# Fresnel R/T vs Tidy3D golden (tiny 2D-like slab interface)

<span class="validation-badge">validated · 2026-08-28</span>

<div class="case-meta"><span>forward</span><span>resolution 1</span><span>parity</span><span>p1</span><span>fresnel</span><span>tidy3d_golden</span><span>high_yield</span></div>

This case compares a local FDTDX result with pinned reference data generated by Tidy3D.

```{figure} ../../_static/generated/plane_wave_field.png
:alt: Technical schematic associated with Fresnel R/T vs Tidy3D golden (tiny 2D-like slab interface)

Representative field across a layered material geometry.
```

## What is exercised

Same physics as analytical_dielectric_slab but reference R,T from a small Tidy3D cloud sim (cached under goldens/). High-yield normalization/parity glue.

This case runs at its declared full benchmark resolution (resolution factor 1.0).



## Recorded result

| Metric | Observed / error | Reference / limit | Error | Effective budget | Result |
|---|---:|---:|---:|---:|---:|
| `reflectance_R` | 0.0407569 | 0.0434504 | 0.00269345 | 0.00847603 | <span class="metric-pass">PASS</span> |
| `transmittance_T` | 0.959243 | 0.956552 | 0.00269059 | 0.0578276 | <span class="metric-pass">PASS</span> |

The table is rendered from `progress.json`; it is not a hand-written success claim. For metrics that report an error directly, the “observed” column repeats that error and the reference column is the acceptance threshold.

## Reproduce

```bash
uv run fdtdx-bench run --case parity_fresnel_interface
```

Implementation and provenance: `benchmarks/cases/parity_fresnel_interface/case.yaml`, `benchmarks/cases/parity_fresnel_interface/run.py`, `benchmarks/goldens/parity_fresnel_interface.npz`.

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