Selective reflection (SR) spectroscopy is usually performed in vapour cells with a wedged window. This prevents contributions to the SR signal due to interference with the beam reflected from the outer side of the window. We report, for the first time to our knowledge, SR spectroscopy measurements made with a cell having highly parallel windows. The recorded spectra are dramatically affected by the interference in the window [1]. When the two reflected beams interfere constructively (reflection maximum), the spectra display sub-Doppler features associated with the real part of the atomic response, as is usual in SR spectroscopy. By contrast, when the reflected beams interfere nearly destructively, the spectra are characterised by absorptive lineshapes, indicating that the imaginary part of the atomic response is dominant. Indeed, the use of a parallel window allows homodyne detection of the radiated atomic field. Phase tuning is achieved by choosing the order of the interference, here using temperature control of the window [2]. We first present the spectra, then give a qualitative explanation of this behaviour, and present a calculation of the reflected intensity, which takes into account, to all orders, the interference in the window. The model, with no adjustable parameter except the exact window thickness, shows excellent agreement with the spectra recorded on the caesium D2 line [1]. The interest of this experimental technique is that one can switch from one regime to the other very easily by changing the window temperature by a few degree Celsius (about 17°C in the case of our 0.5mm thickness sapphire window). It also allows tuneable laser frequency locking with a slope of about 2MHz/°C in our case. References: [1] A. V. Papoyan, G. G. Grigoryan, S. V. Shmavonyan, D. Sarkisyan, J. Guéna, M. Lintz and M.-A. Bouchiat, Eur. Phys. J. D30 (2004) 265-273 [2] E. Jahier, J. Guéna, Ph. Jacquier, M. Lintz, A. V. Papoyan and M.-A. Bouchiat: "Temperature-tunable sapphire windows for reflection loss free operation of cesium cells", Appl. Phys. B71 (2000) 561-565.