Incident Light Sensor

Enabling Representation of Precise Reflectance Values for Mini-MCA 4, 6 or 12 Systems

 
 

Tetracam's patented Mini-MCA Incident Light Sensor is an optional accessory available for Mini-MCA4, 6 or 12 systems.  This device is incorporated into one of the Mini-MCA’s existing channels.  While the balance of the Mini-MCA’s five channels continue to capture up-welling radiation reflected from the area under study below, the ILS is pointed in the direction of the illuminating source. Fitted with narrow band filters with the same center band values and bandwidths as those contained in the Mini-MCA, the ILS gathers down-welling radiation at wavelengths that are identical to the up-welling reflected radiation monitored by the system’s remaining channels.  This enables system software to calculate precise reflectance values as a fraction of the detected incident radiation.

 

 
 

Physically, the ILS consists of a Matching Band Pass Filter Array at the terminus of a fiber-optic bundle (see picture at right). The Matching Band Pass Filter Array holds changeable filters that precisely match the bandwidth and wavelength values of the changeable narrow band filters fitted into each of the Mini-MCA's remaining camera channels . Each fiber in the ILS fiber-optic bundle conveys the light from one filter in the ILS array to the dedicated ILS image sensor in the Mini-MCA. The result is an array of pools of illumination on the ILS image sensor that represent incident light illumination for each Mini-MCA band. 

Beginning with version 1.0.7.5, Tetracam's PixelWrench2 image processing software (included with all Tetracam cameras) fully supports the ILS features.   When a Mini-MCA system is equipped with an ILS, PixelWrench2 lets users view the captured image either as it is generally shown as a function of the system's configurable exposure settings or as a precise fraction of the incident light detected by the ILS.

 

 
 

This latter view provides a more accurate means of comparing images of like areas of interest that are captured at different times.  This is due to the fact that the percentage of light reflected by a subject under study is not dependent on the amount of light impinging on that subject.  Rather it is a function of the subject's physical and chemical characteristics.  Even though incident light may vary from one day to another, if there is no change in the subject then the ratio of reflected light to incident light will stay the same.  Capturing reflectance values provides a better day-to-day comparison between images of identical areas as variations in these values indicate physical or chemical changes to the reflecting bodies themselves. 

An Incident Light Sensor simplifies the job of a user tasked with capturing multi-spectral imagery under the same viewing conditions on different viewing occasions.  The ratio of upwelling reflected radiation to down-welling incident light for each pixel in the image captured by each channel is expressed as some fraction of either 255 for an 8-bit image or 1023 for a 10-bit image. The resulting image accurately represents the reflectance value of the radiation at each pixel in the image.   Images that represent reflectance values typically appear darker than those that are optimized to display best overall image exposure by the user (see sample images below).

 

 
 

 
 

Example Mini-MCA6 Image without ILS

Example Mini-MCA6 Image with ILS

 
 

The ILS umbilical cable that connects the down-looking cameras to the up-looking ILS plate may either be snaked through an aircraft or routed along the outside of the craft.  The ILS plate measures approximately 1.4 inches on each side or about 1.7 inches in diameter.  The minimum bend radius of the umbilical cable is 1.5 inches. Tighter bends may permanently damage the umbilical cable.  

The weight that an Incident Light Sensor adds to a Mini-MCA is made up of the weight of the ILS plate with its filters and the weight added by the fiber-optic umbilical.  The latter depends upon its length.  The weight of a five channel ILS with five filters is 40 grams.  The weight added by the umbilical is 40 grams per meter.  The weight of a Mini-MCA6 ILS with 5 filters and with a 25 cm umbilical would be 50 grams.  The weight of a Mini-MCA12 ILS plate with 11 filters would be 88 grams.  The umbilical is a double cable which adds 80 grams (2 x 40 g) per meter.   The weight of a Mini-MCA12 ILS with 11filters and with a 25 cm umbilical would be 108 grams.  The total umbilical length for an ILS may range from 1/4 meter to two meters.

5 Channel Mini-MCA with 5 Channel Incident Light Sensor 11 Channel Mini-MCA with 11 Channel Incident Light Sensor

The Incident Light Sensor may be ordered with new Mini-MCA systems or existing system owners may contact Tetracam, obtain an RMA, ship their Mini-MCA systems to our Chatsworth facility and have an ILS added to their system.  Contact our sales office at 818-397-0469 or send email to info@tetracam.com for current pricing or further details.