# CIE 1931 blackbody LUT

`cie1931_2deg_xyz_1024.grbblut` is the sole repository LUT. It contains 1,024
XYZ nodes uniformly spaced in `log(T)` over `[670.146556, 101408.88] K`.
Wolfram integrates the Planck spectrum against the piecewise-linear 1 nm data
in `assets/CIE_xyz_1931_2deg.csv` at 40-digit working precision and rounds each
stored value once to little-endian IEEE binary64.

The 160-byte `GRBBLUT3` header records the CIE 1931 2-degree, 360--830 nm,
1 nm-linear reference definition, node count, temperature limits, physical
constants, generator description, and payload FNV-1a checksum. The C loader
validates the header, exact payload size, and checksum, and rejects trailing
bytes. Runtime table generation and the former `GRBBLUT1`/`GRBBLUT2` formats are
not supported. The evaluator uses four-point cubic Lagrange interpolation in
`log(T)`.

- file size: 24,736 bytes
- payload FNV-1a: `0e39867b0e70a809`
- file SHA-256:
  `850446663212fe488c88fca3ac2f54d6f8119a04801dda84fc1a62acdec37d63`
- all 1,023 interval midpoints versus independent Gauss-16 integration:
  max XYZ-channel relative `5.4633e-5`, no negative XYZ

The LUT interpolation acceptance limit is `1e-4`. A separate `5e-4` overall
relative error budget accounts for the estimated error of linearly
interpolating the 1 nm CIE samples; the LUT's much smaller internal error must
not be mistaken for physical-reference accuracy.

Outside the stored range, the same CSV reference supplies both models. Above
101408.88 K the evaluator uses three integrated Rayleigh--Jeans terms
`A T+B+C/T` (maximum validated XYZ-channel relative error `1.6635e-5`
through `10*Tmax`). Below 670.146556 K it uses a six-segment, degree-eight
inverse-temperature/log-XYZ Chebyshev--Lobatto crossover down to 4.097321 K
and channel-specific endpoint forms below that. Their maximum sampled XYZ
relative errors are `8.16e-5` and `6.69e-5`.

The Wolfram generation/validation scripts that produced this table, together
with the bounded performance and accuracy evidence, are preserved on the
experiment branch `codex/blackbody-cost-experiment` (tag
`blackbody-cost-experiment-2026-09-12`) and are not part of the mainline tree.