Macaw

Recommendations for

Band Pass Filter Selection

The cameras in Macaw systems each contain a camera lens, a changeable narrow-band filter and a 1.3 mega-pixel CMOS sensor.  Users select filters to insert into each camera between the lens and the sensor in order to restrict the radiation that contacts the sensor to a narrow band of wavelengths.

The combination of filters selected by the user enables the system to sense a unique spectral signature.  This can be used to identify one or more plants, plant conditions or other compounds.

TECHNICAL PARAMETERS AFFECTING MACAW BAND-PASS FILTER SELECTION

Electric current is developed in an image sensor in response to the electromagnetic radiation that contacts it.  The amount of current generated depends upon the brightness and wavelength of the radiation that hits the sensor.  For a given brightness, Macaw output is greatest in response to wavelengths in the area of 800nm. The output drops in a smooth curve to an approximate 20% peak output at 450 nm in the visible spectrum and 1050 nm in the near-infrared at the limits of its range.  A graph of the sensitivity of the camera's image sensor to impinging radiation is shown below. 

The spectroscopic filters used in Macaw systems are supplied by Andover Corporation. These are placed between each Macaw camera's lens and sensor.  The filters restrict the radiation that is able to contact the sensor to a narrow band of wavelengths.   Andover filter designations indicate the filter's center wavelength, band width and the size of the aperture they fit.  All Macaw systems use filters with a 25 mm diameter.

The amount of light that is allowed to hit the sensor increases as filter bandwidth increases.  As a rule, 10 nm bandwidths provide optimum multi-spectral imagery in Macaw systems.  Wider bandwidths tend to over saturate the sensor.  The exceptions to this occur at the limits of the sensor's sensitivity range.  Here, the greater amount of light that broader bandwidth filters pass is offset by the sensor's decreased sensitivity.

ORDERING AND REPLACING MACAW BAND-PASS FILTERS

Macaw filters are customer-designated at the time of order.  The filters are field-replaceable using standard 25mm spectrometer filters so these systems may be reconfigured in the field to sense entirely new spectral combinations.  Field replacement of filters is limited by the following considerations:

Since wavelength impacts focus, Macaws are initially set up in the factory for optimum focus based on the wavelengths of the filters initially selected by the user.  Changing filters in the field, changes the camera's focal point.  To insure the Macaw cameras provide optimum focus, users should only replace a Macaw filter that is near the wavelength of the filter originally inserted into that camera. 

In the visible spectrum, replacing a filter with a new one that is within plus or minus 100 nm of the original is acceptable.  Beyond 700 nm, replacement filters should be within plus or minus 50 nm of the original.  Replacement of filters beyond this range requires factory re-focusing of the unit.

SELECTING OPTIMUMMACAW BAND-PASS FILTERS FOR YOUR APPLICATION

Both Andover's standard and custom filters may be used for Macaw applications.  The filters we select to include in our standard filter sets are chosen based on their ability to expose conditions able to be derived from vegetation spectral reflectance curves such as the one shown below.

 The selection criteria we use are based on the following factors:

• Bands able to deliver best image results in Macaw Cameras for the best price

• Bands used in common vegetation indices

• Bands recommended by experts or cited in references such as "Hyperspectral Remote Sensing of Vegetation" by Dr. P. Thenkabail,

• Bands cited in prior work documented in scientific papers and text (See Tetracam Multispectral Crop and Application Database).

We select filters for our standard filter set from Andover's list of Standard Filters.  This is available on the Andover Corporation website.  Customers may pick any filter from Andover's Standard List to substitute for a filter in our Standard Filter Set with no change in the product's price.  The price of custom filters are quoted at the time of their order. 

Below is a list of filters in the Standard Filter Set for each Macaw model with a description of each filter's uses

MACAW STANDARD FILTERS AND THEIR USES

   ¹   Band Combinations (James Quinn)  University of Northern Iowa

    ²   Evaluation of Narrowband and Broadband Vegetation Indices for Determining Optimal Hyperspectral Wavebands for Ag Crop Characterization

    ³   Relationships between leaf pigment content and spectral reflectance across a wide range of species, leaf structures and developmental stages

    ⁴   Assessing canopy PRI for water stress detection with diurnal airborne imagery

    ⁵   Vegetation indices as indicators of damage by the sunn pest (Hemiptera: Scutelleridae) to field grown wheat

    ⁶   Soil Backgrounds Impact Analysis on Chlorophyll Indices Using Field, Airborne and Satellite Hyperspectral Data

    ⁷   Hyperspectral vegetation indices and novel algorithms for predicting green LAI of crop canopies

    ⁸   Spectral reflectance characteristics of eucalypt foliage damaged by insects

    ⁸  Reference ENVI Users Guide - Page 1243