be a concern when coatings are used. Thus, the calibration system is even more important for the coated aluminum integrating sphere132.

As stated previously, the collimating mirrors142beam the light to the integrating sphere132as shown in the schematic ofFIG.7. A xenon/IR lamp116and a blackbody radiator118are also each located separately in a position to beam light to the integrating sphere132as shown inFIG.7. The housing of the xenon/IR lamp116can be directly attached to the integrating sphere132. The integrating sphere132itself then beams the combined light onto the sensor UUT138through the beam shaping and profiling optics134, located directly below the integrating sphere132in this embodiment. The beam shaping optics134will provide a beam that is as uniform as possible in amplitude and as flat as possible in phase.

FIG.8shows a perspective view of one embodiment of the solar spectrum simulator152of the present application. In the embodiment, vertical poles156and horizontal poles158are positioned around the integrating sphere132which is at the top of the solar spectrum simulator152. The tops of the vertical poles156are connected perpendicularly to the horizontal poles158which radially extend from the connecting point with the vertical poles156into the integrating sphere132. The solar spectrum simulator152extends around and above a 3-axis motion table150on which can be placed an object to be tested. The spectrum composition optimizer software110is connected to and controls the activity of the spectrum controller software112which in turn is connected to and controls the activity of the emitter driver software114, all of which software is located outside of the solar spectrum simulator152. The emitter driver software114is in communication with the xenon/IR lamp116, the blackbody radiator118, and the laser emitters148in the solar spectrum simulator152. In order to be able to generate a customizable spectrum, several sources of light are employed. By using base spectrum emitters such as xenon/IR lamps116and high temperature cavity black body radiator sources118, a base spectrum is achieved. To achieve the desired resolution and amplitude of very hot objects (like the sun) without using an enormous power source, the present application adds to this base spectrum a set of laser emitters148. These include laser diodes such as UV lasers120, visible lasers122, NIR lasers124, SWIR lasers126, MWIR lasers128and LWIR lasers130, as well as at least one QCL laser source154. The laser emitters148therefore include both QCL lasers154and laser diodes. As shown, the laser emitters148are in the form of small units suspended directly below the beam collimator units142. The beam collimator units142are positioned at the top of the vertical poles156located around the solar spectrum simulator152. The light from the laser emitters148is beamed upward to the beam collimators142where it is then beamed from the beam collimators142radially to the integrating sphere132. This is done concurrently with the beaming of light from the xenon/IR lamp116and the blackbody radiator118to the integrating sphere132. The black body radiator118and xenon/IR lamps116are also each positioned at the top of vertical poles156located around the circumference of the solar spectrum simulator152.

In one embodiment, as shown inFIG.8, horizontal tubes158extend between the beam collimators142and the integrating sphere132. The light from the beam collimators142is beamed through the horizontal tubes158to the integrating sphere132. In an alternative embodiment, the light from the beam collimator142is beamed directly to the integrating sphere132without the horizontal tubes158.

Similarly, in one embodiment, also as shown inFIG.8, the xenon/IR lamp116and the blackbody radiator118each beam light through horizontal tubes158extending radially to the integrating sphere132. In another alternative embodiment, the xenon/IR lamp116and blackbody radiator118beam light directly to the integrating sphere132without the horizontal tubes158.

As shown inFIG.8, after receiving light directly from the xenon/IR lamp116, and the blackbody radiator118as well as from the laser emitters148via the beam collimators142, the integrating sphere132in turn beams light down vertically to the beam profiler134. The beam profiler134in turn beams light to the 3-axis motion table150. The 3-axis motion table150is situated to be able to contain an item to be tested on its surface.

FIG.9shows two separate sample spectra, an upper spectrum160and a lower spectrum162, taken from actual solar light and combined in a graph plotting wavelength (tens of micrometers) vs. irradiance (milliwatts per centimeter squared). As shown inFIG.9, the two sample spectra are superimposed on each other. These solar-light spectra inFIG.9are a model for the artificial spectra shown inFIG.10.

