Photochemical Reflectance Index (PRI) measures photosynthetic efficiency and plant stress by analyzing the reflectance of light related to xanthophyll cycle activity, which is linked to photosynthetic energy conversion. It helps detect changes in photosynthetic activity and plant health. PRI values range from -1 to +1, where higher values indicate more efficient photosynthesis and healthy vegetation, while lower values suggest stress, reduced photosynthetic activity, or senescence.

Firefly = (B017-B029)/(B017+B029)