This is the same Planck-spectrum photon accounting as the 3D version, drawn as a live spectral diagram instead of a 3D photon flight between two plates. A hot emitter at temperature T radiates photons following Planck's law for spectral radiance:
B(λ,T) = (2hc²/λ⁵) · 1/(exp(hc/λkT) − 1)
The orange curve on the right is that function, plotted on a logarithmic wavelength axis from 150 nm to 16 μm. Each falling dot is a real photon: its horizontal landing position on the curve is drawn by inverse-transform sampling the Planck distribution itself (same numerical method as a Monte-Carlo photon source), not placed by hand. A photon's energy E = hc/λ decides its fate the instant it lands: E ≥ Eg (left of the dashed cutoff line) converts Eg of it to electricity — the excess (E − Eg) thermalizes into heat within picoseconds — while E < Eg (right of the cutoff) can't cross the bandgap at all.
- Emitter temperature — reshapes the whole curve; by Wien's law the peak sits at λmax ≈ 2898 μm·K / T, so hotter emitters push more of the curve's area left of the cutoff.
- Bandgap Eg — moves the dashed cutoff line at λg = 1240 eV·nm / Eg. Real TPV cells use low-bandgap III-V alloys (InGaAs ≈ 0.6 eV, GaSb ≈ 0.72 eV) to match 1000–2000 K heat sources.
- Spectral filter (photon recycling) — with the filter on, photons landing right of the cutoff bounce back up into the blue "Recycled" bar instead of the gray "Waste heat" bar, mirroring a real selective emitter or back-surface filter that returns unusable long-wavelength photons to reheat the emitter rather than losing them.
The three accumulator bars along the bottom are a running, exponentially-smoothed energy ledger of every photon sampled — the same tally the 3D scene keeps, just drawn as bars building up instead of colored flashes on a 3D plate.