FIG.10shows a spectrum from the solar simulator of the present application in a graph plotting wavelength (tens of micrometers) vs. irradiance (milliwatts per centimeter squared). This spectrum ofFIG.10artificially approximates an actual solar spectrum. Specifically, it shows the results of superposing a base spectrum166and a laser spectrum164in an integrating sphere132. The spectrum ofFIG.10additionally shows filtered portions168in the UV to LWIR spectrum which have been achieved with absorptive filters (not pictured) and have been created to simulate atmospheric conditions.

In the solar simulator system of the present application, each of these xenon/IR lamps116, high temperature cavity blackbody radiator sources118, and laser emitters148are individually controlled to provide a specific spectrum in each wavelength region. A set of filters (not pictured) can optionally be used to create absorption bands in one or more regions, when needed, in order to create simulated atmospheric conditions or replicate specific amplitudes in light spectra. This is done by placing filters (not pictured) between at least one of a) the xenon/IR lamp116and the integrating sphere132; b) high temperature cavity blackbody radiator sources118and the integrating sphere132; and c) laser emitters148and beam collimating mirrors142. Analogously, absorption bands are created in real sunlight when actual atmospheric conditions cause it.

Each laser produces “in band radiance” (i.e. radiance computed from a specific light wavenumber to infinity for a blackbody at a given temperature) to replicate the level of solar irradiance that occurs in the upper atmosphere. One of the main advantages of using QCLs154is that ideally these lasers can be designed to output approximately any wavelength.

In the present application, the individual parts of the spectrum are superposed together by the integrating sphere132to form a single spectrum. In an embodiment, the integrating sphere132includes at least one diffraction grating170that allows several sources of light to be superposed together to form one broad spectrum. The integrating sphere132with the at least one diffraction grating170is designed to add together light sources having a sub micron bandwidth (i.e., below 1 micron). The combined beams are then shaped by a beam profiler134to produce a “top hat” profile to better simulate a uniform source. The optics output from the beam profiler134is split. A part of the optics output is sent to a spectrometer136to verify that the outputted spectrum matches the desired spectrum for the given sensor UUT138. The other part of the optics output, in the form of the outputted light spectrum, goes to the sensor UUT138being tested. The spectrum analyzer data from the spectrometer136is sent to the spectrum amplitude controllers110. The spectrum amplitude controllers110then modify the amplitude of the beams, if necessary, to conform to the requirements of the desired spectrum.

The present application can be used for the simulation and testing of the U.S. military's existing and future missiles, as well as for testing the IR sensors used for guided missiles. Among other things, the present application can also be used for testing solar panels, for medical uses, and for producing customized spectra for use with sensors that operate in one or more of UV, visible and IR spectra. For such sensors, it is often the case that they need to be tested with a spectrum that has a precise shape and amplitude.

The present application provides a flexible method to arbitrarily produce on demand a precise solar-like spectrum which extends from UV to LWIR. The spectrum in the present application can also be user controlled to change with time. The present application also uses several different light sources efficiently and superposes different beams of light into a whole, undivided spectrum rather than using a subtractive method.

The present application provides programmable, arbitrary spectra in the context of a simulated solar spectrum. The present application also avoids the difficulties associated with outdoor testing by providing a realistic spectral exposure in a laboratory environment.

The present application relates to a solar spectrum simulator152capable of producing a light spectrum including wavelengths from 200 nm to 20 microns. The simulator comprises: a) base spectrum emitters beaming light into an integrating sphere, the base spectrum emitters including: at least one xenon/infrared (IR) lamp116and at least one blackbody radiator118; b) laser generation emitters148beaming light into collimating mirrors142, the laser generation emitters148including at least one ultraviolet (UV) laser source120, at least one visible laser source122, at least one near infrared (NIR) laser source124, at least one short wavelength (SW) laser source126, at least one medium wavelength (MW) laser source128, at least one long wavelength (LW) laser source130, and at least one quantum cascade laser source (QCL)154; c) collimating mirrors142receiving beamed light directly from at least one of the laser generation emitters148; d) software including: i) amplitude optimization software110; ii) a spectrum controller112; ili) an emitter controller114, the emitter controller114being controlled by the amplitude optimization software110and the spectrum controller112; the emitter1controller114controlling the base spectrum emitters and the laser generation emitters148; e) the integrating sphere132including at least one diffraction grating170, the integrating sphere132receiving light from both the base spectrum emitters and the collimating mirrors142; f) a beam profiler134receiving beamed light from the integrating sphere132; g) a surface150providing space for an item to be tested, the surface receiving beamed light from the beam profiler134; and h) a spectrometer136receiving beamed light from the beam profiler134, the spectrometer136generating and feeding back wavelength information about the beamed light to the spectrum controller112.

