FU Classification - Algorithm Specification |

### Introduction

This algorithm is used for deriving the hue angle and FU (Forel-Ule) value from remote sensing reflectance. The data is used with the weighted linear sum of the remote sensing reflectance in which visual bands plus a correction term for the specific band-setting of each instrument. The algorithm is validated by a set of 603 hyperspectral measurements from inland, coastal and near-ocean waters. The processor supports the the following sensors:

- Sentinel-3 OLCI Level2 W(F/R)R
- Sentinel-2 A/B MSI L1C - For A and B sensor specific polynomial coefficients are used
- MERIS L2
- MODIS (1km LAC OC), MODIS-500 (M[O|Y]D09)
- SeaWiFS
- CZCS

The algorithm is described in the following articles which are available online.

An introduction to the SNAP FU-processor and examples of potential applications
A worldwide investigation of the spectral characteristics of FU- and hue properties.
The derivation of the hue angle for the instruments SeaWiFS, MODIS, MERIS and OLCI are described in:
The algorithms to convert instruments like MSI on Sentinel 2:
The conversion of the hue angle to the Forel-Ule number and the corresponding colour legend is based on:
The conversion of the global CCI ocean colour set to hue angle and FU:
This algorithm connects satellite ocean colour observations to historical in-situ colour observations: