---
title: "Laser vs Lamp vs LED Interactive Lab | Sandbox Physics"
description: "Can a very bright light still make interference fringes with poor contrast? Yes."
canonical_url: "https://sandboxphysics.com/labs/laser-source-comparison/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/laser-source-comparison/"
last_modified: "2026-10-27"
---

L01 · Laser foundations

# Laser vs Lamp vs LED

**Can a very bright light still make interference fringes with poor contrast?** Yes. Turning up the power scales the bright and dark fringes alike, so their contrast stays the same. Contrast comes from coherence, which depends on how narrow the spectrum is and on how well points across the beam stay in step. A bright, tightly collimated beam is no proof of a laser.

Move a detector along the beam, then examine the same source with a spectrometer, a double slit, a polarizer and a fast photodiode. A controlled counterexample shows that power, directionality and coherence are separate properties.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

All sources are idealized, conditioned beams at 633 nm, not complete lamp or LED spectra. The beams propagate as Gaussian-Schell beams, and the longitudinal modes have equal power and Lorentzian lines. Beam expansion changes both the beam width and the coherence width. The screen shows relative brightness with a square-root display gamma. The diagnostics are ideal and add no measurement noise. The photodiode passes frequencies below 100 MHz and shows technical amplitude modulation, not photon statistics.

Laser foundations · Light, gain and feedback

[L01 · Compare sources](https://sandboxphysics.com/labs/laser-source-comparison/) [L02 · Count transitions](https://sandboxphysics.com/labs/light-matter-transitions/) [L03 · Build inversion](https://sandboxphysics.com/labs/population-inversion/) [L04 · Amplify once](https://sandboxphysics.com/labs/single-pass-amplifier/) [L05 · Add feedback](https://sandboxphysics.com/labs/passive-optical-resonator/) [L06 · Start from noise](https://sandboxphysics.com/labs/laser-noise-startup/) [L07 · Build and debug](https://sandboxphysics.com/labs/build-debug-laser/) [L08 · Measure and diagnose](https://sandboxphysics.com/labs/laser-diagnostic-bench/) [L09 · Select axial modes](https://sandboxphysics.com/labs/gain-longitudinal-modes/) [L10 · Confine the beam](https://sandboxphysics.com/labs/stable-resonator-geometry/) [L11 · Select spatial modes](https://sandboxphysics.com/labs/transverse-laser-modes/) [L12 · Propagate and measure](https://sandboxphysics.com/labs/gaussian-beam-quality/) [L13 · Align and match](https://sandboxphysics.com/labs/alignment-mode-matching/) [L14 · Optimize extraction](https://sandboxphysics.com/labs/output-coupler-optimization/) [L15 · Drive the dynamics](https://sandboxphysics.com/labs/laser-dynamics-relaxation/) [L16 · Store and release](https://sandboxphysics.com/labs/q-switched-giant-pulse/) [L17 · Lock phases into pulses](https://sandboxphysics.com/labs/mode-locking-frequency-comb/) [L18 · Calibrate the comb](https://sandboxphysics.com/labs/optical-frequency-comb/) [L19 · Inject carriers](https://sandboxphysics.com/labs/semiconductor-laser-diode/) [L20 · Compare gain media](https://sandboxphysics.com/labs/laser-medium-zoo/) [L21 · Distributed gain](https://sandboxphysics.com/labs/fiber-fbg-laser/) [L22 · Choose feedback](https://sandboxphysics.com/labs/laser-feedback-architectures/) [L23 · Balance ultrafast pulses](https://sandboxphysics.com/labs/ti-sapphire-oscillator/) [L24 · Stretch and compress](https://sandboxphysics.com/labs/chirped-pulse-amplification/) [L25 · Trap and extract](https://sandboxphysics.com/labs/regenerative-multipass-amplifier/) [L26 · Convert frequencies](https://sandboxphysics.com/labs/nonlinear-frequency-conversion/) [L27 · Scatter and cascade](https://sandboxphysics.com/labs/raman-brillouin-lasers/) [L28 · Synchronize a slave](https://sandboxphysics.com/labs/injection-locking/) [L29 · Lock to a reference](https://sandboxphysics.com/labs/pdh-frequency-lock/) [L30 · Measure phase noise](https://sandboxphysics.com/labs/laser-linewidth-phase-noise/) [L31 · Scatter for feedback](https://sandboxphysics.com/labs/random-laser/) [L32 · Diagnose thermal scaling](https://sandboxphysics.com/labs/thermal-lensing/) [L33 · Combine coherent fields](https://sandboxphysics.com/labs/coherent-beam-combining/) [L34 · Sustain a Kerr pulse](https://sandboxphysics.com/labs/microresonator-kerr-comb/) [L35 · Return a delayed field](https://sandboxphysics.com/labs/external-feedback-chaos/) [L36 · Scan without mode hops](https://sandboxphysics.com/labs/tunable-laser-scan/)

Physical basis: [MIT: coherence and spectrum](https://ocw.mit.edu/courses/res-6-006-video-demonstrations-in-lasers-and-optics-spring-2008/resources/coherence-length-and-source-spectrum/) · [Rate equations](https://ocw.mit.edu/courses/6-977-ultrafast-optics-spring-2005/resources/chapter2/) · [Pumping and gain media (§7.2)](https://ocw.mit.edu/courses/6-974-fundamentals-of-photonics-quantum-electronics-spring-2006/bf3bdeb971819ddc9c68719d28bb2745_classnotes.pdf) · [Laser dynamics](https://ocw.mit.edu/courses/6-977-ultrafast-optics-spring-2005/resources/chapter4/)

Physics tutorial

## Measure the properties separately

**Background** A light source can be bright, well directed, spectrally narrow, spatially coherent, polarized or quiet. Each of these is a separate measurement, and the name of the source does not tell you the values.

