Abstract:
A spectral reflectance sensor including: a light source for emitting a modulated beam of red light; a light source for emitting a modulated beam of near infrared light; a receiver for receiving reflected light produced by either the red source or the near infrared source; a receiver for receiving incident light from either the red source or the infrared source; a signal conditioner responsive to the modulation such that the signals produced by the receivers in response to reflected and incident light from the source can be discriminated from signals produced by ambient light; and a microprocessor having an input such that the microprocessor can determine the intensities of incident red light, reflected red light; incident near infrared light; and reflected near infrared light. From these intensities, and by knowing the growing days since emergence or planting, the sensor can calculate the mid-growing season nitrogen fertilizer requirements of a plant.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to a sensor for use in precision farming. More particularly, but not by way of limitation, the present invention relates to an optical spectral reflectance sensor and controller for use in a site specific fertilization system.  
           [0003]    2. Background  
           [0004]    “Precision fanning” is a term used to describe the management of intrafield variations in soil and crop conditions. “Site specific fanning”, “prescription farming”, and “variable rate application technology” are sometimes used synonymously with precision farming to describe the tailoring of soil and crop management to the conditions at discrete, usually contiguous, locations throughout a field. The size of each location depends on a variety of factors, such as the type of operation performed, the type of equipment used, the resolution of the equipment, as well as a host of other factors. Generally speaking, the smaller the location size, the greater the benefits of precision farming, at least down to approximately one square meter.  
           [0005]    Typical precision farming techniques include: varying the planting density of individual plants based on the ability of the soil to support growth of the plants; and the selective application of farming products such as herbicides, insecticides, and, of particular interest, fertilizer.  
           [0006]    In contrast, the most common farming practice is to apply a product to an entire field at a constant rate of application. The rate of application is selected to maximize crop yield over the entire field. Unfortunately, it would be the exception rather than the rule that all areas of a field have consistent soil conditions and consistent crop conditions. Accordingly, this practice typically results in over application of product over a portion of the field, which wastes money and may actually reduce crop yield, while also resulting in under application of product over other portions of the field, which may also reduce crop yield.  
           [0007]    Perhaps even a greater problem with the conventional method is the potential to damage the environment through the over application of chemicals. Excess chemicals, indiscriminately applied to a field, ultimately find their way into the atmosphere, ponds, streams, rivers, and even the aquifer. These chemicals pose a serious threat to water sources, often killing marine life, causing severe increases in algae growth, leading to eutrophication, and contaminating potable water supplies.  
           [0008]    Thus it can be seen that there are at least three advantages to implementing precision farming practices. First, precision farming has the potential to increase crop yields, which will result in greater profits for the farmer. Second, precision farming may lower the application rates of seeds, herbicides, pesticides, and fertilizer, reducing a farmer&#39;s expense in producing a crop. Finally, precision farming will protect the environment by reducing the amount of excess chemicals applied to a field which may ultimately end up in a pond, stream, river, and/or other water source.  
           [0009]    Predominately, precision farming is accomplished by either: 1) storing a prescription map of a field wherein predetermined application rates for each location are stored for later use; or 2) by setting application rates based on real-time measurements of crop and/or soil conditions. In the first method, a global positioning system (GPS) receiver, or its equivalent, is placed on a vehicle. As the vehicle moves through the field, application rates taken from the prescription map are used to adjust variable rate application devices such as spray nozzles. A number of difficulties are associated with the use of such a system, for example: due to the offset between the GPS receiver and the application element, the system must know the exact attitude of the vehicle in order to calculate the precise location of each nozzle or application element, making it difficult to accurately and precisely treat the target area; soil and plant conditions must be determined and a prescription developed and input prior to entering the field; and resolving a position with the requisite degree of accuracy requires relatively expensive equipment.  
           [0010]    In the latter method, a sensor is used to detect particular soil and plant conditions as the application equipment is driven through the field. The output of the sensor is then used to calculate application rates and adjust a variable rate applicator in real time. Since the physical relationship between the sensor and the applicator is fixed, the problems associated with positional based systems (i.e., GPS) are overcome. In addition, the need to collect data prior to entering the field is eliminated, as is the need for a prescription map.  
