Absorption Feature Position (AFP)

The Absorption Feature Position (AFP) is a spectral analysis index designed to identify the precise wavelength at which the reflectance of a material reaches its minimum within a defined absorption feature. This corresponds to the maximum absorption and is often linked to specific mineralogical or chemical characteristics.

AFP is particularly useful in hyperspectral and multispectral imaging, where materials exhibit diagnostic absorption features in narrow spectral regions.

How it works #

For a selected range of spectral bands (defined by a minimum and maximum wavelength), the AFP algorithm performs the following steps:

1. Continuum Removal: Each band is normalized by its continuum — a straight line interpolated between the “shoulder” bands at the edges of the range — to isolate the absorption feature from the general reflectance trend. ${\mathrm{CR}}_{\lambda }=\frac{R_{\lambda }}{R_{\text{continuum}}\left(\lambda \right)}$ where $R_{\lambda }$ is the reflectance at wavelength $\lambda$, and $R_{\text{continuum}}$ is the interpolated continuum reflectance.
2. Absorption Strength Estimation: The algorithm computes the absorption as: $A_{\lambda }=1-{\mathrm{CR}}_{\lambda }$
3. Weighted Mean Calculation: Instead of simply selecting the band with the lowest reflectance, AFP uses a weighted mean of the wavelengths, weighted by their absorption strength: ${\lambda }_{\text{AFP}}=\frac{\sum (A_{\lambda }·\lambda )}{\sum A_{\lambda }}$ This results in a continuous value per pixel, representing the estimated wavelength (in microns) of maximum absorption.
4. Clamping: To ensure physical validity, the result is clamped within the spectral range selected by the user — it cannot go below or above the input wavelength bounds.

Use cases #

• Mineral mapping (e.g., identifying clays, iron oxides, or carbonates).
• Vegetation analysis, where absorption features relate to water or pigment content.
• Environmental monitoring (e.g., identifying pollutants based on their spectral fingerprint).

AFP provides sub-band precision, making it especially valuable when materials exhibit absorption features that shift slightly depending on composition, grain size, or moisture.