In an embodiment of the solar spectrum simulator152, additional laser generation emitters148beam light directly to the integrating sphere132through optical/hollow fibers146, the light bypassing the collimating mirrors142.

In another embodiment of the solar spectrum simulator152, the solar spectrum simulator152has a spectral resolution of at most approximately 0.1 micrometer increments.

In still another embodiment of the solar spectrum simulator152, the xenon/IR lamp116, the blackbody radiator118and the laser generation emitters148are individually controlled.

In yet another embodiment of the solar spectrum simulator152, the solar spectrum simulator152is programmed to change spectra after a specific time interval of one microsecond or more.

In another embodiment of the solar spectrum simulator152, absorptive filters (not pictured) are inserted between at least one of a) the xenon/IR lamp116and the integrating sphere132, b) the blackbody radiator118and the integrating sphere132, and c) the laser generation emitters148and the collimating mirrors142for the purpose of simulating atmospheric absorption or for the purpose of modifying light amplitude to match a desired spectrum.

In still another embodiment of the solar spectrum simulator152, each laser of the laser generation emitters148produces at least approximately: 1)1W in the NIR and MWIR and 2) 50 mW in the LWIR, each of which is used to replicate the level of solar irradiance in the upper atmosphere.

In still another embodiment, each laser of the laser generation emitters148of the solar spectrum simulator152produces radiance from 1.0 to 100 W sufficient to replicate the level of solar irradiance in the upper atmosphere.

In yet another embodiment of the solar spectrum simulator152, the diffraction grating170in the integrating sphere132diffracts light sources into beams having a bandwidth less than 2 microns.

The present application also relates to a method of testing an item with a spectrally precise, artificial solar spectrum including wavelengths from 200 nm to 20 microns produced by a solar spectrum simulator152. The method comprises the steps of: a) directing beams of light from laser generation emitters148to collimating mirrors142; the laser generation emitters148including: at least one ultraviolet (UV) laser source120, at least one visible laser source122, at least one near infrared (NIR) laser source124, at least one short wavelength (SW) laser source126, at least one medium wavelength (MW) laser source128, and at least one long wavelength (LW) laser source130; the directing being accomplished by controlling the laser generation emitters148with spectrum optimizer software110, a spectrum controller112, and an emitter controller114; b) directing by collimating mirrors142the beams of light from laser generation emitters148to an integrating sphere132including a diffraction grating170; c) directing beams of light from base spectrum emitters to the integrating sphere132, the base spectrum emitters including at least one xenon/infrared (IR) lamp116and at least one blackbody radiator118; d) combining the directed beams of light with the integrating sphere132; e) profiling the combined beams of light with a beam profiler134to produce a profiled beam simulating a uniform source; f) measuring wavelengths of the profiled beams with a spectrometer136, the spectrometer136feeding back the wavelength of the profiled beams to the spectrum controller112; g) adjusting, if necessary, with the spectrum controller112the wavelength of the combined profiled beams, based on the feedback from the spectrometer136; h) beaming the combined beams onto a surface150on which is provided space for an item to be tested.

In an embodiment of the method, additional laser generation emitters148beam light directly to the integrating sphere132through optical/hollow fibers146, the light bypassing the collimating mirrors142.

In yet another embodiment of the method, the solar spectrum simulator152has spectral resolution of at most approximately 0.1 micrometer increments.

In still another embodiment of the method, at least one of the directing steps of the method is done by individually controlling the xenon/IR lamp116, the blackbody radiator118and the laser generation emitters148.

In another embodiment of the method, the solar spectrum simulator152is programmed to change spectra after a specific time interval of 1 microsecond or more.

