Overview
The Micro-MCA is the latest incarnation of Tetracam's two decade old line of MCA (Multiple Camera Array) multi-spectral imaging systems. Throughout the years, Tetracam's MCA systems have been a consistent choice among remote sensing professionals around the world. This is due to the user-configurable bands of radiation they are able to sense and the high-resolution multi-spectral images they are able to deliver. Today's Micro-MCA is lighter, more compact and faster than the company's market-leading Mini-MCA that immediately preceded the current system.
The Micro-MCA is available in two versions, an economically-priced standard Micro-MCA system and a Micro-MCA Snap system. The latter comes with cameras equipped with lightning-quick global snap sensors, specifically designed for gathering distinct, distortion-free, stop-action imagery from unmanned aerial vehicles flying low and fast over forests or farmland.
Both the standard Micro-MCA and Micro-MCA Snap systems are provided with 16 GB micro SD memories rather than the 2GB compact flash memories supplied with Mini-MCAs. The larger memories enable users to collect up to 12,000 images from each camera in the array during any single remote sensing mission.
Both systems are also delivered with an enhanced USB 2.0 interface. This provides the principal conduit for moving images out of the system's cameras for processing. The USB 2.0 interface supplied with the Micro-MCA and Micro-MCA Snap provides data transfer rates that are ten times faster than the USB interface supplied with the Mini-MCA.
Like the Mini-MCA, the Micro-MCA and Micro-MCA Snap systems are available in three models, one with an array of four cameras, one with six and one with twelve separate cameras. Each down-facing camera in the array is precisely synchronized with the other cameras so that each is able to capture the exact same scene at the exact same instant in time. Each camera is registered with the other cameras in the array with sub-pixel accuracy so all of the images are in perfect alignment.
Each of the system's cameras contain a customer-specified narrow-band filter that is inserted between the lens and sensor. With each exposure, 4, 6, or 12 separate bands of visible or near-infrared radiation move through each lens and filter to form a separate monochromatic image on each sensor. The images are simultaneously transferred from each sensor to each camera's image memory for later processing by the user.
Micro-MCA Sensors and Filters
The cameras in both the Micro-MCA and Micro-MCA Snap system arrays represent optical channels that gather multi-spectral information on a pixel-by-pixel basis for later processing by system software. Each camera consists of a lens, filter, and sensor. Users select the filters that they insert 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 that identifies one or more plants, plant conditions or other compounds. Also, because Micro-MCA filters may be replaced by users in the field, these systems may be re-purposed again and again to detect different wavelengths at different times for different purposes.
The sensors in both the Micro-MCA and Micro-MCA Snap systems are 1.3 mega-pixel CMOS sensors. These provide images made up of 1280 X 1024 pixels. Images may be configured via the system menu to sub-sample the image at smaller sizes (1024 x 768 or 640 x 512) in order to reduce the time interval required by the camera to capture successive images.
The Micro-MCA image sensor's output is optimized for receipt of wavelengths at approximately 800nm dropping in a smooth curve to 20% peak output at 450 nm in the visible spectrum and 1000 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 Snap image sensor's output is optimized for receipt of wavelengths at approximately 650 nm. It has a high output response at 400 nm at the low end of the visible spectrum and provides good response into the near-infrared at the limits of its range. A graph of the sensitivity of the Snap image sensor is shown below.
By choosing a specific narrow band optical filter that is within the sensor's range to place in front of each channel's sensor, the radiation that is able to reach each sensor may be restricted to a specific narrow band of wavelengths. In this way, specific filter combinations may be selected that allow the Micro-MCA to expose a variety of plant conditions identifiable by their unique spectral signature. For example, if the spectral response of various plants are known in the region of the spectrum monitored by the Micro-MCA, filters may be selected that will help differentiate one species from another (see graphic below). This requires the user to know which filters to select in advance in order to expose the sought-after differences. Note: For an excellent resource on spectral signatures of vegetation, reference 'Hyperspectral Remote Sensing of Vegetation' by Dr. Prasad Thenkabail, et al. available from Tetracam. Other references may be found on our web site in our Multispectral Crop and Application Database.
Alternatively, the Micro-MCA may be set up to monitor specific bands of wavelengths from which vegetation indices may be extracted. These provide information needed for extraction of NDVI, SAVI and other indices as well as information that is able to be deduced from these. The band pass filters provided with the unit are customer designated at the time of order. These may be easily changed in the field in order to re-configure the system to look for different spectra to expose different conditions. Standard filters (shown below) are included in the price of each system. Alternative special or custom band pass filters are available upon request with prices provided via a quotation. Additional information regarding band pass filter selection is available.
Micro-MCA Lenses
Micro-MCA cameras use fixed 9.6 mm x-mount lenses. Due to the physical constraints inside each camera, these are not interchangeable with lenses that have different focal lengths. Micro-MCA lenses are set up at the factory to be in focus from approximately ten feet (3 meters) to infinity. Tetracam supplies an MCA alignment file with each Micro-MCA. This provides registration alignment information which the Micro-MCA uses to align its cameras. The factory sets this up based on perfect registration at infinity (which occurs at some hundreds of meters).
PixelWrench2 contains a Field of View or FOV utility. In this, users enter the distance between the camera and the target. When processing images, PixelWrench2 can be instructed to pick up the target distance based on the entries in the FOV utility. This supplies new values to the MCA alignment file which the Micro-MCA uses to correct the registration for the exact FOV-supplied distance.
Tetracam Micro-MCA lenses have a maximum 2% barrel/pincushion distortion. We offer a Camera Calibration Service that identifies lens distortions and provides a detailed report that photogrammatic aerial mapping software such as Icaros OneButton or Pix4Dmapper can use for optimizing the accuracy of its imagery.
