Thin-Film Optical Coating Calculator

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Refractive index of the medium the light travels through before hitting the film. Air is 1.0, glass is typically 1.5.
Refractive index of the thin-film coating. MgF₂ (1.38) is a common anti-reflective coating; TiO₂ (2.32) is a high-index material.
Refractive index of the material underneath the film (the substrate). Crown glass is about 1.52.
Wavelength of the light in vacuum (or air). Visible light spans roughly 380-700 nm; 550 nm is the peak sensitivity of human vision.
nm
Physical thickness of the thin-film layer in nanometers. Quarter-wave thickness for MgF₂ at 550 nm is about 99.6 nm.
nm
Angle between the incoming light ray and the surface normal (0° = perpendicular/normal incidence). At 0° s and p polarizations are identical.
deg
s-polarized reflectance (Rₛ)Destructive interference (anti-reflective)
1.26%

Fraction of s-polarized intensity reflected by the coated surface

p-polarized reflectance (Rₚ)1.26%
Average reflectance (Rₐᵥᵧ)1.26%
s-polarized transmittance (Tₛ)98.74%
p-polarized transmittance (Tₚ)98.74%
Optical path difference (OPD)274.9nm
Round-trip phase shift (δ)179.9deg
Interference conditionDestructive
Min AR-coating thickness (dₘᵢₙ)99.64nm
Ideal quarter-wave index1.233
Rₛ (s-pol)1.26%
Rₚ (p-pol)1.26%
Rₐᵥᵧ (avg)1.26%
Tₛ (s-pol)98.74%
Tₚ (p-pol)98.74%

Destructive interference - 1.26% average reflectance

  • The coating produces 1.26% s-polarized and 1.26% p-polarized reflectance at the current thickness.
  • The film is operating near a destructive interference condition, which suppresses reflection and maximises transmission - ideal for anti-reflective coatings.
  • Your film thickness (99.6 nm) is very close to the optimum AR thickness (99.6 nm).
  • For zero reflectance at normal incidence the ideal film index would be 1.233 (the geometric mean of n₁ and n₃). Your film uses n₂ = 1.38.

Next stepTo further reduce reflectance, verify that n₂ equals sqrt(n₁ × n₃) and that the film thickness is exactly λ/(4n₂).

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