           [0011]    The limiting factor, thus far, in the later method has been the degree to which sensors are available which provide meaningful information concerning conditions within the field. For example, U.S. Pat. No. 5,585,626 issued to Beck et. al., and U.S. Pat. No. 5,763,873, likewise issued to Beck et al., discloses a sensor which detects plants in a field so that herbicide may be selectively applied to unwanted plants. Unfortunately, these devices discriminate only between soil and a plant. Thus, as a sprayer is passed over areas where there should only be bare soil, herbicide will automatically be applied to any plants detected. In practice, the sensors of the Beck &#39;626 and &#39;873 patents have proven to be temperature sensitive and thus, to require nearly continuous monitoring and regular re-adjustment while being used. Furthermore, due to the nature of these devices, the distance between the sensor and the ground must be maintained with a relatively high degree of precision. Another limitation is that presently, no such sensor exists for the application of nitrogen fertilizer.  
           [0012]    Thus it is an object of the present invention to provide a sensor for use in precision farming which provides an output indicative of one or more growing conditions over a relatively small area, which may be used for the selective application of a farming product or used in the development of a prescription map.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention provides a sensor for use in precision farming which satisfies the needs and overcomes the problems discussed above. The sensor measures the reflectance of a target at one or more wavelengths of light and, based on known reflectance properties of the target, produces an output indicative of the need for a given farm product.  
           [0014]    In a preferred embodiment, the sensor comprises: a light emitter which provides one or more light sources, each source producing light at a specific wavelength; a modulator for modulating each light source at a particular frequency, a reflected light receiver for receiving, detecting, and discriminating each wavelength of light; a direct receiver for receiving light directly from each source; and a processor for gathering information from the receivers and processing such information to determine reflectance of a plant and to determine the need for a given product based on the reflectance information.  
           [0015]    The reflectance properties of a target are-known to vary based on the amount of nitrogen available to the plant. By observing the reflected light, at particular wavelengths, preferably in the ranges of red and near infrared, and the intensity of the light source at the same wavelengths, it is possible to predict, with a reasonable degree of certainty, the expected crop yield with the present level of available nitrogen and the maximum crop yield if an ideal amount of nitrogen fertilizer is added. This information may be used in real time to control a variable rate applicator for applying a mid-growing season nitrogen fertilizer or, alternatively, used to develop a prescription map for later application of mid-growing season nitrogen fertilizer to a field. Although the inventive sensor could be easily adjusted for any particular target size, the preferred embodiment allows precision farming of sites having an area of approximately four square feet.  
           [0016]    Another feature of the inventive device is that the accuracy of the resulting measurement is relatively independent of the height of the sensor above the ground. Thus, as a vehicle equipped with the sensor moves through a field, the output of the sensor is consistent regardless of the terrain and its effect on the height of the sensor.  
           [0017]    Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 provides a side view of a farming vehicle having an inventive spectral reflectance sensor mounted thereon.  
         [0019]    [0019]FIG. 1A provides a top view of the farming vehicle showing a typical arrangement of the inventive sensors in use to control the selective application of a farm product.  
         [0020]    [0020]FIG. 2 provides a perspective view of a preferred embodiment of the spectral reflectance sensor.  
         [0021]    [0021]FIG. 3 provides a cutaway back view of the spectral reflectance sensor.  
         [0022]    [0022]FIG. 4 provides a cutaway side view of the spectral reflectance sensor.  
         [0023]    [0023]FIG. 5 provides a schematic representation of a preferred emitter circuit as employed in the inventive sensor.  
         [0024]    [0024]FIG. 6 provides a block diagram of a preferred receiver circuit employed in the inventive sensor.  
         [0025]    [0025]FIG. 7 provides a block diagram of the preferred circuitry of the circuit board employed in the inventive sensor.  
         [0026]    [0026]FIG. 8 provides a cutaway perspective view of a preferred multiple sensor embodiment of the inventive reflectance sensor.  
