ToA Reflectance: ToA Radiance Correction: Atmospheric Correction: Q GIS: Without using Raster Calculator.
This video walks through the conversion of Landsat 8 products, provided by the USGS EROS Center to estimate ToA Reflectance: ToA Radiance Correction: Atmospheric Correction using RS&GIS Tool performs well-defined algorithms in Q GIS.

Figure 2. Illustration of radiance, reflectance, and TOA. Source: United States geological Survey (USGS).

Land Surface Temperature (LST) : Q GIS
:    • Land Surface Temperature in QGIS 3  
Radiometric correction in QGIS:
:   • Radiometric correction in QGIS  
Radiometric Correction with Raster Calculator in Qgis
:   • Radiometric Correction in QGIS : Rast...