A new automatic method is proposed for the design of thin- film optical coatings. The principle of this method consists in increasing gradually the layer count of a coating by adding thin layers at one end of the design. Each time a new layer is added, the design is optimized. However, this optimization is performed using an external medium whose refractive index corresponds to the index of the next layer to be added. This procedure, for which the external medium is regularly alternated, first degrades the optical properties but permits to improve them afterwards, while this may not be possible using the real external medium of the design. This real external medium is only taken into account at the end of the synthesis work. A major advantage of this method is that the starting design can be restricted to the list of materials used both for substrate and layers. No starting thickness is required. An other advantage is that the final layer count of the design does not seem limited.