In the world of precision optics, these specifications define how well a component (like a lens or mirror) preserves the quality of light passing through it or reflecting off it. These values are typically measured using interferometry and profilometry.
1. Transmitted Wavefront Error (TWE)
TWE measures the distortion an optical element introduces to a light wave as it passes through. It accounts for both surface inaccuracies and internal material inhomogeneities (like variations in the refractive index).
- Standard Spec: Usually expressed in fractions of a wavelength (λ), typically at 632.8 nm (HeNe laser).
- Commercial Grade: 1λ to λ/2.
- Precision Grade: λ/4 to λ/10.
- High Precision/Laser Grade: Better than λ/20.
2. Peak-to-Valley (PV)
PV is a measure of the "flatness" or "form" of a single surface. It represents the vertical distance between the highest point and the lowest point on the surface profile relative to the ideal shape.
The Formula:
PV=zmax−zmin
- Standard Spec: Like TWE, this is measured in waves (λ).
- Commercial: λ/2.
- Precision: λ/4 to ()λ/8.
- Research/UV Grade: λ/10 to λ/20.
- Note: PV is often paired with RMS (Root Mean Square), which provides a statistical average of the entire surface, whereas PV only looks at the two most extreme points.
3. Surface Roughness
While PV measures the overall "hills and valleys" of the surface shape (low-frequency errors), roughness measures the microscopic texture (high-frequency errors). This affects how much light is scattered (haze) rather than refracted or reflected cleanly.
- Standard Spec: Measured in Angstroms (Å ) or Nanometers (nm).
- Commercial: 50Å −80Å RMS.
- Precision: 10Å −20Å RMS.
- Super-polished: <5A˚ RMS (common in high-energy laser systems or X-ray optics).
Comparison Summary
| Specification | Measures | Units | Impact |
| TWE | Full system distortion | λ | Image resolution/focus |
| PV | Surface form error | λ | Alignment and aberration |
| Roughness | Micro-texture | Å or nm | Scattering and contrast |
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