         [0027]    [0027]FIG. 9 provides a perspective view of the multiple sensor embodiment.  
         [0028]    [0028]FIG. 10 provides a cutaway side view of an improved reflectance sensor.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the construction illustrated and the steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.  
         [0030]    Referring now to the drawings, wherein like reference numerals indicate the same parts throughout the several views, a preferred embodiment of the inventive reflectance sensor  20  is shown in its general environment in FIGS. 1 and 1A. In one application, a plurality of sensors, of which sensors  20  are representative, are disposed along boom  22  at substantially equal spacings. Preferably, boom  22  extends laterally from vehicle  24 . Spray nozzles, of which nozzle  26  is representative, are also disposed along boom  22  preferably such that a nozzle  26 , or group of nozzles, corresponds to each sensor  20 . As the vehicle  24  travels along a crop row, boom  20  projects over the plants such that each sensor  20  evaluates the plant or plants in its immediate view, determines the extent to which nitrogen fertilizer is needed, and controls the rate of application of fertilizer through its corresponding nozzle  26 .  
         [0031]    Referring next to FIGS. 2 and 3, reflectance sensor  20  comprises: a housing  28 ; an emitter lens  30  located on the bottom of housing  28 ; and a receiver lens  32  likewise located on the bottom of housing  28 . Preferably lens  32  and lens  30  will be selected such that the light emitted by sensor  20  will illuminate an area of consistent size and shape over a range of heights above the ground and likewise, sensor  20  will detect the reflected light consistently from approximately the same area as is illuminated by the sensor.  
         [0032]    As best seen in FIGS. 3 and 4, housed within housing  28 , sensor  20  further includes parabolic reflector  34  secured to divider  36 , for focusing light received through receiver lens  32  onto photodetector  38 . Reflected light photodetector  38  projects through aperture  116  in divider  36  to receive only light received through receiver lens  32 .  
         [0033]    While not critical to the operation of the present sensor, it should be noted that in the preferred embodiment, the light source LEDs  44   a - e  and  46   a - e  are located in close physical proximity to the detector  38 . This feature minimizes the effect of sensor height on the location of the illuminated surface relative to the field-of-view of the detector  38 . In addition, the LED&#39;s  44   a - e  and  46   a - e  are distributed along a row and, furthermore, the light is projected through a cylindrical lens  30  oriented parallel with the row of LEDs  44   a - e  and  46   a - e  to restrict the spread of light in the direction of travel to further minimize the effects of variations in sensor height. Finally, the receiver employs a parabolic reflector  34  with the detector  38  located at the point of focus of the reflector  34  to establish a field-of-view of the detector  38  which is relatively independent of sensor height. Preferably the reflector  34  is in the form of an offset section of a 3-dimensional parabaloid such that detector  38  may be located outside of the field-of-view. This construction results in a sensor which produces consistent results relatively independent of sensor height, particular when compared to prior art sensors.  
         [0034]    Located on the opposite side of divider  36 , sensor  20  includes circuit board  40  and light pipe  42 . Circuit board  40  includes a first light source for producing red light made up of a row of LEDs,  44   a - e  which are positioned to project light downward through lens  30 ; and a second light source for producing light in the near infrared range made up of a row of LEDs  46   a - e,  preferably arranged such that LEDs  44   a - e  and  46   a - e  are aligned in a row wherein adjacent LEDs alternate between red and near infrared. Preferably, LEDs  44   a - e  produce red light having a wavelength of approximately 670 nanometers while LEDs  46   a - e  produce light in the near infrared range (“NIR”), having a wavelength of approximately 780 nanometers. Light pipe  42  receives and collects incident light from LEDs  44   a - e  and  46   a - e  and transmits such light to photodetector  48 . Detector  48  thus provides a means for directly measuring the intensity of the light produced by LEDs  44   a - e  and  46   a - e.  Since light pipe  42  collects light from all of the LEDs  44   a - e  and  46   a - e,  the sensor can measure true reflectance from its field-of-view. In addition, by storing a baseline intensity in nonvolatile memory, the sensor can determine when an LED fails based on a reduced intensity of incident light.  
