The Ellipsometry Calculator uses the values P1, A1, P2, and A2 to calculate delta and psi. If the numbers input for these values are out of range or non-numeric characters are included then the calculator will not be able to calculate delta and psi. After finding delta and psi the ellipsometry calculator searches through multiple look up tables (LUTs) comparing the values of delta and psi to those in the table. When it finds the entry closest in value to delta and psi it can use that same entry number to find the corresponding index and thickness in other LUTs. Different LUTs are used depending on the chosen angle of incidence. The thickness range is necessary because on some materials like silicon nitride there may be two possible thicknesses the calculator could return based on its search and compare algorithm. After finding the right index the ellipsometry calculator then searches through the thickness range selected using the same method described above with delta and psi. It is common for the calculator to be off by one in the least significant decimal place such as a thousandth on the index or a tenth of a nanometer on the thickness. It is very rare for the calculator to be off by more than this, but it can occur so if high accuracy is needed then results should be double checked with another program. Please note that the ellipsometry calculator can only work for thicknesses between 50 nm and 250 nm and indexes between 1.3 and 2.11. If your result seems to be on the upper or lower of these bounds then it might be out of the range of the calculator.