Optical Parameters

The Optics page defines illumination conditions, solver range, and output targets. This chapter covers four areas:

1. setup rules for wavelength mode, sampling, incident angle, polarization ratio, and cone-angle averaging;
2. activation conditions for basic detectors, incident spectrum, visible color, ellipsometry, depth-resolved outputs, and dispersion detectors;
3. configuration of dispersion numerical quality and interpolation recommendations;
4. the most common validation conflicts that should be cleared before pressing Run.

Page Structure and Setup Order

The Optics page is divided into six functional regions:

The recommended setup order is:

1. Choose the wavelength mode first.
2. Enter the wavelength parameters.
3. Set incident angle and polarization ratio.
4. If cone-angle averaging is needed, enable and configure it.
5. Select detectors according to the required outputs.
6. Enable incident spectrum, visible color, depth resolution, or dispersion only when the analysis requires them.
7. Check the footer validation status before running.

This sequence minimizes rework because most rules on this page are coupled rules involving wavelength mode, detector selection, and the current structure.

Baseline Illumination Conditions: Wavelength, Angle, and Polarization

Common Parameters is the base control block of the page. Its settings directly affect which detectors remain available and which advanced options are disabled.

Wavelength Modes and Sampling Rules

The three wavelength modes differ as follows:

The corresponding input rules are:

In practice:

1. Use Single for a quick point check.
2. Use Sweep for standard spectral analysis.
3. Use Average only when you care about band-averaged behavior rather than spectral shape.

For first-pass modeling, Sweep remains the safest default.

Incident Angle and Polarization Ratio

Incident angle and pRatio define the direction and polarization state of the incoming light. Both are controlled by a slider and a numeric input; changing either one updates the other.

If no specific polarization condition is required, pRatio = 0.5 is the safest baseline choice.

Cone Angle

The Cone Angle section averages simulation results over a cone of incident angles centered on the current Incident Angle. This models situations where the incoming light has a finite angular spread, such as focused beams or light collected through a lens with a specific numerical aperture.

Input Modes

The cone half-angle can be entered in three equivalent ways:

All three modes set the same underlying half-angle; only the input representation differs.

Distribution and Ring Count

The effective number of rays is 1 + 12 × ringCount. For example, the default ringCount = 10 uses 121 rays.

Scope of Cone-Angle Averaging

Cone-angle averaging only affects the following detectors and derived outputs:

• R, T, A, Layer Absorption
• Incident-spectrum-weighted spectra
• Visible-color calculations

All other detectors (Ellipsometry, Depth Distribution, Dispersion) continue to use the single Incident Angle value.

Basic Detectors, Incident Spectrum, and Visible Color

The R · T · A panel controls the most common spectral outputs and also contains the prerequisite settings for incident spectrum and visible-color calculation.

Basic Detectors

The four basic detectors are:

Use:

1. For a first calculation, keep at least R, T, and A enabled.
2. Enable Layer Absorption when you need to identify which layer dominates absorption.
3. If only one result family matters, there is no need to keep the entire set enabled.

When Incident Spectrum Can Be Enabled

Incident spectrum applies spectral weighting to the basic-detector outputs. It is available only when both conditions below are satisfied:

1. The current wavelength mode is not Single.
2. At least one basic detector is enabled (R, T, A, or Layer Absorption).

If either condition fails, the incident-spectrum toggle becomes disabled or hidden.

Incident-Spectrum Sources and File Requirements

The available spectrum sources are:

For a custom file, the data must satisfy all of the following:

1. It contains at least one data point.
2. Wavelength and intensity arrays have the same length.
3. Wavelength values are strictly increasing.
4. Every wavelength is greater than 0.
5. Every intensity is non-negative.

If a file appears to import but validation still fails, inspect the source data first and the page settings second.

Wavelength Range vs Source Coverage

Once incident spectrum is enabled, the current wavelength range must stay inside the available coverage of the selected source:

The wavelength range must satisfy:

1. From is lower than the source minimum.
2. To is higher than the source maximum.

These are the most common validation conflicts after incident spectrum is enabled.

Visible Color Calculation

Enable Color Calculation enforces stricter wavelength-sampling conditions. The required settings are:

1. From = 360 nm
2. To = 780 nm
3. Step <= 1 nm

Visible-color output therefore applies only to the visible band with sufficiently fine sampling.

The Apply Preset button writes 360 / 780 / 1 automatically. When the goal is color analysis, this preset should be used first instead of entering the values manually.