         [0035]    The circuitry of circuit board  40  is shown in FIGS.  5 - 7 . Beginning with FIG. 7, circuit board  40  includes: microprocessor  50 ; an emitter circuit  52  having a color input  54  and an intensity input  56 ; a receiver circuit  68  having an input  70  to select between the signal representing the light striking the reflected light photodetector  38  or the signal representing the light striking the direct photodetector  48 , and an analog output  72  which is connected to track and hold analog to digital converter  58 . In the preferred embodiment, analog to digital converter  58  provides 12 bits of resolution although analog to digital converters of more or less resolution are suitable for use with the present invention. Microprocessor  50  includes outputs  60 ,  62 , and  64  for driving variable rate applicator  66  which will be discussed in more detail, hereinbelow.  
         [0036]    The term “microprocessor” is used in its broadest sense to describe any computing device. In addition to devices generically known as microprocessors, the term includes, by way of example and not limitation, microcontrollers, RISC devices, ASIC devices manufactured to provide logical and mathematical functions, FPGA devices programmed to provide logical and mathematical functions, computers made up of a plurality of integrated circuits, and the like.  
         [0037]    Referring next to FIG. 5, preferably emitter  52  comprises: an oscillator  74  which produces a periodic output having a frequency of approximately  40  kilohertz; switch  76 ; amplifiers  78 ,  80 ,  82 , and  84 ; LEDs  44   a - e  and  46   a - e;  and current mirrors  86  and  88 .  
         [0038]    Analog switch  76  includes a color input  54  and an intensity input  56 . When color input  54  is at a first binary state, the output of oscillator  74  is directed to either amplifier  78  or amplifier  80 , depending on the state of intensity input  56 . When color input  54  is at its second binary state, the output of oscillator  74  is instead directed to either amplifier  82  or amplifier  84 , likewise depending on the state of intensity input  56 . Thus, input  54  allows selection of the color of the lighted emitted, either red, from LEDs  44   a - e  when input  54  is in its first binary state, or near infrared, from LEDs  46   a - e  when input  54  is in its second binary state.  
         [0039]    Amplifiers  78 ,  80 ,  82 , and  84  are each a transistor, wired in an emitter-follower configuration. Depending on the states of inputs  54  and  56 , only a single amplifier  78 - 84  will receive an input from oscillator  74  at any given time. Preferably resistor  96  will be selected such that its impedance is approximately twice that of resistor  94  and resistor  100  will be selected such that its impedance will be twice that of resistor  98 . The outputs of amplifiers  78  and  80  are directed to current mirror  86  while the outputs of amplifiers  82  and  84  are directed to current mirror  88 . The current which flows through transistor  102   a  will be roughly proportional to the current flowing through transistor  102   c.  Similarly, the current flowing through transistor  104   a  will be roughly proportional to the current flowing through transistor  104   c.  Accordingly, a greater current will flow through LEDs  44   a - e  when amplifier  78  is selected than when amplifier  80  is selected, and in a similar manner, more current will flow through LEDs  46   a - e  when amplifier  82  is selected than when amplifier  84  is selected. Thus, by manipulating inputs  54  and  56 , microprocessor  50  can select a color of light, between red or near infrared, and can select, between two choices, the intensity of the light produced.  
         [0040]    Preferably, oscillator  74  will produce a waveform which approximates a sine wave, thus having harmonic content substantially less than that of a square wave. When LEDs  44   a - e  and  46   a - e  are driven in this fashion, the electrical current flowing through the LED&#39;s will have a harmonic content far below that of a square wave current. Accordingly, the light produced by each individual LED  44   a - e  or  46   a - e,  will result in a modulated light beam having substantially less harmonic energy than would be produced if modulated with a square wave.  
