Distance measuring apparatus with phase detection

Distance measuring apparatus comprising a photoelectric light receiver (12) and a photoelectric light transmitter (11) which is arranged alongside the photoelectric light receiver (12), said light transmitter (11) comprising two opto-electronic light transmitting elements (18, 19), which can be electronically switch in a complementary manner, said light transmitting means being fed by an oscillator device (43, 44, 45, 46) and alternately transmitting high frequency modulated light wave trains to the light receiver (12) via a measurement path (14) and a reflective target (13), wherein the distance of the target from the light transmitter (11) to light receiver (12) is to be measured via reference path (15) of known length between the light transmitter (11) and the light receiver (12), within the housing (17) and an electronic evaluation circuit which deduces, from the phase of the alternately received light wave trains, the spacing of the light transmitter (11) to light receiver (12) from the target (13), as the speed of light and the known length of the reference path (15) is considered, characterised in the light receiver (12) containing only one single opto-electronic light receiving element (20), which receives both the light wave trains from the measurement path (14) and also the light wave trains from the reference path (15), said light receiving element being connected to one input of a multiplicatively operating mixing stage (47) belonging to the evaluation circuit, the other input of said mixing stage (47) being supplied with one output wave train signal P' of a digital phase shifter (46) which generates temporally one after the other four phase pairs with modulation frequency, the phase pairs being adjustable in a 90.degree. raster, the other output wave train signal P of the digital phase shifter (46) being supplied to the light transmitter (11), the light wave trains having a wavelength on the order of magnitude of the distance to be measured, the electronic evaluation circuit measuring the phase difference of the modulation of the light wave trains which have passed along the measurement path (14) and the light wave trains which have passed along the reference path (15) by connecting an approximation filter (48) with a particular narrow band design to the output of the mixing stage (47), the approximation filter (48) forming the integral (P.times.P') of the two wave train signals (P, P'). and the signal occurring at the output of the approximation filter (48) being evaluated in a microprocessor (16) belonging to the evaluation circuit for obtaining the difference of the transit times of the light wave trains via the measurement path (14), on the one hand and the transmit time via the reference path (15), on the other hand.