dn2reflectance

Convert digital number to reflectance

Since R2020b

Syntax

newhcube = dn2reflectance(hcube)

newhcube = dn2reflectance(hcube,'BlockSize',blocksize)

Description

newhcube = dn2reflectance(hcube) converts the pixel values of the hyperspectral data cube from digital number (DN) to reflectance values. The function returns a new hypercube object and the pixel values of the data cube are the top of atmosphere (TOA) reflectance values. For details on TOA reflectance values, see Compute TOA Reflectance values from DNs.

newhcube = dn2reflectance(hcube,'BlockSize',blocksize) specifies the block size for block processing of the hyperspectral data cube by using the name-value pair argument 'BlockSize'.

The function divides the input image into distinct blocks, processes each block, and then concatenates the processed output of each block to form the output matrix. Hyperspectral images are multi-dimensional data sets that can be too large to fit in system memory in their entirety. This can cause the system to run out of memory while running the dn2reflectance function. If you encounter such an issue, perform block processing by using this syntax.

For example, dn2reflectance(hcube,'BlockSize',[50 50]) divides the input image into non-overlapping blocks of size 50-by-50 and then computes the reflectance values for pixels in each block.

Note

To perform block processing by specifying the 'BlockSize' name-value pair argument, you must have MATLAB^® R2021a or a later release.

Note

This function requires the Hyperspectral Imaging Library for Image Processing Toolbox™. You can install the Hyperspectral Imaging Library for Image Processing Toolbox from Add-On Explorer. For more information about installing add-ons, see Get and Manage Add-Ons.

The Hyperspectral Imaging Library for Image Processing Toolbox requires desktop MATLAB, as MATLAB Online™ or MATLAB Mobile™ do not support the library.

Examples

Convert Digital Number to Reflectance

Read hyperspectral data into the workspace.

hcube = hypercube('EO1H0440342002212110PY_cropped.hdr');

Determine the bad spectral band numbers using the BadBands parameter in the metadata.

bandNumber = find(~hcube.Metadata.BadBands);

Remove the bad spectral bands from the data cube.

hcube = removeBands(hcube,'BandNumber',bandNumber);

Convert digital numbers to top of atmosphere (TOA) reflectances. The pixel values in the output data cube are the TOA reflectances.

newhcube = dn2reflectance(hcube);

Read and display the 80th spectral band image in the input and the output reflectance data cubes.

inputBand = hcube.DataCube;
reflectanceBand = newhcube.DataCube;
band = 80;
figure
subplot(1,2,1)
imagesc(inputBand(:,:,band))
title('Input Band')
axis off
subplot(1,2,2)
imagesc(reflectanceBand(:,:,band))
title('Reflectance Band')
axis off
colormap gray   

Input Arguments

hcube — Input hyperspectral data

hypercube object

Input hyperspectral data, specified as a hypercube object. The DataCube property of the hypercube object stores the hyperspectral data cube. The MetaData property of the hypercube object must contain reflectance gain values.

blocksize — Size of data blocks

2-element vector of positive integers

Size of the data blocks, specified as a 2-element vector of positive integers. The elements of the vector correspond to the number of rows and columns in each block, respectively. The size of the data blocks must be less than the size of the input image. Dividing the hyperspectral images into smaller blocks enables you process large data sets without running out of memory.

• If the blocksize value is too small, the memory usage of the function reduces at the cost of increased execution time.

• If the blocksize value is large or equal to the input image size, the execution time reduces at the cost of increased memory usage.

Example: 'BlockSize',[20 20] specifies the size of each data block as 20-by-20.

Output Arguments

newhcube — Output hyperspectral data

hypercube object

Output hyperspectral data, returned as a hypercube object. The pixel values of the data cube returned at the output specifies the top of atmosphere (TOA) reflectance values.

More About

Compute TOA Reflectance values from DNs

Given a digital number (DN), the TOA reflectance is computed by using the reflectance gain (R_Gain) and reflectance offset (R_Offset) of each spectral band in the data cube.

$$\text{Reflectance }{\rho }_{\lambda }=\left(DN\times R_{Gain}\right)+R_{Offset}$$

The reflectance gain and reflectance offset values of each spectral band are stored in the header file.

Alternatively, the TOA reflectance values can be estimated from digital numbers (DN) by using these two steps:

1. Compute the radiance values from the digital number (DN).

   $${\text{Radiance L}}_{\lambda }=\left(DN\times Gain\right)+Bias$$

   Gain_λ and Bias_λ are the gain and offset values for each spectral band (λ) respectively. The Metadata property of hypercube object contains the gain and offset values.

2. Compute the TOA reflectance values from the radiance values.

   $$\text{Reflectance }{\rho }_{\lambda }=\frac{\pi d^2{L}_{\lambda }}{ESU{N}_{\lambda }{\theta }_E}$$

   d is the earth-sun distance in astronomical units, ESUN_λ is the mean solar irradiance for each spectral band, and θ_E is the sun elevation angle.

Version History

Introduced in R2020b