In another embodiment of the method, absorptive filters (not pictured) are placed between at least one of a) the xenon/IR lamp116and the integrating . . . sphere132; b) the blackbody radiator118and the integrating sphere132; and c) the laser generation sources148and the collimating mirrors142, for the purpose of simulating atmospheric absorptions or for the purpose of modifying amplitude of the light to match a desired spectrum.

In still another embodiment of the method, each laser of the laser sources produces at least approximately 1)1W in the NIR and MWIR; and 2) 50 mW in the LWIR to replicate the level of solar irradiance in the upper atmosphere.

In another embodiment of the method, each laser of the laser generation sources produces radiance from 1.0 to 100 W sufficient to replicate the level of solar irradiance in the upper atmosphere.

In yet another embodiment of the method, the diffraction grating170in the integrating sphere132diffracts light sources into beams having a bandwidth less than 2 microns.

The present application also relates to a solar spectrum simulator system152which is capable of producing a light spectrum, including wavelengths from 200 nm to 20 microns. The system comprises: a) base spectrum emitters beaming light into an integrating sphere132, the base spectrum emitters including: at least one xenon/infrared (IR) lamp116and at least one blackbody radiator118; b) laser generation emitters beaming light into collimating mirrors142, the laser generation emitters148including at least one ultraviolet (UV) laser source120, at least one visible laser source122, at least one near infrared (NIR) laser source124, at least one short wavelength (SW) laser source126, at least one medium wavelength (MW) laser source128, at least one long wavelength (LW) laser source130, and at least one quantum cascade laser source (QCL)154; c) collimating mirrors142receiving beamed light directly from at least one of the laser generation emitters148; d) software including i) amplitude optimization software110; ii) a spectrum controller112; iii) an emitter controller114, the emitter controller114being controlled by the amplitude optimization software110and the spectrum controller112, the emitter controller114controlling the base spectrum emitters (xenon/infrared (IR) lamp116and/or blackbody radiators118) and the laser generation emitters148; e) the integrating sphere132including at least one diffraction grating170, the integrating sphere132receiving light from both the base spectrum emitters and the collimating mirrors142; f) a beam profiler134receiving beamed light from the integrating sphere132; g) a three-axis motion table150providing space for a sensor unit under test (UUT)138, the table150receiving beamed light from the beam profiler134; and h) a spectrometer136receiving beamed light from the beam profiler134, the spectrometer136generating and feeding back wavelength information about the beamed light to the spectrum controller112.

In an embodiment of the solar spectrum simulator system152, additional laser generation emitters148beam light directly to the integrating sphere132through optical hollow fibers146, the light bypassing the collimating mirrors142.

EXAMPLES

Example 1

Several spectral characteristics of laser diodes and QCL154have been measured, and potential beam combiner options have been evaluated. A small gold coated reflective integrating sphere132, including diffraction gratings170, has been devised and used successfully to combine two IR sources to produce a combined spectrum.

Example 2

Several devices including blackbody radiators118, IR lamps116, laser diodes and QCL lasers154were tested and used for their spectral characteristics in the solar spectrum simulator152of the present application. Blackbody radiators118, laser diodes and QCL lasers154have also been tested and found to have characteristics that are useful in a solar spectrum simulator152with a larger integrating sphere132, including diffraction gratings170, for specific wavelengths. IR lamps116have not yet been tested for their usefulness in a larger integrating sphere132.

Example 3

Tests were performed that showed that a preferred way to add visible spectrum to the IR spectrum is to add aluminum (or “aluminum-like” metal which does not add color) to reduce the gold tint in the visible spectrum that occurs when gold is used in the integrating sphere132.

Example 4

Different diffraction gratings170have been tested in a smaller gold coated reflective integrating sphere132and in a larger integrating sphere170. The results show the effectiveness of diffraction gratings170, which function to combine multiple sources of light and feed them into a single port.

Example 5

When missile sensors were tested using the present application, some testing was conducted in a controlled environment with, as much as possible, the precise spectra that sensors may be exposed to in a given real environment. In several cases, the tested spectra were able to replicate the imageries obtained with a given sensor in natural sunlight.

While several embodiments have been described in detail, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting.