Core Concept Analysis of Fringes, Local Fringes, PV and RMS
Fringes, Local Fringes, PV and RMS's concept
Apr 14th,2026
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Evaluation of Optical Surface Figure Accuracy: Core Concept Analysis of Fringes, Local Fringes, PV and RMS
One Article to Master Fringes, Local Fringes, PV and RMS
Optical surface figure accuracy is a core indicator for measuring the quality of optical components. Today we will analyze these confusing terms:
Fringe Number (N): Describes the deviation between the tested surface and the reference surface. The more fringes, the greater the surface figure deviation.
Local Fringes (ΔN): Evaluates the irregularity of local areas and reflects surface smoothness. Excessive local fringes will degrade imaging quality.
Peak-to-Valley (PV or PT): The vertical distance between the highest and lowest points, reflecting the overall maximum fluctuation of the surface, while being sensitive to defects.
Root Mean Square (RMS): The statistical average of height data at all sampling points, which better characterizes the overall surface roughness.
Relationship Analysis
1.PV ≈ (6–8) × RMS; some conversions use a factor of 3.
2.There is also a conversion relationship between fringe number N and PV, though they belong to different evaluation systems.
3.Local fringes focus on irregularity within sub-apertures, and local PV is usually stricter than global PV.
Measurement Practice
1.Drawings typically specify requirements using fringe number N and local fringes ΔN, while modern testing adopts PV and RMS values.
2.Fringes reported by instruments are based on the best-fit sphere, which may not fully match the reference surface of the drawing template, yet the principle remains consistent.
