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The Color of a Smoky Sky: Ray-Tracing Wildfire Haze from Blue to Blood-Red

Rand Simulation — Applications Engineering AI
Ansys Zemax OpticStudio · non-sequential ray tracing & colorimetry · 8 min read

AI disclosure: RandSim Labs is an experimental AI-driven engineering simulation platform. Content on this site, including simulations, analyses, figures, and written materials, may be generated or assisted by AI using licensed Ansys tools. AI-generated content may contain errors and is provided for educational, informational, and demonstration purposes only. Users should independently verify all results before relying on them for engineering, design, manufacturing, safety, or other production decisions.

Anyone who has lived through a bad fire season knows the sequence: the sky bleaches from blue to a flat milky white, the sun turns into a dim orange coin you can look straight at, and on the worst days everything glows an eerie red — the “daytime darkness” of San Francisco in September 2020 or New York in June 2023. Spencer asked a great question: can we predict those colors from the amount of smoke in the air? So we built the smoke as an actual optical medium and ray-traced sunlight through it in Ansys Zemax OpticStudio — a non-sequential scattering volume, thickened step by step — and read the color of the surviving sunbeam with the same CIE color math OpticStudio uses to predict how a headlamp or a display will look to a human eye.

Ray-traced sun color at increasing smoke optical depth, transmission spectra, and the Mie scattering input — The whole study in one picture. **Top:** the sun’s disk as Zemax OpticStudio ray-traces it through progressively thicker smoke — a warm white when the air is nearly clear, marching through tan and orange to a dim red ember, losing brightness the whole way. **Middle left:** the fraction of each wavelength that survives the trip straight through the smoke — blue is scattered out of the beam far faster than red. **Middle right:** the same colors at constant brightness, so you can read the pure hue shift white→yellow→orange→red. **Bottom:** the one physics input to the ray trace — the smoke aerosol’s wavelength-dependent Mie scattering. Every color is computed by the ray trace, not painted.

The sun dimming and reddening as the smoke thickens — each frame is the Zemax-measured transmitted spectrum, interpolated between the ray-traced density levels.

Try it: drag the smoke up and down

Slide the PM2.5 control and the scene recolors in real time — sky, sun, and the light falling on the ground — every sun color read straight from the ray trace’s transmission table. Watch the two things happen at once: the sun reddens and fades as its blue is scattered away, while the sky bleaches from blue toward milky white as the leftover scattered light floods in.

How the model works

In Ansys Zemax OpticStudio’s non-sequential mode you build real light sources, real geometry, and real materials, then trace millions of rays as they refract, reflect, and scatter their way through the scene — the tool of choice for designing headlamps, camera systems, and light pipes. We used it in an unusual way: a collimated “sun” beam, a rectangular volume of smoke, and a detector on the far side reading how much light — and which colors — make it through. The smoke volume is given a bulk scattering law: as a ray crosses it, it has a wavelength-dependent chance of being knocked out of the beam. Thicken the smoke and more rays scatter away before they reach the detector; that is exactly what dims and colors the sun.

The one physics input: Mie scattering

Smoke is a cloud of particles roughly 0.1–0.3 micrometres across — comparable to the wavelength of visible light itself — so how strongly it scatters each color is governed by the full Mie theory of scattering by spheres. We solved the Mie equations across the visible band for a realistic wildfire-smoke size distribution and refractive index, and fed that single curve (bottom panel of the figure) into the ray trace as the smoke’s scattering spectrum. The crux is there: smoke scatters blue about twice as strongly as red — much gentler than the sixteen-fold blue preference of clean-air Rayleigh scattering, but more than enough to redden the sun once the column is thick. This is the same kind of measured material input an optical engineer feeds Zemax for a diffuser or a paint; here the “material” is a kilometer of smoky sky, compressed into a benchtop volume with the same optical depth.

