Photon-counting detection horizons for nanosecond optical and near-infrared laser pulses, across telescope apertures from LaserSETI's 7 cm wide-field cameras up to the proposed ESO 2040s 39 m dedicated SETI mode. The empirical complement to the radio calculator (Tool 23): same EIRP question, different physical window.
Three classes of signal: (1) nanosecond optical/NIR laser pulses easily distinguishable from any natural astrophysical source — the original Schwartz & Townes (1961) proposal; (2) narrowband continuous-wave lasers visible as anomalously narrow spectral lines in stellar spectra — the APF Breakthrough Listen approach (Lipman et al. 2023); (3) all-sky transient surveys catching one-shot pulses with wide-field photon-counters (LaserSETI). All three exploit the same fact: lasers produce photon arrival statistics no natural process can replicate.
A 1064 nm photon carries E_γ = hc/λ ≈ 1.87×10⁻¹⁹ J — about ~10⁵× more energy than a 1.4 GHz radio photon (~10⁻²⁴ J). For a fixed transmitter EIRP, an optical beam produces ~10⁵× fewer photons per second, but each photon is individually detectable. A 1 ns optical pulse can deliver thousands of photons to a 10 m aperture at 100 ly using an EIRP a million times lower than radio would require for the same SNR — because optical photon counting against zero background beats noise-temperature-limited radio sensitivity by enormous factors.
Diffraction-limited optical beams are very narrow: a 10 m transmitter at 1 μm produces a beam ~10 μas wide, vs. ~arcminute for 1.4 GHz radio from the same aperture. High gain means high EIRP at low transmitter power — but it also means the transmitter must be aimed at us. The strategy that pays off: target as many stars as possible, and assume some fraction of the galactic civilisation set has chosen to aim at our solar system in the optical band.
For nanosecond gates, even bright sky background (~10 photons/m²/s/Å in the visible) gives <<1 photon per pixel per gate — essentially zero. Detection thresholds can be set as low as ~3 photons (above zero background) rather than the noise-floor / radiometer-equation calculations that govern radio. This tool uses N_thr = 3 by default; bumping it to 5 or 10 simulates an RFI-hardened or false-alarm-suppressed detection regime.
Near-infrared (1–2.5 μm) penetrates galactic dust much better than visible light, extending useful detection distance across the Galactic plane and into the bulge. Visible (400–700 nm) is the original SETI band and is well-served by APF, VERITAS, and TMT. UV is effectively blocked by interstellar absorption beyond a few hundred parsecs — too short for galactic SETI. Most NIROSETI signal-search is in the 1.3–1.8 μm window, chosen specifically because it pierces dust toward the galactic centre and ω Centauri.
Tools 23 (radio) and 32 (optical) together establish that, for the EIRP regimes a transcended civilisation might plausibly use, ω Centauri is well within reach of current instruments in at least one window. The non-detection of either radio or optical signals at ω Cen distance therefore puts a real constraint on the population of high-EIRP transmitters there. This tool quantifies the optical half of that constraint: move the EIRP slider until d_max ≈ 17,900 ly and read off the EIRP required for a positive detection at any given aperture.
Photon energy: E_γ = hc/λ with h = 6.626×10⁻³⁴ J·s, c = 3×10⁸ m/s. Photons per pulse from a transmitter of EIRP P over duration τ: N_emit = P·τ / E_γ. Photons per m² at distance d: Φ = N_emit / (4π d²). Photons collected by aperture D: N_det = Φ · π D²/4 = P·τ·D² / (16 E_γ d²). Inverting for the detection horizon at threshold N_thr: d_max = √(P·τ·D² / (16 E_γ N_thr)). The tool assumes background-limited regime (N_bg ≈ 0 over a ns gate), 100% quantum efficiency, perfect pointing, and ignores atmospheric extinction (~30% loss at zenith in visible, less in NIR). Real surveys typically lose another factor 2–10 to these effects.
Schwartz & Townes 1961 (Nature 190:205, original optical SETI proposal); Townes 1983 (PNAS 80:1147, infrared SETI); Howard et al. 2007 (Acta Astronautica, optical SETI overview); Maire et al. 2018 (PASP 130, NIROSETI commissioning); Lipman et al. 2023 (arXiv:2301.06971, APF laser pipeline); Garrett 2025 (arXiv:2512.18903, ESO Optical SETI 2040s).
→ Radio SETI sensitivity (Tool 23) → Falsification hub → Drake Monte Carlo → BZ / Kardashev