Verify against the primary 1931 Kubelka & Munk paper that the theory was framed for paint hiding-power / opacity

Raised while promoting the Kubelka–Munk cross-industry capture (claim-kubelka-munk-theory-devised-1931-for-paint-opacity). That note rests on a single Tier-4 source (Wikipedia, "Kubelka–Munk theory"). The claim is historical/definitional and within the sourcing floor for Tier 3–4, but the 2026-07-06 changelog ruling says the citation should point at the primary the definition rests on rather than at the encyclopedia.

What to confirm against the primary:

1. The paper and date. Paul Kubelka & Franz Munk, "Ein Beitrag zur Optik der Farbanstriche" ("A contribution to the optics of paint coatings"), Zeitschrift für technische Physik 12 (1931): 593–601. Confirm authorship, venue, year, and page range.

2. The framing. Confirm that the paper's stated problem is paint hiding-power / opacity — how thick a coat is needed to obscure a substrate — and that the Wikipedia paraphrase ("the thickness of paint needed to obscure the substrate") is faithful to the original wording.

3. The two-flux structure. Confirm the K (absorption) and S (scattering) coefficient formulation and the K/S–reflectance relationship originate in this paper.

The original is in German and may require a translation or a high-method secondary (e.g. a colorimetry textbook citing it directly). Low-to-medium priority — this is a color-science seed in a vault otherwise centered on ML history; worth a pass if the K–M / CIELAB cluster grows.