Ellipsometry and Depth Distribution

Ellipsometry and Depth Distribution are advanced result groups. Both impose stricter constraints on the wavelength mode and, in some cases, on the coherence state of the current structure.

Ellipsometry (Psi / Delta)

The Ellipsometry panel contains only Psi and Delta. These outputs are available only when the following conditions are satisfied:

As soon as Psi or Delta is enabled, return to Structure and confirm that every enabled layer is coherent, including top-level normal layers and enabled internal layers inside any Layer Group.

Depth-Distribution Detectors

The Depth Distribution panel contains four detectors:

Two points matter:

1. Average disables the entire depth-distribution group.
2. Of the four detectors, the first three are the ones that truly require a fully coherent active structure.

Depth Resolution

Depth resolution becomes relevant only when at least one coherent depth detector is enabled (Poynting Vector, Absorption Density, or Electric Field).

The current rules are:

1. Depth resolution must be >= 0.1 nm.
2. Depth resolution must be smaller than the total thickness of the active structure.

The “total thickness” includes:

1. The sum of all enabled normal-layer thicknesses.
2. The sum of enabled Layer Group internal thicknesses multiplied by the group repeat count.

Therefore, depth resolution is not valid merely because it is positive. For a thin active stack, a value that is too large still fails validation.

From an engineering standpoint, start with a coarse but valid depth resolution, and reduce it only when finer spatial detail is actually needed.

Dispersion Detectors

The Dispersion section provides four detectors for characterizing the phase and group-delay behavior of the thin-film stack:

Prerequisites

Dispersion detectors have the strictest activation conditions on the Optics page:

Numerical Quality

GD, GDD, and DGD are computed via numerical differentiation. The Numerical Quality setting controls the internal sampling density:

Phase itself does not require differentiation and is not affected by this setting.

Sparse Output Warning

If the number of output wavelength points is below the recommended threshold, a warning appears:

Increase the wavelength range or decrease the step size to obtain enough output points.

Interpolation Recommendation

When dispersion detectors are enabled, the panel may display a recommendation to change the refractive-index interpolation method from Linear to PCHIP or CubicSpline. Two convenience buttons are provided:

• Use PCHIP Interpolation for Layers — sets all file-based layers to PCHIP interpolation
• Use CubicSpline Interpolation for Layers — sets all file-based layers to CubicSpline interpolation

Smoother interpolation reduces numerical artifacts in GD and GDD. For theory and usage details, see the Dispersion Theory chapter and the Dispersion Results chapter.

Common Conflicts and Pre-Run Checks

The most common failures on this page can be summarized as follows:

Before pressing Run or moving on to the next chapter, verify the following:

1. The wavelength mode matches the intended analysis.
2. The wavelength inputs satisfy the current mode.
3. Incident angle and pRatio are within sensible ranges.
4. If cone angle is enabled, incidentAngle + halfAngleDeg < 89.9.
5. At least one detector is enabled.
6. If incident spectrum is enabled, the wavelength interval stays inside source coverage.
7. If visible color is enabled, 360 / 780 / <= 1 nm is already satisfied.
8. If ellipsometry or coherent depth detectors are enabled, the structure contains no enabled incoherent layer.
9. If dispersion detectors are enabled, the mode is Sweep and all layers are coherent.

Baseline Setup Procedure

For a standard spectral calculation, use the following baseline configuration:

1. Keep Sweep mode.
2. Use 400 ~ 900 nm with a 5 nm step.
3. Keep incident angle at 0°.
4. Keep pRatio = 0.5.
5. Enable R, T, and A.
6. Leave incident spectrum disabled.
7. Run once and inspect the basic result pages first.

If the next task is visible-color analysis:

1. Enable incident spectrum.
2. Choose D65 or the required custom source.
3. Enable visible color.
4. Click Apply Preset.
5. Run again and inspect the spectrum and color pages.

If the next task is depth-resolved analysis:

1. First confirm that Structure contains no enabled incoherent layer.
2. Select Poynting Vector, Absorption Density, or Electric Field.
3. Set a valid depth resolution.
4. Run again and inspect the depth-distribution outputs.

If the next task is dispersion analysis:

1. Set wavelength mode to Sweep.
2. Confirm that Structure contains no enabled incoherent layer.
3. Select one or more of Phase, GD, GDD, DGD.
4. Set Numerical Quality as needed (default Medium is usually sufficient).
5. If an interpolation recommendation appears, consider applying PCHIP or CubicSpline.
6. Run and inspect the Dispersion Results.

After these conditions are met, continue with the next chapter: Parameter Sweep.