# Black Light

> Author: Black Light Research Group
> Last Modified: 2026-10-08
> Canonical URL: https://alexspac.es/blacklight-encyclopedia/index.html
> Machine Endpoints: /data/all-data.json, /llms.txt, /llms-full.txt

## Definition
A black light (also termed a Wood's light or UV-A lamp) is an optical emission source that radiates electromagnetic radiation in the longwave Ultraviolet-A spectrum (predominantly 315–400 nm, peaking at 365 nm or 395 nm) while emitting minimal visible light.

When invisible UV-A photons strike materials containing fluorescent molecular compounds (fluorophores), electrons undergo quantum excitation and re-emit photons at longer, visible wavelengths via the Stokes Shift, causing target substances to glow brilliantly in total darkness.

## 1. Fundamental Photophysics & The Electromagnetic Spectrum
Ultraviolet (UV) radiation occupies the electromagnetic band between visible violet light and ionizing X-rays, spanning wavelengths from 100 nm to 400 nm (ISO 21348):
- UV-A (Longwave / Black Light): 315–400 nm (3.10–3.94 eV photon energy). Peak excitation for fluorescence with zero immediate erythema.
- UV-B (Midwave / Erythemal): 280–315 nm (3.94–4.43 eV). High biological activity, causes sunburn and DNA dimerization.
- UV-C (Shortwave / Germicidal): 100–280 nm (4.43–12.4 eV). Lethal to microorganisms at 254 nm; severe corneal hazard.
- Vacuum UV (VUV): 10–200 nm. Generates toxic ozone below 242 nm.

Planck-Einstein Relation: E = hc/λ
- 365 nm: E ≈ 3.40 eV
- 395 nm: E ≈ 3.14 eV
- 254 nm: E ≈ 4.88 eV

## 2. Quantum Mechanism: Fluorescence, Phosphorescence & The Stokes Shift
- Fluorescence: Spin-allowed transition from singlet excited state (S1) to ground state (S0). Radiative decay occurs within 10^-9 to 10^-7 seconds. Ceases instantaneously when excitation ceases.
- Phosphorescence: Spin-forbidden transition from triplet state (T1) to ground state (S0) following intersystem crossing. Lingers for milliseconds to hours.
- Stokes Shift: Named after Sir George Gabriel Stokes (1852). Wavelength of emitted fluorescence is always longer than excitation wavelength due to non-radiative vibrational relaxation.

## 3. Robert Williams Wood & Wood's Glass
Invented in 1903 by Professor Robert Williams Wood (Johns Hopkins University). Wood's glass is a barium-sodium silicate glass doped with ~9% nickel oxide that absorbs visible light (>400 nm) while passing 320–400 nm UV-A.