Abstract:
A flux diffuser for radiometrically calibrating an imaging sensor using the sun as a calibration light source, the flux diffuser including a fiberglass cloth having input and output surfaces. The input surface receives solar irradiance, and the output surface provides diffused scattered light to a radiometer. A layer of mylar may be disposed on top of the input surface. A layer of PTFE or Spectralon™ may be disposed on top of the output surface, and another layer of mylar may be disposed on top of the layer of PTFE or Spectralon™.

Description:
TECHNICAL FIELD 
     The present invention relates, in general, to the field of radiometry and, more specifically, to a system and method for radiometric calibration of remote sensors in space employing solar radiation as a source of radiation. 
     BACKGROUND OF THE INVENTION 
     Planetary imagers are useful for remote sensing of atmospheric compositions, crop assessments, weather prediction and other types of monitoring activities. Monochromatic and multispectral satellite-based, remote sensors are able to measure properties of the atmosphere above the earth, when their detector arrays are properly calibrated for radiometric response. 
     A method of calibrating the radiance measured by these remote sensors is to create a reference radiation using a known source of spectral irradiance, such as the sun. The radiation from the sun may be used as a reference signal to a diffusive reflector which, in turn, may provide a known radiance to a remote sensor for calibrating its detector arrays. 
     The output of the detector arrays may be measured as the remote sensor receives the known diffusely reflected energy from the diffusive reflector. This radiance calibration method provides sufficient information to correctly measure and calculate other types of radiance incident on the remote sensor during normal operation, when using the output of the remote sensor, as the remote sensor views the earth or other target of interest. 
     The spectral reflectance characteristics of the diffusive reflector, or diffuser panel, however, may change with time due to degradation of the diffuser panel. Since the diffuser panel is employed as the reference source, any change, i.e., degradation of the diffusive surface material, results in a distortion in the measurements of the remote sensor. 
     Currently, a common diffuser used in on-orbit radiometric calibration is reflective in nature. Typically, such diffuser is made from PTFE or Spectralon™. The PTFE is a pressed polytetrafluoroethylene material. The PTFE, for example, may be sold under the trade name of Algoflon®, which is manufactured by Solvay Solexis of Thorofare, N.J. Another type of material used for a diffusive surface is a thermoplastic resin material sold under the trade name of Spectralon™, manufactured by Labsphere of North Sutton, N.H. An integrating sphere lined with the PTFE or Spectralon™, or lined with barium sulfite (BaSO 4 ) may also be used as a diffuser. 
     The aforementioned diffusers are of the reflective type. There are also transmissive diffusers. Transmissive diffusers may be made from ground or frosted glass; they may also be made from opal glass or small particulate scatterers placed in a transparent matrix. Transmissive diffusers may also be made from screens or pinhole arrays. Still another type of diffuser may be a diffractive diffuser, such as diffractive scatterers formed from micro-lens arrays or holographic material. 
     All reflective diffusers have disadvantages. One such disadvantage is that reflective diffusers must have a view of the sun at a limited angle, so that the reflected light from the diffuser is seen by the remote sensor. At very high incidence, or exit angle, a reflective diffuser becomes difficult to characterize. Materials, such as BaSO 4 , are brittle or fragile, which increases the probability of failure due to rocket launch vibrations. 
     Transmissive diffusers also have disadvantages. For example, ground or frosted glass has a limited spectral range. In addition, this glass is limited to output angles over which the sunlight scatter may occur. Ground or frosted glass also has a tendency to be heavy and fragile, resulting in drawbacks for on-orbit use due to rocket launch vibration risks. Similarly, opal glass or small particulate scatterers in a transparent matrix have a limited spectral range, due to particulate or inclusion size distribution. Since opal is glass, it also tends to be heavy and fragile, resulting in a drawback for on-orbit use, due to launch vibration risk. 
     Screens or pinhole arrays also have disadvantages, because their regular geometries may cause undesired diffraction effects. Screens are also difficult to calibrate over ranges of angles, due to the three-dimensional nature of the screens, which may cause internal shadowing. Furthermore, pinholes are subject to clogging from extraneous minute particles. Finally, diffractive scattering material, such as micro-lens arrays or holographic diffusers, have deficiencies in that they have a limited spectral range and a limited range of output angles over which the scatter may occur. Since they are made from glass, they tend to be heavy and fragile, resulting in drawbacks for on-orbit use, due to launch vibration risks. 