         [0041]    Referring now to FIG. 6, receiver  68  preferably comprises: a first amplifier  106  for amplifying a signal produced by direct light photodetector  48 ; a second amplifier  108  for amplifying a signal produced by reflected light photodetector  38 ; amplifier  114  providing ambient light compensation means for reducing the effects of ambient light on photodetector  38 ; analog selector  110  for selecting either the direct light signal or the reflected light signal as directed by input  70 ; and signal conditioner  112 . Signal conditioning is generally known in the art and typically includes filtering and, if necessary, amplification of the signal. Signal conditioner  112  can also be thought of as a discriminator. Preferably signal conditioner  112  includes a second order band pass filter centered about  40  kilohertz. Since LEDs  44   a - e  and  46   a - e  are modulated at  40  kilohertz, signal conditioner  112  will discriminate between the signal resulting from the light produced by LEDs  44   a - e  and  46   a - e  and “noise” resulting from the signal produced by other light sources. As will be apparent to those skilled in the art, a variety of other methods, such as synchronous demodulation, are available to discriminate between the reflected, modulated light, and unwanted ambient light.  
         [0042]    It should be noted that the light produced by LEDs  44   a - e  and  46   a - e  will contain a DC component if the signal from oscillator  74  (FIG. 5) is superimposed on a DC voltage. For the purposes of this invention, the steady-state light so produced is considered part of the ambient light and the low frequency affects of such light, particularly as the sensor height varies or from terrain variations, on reflected light detector  38  will also be compensated for by amplifier  114  and ultimately removed by signal conditioner  112 .  
         [0043]    In particular, amplifier  114  allows detector  38  to operate over a wider range of ambient lighting conditions without saturating. In addition, capacitors  116  and  118  AC couple the outputs of photodetectors  48  and  38 , respectively, to eliminate DC offset voltages from the signals produced by detectors  48  and  38  which could otherwise cause the outputs of amplifiers  106  and  108  to saturate. In general, ambient light compensation means includes any of these techniques which reduce the degree to which ambient light may degrade or impede reception of the reflected light from light sources  44  and  46 , or any other technique for improving the signal to noise ratio of the received signal.  
         [0044]    Receiver circuit  68  provides output  72  which produces an amplified and filtered version of the signal received by one of the two detectors  38  or  48 , as selected by input  70 . The operation of analog to digital converter  58  is synchronized with the oscillator  74  so that conversions are performed on the peaks of the signal present at output  72 .  
         [0045]    In operation, the sensor is passed over crops such that modulated light from the emitter, both red and near infrared, is reflected back through the receiver lens and focused by the parabolic reflector onto the photodetector. The microprocessor  50  directs analog to digital converter  58  to read the received signal at the peaks of the waveform output by signal conditioner  112  for both the output of the reflected light detector  38  and the direct light detector  48 . By calculating the ratio of the light reflected at each wavelength, i.e. the reflected intensity divided by the source intensity, the reflectance of the crop at each wavelength can be determined.  
         [0046]    In a typical system, multiple sensors  20  will be used to scan a contiguous strip across one or more rows of plants. In such a system, microprocessor  50  receives synchronization timing information from a central source and uses that timing information to synchronize emission from emitter circuit  52 . This process assures that light from adjacent sensors  20  are synchronized and consequently that such sensors do not interfere with each other. In a preferred method for synchronizing multiple units, microprocessor  50  includes a controller area network (CAN) interface. Such networks are well known in the art. To synchronize the emitters, a message is periodically transmitted on the network from the central source. Upon receipt of this message, each sensor sets the output of oscillator  74  to a known position in its periodic waveform, thereby synchronizing all sensors connected to the network.  
         [0047]    Once the microprocessor  50  has gathered reflectance information from receivers  38  and  48 , it is necessary to process the information to determine the need for nitrogen. One method for using reflectance information to determine such a need is disclosed in co-pending U.S. patent application, Ser. No. ______, entitled “A Process for In-Season Fertilizer Nitrogen Application Based on Predicted Yield Potential,” filed contemporaneously herewith, which is incorporated herein by reference.  
