Lamps — Abridged Guide

Quick-reference guide to broadband lamp sources — thermal emitters, gas discharge lamps, blackbody radiation, spectral characteristics, étendue, and selection. For full derivations and worked examples, see the Comprehensive Guide.

Comprehensive Lamps Guide →

1.Introduction to Lamp Sources

Lamps are broadband, incoherent light sources that span the UV through mid-infrared, divided into thermal emitters (tungsten-halogen, Globar, Nernst glower) and gas discharge sources (deuterium, xenon arc, mercury arc, metal halide, xenon flash). They remain essential in spectroscopy, microscopy, and photolithography.

When selecting a lamp, start with the required spectral range — this eliminates most options immediately. No single lamp covers the full UV-to-mid-IR spectrum.

2.Lamp Classification and Types

Lamp Type	Range	Best For
Tungsten-halogen	320–2400 nm	Vis-NIR spectroscopy, calibration
Globar (SiC)	2–25 µm	FTIR mid-IR spectroscopy
Deuterium (D₂)	160–400 nm	UV absorption spectroscopy, HPLC
Xenon arc	190–1100 nm	Fluorescence, spectrofluorometry
Mercury arc (HBO)	250–600 nm (lines)	Fluorescence microscopy (line excitation)
Metal halide	300–700 nm	Fluorescence microscopy (broadband)
Xenon flash	190–1100 nm (pulsed)	Time-resolved fluorescence

Thermal sources produce Planck-like continuum spectra; gas discharge sources range from pure continuum (D₂) to line-dominated (Hg) depending on gas species and pressure.

3.Blackbody Radiation and Thermal Emitters

Planck Spectral Radiance

B_\lambda(T) = \frac{2hc^2}{\lambda^5} \cdot \frac{1}{e^{hc / \lambda k_B T} - 1}

Wien Displacement

\lambda_{\max} = \frac{2.898 \times 10^{-3}}{T} \text{ m}

Stefan-Boltzmann

M = \sigma T^4 \quad (\sigma = 5.670 \times 10^{-8} \text{ W·m}^{-2}\text{·K}^{-4})

Graybody Correction

B_{\lambda,\text{gray}} = \varepsilon(\lambda) \cdot B_\lambda(T)

The peak emission wavelength is inversely proportional to temperature. A tungsten-halogen filament at 3200 K peaks at 906 nm (NIR); a Globar at 1500 K peaks at 1.93 µm (mid-IR). Doubling the temperature increases total radiated power 16-fold.

For a quick estimate of whether a thermal source has useful output at your wavelength, compute λ_max from Wien's law. If your target wavelength is more than 5× shorter than λ_max, the source output will be negligibly small.

4.Gas Discharge Lamp Physics

Gas discharge lamps produce light via electrical excitation of a pressurized gas. Low pressure → sharp atomic emission lines. High pressure → broadened lines plus strong continuum background. Deuterium lamps use a molecular dissociation mechanism that produces a uniquely smooth UV continuum.

Mercury and metal halide lamps need 5–15 minutes to warm up and stabilize — do not turn them off between measurements during a session. Xenon lamps stabilize faster (2–5 min).

5.Spectral Characteristics by Lamp Type

λ (nm)	Line	Use
365	i-line	DAPI, photolithography
405	h-line	Alexa 405, lithography
436	g-line	CFP, lithography
546	e-line	Rhodamine, Cy3, calibration
579	—	MitoTracker Red

Xenon arc lamps produce the most solar-like spectrum (~6000 K color temperature) with a nearly flat visible continuum. Mercury lamps concentrate output into intense discrete lines. Metal halide lamps combine broadened mercury lines with enhanced continuum fill between the lines.

If your fluorophore absorbs between 450 and 530 nm (fluorescein, EGFP family), a mercury lamp is a poor match — use xenon arc or metal halide instead. Mercury's output is weakest in exactly this excitation band.

6.Radiometric Performance and Étendue

Étendue

G = A \cdot \Omega

Solid Angle

\Omega = 2\pi(1 - \cos\theta)

Throughput

\Phi_\lambda = L_\lambda \cdot G_{\text{lim}}

where G_lim = min(G_source, G_system)

Étendue (G = area × solid angle) is conserved through any optical system. The radiant flux coupled from a lamp into a fiber or monochromator equals the source radiance times the limiting étendue — whichever is smaller, source or system. A compact-arc source (small area, high radiance) outperforms a large-area source for small-étendue receivers like fibers and narrow slits.

