How to enter glass data at specific wavelengths

This article explains the details and format of the table glass. It also discusses the benefit and simplicity of the table glass.

Authored By Dan Hill

Introduction

OpticStudio is supplied with the glass catalogs of many major manufacturers, but there are times when you still need to add your own glass data. There are several methods to do this in OpticStudio.

When you have refractive index data only at a few data points, it is usually best to use the table glass feature to enter your data into OpticStudio. Table glasses are defined by data stored in an ASCII file wherein you can specify the refractive index and transmission of the glass at specified wavelengths for OpticStudio to use. If refractive index values are needed at intermediate wavelengths, OpticStudio automatically fits the data to a smooth spline curve to model the dispersion of your glass.

The Details and Syntax of Table Glass

The table glass is ideal in cases where certain wavelengths must have specific corresponding indices of refraction. One or more data points are required, and the spline-fitting performed by OpticStudio is simply used to create a smooth transition from one data point to the next. When using the table glass, you can be assured that the index of refraction for a given wavelength will be exactly as you define in the table glass; there is no fitting function which may deviate from the data being fitted by the data points.

The table glass file is a simple ASCII list, and includes the density of the glass (optional), wavelength-index pairs, and transmission data. The file format is as follows:

file format

The ASCII table glass file must end in the extension “.ZTG” (for Zemax Table Glass) and must be placed in the “Glass” subdirectory as defined in File...Project Preferences...Folders dialog (by default the directory is {Zemax}/Glasscat).  The name of the file, including the extension, must not exceed 20 characters in length.

Please also note the following (this is also covered in “Using Table Glasses” section of the OpticStudio Help System):

  • For the table glass, any blank line or a line starting with the “!” symbol is assumed to be a comment and is ignored by OpticStudio.
  • The DENSITY data line refers to the density of the glass in grams per cubic centimeter.  If this line is omitted, then the density is assumed to be zero.
  • The wavelength values must always be in units of micrometers, and should be listed in ascending order
  • The index of refraction and transmission values must also be positive values.
  • The thickness values of the glass are in millimeters (regardless of the system lens units) for the specified internal transmittance.
  • Both the transmission and thickness values are used to determine the internal transmission per unit length. If both of these values are excluded from the table glass file, then the material is assumed to be 100% transmitting.
  • Up to 1200 lines of wavelength data may be defined per file. At least 5 data points should be defined to provide a reasonable spline fit.

The Use and Functionality of the Table Glass

To demonstrate the use and functionality of the table glass, we will create a fictitious glass for use in the infrared (3-4μm in this case).  Note that the dispersion data for this fictitious glass has been grossly exaggerated (nowhere near realistic) to demonstrate the spline-fitting of the data performed by OpticStudio.  Assume that the information used in this example is available for the glass we wish to model.

Using the file format from the Help system as a guide, we may enter the dispersion data into any desired text editor (such as Notepad).

Notepad

Once the file has been given an appropriate file name and has been saved to the Glass Path directory, this glass may be readily used within OpticStudio.

Using the Table Glass in Design and Verifying Dispersion Data

To use the table glass, enter the file name, including the extension, into the glass column of the Lens Data Editor (or the Material column in Non-Sequential Component Editor) for the desired surface (object).

Lens_data

OpticStudio will internally recognize the dispersion data in the appropriate table glass file in order to calculate the proper index of refraction at each design wavelength. When computing the data for a specific wavelength, OpticStudio uses a cubic spline interpolation if the wavelength is between the shortest and longest wavelengths defined in the ZTG file.

To confirm the accuracy of the spline fit, it is important to check the data via the Dispersion Diagram in OpticStudio (Libraries...Materials Analyses...Dispersion Diagram).

From the settings of the Dispersion Diagram, select the desired table glass, and enter the minimum and maximum wavelength range to be plotted. For the current example, the following settings may be used:

Dispersion Diagram

Note the cubic-spline fit between the ten dispersion data points used to define the TABLETEST_2 table glass. The defined wavelength region in the current example was 3-4 micrometers. Beyond these wavelengths, OpticStudio uses the index of refraction and transmission data of the nearest data point; no extrapolation of the data is performed. Therefore, for wavelengths beyond the range of the defined table glass, no fitting is performed.

Dispersion_Diagram_plot

What is Unique About the Table Glass?

What’s unique and advantageous about the table glass is that the index at the defined points is exactly the index specified in the table. This is unlike the fitted function, which may deviate from the data being fit at the data points. To prove this, open the text of the Dispersion Diagram and compare the wavelength vs. index listing to that of the original defined ASCII table glass file.

In this demonstration, change the “Min Wave” and “Max Wave” to 3 and 4 respectively. Note that at each wavelength defined in the TABLETEST_2.ZTG file, the index for that wavelength is identical to what is reported in the text listing of the Dispersion Diagram.  A few points for comparison are shown below:

Dispersion_Diagram_data

KA-01440

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