2. The Science: Pulse Width and Color Formation
Thin-Film Interference Explained
When the laser creates an oxide layer on stainless steel, that layer acts as a thin film. Light hitting the surface partially reflects off the top of the oxide and partially off the metal beneath. These two reflected beams interfere — constructively for some wavelengths (those colors appear bright) and destructively for others (those colors are suppressed).
The oxide layer thickness determines which wavelengths constructively interfere:
| Approximate Oxide Thickness | Perceived Color |
|---|---|
| ~80–100 nm | Gold / Yellow |
| ~100–120 nm | Orange / Red |
| ~120–140 nm | Magenta / Purple |
| ~140–170 nm | Blue |
| ~170–200 nm | Green |
| ~200–250 nm | Second-order colors (lighter gold, pink) |
A difference of just 10–20nm in oxide thickness can shift the color noticeably. This is why precise pulse control is essential — and why MOPA’s adjustable pulse width is the key enabling technology.
The Pulse Width-to-Color Relationship
While the relationship isn’t a simple linear mapping (it interacts with frequency, speed, and material), the general principle holds:
- Shorter pulse widths (2–30ns) → less heat per pulse → thinner oxide → warmer colors (gold, yellow)
- Medium pulse widths (30–80ns) → moderate oxide → red, magenta, purple
- Longer pulse widths (80–200ns) → thicker oxide → blue, green
Critical note: These relationships interact with frequency and marking speed. A given pulse width at 20kHz produces a different thermal result than the same pulse width at 80kHz because pulse overlap changes cumulative heating.
