> ## Documentation Index
> Fetch the complete documentation index at: https://docs.plantpredict.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Horizontal Irradiance Processing Overview

export const PlaneOfArrayIrradiance = () => <Tooltip tip="Solar irradiance on the tilted module surface (beam + diffuse + reflected); drives PV power generation.">
    plane-of-array irradiance
  </Tooltip>;

export const ClosureEquation = () => <Tooltip tip="GHI = DNI × cos(θz) + DHI; relates the three horizontal irradiance components.">
    closure equation
  </Tooltip>;

export const DecompositionCap = () => <Tooltip tip="Separating GHI into direct (DNI) and diffuse (DHI) components using empirical models.">
    Decomposition
  </Tooltip>;

export const Decomposition = () => <Tooltip tip="Separating GHI into direct (DNI) and diffuse (DHI) components using empirical models.">
    decomposition
  </Tooltip>;

export const ClearnessIndex = () => <Tooltip tip="Ratio of measured GHI to extraterrestrial irradiance; quantifies atmospheric transmittance.">
    clearness index
  </Tooltip>;

Horizontal Irradiance Processing models prepare input irradiance data (measured or modeled) for use in energy production calculations. These models handle component separation, clear-sky estimation, and—when <PlaneOfArrayIrradiance /> is the input—reverse decomposition to derive horizontal components.

## Models in This Section

### [Horizontal Irradiance Components Pre-Processing](horizontal_irradiance_components)

Validates input irradiance data using the <ClosureEquation /> (GHI = DHI + DNI × cos θz) and applies quality control checks to ensure physical consistency.

### [GHI Diffuse-Direct <DecompositionCap />](ghi_diffuse_direct_decomposition)

Estimates DHI from GHI, then calculates DNI using the <ClosureEquation />. This is the standard decomposition path, applied when DHI or DNI measurements are unavailable or when the user chooses to override measured values. Three model options:

* **Erbs**: Simplest model; uses only GHI and extraterrestrial irradiance via a <ClearnessIndex /> correlation
* **Reindl**: Adds air temperature and optionally relative humidity for improved accuracy under varying atmospheric conditions
* **DIRINT**: Uses adjacent timesteps to detect rapid irradiance changes (e.g., cloud transients), providing better accuracy for variable sky conditions

### [POAI Diffuse-Direct <DecompositionCap />](poai_diffuse_direct_decomposition)

Reverse-decomposes measured <PlaneOfArrayIrradiance /> (POAI) into GHI, DNI, and DHI using the GTI-DIRINT algorithm. This path is activated by the **Frontside POAI** toggle in [Simulation Settings](/user-guide/ui/simulation-settings) and bypasses the standard GHI decomposition. The derived horizontal components feed into the normal transposition pipeline for each DC field.

### [Bird Clear Sky Model](bird_clear_sky_model)

Calculates theoretical clear-sky irradiance components using atmospheric transmittance factors. Used for spectral correction calculations.

## Processing Workflow

PlantPredict supports two entry points for irradiance data, selected via the Frontside POAI toggle:

1. **GHI-based (standard path)** — GHI is validated, then DHI and DNI are resolved based on user settings (see [Horizontal Irradiance Components](/models/horizontal-irradiance/horizontal_irradiance_components) for the full processing sequence):
   * **DHI**: calculated from GHI when a decomposition model is selected, or read from the weather file when decomposition is set to None
   * **DNI**: read from the weather file when the Weather File DNI toggle is on, or derived from the closure equation otherwise

2. **POAI-based (alternative path)** — Measured POAI is reverse-decomposed into GHI, DNI, and DHI using GTI-DIRINT, bypassing standard GHI processing.

Both paths produce validated horizontal components (GHI, DNI, DHI) that feed into the transposition models for each DC field (Hay-Davies or Perez). In either case, the Bird clear-sky model runs independently to provide reference irradiance for spectral corrections.