A method of performing an optical time domain measurement by an optical time domain reflectometer (OTDR) means and a wavelength-dependent reflective element in characterizing one or more parameters of a selected optical path of an optical network, said OTDR and said HRD element being connected to said selected optical path at respective first and second mutually-spaced points, said HRD element being highly-reflective at one of two predetermined wavelengths (λ1,λ2) and significantly less reflective at the other of the two predetermined wavelengths, neither of the two wavelengths corresponding to a normal operating wavelength of the optical path, the method comprising the steps of: using said optical time domain reflectometer means connected to said first point of said selected optical path: to launch into the selected optical path at said first point light at each of said two wavelengths (λ1) and (λ2), to detect at said first point corresponding backreflected light as a function of time at each of said two wavelengths (λ1,λ2) and obtain therefrom first and second OTDR traces (OTDR-λ1,OTDR-λ2), respectively, each representing backreflected light as a function of optical distance; to compare the first and second OTDR traces to distinguish a peak corresponding to said HRD element from peaks corresponding to wavelength-independent localized reflectances, and to derive from said peak a value of one or more parameters of either or both of said HRD element and said selected optical path between said first and second mutually-spaced points; characterized in that said optical network has at least one other wavelength-dependent reflective element which is highly-reflective at the one of two predetermined wavelengths (λ1,λ2) and significantly less reflective at the other of the two predetermined wavelengths, and which is connected to a different optical path of the network at a known distance from said first point at which said OTDR light is launched, the method comprising taking into account, in the comparison of the first and second OTDR traces (OTDR-λ 1,OTDR-λ 2), amplitudes and said known distance to distinguish the peak produced by the HRD element from another peak produced by said other wavelength-dependent reflective element. using said optical time domain reflectometer means connected to said first point of said selected optical path: to launch into the selected optical path at said first point light at each of said two wavelengths (λ1) and (λ2), to detect at said first point corresponding backreflected light as a function of time at each of said two wavelengths (λ1,λ2) and obtain therefrom first and second OTDR traces (OTDR-λ1,OTDR-λ2), respectively, each representing backreflected light as a function of optical distance; to compare the first and second OTDR traces to distinguish a peak corresponding to said HRD element from peaks corresponding to wavelength-independent localized reflectances, and to derive from said peak a value of one or more parameters of either or both of said HRD element and said selected optical path between said first and second mutually-spaced points; characterized in that to launch into the selected optical path at said first point light at each of said two wavelengths (λ1) and (λ2), to detect at said first point corresponding backreflected light as a function of time at each of said two wavelengths (λ1,λ2) and obtain therefrom first and second OTDR traces (OTDR-λ1,OTDR-λ2), respectively, each representing backreflected light as a function of optical distance; to compare the first and second OTDR traces to distinguish a peak corresponding to said HRD element from peaks corresponding to wavelength-independent localized reflectances, and to derive from said peak a value of one or more parameters of either or both of said HRD element and said selected optical path between said first and second mutually-spaced points; characterized in that said optical network has at least one other wavelength-dependent reflective element which is highly-reflective at the one of two predetermined wavelengths (λ1,λ2) and significantly less reflective at the other of the two predetermined wavelengths, and which is connected to a different optical path of the network at a known distance from said first point at which said OTDR light is launched, the method comprising taking into account, in the comparison of the first and second OTDR traces (OTDR-λ 1,OTDR-λ 2), amplitudes and said known distance to distinguish the peak produced by the HRD element from another peak produced by said other wavelength-dependent reflective element.