The invention relates to a method and a device for obtaining a low-noise optical signal.According to the method, a luminous beam is injected through two apertures and after detection respectively a basic optical signal (21) and a corrective optical signal (22) are generated. Both optical signals obtained (21, 22) are subtracted, so that a resulting optical signal is generated, forming the low-noise optical signal. The apertures are preferably two slits of a spectroscope, the optical signals being expressible relative to the wavelength.