**Why it matters** Before learning how a laser works, it helps to know which measurements describe its light and which ones cannot show that lasing is happening.

Start with the essentials

- **Focus question**: Does raising the power improve the normalized fringe visibility?
- **One-sentence intuition**: Power scales the signal, and coherence sets the contrast of the fringes. Temporal coherence must match the spectrum; spatial coherence describes how well points across the beam are correlated.

### Core mathematical model

#### Power and irradiance

$$
I(r,z)=\frac{2P}{\pi w^2(z)}e^{-2r^2/w^2(z)}
$$

Integrating the irradiance across the beam gives the mean power. The screen is 40 mm across and can miss the faint tails. The peak irradiance readout is still a linear physical quantity.

#### Beam spread in the Gaussian-Schell model

$$
w^2(z)=w_0^2+\theta_d^2z^2,\quad\theta_d=\frac{\lambda}{\pi}\sqrt{w_0^{-2}+\xi^{-2}}
$$

In the Gaussian-Schell model, a narrower coherence width means a larger angular spread. The beam expander makes both the waist and the coherence width ten times larger and keeps the power the same. The drawn beam outline is schematic; no lens train is simulated.

#### Spectrum determines temporal coherence

$$
g^{(1)}(\tau)=e^{-\pi\Delta\nu|\tau|}\sum_j p_j e^{2\pi i\nu_j\tau},\quad\sum_jp_j=1
$$

Every spectral component has the same Lorentzian full width at half maximum. Five longitudinal modes can make the correlation revive. Technical modulation of the field envelope adds sidebands to the same list of components, so the time trace always stays tied to the spectrum.

#### Fringe visibility from two equal slits

$$
V=e^{-d^2/(2\xi^2)}|g^{(1)}(\Delta L/c)|,\qquad d=0.5\,\mathrm{mm}
$$

The two slits sample the beam at the source exit plane, after any beam expansion. The fringe position is counted in periods, and both arms carry equal intensity. Equal optical paths remove the time delay, but they cannot make up for low spatial coherence.

### Common difficulties

#### A source name does not fix its properties

**Typical misconception** Every laser is perfectly monochromatic and every LED is completely incoherent.

**Better mental model** The presets are idealized examples for comparison. Real sources have finite coherence and may run on several modes, and their light can be filtered or spatially conditioned.

#### Display brightness is not a measurement

**Typical misconception** If two detector spots look alike on screen, they have the same irradiance.

**Better mental model** The screen shows relative brightness through a display gamma. Compare the calibrated irradiance readouts and profiles instead of judging by screen brightness alone.

### Run the experiment

1. 01

   #### More power does not mean more contrast

   Choose Lamp, turn on Beam expander, and raise the power to 80 mW or more. Open the Double slit diagnostic and press Check counterexample.

   **What to observe:** The irradiance goes up, but the normalized visibility stays low. Collimating the beam changes the spatial coherence that the fixed slits sample, but it does not narrow the spectrum.

2. 02

   #### Separate spatial and temporal coherence

   Choose Single mode. Turn off High spatial coherence and turn on Equal optical paths. Then turn High spatial coherence back on, switch off Equal optical paths and Narrow spectral lines, and keep a nonzero path difference.

   **What to observe:** Losing either spatial or temporal correlation is enough to wash out the fringes. The readouts show which one limits this measurement.

3. 03

   #### A spectrum is more than one width

   Choose Multiple modes and open the Spectrum diagnostic. Sweep the path difference and look for revivals of the correlation. Toggle Technical intensity variation and compare the photodiode trace with the spectral sidebands.

   **What to observe:** The five modes are 500 MHz apart. Beats between the modes lie above the 100 MHz cutoff of the ideal photodiode, so it does not show them. It does show the technical 20 MHz modulation and its intensity harmonic.

4. 04

   #### Check polarization on its own

   Open the Polarizer diagnostic and rotate the analyzer. Repeat with Linear polarization turned off, then drag the detector in the apparatus.

   **What to observe:** Unpolarized light passes half its mean power at every analyzer angle. Moving the detector changes the beam size and the peak irradiance, but the total source power stays the same. A wide beam can spill past the edge of the screen, so less power is collected.

Physical conclusion

A bright or well-collimated beam does not by itself prove that the light comes from a laser oscillator. Gain, feedback and mode selection are separate mechanisms, covered later in this series.

Waves & optics · Starter path

## From one ray to interference measurement

Start with refraction, find where the ray picture stops working, then use interference to measure tiny distances.

[Start here → **Optical Boundary Explorer** Turn the incident angle and watch reflected and transmitted power trade off. Then switch to the critical-angle scene. Observe · 11 min](https://sandboxphysics.com/labs/optical-boundary-explorer/)

1. [1 Optical Boundary Explorer](https://sandboxphysics.com/labs/optical-boundary-explorer/)
2. [2 Rainbow Caustic Observatory](https://sandboxphysics.com/labs/rainbow-laboratory/)
3. [3 Camera Obscura](https://sandboxphysics.com/labs/camera-obscura/)
4. [4 Coherent Wave Composer](https://sandboxphysics.com/labs/coherent-wave-composer/)
5. [5 Young Double-Slit Bench](https://sandboxphysics.com/labs/young-double-slit-bench/)
6. [6 Michelson Precision Lab](https://sandboxphysics.com/labs/michelson-precision-lab/)

[View the full path →](https://sandboxphysics.com/topics/waves-and-optics/#starter-path)