Optical Coating Thickness Calculator

Calculator Inputs

Coating mode

Quarter-wave uses odd quarter-wave orders.

Design wavelength (nm)

Common values include 450, 550, 632.8, and 1064 nm.

Coating refractive index

Enter the thin-film refractive index at the design wavelength.

Ambient refractive index

Use 1.00 for air unless your process uses another medium.

Incident angle (degrees)

The calculator applies Snell’s law for internal propagation angle.

Design order

Quarter-wave order 1 = λ/4, order 2 = 3λ/4.

Custom optical fraction (λ units)

Used only in custom mode. Example: 0.375 means 0.375λ per order.

Layer count

This estimates total stack thickness for repeated identical layers.

Thickness tolerance (%)

Used for tolerance band and graph envelope.

Measured thickness (nm)

Optional process check against the calculated target thickness.

Graph sweep minimum wavelength (nm)

Start of the wavelength range for the plot.

Graph sweep maximum wavelength (nm)

End of the wavelength range for the plot.

Example Data Table

Case	Mode	Wavelength (nm)	Index	Angle (°)	Order	Custom Factor	Result Thickness (nm)
AR layer	Quarter-wave	550	1.38	0	1	—	99.638
Laser optic	Half-wave	1064	1.45	0	1	—	366.897
Oblique design	Quarter-wave	632.8	2.10	15	1	—	75.962
Custom phase film	Custom	450	1.85	30	2	0.375	189.388

Formula Used

Snell’s law for the coating angle
θt = asin((n0 / n) × sin(θi))

Optical thickness factor
Quarter-wave: F = (2m - 1) / 4
Half-wave: F = m / 2
Custom: F = m × f

Optical thickness target
OT = F × λ

Required physical thickness
d = OT / (n × cos(θt)) = (F × λ) / (n × cos(θt))

Phase thickness at the design wavelength
δ = 2π × n × d × cos(θt) / λ = 2πF
Phase in degrees = 360 × F

Total repeated stack thickness
Total stack = d × layer count

Equivalent center wavelength from measured film thickness
λeq = (n × dmeasured × cos(θt)) / F

Variable meanings:

λ = design wavelength in nanometers.
n = coating refractive index at the design wavelength.
n0 = ambient refractive index before the coating.
θi = incident angle in the ambient medium.
θt = internal angle inside the coating layer.
m = design order.
f = custom optical fraction in wavelength units.
OT = optical thickness target.

How to Use This Calculator

Choose the coating mode: quarter-wave, half-wave, or custom optical fraction.
Enter the design wavelength for the optical system in nanometers.
Provide the coating refractive index and the ambient refractive index.
Set the incident angle, then enter the design order for the coating.
Use the custom optical fraction only when custom mode is selected.
Enter layer count if you want the total repeated stack thickness estimate.
Add tolerance and measured thickness to evaluate manufacturing deviation.
Define the graph wavelength sweep, then press Calculate Thickness.
Review the result block above the form, inspect the plot, and export CSV or PDF if needed.

Frequently Asked Questions

1. What does quarter-wave thickness mean?

Quarter-wave thickness means the layer has an optical thickness of one quarter of the design wavelength. At normal incidence, physical thickness equals wavelength divided by four times refractive index.

2. Why does incident angle change coating thickness?

Oblique incidence changes the propagation angle inside the coating. That changes the cosine term in the optical path, so the physical film thickness must adjust to keep the same optical thickness.

3. What is design order in thin-film coatings?

Design order is the multiple of the base interference thickness. In quarter-wave mode, order two means three-quarter-wave optical thickness, not one-half-wave thickness.

4. When should I use custom optical fraction mode?

Use custom mode when your design needs a nonstandard optical thickness, such as 0.375λ or 0.625λ. It helps model phase-sensitive films outside simple quarter-wave or half-wave cases.

5. Does this calculator account for wavelength-dependent refractive index?

This version uses one refractive index value at the design wavelength. For dispersive materials, enter the index corresponding to your selected wavelength for better engineering accuracy.

6. What does equivalent center wavelength mean?

Equivalent center wavelength estimates where a measured thickness would place the coating response if the same design factor and incidence geometry stay unchanged. It is useful for process drift checks.

7. Can I use this for multilayer stacks?

You can estimate repeated identical layer thickness totals. For full multilayer performance, such as reflectance spectrum or admittance, you still need stack-specific thin-film matrix analysis.

8. Why are tolerance limits important?

Tolerance limits show how deposition variation changes the accepted film thickness window. Small errors can shift phase thickness and move the spectral response away from the intended design wavelength.