Field based spectral radiometer

Details Field based spectral radiometer Field based spectral radiometer

2002-02-13

2004-03-30

Porta, David (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

C250S339010

Reexamination Certificate

active

06713764

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a spectral radiometer designed to be left in the field on a stand-alone basis for prolonged periods of time (months to years) to measure the spectral characteristics of various earth surface targets in two bands.
BACKGROUND OF THE INVENTION
For several decades, spectral radiance data have been used to help map and monitor the earth's surface. The advent of civilian satellite imaging systems in the early 1970s propelled this technology into widespread use both in image format and in situ field measurements. Digital imaging systems carried on-board earth-orbiting satellites collect images using optical systems that record the earth surface spectral characteristics in various bands (e.g., the brightness/color in visible and near-infrared (NIR) spectral bands). Current satellite imaging systems have expanded spectral band coverage compared to those used in the 1970s and early 80s (e.g., short-wave IR—SWIR), resulting in the need for more sophisticated field instruments with increased spectral measurement capabilities. In order to help identify the design of future satellite imaging sensors and to collect spectral radiance data in the field for use with current state-of-the-art satellite and airborne imaging systems, the need to design and build more complex and sophisticated spectral radiometers for in-the-field use has driven both the cost and constraints of such an instrument quite high. The cost of spectral radiometers now range from $12K to $80K for field instruments that can collect data up to 1024 spectral bands. For applications that require only a few bands (e.g., 2 to 6), with a high temporal resolution over a prolonged period of time, using a current state-of-the-art spectral radiometer is out of the question. Both in cost and design the currently available spectral radiometers are not made for long-term stand-alone field use. Therefore, they are used only for short-term studies and applications while the user is in the field, meaning that they cannot provide the high temporal resolution needed for long-term studies and operational monitoring of the earth's surface.
Spectral radiometers were developed for in-the-field use for remote sensing in the early 1970s. Radiometers with four broad spectral bands were designed first, then the number of spectral bands began to increase and the band width to decrease to allow better total spectral measurements to be collected of the cover types of interest (i.e., soils, vegetation, or water). In the late 1980s and early 1990s, spectral radiometers having over 250 narrow spectral bands were designed and built and now ones with 512 to 1024 bands (hyper-spectral) are becoming the standard. However, there are a number of problems with the use of current hyper-spectral radiometers, including:
1) The amount of data they are designed to collect is typically an “overkill” for many applications and operational monitoring of the earth's surface (e.g., surface waters and general vegetation cover). That is, over 500 bands of spectral radiance data are not needed for many applications, especially for non-research operational and monitoring uses.
2) The cost for the much more complex and sophisticated instruments now on the market ranges from $12,000 to $80,000. This cost is quite high for most non-research applications, especially if several to tens of them are needed for good spatial monitoring over a regional area.
3) The spectral radiometers currently on the market are designed to be used in the field for a relatively short amount of time by a person while doing fieldwork. Current radiometers are not designed to be left in the field for long periods of time (i.e, months to years) in a stand-alone node to automatically collect spectral radiance data with a high temporal resolution.
4) The current spectral radiometers with over 500 bands have narrow bandwidth, so the overall signal-to-noise ratio is low compared to the more broadband width radiometers. Therefore, for low radiance targets, such as surface waters, the noise levels will typically be higher than for those collected by a less complex broadband radiometer.
5) A spectral radiometer with four bands was developed in the early 1970s by Exotech, Inc., for use in the field while the operator was present; it is not designed to be left in the field on a stand-alone basis for prolong periods of time.
Satellite image data can be used to monitor various features on the earth's surface with additional spectral windows. However, major problems with using satellite image data to monitor surface water parameters, and on-land vegetation cover, are that the combination of temporal and spatial resolutions often needed are well beyond the capability of current satellite imaging systems. To obtain the temporal resolution needed of minutes to hours, and a spatial resolution of one to three meters required to see the surface waters of rivers, or for daily monitoring of vegetation within a small area, a field based instrument is required.
A number of other types of radiometers have been disclosed which have been used for a variety of purposes.
Goetz et al., in U.S. Pat. No. 4,345,840, disclose a hand held, self-contained dual beam rationing radiometer for identifying selected materials that reflect radiation within a predetermined band, preferably in the IR or visible range. The apparatus includes two pivoting optical trains directed toward the same target. Each train has a separate filter for selection of the narrow spectral bands to be ratioed, by means of a dividing circuit, to identify a particular substance based on its known spectral characteristics.
Spiering et al., in U.S. Pat. No. 6,020,587, disclose a device for measuring plant chlorophyll content by collecting light reflected from a target plant. A beam splitter separates the light into distinct wavelength bands or channels. Photo-detectors and amplifiers within the device then process the bands, converting them into electrical signals.
Gupta discloses, in U.S. Pat. No. 4,996,430, a device for distinguishing target objects having substantially identical reflectance ratios for two separated wavelengths (lambda-1, lambda-2), from background objects possessing different reflectance values for the same two wavelengths. This device includes an active optical sensor with first and second transmitters that transmit signals at wavelengths of lambda-1 and lambda-2, respectively. When the transmitted signals reflect off of an object, a receiver senses the reflected signals, which are then processed through a high-speed preamplifier and amplifier, producing voltages V1 and V2 at the receiver's output. A conventional ratio calculating circuit then calculates a ratio of V1 and V2, which is then compared to a predetermined threshold value.
Levin et al., in U.S. Pat. No. 6,031,233, disclose a handheld spectrometer for identifying samples based upon their IR reflectance. The device includes a window and adjacent optical bench. The optics, which align directly with the sample under investigation, consist of a broad band IR light shining onto an acousto-optic tunable filter crystal, the latter passing narrow band IR light with a swept frequency; a lens focusing the IR light through the window onto the sample; and a reflectance detector aligned with the housing window to detect light reflected from the sample. A computer mounted in the housing compares the reflectance spectrum with stored data and identifies the sample material.
Novison, in U.S. Pat. No. 5,527,062, discloses a portable IR spectrophotometer for testing samples of organic construction materials. A single source of IR light is divided into two beams. The first beam is directed to the surface of the target sample, from which the light is reflected at least four times. The second reference beam is directed toward a neutral surface. The two beams are then combined and focused onto a detector element. The detector output is proportional to the energy absorbed by the test sample. A pen recorder attached to the apparatus generates a

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