IR Blocking & Blackbody Flux Calculator

A static, browser-only LCARS-themed calculator for the blackbody photon flux landing on a detector pixel through a three-stage cryogenic filter stack. A 300 K source radiates through three cold stages (default 65 K / 4 K / 800 mK) onto the pixel; the tool plots counts/nm vs wavelength and reports the integrated counts/sec.

Features

• Reactive optical-bench schematic with the controls laid out per stage (temperature, aperture diameter, axial distance, substrate + thickness, coating).
• Full radiative model: the 300 K source and each cold stage emit as graybodies, each attenuated by the colder downstream filters; cold-stop geometry sets the étendue onto the pixel.
• Thickness-scaled substrate materials computed from first principles: N-BK7, fused silica (Suprasil / Infrasil), sapphire, MgF₂, CaF₂.
• Coatings from a measured-curve library (DARKNESS, PICTURE-C, ASAHI YSC1100, ASAHI YSC0750, ITO, M254C cold mirror), plus CSV import and a Custom coating (ideal top-hat or a designed Ta₂O₅/SiO₂ multilayer).
• Coating designer: synthesize a real Ta₂O₅/SiO₂ stack from a passband target (transfer-matrix forward model + bounded refinement), edit the layer list by hand, and see the designed filter's throughput overlaid on the flux plot on a log right-hand axis.

Run locally

The app must be served over HTTP — ES-module imports and fetch() of the filter JSON do not work from file://.

python3 -m http.server 8000   # or: npm run serve
# open http://localhost:8000/

Tests

npm test        # node --test over test/*.test.js

The calculation

Photon spectral radiance L_ph(λ,T) = (2c/λ⁴)/(exp(hc/λk_BT) − 1); each emitter k contributes A_pix · QE · Ω_k · ε_k(λ) · (∏ downstream T_j) · L_ph(λ,T_k), where Ω_k is the projected solid angle of the limiting (cold-stop) aperture between plane k and the pixel. The per-nm spectrum is integrated (trapezoid) to counts/sec. Units and assumptions are documented inline in js/physics.js.

Optical model

Coatings and substrates share one swappable OpticalElement interface (transmission/reflection over a wavelength grid), so the flux engine never cares how an element's curve was produced:

• Substrates (js/materials.js) are computed from physical thickness: T = (1−R)²·τ(λ,d), ε = (1−R)(1−τ), with R from the Sellmeier refractive index and bulk τ from either a tabulated absorption coefficient (fused silica, sapphire, MgF₂, CaF₂) or the measured N-BK7 curve scaled from its 10 mm reference (Beer–Lambert). Each stage has a Thickness (mm) input.
• Coatings are measured T(λ) curves (js/filters.js), the analytic Custom top-hat (CustomCoating), or a designed multilayer (MultilayerCoating).
• Multilayer designer (js/tmm.js, js/synthesis.js): a transfer-matrix forward model for Ta₂O₅/SiO₂ stacks, plus a synthesizer that builds a chirped long-wave-blocking edge filter from the Custom passband and refines the layer thicknesses (asymmetric objective: maximize in-band, suppress long-wave leakage out to the ~3.4 µm glass cutoff, leave the short side free). Layer count is adjustable; the result is demonstration-grade (see js/synthesis.js for the limits).

Filter & material library

The bundled coating curves in data/filters/*.json are checked in and are all the app needs. They were converted from raw manufacturer/measured source curves (kept outside this repo) with the py313

• numpy scripts in tools/; tools/notebook_parity_reference.py regenerates the physics parity fixture from the bundled curves. Substrate material data (Sellmeier coefficients + absorption tables, with citations) lives inline in js/materials.js. You can also import your own coating/substrate curve at runtime from a 2-column file (fraction / percent / OD).

Deploy (GitHub Pages)

Settings → Pages → Deploy from branch → main, / (root). All files are static at the repo root; no build step.