     The present invention solves the above disadvantages, by providing a unique transmissive diffuser, as will be described, for on-orbit radiometric calibration. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a flux diffuser for radiometrically calibrating an imaging sensor using the sun as a calibration light source. The flux diffuser comprises a fiberglass cloth including input and output surfaces. The input surface receives solar irradiance, and the output surface provides diffused scattered light to a radiometer. 
     An aspect of the invention includes a layer of mylar disposed on top of the input surface. Another aspect of the invention includes a layer of mylar disposed on top of the output surface. 
     Yet another aspect of the invention includes a layer of PTFE or Spectralon™ disposed on top of the output surface. 
     Still another aspect includes a layer of mylar disposed on top of the input surface, a layer of PTFE or Spectralon™ disposed on top of the output surface, and another layer of mylar disposed on top of the layer of PTFE or Spectralon™. 
     The fiberglass cloth is supported by a clear substrate. The fiberglass cloth may be supported by wires, or by opposing tension rollers cooperating to support the fiberglass cloth. 
     The fiberglass cloth includes fiber that is regularly woven or randomly woven. 
     A further embodiment includes an optical system for calibrating an imaging sensor array using the sun as a calibration light source comprising a port for viewing sunlight; an imaging sensor array for imaging a scene; and a flux diffuser, located between the port and the imaging sensor array, for illuminating the imaging sensor array with diffused sunlight. The flux diffuser includes a fiberglass cloth including input and output surfaces, where the input surface receives sunlight, and the output surface provides diffused scattered light to the imaging sensor array for radiometric calibration. 
     The mylar may be pressed onto respective surfaces of the fiberglass cloth for providing protective layers, and the layer of PTFE or Spectralon™ may be a thin layer of powder for binding the fiberglass cloth. 
     Another embodiment of the invention is a method of calibrating an imaging sensor array using the sun as a calibration light source in an optical system. The method includes the steps of: (a) receiving sunlight by an input surface of a fiberglass cloth; (b) diffusing the received sunlight; and (c) outputting the diffused sunlight, from an output surface of the fiberglass cloth, to the imaging sensor array for radiometric calibration. 
     An aspect of the method includes the step of protecting the input surface of the fiberglass cloth by adding a layer of mylar onto the fiberglass cloth. 
     Another aspect of the method includes the step of binding the fiberglass cloth by adding a layer of PTFE or Spectralon™ onto the fiberglass cloth. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures: 
         FIGS. 1A and 1B  are side views of a transmissive diffuser for radiometric calibration, in accordance with different embodiments of the present invention; 
         FIG. 2  is a schematic representation of the transmissive diffuser employed in  FIGS. 1A and 1B  receiving irradiance from the sun, or the earth, and providing diffused output light to a radiometer for calibration purposes, in accordance with an embodiment of the present invention; 
         FIG. 3A  is a schematic representation showing a method of mounting the transmissive diffuser employed in  FIGS. 1A and 1B ; 
         FIG. 3B  is a schematic representation showing an alternative method of mounting the transmissive diffuser employed in  FIGS. 1A and 1B ; and 
         FIG. 4  is a schematic diagram of an earth imaging system employing a radiometric calibration assembly, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a unique transmissive diffuser for on-orbit radiometric calibration. In general, a transmissive diffuser may use solar angles that are unavailable to a reflective diffuser. A transmissive diffuser, furthermore, may be made from lightweight, low outgassing, none brittle, radiation stable material that lowers risk of device failure prior to satellite launch and after satellite launch. Moreover, the materials used for the transmissive diffuser includes material already approved for the space environment. 
     Referring first to  FIG. 1A , there is shown a transmissive diffuser, generally designated as  10 . As shown, the transmissive diffuser of the present invention includes a thin fiberglass cloth, generally designated as  12 . In the exemplary embodiment shown in  FIG. 1A , fiberglass cloth  12  is sandwiched between a first layer of mylar, generally designated as  16 , and a thin layer of PTFE/Spectralon™ material, generally designated as  14 . Also shown is a second layer of mylar, generally designated as  18 . 
     It will be appreciated that the present invention may operate with a single layer of fiberglass cloth  12  and no other layers. Fiberglass cloth  12  is effective in receiving solar irradiance  20 , diffusing the solar irradiance, and providing a diffused scattered light output  22  to a radiometer (not shown). The solar irradiance may be received from a wide range of sun angles. 
