Gray Scale Calibration

Accurate dynamic range calibration of a multispectral sensor system requires one or more targets of known levels of reflectivity be placed within or close to the subject area from which image data is to be acquired. This establishes a known point or points which can be used during image processing to correct and compensate the image data for independent variables that affect the incident irradiance downwelling on the scene. Some of these independent variables are solar geometry (time of day and latitude), atmospheric contaminants (particulates, aerosols, gasses), relative humidity and cloud cover. The calibration panels need to be spectrally neutral (gray) across as wide a spectral domain as possible and provide a non-directional return as a function of the acquisition geometry from which the target is imaged. This minimizes the variation of the reflectivity as a function of the multispectral wavelength sensitivity of the sensor and the actual acquisition geometry.

Group 8 Technology, Inc. (G8T) manufactures fabric gray scale targets that meet spectral and diffuse reflectance criteria using custom coated Type 822 polyester fabric. The three parameters that must be defined by a user are: (1) panel size, (2) the number of reflectivity steps that ensure adequate calibration across the dynamic range of the sensor, and (3) the reflectivity of each of the steps in the gray scale. The first parameter, panel size, is defined by the GSD of the imaging system, the pixel sampling statistics needed to provide sufficient confidence in the calibration and also financial budgets. The second parameter, number of gray scale steps, is defined by the system dynamic range linearity and may be a single gray level to as many as ten steps. The third parameter is the reflectivity level of each panel. Panel reflectivity is determined by the nominal reflectivity or albedo of the areas to be imaged.

Panels of any size may be fabricated by G8T, however the standard sizes are listed in the table below. The smallest standard panel is 1.2 meters per side – this being one web width of fabric. The most common sizes used with UAV and medium altitude airborne sensors are 3 to 6 meters per side. High altitude or satellite borne sensors may require panels as large as 10 to 20 meters per side. In the latter case, the target is comprised of two 10 meter x 20 meter panels which are joined along a common hem line during deployment.

Multispectral or hyperspectral imaging systems which have a linear dynamic range are generally calibrated with a one or two level calibration, although a three point calibration will strengthen the linearity assertion. Standard band average reflectivity levels coated by G8T are 3%, 6%, 12%, 24%, 36%, 48% and 56% to 60%. G8T can coat fabric to a band average reflectivity of 84% (white), but a calibration point at that reflectivity level is not often used because of detector saturation. Custom reflectivity levels can also be coated for special applications or for fabrication of large targets. The selection of the target reflectivity to be used is governed by the sensor performance and the reflective properties of the area to be imaged. High reflectivity areas (sand, snow playas) may require calibration points at the higher end of the dynamic range. Heavy foliage areas may require calibration points in the lower end (3%) of the scale, but the high Near-IR reflectivity of living vegetation may require an additional reference calibration point at the 56%-60% level. A two point mid-range calibration is commonly accomplished using 6% and 48% calibration panels.