Schlieren Optics
Schlieren photography makes the invisible visible. Density gradients in air — from heat, sound, or gas flows — bend light rays ever so slightly. A knife edge placed at the focal point of a mirror converts these tiny deflections into dramatic patterns of light and shadow. Click on the test area to place heat sources and watch turbulent convection plumes appear.
n(T) ≈ 1 + (n0 − 1) · T0/T | refractive index decreases with temperature
Optical Setup (Schematic)
Schlieren Image — click to place heat sources
What is schlieren photography?
Schlieren (German for "streaks") photography is an optical technique for visualizing refractive index variations in transparent media. First developed by August Toepler in 1864, it has become essential in aerodynamics, combustion research, and fluid dynamics for revealing phenomena invisible to the naked eye: heat convection, shock waves, gas mixing, and sound waves in air.
How it works
A point light source is placed at the focus of a concave mirror (or lens), producing a parallel beam that passes through the test area. A second mirror refocuses the beam onto a knife edge placed precisely at the focal point. In undisturbed air, exactly half the light passes the knife edge. Any density gradient in the test area deflects light rays: rays deflected toward the knife edge are blocked (dark regions), while rays deflected away pass through (bright regions). The result is a grayscale image where brightness encodes the density gradient in one direction.
The physics of refraction
Air's refractive index depends on density: n ≈ 1 + 0.000293 · (ρ/ρ0). Since hot air is less dense, it has a lower refractive index. The deflection angle of a light ray is proportional to the gradient of the refractive index integrated along the beam path: θ ∝ ∫ (∂n/∂y) dz. This means schlieren images show the first spatial derivative of the density field — not the density itself, but its gradient.
Knife edge orientation
A vertical knife edge is sensitive to horizontal density gradients (dn/dx), while a horizontal knife edge detects vertical gradients (dn/dy). For convection plumes rising vertically, a horizontal knife edge reveals the sharpest structures at the edges of the plume, while a vertical knife edge shows the left-right asymmetry of turbulent eddies.
What you see in this simulation
Each heat source creates a plume of hot, buoyant air that rises and develops turbulent structures. The simulation models temperature diffusion and buoyancy-driven convection with Perlin-like noise for turbulence. The schlieren image is computed by taking the directional gradient of the temperature field (a proxy for refractive index) and mapping it to brightness. Bright regions represent density gradients deflecting light past the knife edge; dark regions represent gradients deflecting light into the knife edge.