Multiple-Beam Fringes of Equal Chromatic Order* Part I Phase Change Considerations

An equation of the form nλ=2t+fλ is necessary to represent multiple-beam fringes of equal chromatic order. Experimental values of f are reported for several silver coatings of various reflectivities and ages in the working range. The experimental values of f for new coatings are independent of λ in the range 4500A to 6300A. The experimental values of f for aged coatings with reflectivities greater than 60 percent agree reasonably well with a value estimated from the known optical constants of bulk silver. The systematic error inherent in the result of determining a thickness or a change in thickness when using the elementary equation nλ=2t is of the order of one percent in the working range of age, reflectivity, and order of interference.