         [0048]    To summarize the process, data from the sensor is used to predict the potential yield that can be achieved with additional, mid-growing season, nitrogen fertilization based on an in-season response index given by the equation:  
         
       YP 
       N 
       =YP 
       0 
       *RI 
       NDVI  
     
         [0049]    where:  
         [0050]    YP N  is the predicted or potential yield that can be achieved with additional fertilizer;  
         [0051]    YP 0  is the predicted or potential yield based on growing conditions up to the time of sensing, that can be achieved with no additional nitrogen fertilizer;  
         [0052]    RI NDVI  is the In-Season-Response-Index computed as NDVI from Feekes 4 to Feekes 6 from a non-N-limiting fertilized strip divided by NDVI from Feekes 4 to Feekes 6 in the farmer&#39;s field fertilized in the common practice employed by the farmer, located adjacent to the non-N-limiting strip; and  
         [0053]    NDVI is the normalized difference vegetation index calculated as (NIR−red)/(NIR+red) where NIR and red are reflectance values measured by the inventive sensor.  
         [0054]    YP N  can then be used to predict the percent of nitrogen (PNG) contained in the crop calculated as:  
           PNG=− 0.1918 *YP   N +2.7836 ( PNG  in %  N );  
         [0055]    the predicted grain nitrogen uptake (GNUP) is calculated as:  
           GNUP=PNG *( YP   N /100);  
         [0056]    the predicted forage nitrogen uptake (FNUP) calculated as:  
           FNUP   NDVI =14.76+0.7758 e   5.468*NDVI ;  
         [0057]    and finally, the in-season mid-growing season fertilizer nitrogen requirement (FNR) is calculated as:  
           FNR =( GNUP−FNUP )/0.70  kg/ha.    
         [0058]    As will be apparent to those skilled in the art, the above series of calculations may easily be performed in microprocessor  50  and FNR can then be used to control the rate of fertilizer application through a variable rate applicator  66 .  
         [0059]    Due to variations in various components employed in the inventive sensor, it may be desirable to provide a calibration procedure. Typically, the, sensor may be calibrated by shining the emitter at an object having known reflectance properties and comparing the signal produced by photodetector  38  to an expected value. The response of the entire system may be determined independently for both red and near infrared and may be determined at each of the two selectable intensities. Constants may then be calculated and stored in non-volatile memory within microprocessor  50  for later use to correct subsequent measurements of plant reflectance.  
         [0060]    One advantage of the present invention is that each individual sensor  20  may be independently calibrated. When multiple sensors are employed, for example along a boom, and each sensor has been properly calibrated, the amount of nitrogen available to each plant will be consistent throughout the field, regardless of which particular sensor scans any given plant.  
         [0061]    Referring again to FIG. 5, emitter  52  can be adapted to output more light by adding additional LED&#39;s. Preferably, additional LEDs  44   f - j  and  46   f - j  would be driven by adding additional current mirror transistors  102   b  and  104   b,  respectively. In this way, the electrical current flowing through LEDs  44   a - e  would closely approximate the current flowing through LEDs  44   f - j  and similarly, the electrical current flowing through LEDs  46   a - e  would closely approximate the current flowing through LEDs  46   f - j,  resulting in consistent brightness throughout the individual LEDs of a given color. As will be apparent to those skilled in the art, additional groups of LEDs could be added to achieve any desired level of intensity. In addition, the number of red LED&#39;s  44  need not equal the number of infrared LEDs  46 .  
         [0062]    In a similar vein, it should also be noted that additional wavelengths of light may also be produced by sensor  20 . A switch similar to switch  76  could be employed which provides the desired number of individual switches to accommodate the desired additional wavelengths of light. For each wavelength emitted, additional amplifiers, current mirrors, and LEDs are employed in the same configuration as those presently employed. Thus, the present invention is suitable for measuring the reflectance at any number of wavelengths. Incorporating additional wavelengths of light allows the sensor to separate confounding factors from the estimate of nitrogen as well as increase the number of possible applications for the sensor. When more than two wavelengths of light are produced, it may be more practical to arrange the light emitting diodes into additional parallel rows with appropriately configured lenses such that the gap between individual LEDs of the same color does not become large enough to leave unilluminated holes in the field-of-view. For purposes of this invention, the parallel rows are viewed collectively as a single row having a width greater than one LED.  
