NBR - Normalized Burn Ratio

The Normalized Burn Ratio (NBR), which measures burn intensity, is used to identify burned areas. Traditionally, it is calculated as a ratio between the NIR and SWIR values. According to Keeley (2009), fire intensity is a representation of the energy generated during the combustion of organic matter. It also describes the fire's intensity when it is burning. On the other side, burn severity indicate how the fire intensity affects the functioning of the ecosystem in the area that has been burnt. Within a region and between several ecosystems, the observed impacts frequently differ (Keeley, 2009). The degree to which a space has been changed or disrupted by the fire can also be used to indicate the severity of a burn. An instance of the  difference between fire intensity and burn severity is shown in the figure below: 

A measure called the Normalized Burn Ratio (NBR) is used to draw attention to burned areas in expansive fire zones. Except that it uses both near infrared (NIR) and shortwave infrared (SWIR) wavelengths, the formula is similar to NDVI. In contrast to regions that have been severely burned, healthy vegetation exhibits very high reflectivity in the NIR and low reflection in the SWIR region of the spectrum (Figure 2). The difference between the spectral responses of healthy vegetation and burnt areas peaks in the NIR and SWIR portions of the spectrum. Recently burned areas exhibit low reflectance in the NIR and high reflectivity in the SWIR. 

o benefit from the magnitude of spectral difference, NBR uses the ratio between NIR and SWIR bands, according to the formula shown below. A high NBR value indicates healthy vegetation while a low value indicates bare ground and recently burnt areas. Non-burnt areas are normally attributed to values close to zero.

Generic Formula

Formula by Sentinel-2 bands 

References:

• Keeley, J. E. (2009). Fire intensity, fire severity and burn severity: A brief review and suggested usage. International Journal of Wildland Fire, 18(1), 116–126.Van Wagtendonk, J. W., Root, R. R., & Key, C. H. (2004). Comparison of AVIRIS and Landsat ETM+ detection capabilities for burn severity. Remote sensing of environment, 92(3), 397-408.