     In another embodiment of the present invention, fiberglass cloth  12  may include a layer of PTFE/Spectralon™  14 , as shown in  FIG. 1A , without first or second mylar layers  16  and  18 . It will be appreciated that the PTFE/Spectralon™ layer may be used as an optional thin layer, or as a powder for binding fiberglass cloth  12 . Accordingly, layer  14  may actually be part of layer  12 , in which the PTFE is embedded into fiberglass cloth  12 . An embodiment in which the PTFE is embedded/impregnated into a fiberglass matrix is shown in  FIG. 1B , and is generally designated as  12   a.    
     In yet another embodiment of the present invention, fiberglass cloth  12  and PTFE/Spectralon™ layer  14  may be sandwiched between first mylar layer  16  and second mylar layer  18 . It will be appreciated that first and second mylar layers  16  and  18  may be used as a sealer to protect the fiberglass cloth. The mylar layers may be pressed, possibly hot pressed, into fiberglass cloth  12  and/or PTFE/Spectralon™ layer  14 . 
     It will be understood that wherever PTFE is used herein it may be either powder, pressed powder, a thin sheet, or PTFE embedded in fiberglass. PTFE embedded/impregnated into a fiberglass matrix may be obtained, for example, from the following supplier: Sheldahl (supplier of Beta cloth), 1150 Sheldahl Road, Northfield, Minn., 55057, 507-663-8000, Fax 507-663-8545. This is a space qualified product called Beta cloth which is typically aluminized and used for spacecraft thermal control surfaces, but it may be obtained without the aluminization layer. 
     Referring next to  FIG. 2 , there is shown transmissive diffuser  10  in operation. As shown schematically, transmissive diffuser  10  is placed in front of radiometer  28  (or remote imaging sensor  28 ), so that sunlight or earth light, or any other light, may be diffused and transmitted as diffused light onto radiometer  28 . In operation, transmissive diffuser  10  changes collimated sunlight into a diffused illumination source for viewing by radiometer  28 . 
     Referring next to  FIGS. 3A and 3B , there are shown two different implementations for supporting transmissive diffuser  10 , so that it is disposed between the received solar irradiance and the input to the radiometer. As shown in  FIG. 3A , transmissive diffuser  10  is supported by wires  26  that are attached to rectangular (or any other geometry) frame  24 . Tension to maintain transmissive diffuser  10  in a flat configuration may be achieved by sewing the ends of the fiberglass cloth of transmissive diffuser  10  onto stabilizing frame  24 . 
     Alternatively, as shown in  FIG. 3B , transmissive diffuser  10  may be maintained in a flat configuration by clamping ends of transmissive diffuser  10  directly onto rods  30 , or into slots  32  formed in rods  30 . The rods may be mounted with tortional springs, in order to maintain transmissive diffuser  10  under tension. 
     Referring to  FIG. 4 , there is shown another embodiment of the present invention. As shown, earth imaging system  46  is located above the surface of the earth and is pointed generally toward the earth to collect information by way of entrance  50 , leading to remote sensors. In order to calibrate earth imaging system  46 , a radiometric calibration assembly, generally designated as  40 , is incorporated into the interior of earth imaging system  46 . The radiometric calibration assembly may be moved into its calibration position by way of movable control mechanism  48 . 
     After being moved into its calibration position, light  20  from the sun enters earth imaging system  46  when a shutter or door  44  is opened (there may also be a configuration with an open port without any door). Imaging system  46  (or remote sensor system  46 ) includes a sensor array (not shown), which may be accessed by sunlight  20  by way of the entrance port to the sensor array designated as  50 . Light  20  encounters transmissive diffuser  10  which diffuses light  20  to form diffused light  22 . The diffused light then impinges on reference radiometer  28 . The diffused light is used for calibration purposes. Transmissive diffuser  10  is supported by frame  24 , via wires  26 , similarly to the configuration shown in  FIG. 3A  (the configuration shown in  FIG. 3B  may also be used). 
     It will be appreciated that an embodiment of this invention has been described in detail. Other variations may include the following: Most typically earth imaging system  46  may reside on a satellite or a drone. Other imaging systems may also use the described calibration hardware and methodology. For example, lunar based astronomical observatories and earth imagers may also benefit from using the transmissive diffuser of the present invention. Planetary imaging satellites also may benefit from the present invention. 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.