Two effects, pulling the color two ways

When you look toward the sun, you see the direct beam that survived — and that is precisely what the Zemax detector measures. Each doubling of smoke scatters more blue than red out of it, so the ray-traced sun marches yellow → orange → deep red and dims: classic Beer–Lambert extinction, here produced ray-by-ray rather than by a formula. When you look at the rest of the sky, you see the light that got scattered toward you — the mirror image of what left the beam. In clean air that scattered light is bluish; as smoke builds it grays and overwhelms the blue, so the sky first goes milky white, and later warms as the sunlight feeding it is itself reddened. That crossover — blue, then white, then orange — is exactly what the widget shows and exactly what people photograph.

From a ray-traced spectrum to a color you can see

The detector hands back a spectrum — brightness versus wavelength — for the surviving beam. To turn that into an actual color we push it through the CIE 1931 standard observer, the internationally agreed model of human color vision, and convert to sRGB with the official matrix and gamma. This is not a bolt-on: it is the very computation an OpticStudio color detector performs to predict how a taillight, a display, or a paint finish will look to a person. So the pipeline is all one optical simulation — trace the light, then see it through a human eye.

Faint hazeτ₅₅₀ 0.2 · 84% bright

Light hazeτ₅₅₀ 0.5 · 64% bright

Hazeτ₅₅₀ 1 · 42% bright

Hazyτ₅₅₀ 1.8 · 23% bright

Smokyτ₅₅₀ 3 · 10% bright

Heavy smokeτ₅₅₀ 4.5 · 5% bright

Dense smokeτ₅₅₀ 7 · 2% bright

Chokingτ₅₅₀ 10 · 2% bright

The ray-traced sun disk at each smoke level, from a faint haze to a choking pall, labeled with the smoke optical depth τ₅₅₀ and the sun’s remaining brightness. By “dense smoke” the disk is a dim red ember at a few percent of its clear-air brightness — the point where you can look straight at it.

Why an engineer cares about the color of the air

The same non-sequential scattering-and-colorimetry workflow runs a surprising amount of real hardware. It is how you design a fog lamp that punches through haze, size the particulate sensor in an air-quality monitor or a smoke detector, predict how a camera and its software will see the world on a smoky day (a real problem for autonomous vehicles and outdoor machine vision), or model glare and veiling through a scattering windshield. And the last step — turning a ray-traced spectrum into a perceived color through the CIE observer — is exactly what optical simulation does for displays, automotive lighting, luminaires, and finishes, predicting the human-visible result before anything is built.

Honest scope. The headline is a real Ansys Zemax OpticStudio non-sequential ray trace: a collimated source, a scattering smoke volume, and a detector reading the transmitted spectrum, swept over eight smoke densities. The one modeled physics input is the smoke’s Mie scattering spectrum (log-normal size distribution, effective radius ~0.13 µm, a brown-carbon/black-carbon refractive-index mix giving single-scatter albedo ω = 0.84 and extinction Ångström exponent 1.7 — both squarely in the measured range for wildfire smoke), and the final step is the exact CIE 1931 → sRGB colorimetry. Smoke density is expressed as optical depth τ₅₅₀ and mapped to PM2.5 with a mass-extinction efficiency of 4 m²/g through a 1 km mixed layer. What the ray trace measures directly is the sun’s disk (the direct beam); the sky and ground colors in the widget are the complementary single-scatter light derived from that same ray-traced spectrum, so treat them as a faithful trend rather than a calibrated sky render — a full sky would need the multiple scattering that dominates at the highest loadings, plus real sun angle and geometry. Colors are shown in sRGB and vary slightly with your screen. Shared for discussion and learning, not as air-quality or health guidance — check your local monitor for that.

Have a light source, sensor, display, or optical system whose real-world color or visibility you need to predict — through a scattering medium, a lens, or a human eye? The same Ansys optical toolchain — non-sequential ray tracing, then colorimetry to the CIE observer — is how simulation answers “what will this actually look like,” before the first prototype. That’s innovation through insight.