To maximize lamp throughput: match the source étendue to the receiver étendue. A bigger, faster collection optic only helps until the source étendue is the bottleneck.

Lamp Source Calculator →F-Number & NA Calculator →

7.Stability, Lifetime, and Degradation

Lamp Type	Lifetime	Fluctuation	Warm-Up
W-halogen	2000–5000 h	< 0.1%	< 1 min
Deuterium	1000–2000 h	0.005% p-p	15–30 min
Xe arc	1000–3000 h	0.5–2%	2–5 min
Hg arc (HBO)	200–400 h	2–5%	5–15 min
Metal halide	> 1000 h	1–3%	5–10 min
Xe flash	10⁸–10⁹ pulses	< 1% p-p	N/A

Deuterium lamps have the best short-term stability of any photonics lamp (0.005% fluctuation). Mercury arc lamps have the shortest lifetime (200–400 h) and worst arc stability. Metal halide lamps offer a practical compromise with > 1000 h lifetime and easier bulb replacement.

For quantitative measurements requiring < 0.1% stability, only deuterium (UV) and tungsten-halogen (Vis-NIR) lamps qualify. Xenon and mercury arcs are too unstable without active feedback correction.

8.Optical Coupling and Light Delivery

Fiber Étendue

G_{\text{fiber}} = \pi\left(\frac{d}{2}\right)^2 \cdot \pi \cdot \text{NA}^2

Coupling lamp light into a fiber or monochromator is an étendue-matching problem. A 200 µm / NA 0.22 fiber has G ≈ 4.8 × 10⁻⁹ m²·sr — only a compact-arc source (Xe, Hg) can efficiently fill such a small étendue. Large-core fibers (600 µm+) or liquid light guides are better matched to extended sources.

If coupling efficiency into a fiber is poor, first check the étendue mismatch — no amount of optical refinement can overcome a source étendue that is orders of magnitude larger than the fiber étendue. Switching to a larger-core fiber or higher-NA fiber is often more effective than optimizing the condenser optics.

Fiber Optics Fundamentals →

9.Applications in Photonics

Application	Primary Lamp(s)	Why
UV-Vis spectrophotometry	D₂ + W-halogen (dual)	Full UV-Vis coverage, max stability
Fluorescence microscopy	Hg arc, Xe arc, metal halide	High radiance at excitation λ
FTIR spectroscopy	Globar (SiC)	Broadband mid-IR continuum
Time-resolved fluorescence	Xe flash	Short-pulse, broadband excitation
Photolithography	Hg arc (i/g/h lines)	High intensity at specific UV lines
Wavelength calibration	Hg (546 nm), D₂ (486/656 nm)	Sharp, precisely known lines

The D₂ + tungsten-halogen dual source is the standard for UV-Vis spectrophotometry. Mercury arc lamps are preferred for fluorescence excitation at wavelengths matching their intense emission lines. Xenon arc lamps are chosen when broad, flat excitation is needed.

10.Lamp Selection Workflow

Spectral range

UV → D₂. Vis-NIR → W-halogen. UV-NIR → Xe arc. Mid-IR → Globar.

Radiance / power

High radiance → Xe/Hg arc. High stability → D₂/W-halogen. Pulsed → Xe flash.

CW vs. pulsed

Time-resolved → Xe flash. Steady-state → CW sources.

Stability

< 0.01% → D₂ (UV) or W-halogen (Vis-NIR).

Étendue match

Match source G to system G. Compact arcs for fibers/narrow slits.

Lifetime & cost

Hg arc: 200 h. W-halogen: 5000 h. Factor in replacement and alignment.

Select lamps in this order: (1) spectral range → (2) radiance/power → (3) CW vs. pulsed → (4) stability → (5) étendue match → (6) lifetime and cost. Always check whether a solid-state alternative (LED or LDLS) now outperforms the traditional lamp choice for your application.

For the common case of a UV-Vis spectrophotometer, the answer is almost always D₂ + tungsten-halogen. The decision only gets complex for broadband fluorescence, multi-wavelength excitation, or fiber-coupled systems where étendue matching dominates.

Continue Learning

Comprehensive Guide →Lamp Source Calculator →

The Comprehensive Guide includes 6 worked examples, 5 SVG diagrams, 3 data tables, and 10 references.