Thick substrates: Optical constant determination
Now, since we have R and T of our first substrate, we would like to determine the optical constants n and k of Float A and make them available in the database. We will need the optical constants of all substrates in order to analyze deposited thin films and to design the optical properties of coating products on these substrates.
There are two methods to determine optical constants from R and T spectra of thick glass plates: We could setup an appropriate model for the optical constants in the CODE software and fit the relevant parameters of this model, or we can use the DirectDF tool delivered with CODE. DirectDF can be applied in the case of thick, homogeneous plates only. Knowing the thickness of the plate, the optical constants can be determined directly from R and T, if the transmission is significantly different from zero. Using DirectDFis much faster than fitting an optical constant model, so in this case we will use DirectDF.
The DirectDF manual gives a detailed description of all required actions, and we just follow the sequence. A little problem arises at the point where we have to type in the thickness of the plate: Although the supplier specified a thickness of 5 mm, our mechanical measurement gives a value of 4.8 mm. Very likely the reason for this mismatch is a glass production at the lower limit of the thickness tolerance. Nevertheless, we decide to work with 5 mm thickness in DirectDF. This will lead to absorption coefficients being slightly too low, but if we consistently use 5 mm thickness in all computations later on, the calculated absorption of the glass plate will be correct. Using the nominal 5 mm thickness instead of the true 4.8 mm will avoid a lot of confusion within the company.
We compute the optical constants in DirectDF in the range 200 ... 2500 nm using 1000 points. The program computes the so-called dielectric function which is the square of the complex refractice index n + i k. The result is the following:
After the computation of the optical constants of Float A, we save the configuration of DirectDF to the database (database/software configurations/directdf/float_a.ddf).
The new material in the VirCoC database is tested using an appropriate CODE configuration. R and T are computed based on the database data and compared to the measured spectra. A first comparison is rather disappointing:
After the successful comparison with the measured spectra, the substrate layer stack definition (a thick layer of the material 'Float A' with a thickness of 5 mm) is stored in the database folder 'substrates'. We open the layer stack definition window
and use the command File|Save layer stack definition. The layer stack is saved in the file 'Float A (5 mm)' in the subfolder 'substrates'. The Update database command is used to reload the substrate definitions from the database. From now on the 'Float A (5 mm)' substrate can easily be dragged into any layer stack definition.
In order to clearly indicate the type of float glass we place a kind of logo for 'Float A' in the 'substrate' database folder. The bitmap 'Float A (5 mm).bmp)' is displayed in the layer stack definition window when the stack 'Float A (5 mm)' is selected:
Based on the optical constants of 'Float A' we can vary the thickness of the glass and store other substrate definitions to the database as well. As soon as we will receive samples of these additional thicknesses we will check if the computed spectra match the measured ones. Thicknesses of 4, 6, 8 and 10 mm have been added. Corresponding bitmap files named 'Float A (4 mm).bmp', 'Float A (6 mm).bmp', 'Float A (8 mm).bmp' and 'Float A (10 mm).bmp' are created as well.