Application aux couches minces de la théorie de l'étalon interférentiel par réflexion

Abstract
The theory of the etalon used in refleclion shows that the phase change on reflection from the second mirror does not affect the intensity distribution in the interference pattern. The first film, however, is involved through its transmission coefficient, its reflection coefficients an the angle γ = α + α'- 2β where α, α' are the phase changes on reflection and β is the phase change on transmission. In the simplified theory, valid for semi-transparent, non-absorbing films, one assumes that all the phase changes are 0 or π and therefore γ = o. In the general case of absorbing films we have developed a graphical method allowing to find a limit for γ depending on the absorption of the semi-transparent film. For this purpose, we imagine it to be placed in front of a mirror of reflectivity 1, and we express graphically the fact that the reflection factor of the resulting etalon cannot exceed I. We dérive an algebraical expression for γ for very weak absorption A, [FORMULA] Applications. - The knowledge of an upper limit of γ provides information on the limits of application of the simplified theory of the reflection etalon or of the interference filter with three films used in reflection. It has also allowed us to evaluate the error committed by assuming γ = o in the calculation of the absorption of multiple films of small absorption from the absorption of a single film.