ZEMAX is supplied with the glass catalogs of the major manufacturers (which are built-in to ZEMAX), but there are times when you still need to add your own glass data. There are several methods to do this in ZEMAX.
 
Sometimes, you may have refractive index data at only a single wavelength, and other times you may have only two or three data points available to you.  In these cases, the table glass may be the best way to enter the data into ZEMAX.  Table glasses are defined by data stored in an ASCII file.  Within the ASCII file, you can specify the refractive index and transmission of the glass at specified wavelengths, and then ZEMAX uses these exact values at the specified wavelengths.  If refractive index values are needed at intermediate wavelengths, ZEMAX automatically fits the data to a smooth spline curve to model the dispersion of your 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 ZEMAX is simply used to create a smooth transition from one data point to the next.  In any case, 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:



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

Please aslo note the following (this is also covered in Chapter 18 of the ZEMAX User’s Guide):

• For the table glass, any blank line or a line starting with the “!” symbol is assumed to be a comment and is ignored by ZEMAX.
• 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.

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 ZEMAX.  Assume that the information used in this example is available for the glass we wish to model.

Using the file format from the User’s manual as a guide, we may enter the dispersion data into any desired text editor (such as 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 ZEMAX.

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).



ZEMAX 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, ZEMAX 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 ZEMAX (Analysis > Glass and Gradient Index > 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:



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, ZEMAX 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, as can be seen by the flat regions at the extent of the wavelength region in the plot below:

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:

The table glass is easy to use, and is a very convenient glass modeling tool available in ZEMAX.  Dispersion data for the table glass is entered via and ASCII text file in a specific file format.

ZEMAX internally uses a cubic spline interpolation of the data entered into the ASCII file.  The advantage of the table glass is that the index at the defined wavelength is exactly the index specified in the table, unlike fitted functions which may deviate from the data being fit at the data points. 

References

ZEMAX Optical Design Program User's Guide, ZEMAX Development Corporation