         [0063]    As mentioned hereinbefore, prior art reflectance sensors are available to direct the selective application of herbicide to eliminate unwanted plants in a field. Features of the inventive sensor may be incorporated into such a prior art sensor to improve the performance of such a sensor and to adapt such a sensor for use in the application of other farming products. For example, the sensors described in U.S. Pat. No. 5,763,873 issued to Beck et al., which is incorporated herein by reference, and U.S. Pat. No. 5,789,741 issued to Kinter et al., likewise incorporated herein by reference, could easily be adapted to house the inventive circuitry. As shown in FIG. 10, light source  210  directs a beam of light, as indicated by line  212 , through lense  214  towards a target. Light reflected by the target, as indicated by line  216 , is focused by lense  226  on to reflected light photodetector  218 . In accordance with the present invention, a portion of the light emitted from all of the emitters which collectively comprise light source  210  is collected by light pipe  220  and directed to direct light photodetector  222 . While mechanically, sensor  224  differs from sensor  20  (FIG. 2), the circuitry employed therein may be identical, resulting in a sensor which provides improved accuracy in regard to the measured reflectance over the prior art design and which is, therefore, suitable for purposes beyond the selective application of herbicide.  
         [0064]    As will be apparent to those skilled in the art, the inventive sensor  20  may be scaled upward or downward in size to achieve virtually any desired resolution. Thus, while the preferred embodiment is suitable for sites of approximately four square feet, the invention is not so limited. With presently available light sources and detectors, a resolution of less than 12 square inches per site is possible.  
         [0065]    It should also be noted that, while the inventive sensor can provide an output to directly control the rate of application of fertilizer, it could also be used to create a prescription map for later application of fertilizer. In such a system, positional information is obtained from a GPS receiver, from the vehicle carrying the sensor, or from some other position indicating system. Crop conditions are stored in nonvolatile memory in such a manner that the condition at each specific site within the field may be later recalled for use in developing a prescription map.  
         [0066]    It should be further noted that multiple sensors  200  may be incorporated into a single housing  202  as shown in FIGS. 8 and 9. Typically, each sensor  200  would be constructed in accordance with the description of sensor  20  and have its own microprocessor for performing the calculations to arrive at the required mid-growing season fertilizer. This arrangement is particularly well suited for a use on a spray boom where, ideally, sensors are aligned, end-to-end along the boom. Optionally, instead of directly controlling a variable rate applicator, the sensors may provide reflectance data or growing conditions to a central computer via a communication network, such as a CAN network. The central computer would both direct an array of variable rate applicators and create a log of crop conditions throughout the field for later analysis or to create a history for comparison with those of other growing seasons.  
         [0067]    In a multiple sensor configuration, it is also possible for a single microprocessor to control multiple sensors. When multiple sensors are controlled by a single processor, or when sensors are interconnected via a network, and the resolution is reduced, it is possible to perform image processing within an individual sensor to determine such factors as: missing plants; non-uniform plant stands; chlorophyll concentration; infestation of disease or pests; changes in soil background; etc. In addition, networking of the individual sensors allows conditions to be averaged over a larger area for applications where a larger site is desired and networking enables microprocessor controllers to use information collected from sensors not under their direct control to make treatment decisions.  
         [0068]    Through the proper selection of resolution and appropriate use of reflectance information from various wavelengths of light, the inventive sensor can be used to simultaneously detect and treat multiple anomalies in a single pass over a field. For example, using the techniques described in U.S. Pat. No. 5,789,741 issued to Kinter, previously incorporated herein by reference, the inventive sensor could be used to selectively apply nitrogen at an optimal rate to desirable plants while also selectively applying herbicide between plant rows to eliminate unwanted weeds.  
         [0069]    Finally, it should likewise be noted that, while farming applications of the inventive sensor were discussed in relation to the preferred embodiment, the invention is not so limited. The inventive device could be used to improve the efficiency of plant maintenance in virtually any application, i.e. golf courses, lawn care, landscape maintenance; etc. While the constants in the equations given above may vary from crop-to-crop, the inventive method is otherwise applicable to virtually any type of plant.  
         [0070]    Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the appended claims.