.. _radiance_example:

Basic Radiative Transfer Calculation with TIR
*********************************************

The following example demonstrates a limb radiance calculation for a methane micro window.
By importing ``sasktran_tir.interface``, the TIR classes (engine, optical properties, and climatologies) can be accessed
without importing each class individually.

.. code-block:: python

    import sasktran as sk
    import sasktran.tir.interface as tir
    import numpy as np
    import matplotlib.pyplot as plt

    # select wavelengths
    wavelengths = np.arange(7370, 7380, 0.01)

    # create a limb line of sight
    geometry = sk.VerticalImage()
    # NOTE: the VerticalImage class requires sza and saa to be specified but the solar position has no effect
    #       on TIR radiance calculations
    geometry.from_sza_saa(sza=0, saa=0, lat=45, lon=0, tanalts_km=[20], mjd=56300, locallook=0.0)

    # create an atmosphere containing methane; using the TIR engine's builtin climatologies
    atmosphere = sk.Atmosphere()
    atmosphere.atmospheric_state = tir.ClimatologyAtmosphericState()
    atmosphere['CH4'] = sk.Species(tir.HITRANChemicalTIR('CH4'), tir.ClimatologySpecies('CH4'))

    # create the engine
    engine = tir.EngineTIR(geometry=geometry, atmosphere=atmosphere, wavelengths=wavelengths)

    # do the calculation
    radiance = engine.calculate_radiance()

    # plot the result
    plt.figure()
    plt.plot(wavelengths, radiance)
    plt.xlabel('Wavelength (nm)')
    plt.ylabel('Radiance (photons / (s cm$^2$ sr nm))')

    plt.show()

.. figure:: images/ch4_rad.png
    :align: center