n and k Determination from %T and %R Spectra

If the n & k values of the layer materials used in a process to produce an optical thin film coating are not adequately known, the resulting products will not agree with the designs. This problem can be overcome by depositing a single layer of each of the materials of interest on an appropriately prepared substrate, measuring the spectral results in %T and %R versus wavelength (or wavenumber), and using these spectra to find the n & k values which fit the spectral results. These n & k values can then be used in the designs to better represent what is expected from the deposition process. The process of extracting the n & k values is not always easy for the technician to perform, or he/she may want a second opinion on the n & k values. We offer this service for a reasonable fee per n & k value extraction.

Substrates

Although the n and k of test coatings can be determined on most substrates, the best results are obtained when optical glass is used for the deposition on the test sample. Microscope slide glass, which is commonly used in the industry, has variable indices, which is why the %T spectrum of the actual uncoated substrate used for the deposition is required, if it is not on an optical glass with a table of known n versus wavelength.

The most desirable practice with the witness sample glass is to measure the actual piece to be used for the deposition and then apply the test deposition. Alternatively, the witness glass can be partially masked before coating, so that there is an uncoated area for measurement on the part with the test deposition.


Data Needed for Reasonable Index Determination

Spectrophotometer Make and Model

Data tables of:

100% transmittance (%T) spectrum over the intended range of spectral measurements with no sample in the beam.

0% transmittance spectrum over the range of spectral measurements with an opaque sample in the beam.

%T spectrum over the range of spectral measurements with uncoated substrate in the beam, plus the type of optical glass and thickness of the substrate and its manufacturer.

%T spectrum over the range of spectral measurements with coated substrate in the beam.

Measured or estimated coating thickness, and how measured or estimated.

Additionally:

Dielectric layers must be thick enough to provide at least 3 extrema (max or min) in the spectral range measured.

Metallic and Cermet layers must also include reflectance (%R) spectra tables at near normal incidence (<10°) of:

1) %R of the uncoated substrate,
2) no sample (0%R),
3) the %R from the coated side, and
4) the %R from the uncoated side.
(Such metallic films may not exhibit the extrema of dielectric layers.)

Fee Per n and k Determination:

One Determination of n and k represents one layer from the spectra supplied as listed above.

The price is: $150 per for a Dielectric layer (no Absorption) and $250 per for a Metallic or Cermet layer.

For Determination of the n and k results from the spectra, submit the spectral data as listed above. If the data is appropriate for yielding the n and k, it will be processed. Payment before the return of the results is required by Credit Card, Check, or Wire Transfer.

Phone 1-231-237-9392.

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