IRECI Algorithm Specification |

### IRECI

The Inverted Red-Edge Chlorophyll Index algorithm incorporates the reflectance in four bands to estimate canopy chlorophyll content (Guyot and Baret, 1988; Clevers et al., 2000)
The 'red edge' is the name given to the abrupt reflectance change in the 680 ± 740 nm region of vegetation spectra that is caused by the combined effects of strong chlorophyll absorption and leaf internal scattering.
Increases in the amount of chlorophyll visible to the sensor, either through an increase in leaf chlorophyll content, or Leaf Area Index (LAI), result in a broadening of a major chlorophyll absorption feature centred around 680 nm.
The effect is to cause a movement of the point of maximum slope, termed the red edge position (REP).
The position of the red edge has been used as an indicator of stress and senescence of vegetation (Collins1978, Horler et al. 1983, Rock et al. 1988, Boochs et al. 1990, Jago and Curran 1995)

The IRECI results from the following (Sensor-dependent) equation:

IRECI = (IR_factor * near_IR - red1_factor * red1) / (red2_factor * red2 / red3_factor * red3)

For Sentinel-2 the formula is:
(B7 - B4) / (B5 / B6), where (Central wavelength/Bandwidth): B7 = 783 nm (15 nm), B6 = 740 nm (15 nm), B5 = 705 nm (15 nm), B4 = 665 nm (30 nm)

### IRECI-Flags

Also the processor computes an additional flags band called 'ireci_flags' with the following bit coding:

Bit Position | Description
--- | ---
Bit 0 | The computed value for IRECI is NAN or is Infinite
Bit 1 | The computed value for IRECI is less than -1 (minus one)
Bit 2 | The computed value for IRECI is greater than 1 (one)