cube_refl: Calculate lambertian equivalent bi-hemispherical reflectance

In AlexCast/surfspec: PAE tools for imaging spectroscopy

Calculate lambertian equivalent bi-hemispherical reflectance

Description

An interactive function to calculate the lambertian equivalent bi- -hemispherical reflectance for imagery that contains at least one target of known reflectance.

Usage

cube_refl(cube, dir, fln, rho, method = c("along", "area"), scandir = "x")

Arguments

cube	Calibrated hyperspectral cube.
dir	Path to write reflectance cube. If omitted, data will be saved to the working directory.
fln	Filename for the reflectance cube. If omitted, data will be saved with the same input filename adding the suffix ".refl.envi".
rho	Directional-hemispherical reflectance of reference at the same wavelength grid of the data.
method	Method for reference reflectance. One of "along" or "area". See Details.
scandir	Scan direction of the image. One of "x" or "y".

Details

Although reflectance can be calculated from digital counts, the raw digital counts of the instrument are not recommended since it may contain saturation pixels and spectral or spatial distortions that are corrected in the calibration process. It is recommended to use only with calibrated data.

Since exact position of samples and reference may vary per image, the function is iterative and require the user to point to regions in the image. The user is first prompt to select the LL and UR corners to process. To process all the image, click the LL below and to the left of the image LL and above and to the right to the UR of the image.

There are two methods available for calculating the reflectance. The "along" method requires that the reflectance reference was aligned with the scan direction of the instrument. The exitant radiance of the reference will be averaged per scan line. This is specially recommended if data was acquired with natural illumination since reflectance normalization will be performed per scan line allowing to remove illumination fluctuations during scan. This is the preferred method for data acquisition and processing.

The simpler method "area" is also available, requiring to drawn the surface of the reference on the image. The signal form the reference surface will be averaged and this single value applied to the whole image. This method should only be used for stable and homogeneous illumination.

The lambertian equivalent bi-hemispherical reflectance is calculated by scaling the measured hemispherical-directional reflectance by pi steradians.

Value

A raster stack with the lambertian equivalent bi-hemispherical reflectance (unitless).

AlexCast/surfspec documentation built on July 7, 2022, 9:35 a.m.