Description goes here.
The colours laser-engraved into stainless steel aren't pigment — they're nanometre-thin oxide films that split white light by interference. This chapter traces the physics from photon to colour.
The laser's only job is to grow an oxide film of precisely the right thickness. The colour emerges from the physics of light — the same effect as soap bubbles, oil on water, and butterfly wings.
When the laser heats stainless steel into the right temperature window, oxygen from the air reacts with the chromium in the alloy (SS304 contains 18% Cr) to form a chromium oxide (Cr₂O₃) layer at the surface. This layer is transparent and colourless in isolation — just a few tens of nanometres thick.
White light hitting this film is split: some reflects off the top surface of the oxide, the rest passes through and reflects off the steel below. Those two reflections recombine, but because they have travelled slightly different path lengths (twice the oxide thickness), certain wavelengths cancel each other (destructive interference) and complementary wavelengths reinforce. The remaining light reaches your eye as a vivid structural colour.
Notice the colour sequence: silver → straw → gold → red → copper → purple → deep blue → cyan → grey. This is not arbitrary — it mirrors the sequence of visible wavelengths that are successively cancelled as the film grows thicker. The cycle would repeat (second-order colours) but the oxide layer becomes optically thick enough to absorb before that point.
The oxide grows according to a parabolic rate law — growth slows as the film thickens. The growth rate is exponentially sensitive to temperature, which means a narrow temperature window determines the final colour.
Each oxide molecule must diffuse through the existing film to reach fresh chromium below. As the film thickens, this journey gets longer and the growth rate slows. This gives the parabolic rate law:
This extreme sensitivity is the key insight: oxidation effectively only happens while the surface is above ~600°C. Once the surface cools below ~250°C, k_p approaches zero and growth stops. The final oxide thickness is determined almost entirely by the peak temperature reached and how quickly the surface cools through the high-rate window.
Early growth is fast — the film is thin, diffusion is easy. As it thickens the film itself acts as a barrier, slowing further oxygen transport. This self-limiting behaviour is what makes the colour repeatable: the same temperature history always produces the same thickness.
Below ≈250°C the Arrhenius exponential makes k_p negligibly small — growth stops. The majority of the oxide forms in the first fractions of a second when the surface is above ≈600°C. Cooling rate matters: a slower cool gives more time in the high-k_p window.
Drag the slider to grow the oxide layer and watch two reflected rays fall in and out of phase — directly producing the colour shift.
White light is split at the air–oxide surface: Ray 1 reflects immediately off the top; Ray 2 travels through the film, bounces off the steel, and exits. Because both rays originate from the same incident beam they are coherent — they interfere. The extra optical path Ray 2 travels is 2 × n × d (the film traversed twice, at refractive index n = 2.3).
Ray 1 undergoes a ½λ phase flip at the air–oxide surface. Ray 2 also undergoes a significant phase shift at the oxide–metal surface due to the complex optical properties of steel. These two shifts roughly cancel each other out. With zero net phase flip, the interference condition follows the optical path: a wavelength is reinforced when 2nd = mλ (m ≥ 1), and cancelled when 2nd = (m + ½)λ.
In physics, the "Order" refers to how many half-wavelengths of light are packed into the optical path before reinforcement occurs.
The condition for reinforcement is 2nd = (m + ½)λ, where m is the order. This integer changes the saturation and character of the resulting colour.
The path difference is only half a wavelength. These are the primary colours of laser marking (0–150 nm). Because the film is thin, only one primary wavelength peak is reinforced at a time, resulting in pure, saturated colours like Deep Blue and Vivid Gold.
As the oxide grows thicker (> 200 nm), the path difference matches 1.5 wavelengths. The spectral peaks move closer together, often reinforcing multiple wavelengths simultaneously (e.g., green and violet). This produces pastel or pearlescent colours that look "milky" compared to 1st order.
Drag the peak temperature slider and watch the Arrhenius integral accumulate oxide — and the colour that results. The simulation models exponential surface cooling after the laser passes.
The laser controls surface temperature through power, scan speed, and hatch density. The grid below shows how every power/speed combination maps to a colour at 1000 LPC on polished SS304.
For a 1000 LPC crosshatch fill at 1064 nm on polished SS304, the energy injected into a 1 mm² patch is determined by power and speed. The focal skin model concentrates that energy into the top 40 µm, producing the peak temperature spike that drives oxidation:
| Stage | What happens | Key variable |
|---|---|---|
| 1. Photon absorption | IR beam absorbed by SS304 surface — 38% absorptivity at 1064 nm | Power × LPC / speed |
| 2. Focal skin heating | Top 40 µm reaches peak temperature in microseconds | Peak °C |
| 3. Arrhenius growth | Cr₂O₃ grows while surface > 250°C — rate exponential in temperature | k_p(T) = A·exp(−Ea/RT) |
| 4. Cooling & lock-in | Growth stops as surface cools. Final oxide thickness is frozen | Cooling rate |
| 5. Interference colour | Reflected white light splits — specific wavelengths cancel, rest appears as colour | Oxide thickness (nm) |
| Oxide thickness | Colour | Approx. peak temp (SS304) |
|---|---|---|
| <18 nm | Silver / no colour shift | <450°C |
| 18–48 nm | Straw → Gold | 450–620°C |
| 48–68 nm | Red → Copper | 620–690°C |
| 68–88 nm | Purple → Violet | 690–720°C |
| 88–112 nm | Deep Blue | 720–760°C |
| 112–140 nm | Blue → Cyan | 760–830°C |
| >140 nm | Grey / Burnt (diffusion-limited) | >830°C |