Device for measuring an object moving in a direction of motion along an axis of motion, comprising: - a first sensor arrangement having, - a first self-mixing interference sensor (18.1), for emitting first measuring light beams along a first measuring axis (22.1), for receiving first measuring light beams reflected back from a first working area (26.1) of the first SMI sensor (18.1), and for generating a first measurement signal from the reflected-back first measurement light beams, wherein the first SMI sensor (18.1) is aligned in such a way that the emitted first measurement light beams run at least partially in the direction of movement of the object, - a second SMI sensor (18.2) for emitting second measuring light beams along a second measuring axis (22.2), for receiving second measuring light beams reflected back from a second operating range (26.2) of the second SMI sensor (18.2) and for generating a second measuring signal from the reflected second measuring light beams, wherein the second SMI sensor (18.2) is aligned in such a way that the emitted second measuring light beams run at least partially counter to the direction of movement of the object, wherein the first working area (26.1) has an end (28.1) facing away from the first SMI sensor (18.1) and the second working area (26.2) has an end (28.2) facing away from the second SMI sensor (18.2), and the ends (28.1, 28.2) define a measuring distance with a measuring distance length (I M) parallel to the axis of movement; - a control and evaluation unit for receiving the first measurement signal and the second measurement signal and for determining a velocity (v) of the object along the axis of movement from at least one of the measurement signals (44, 48), wherein the control and evaluation unit is designed to detect - a first characteristic change in the second measurement signal at a first time (T 1) - detect a first characteristic change in the first measurement signal at a second time (T 2) - detecting a second characteristic change of the first measurement signal at a third time (T 3), and determining an object length (I Obj) of the object along the axis of motion using the first time (T 1), the third time (T 3), the velocity (v), and the measurement path length (I M). - a first sensor arrangement having, - a first self-mixing interference sensor (18.1), for emitting first measuring light beams along a first measuring axis (22.1), for receiving first measuring light beams reflected back from a first working area (26.1) of the first SMI sensor (18.1), and for generating a first measurement signal from the reflected-back first measurement light beams, wherein the first SMI sensor (18.1) is aligned in such a way that the emitted first measurement light beams run at least partially in the direction of movement of the object, - a second SMI sensor (18.2) for emitting second measuring light beams along a second measuring axis (22.2), for receiving second measuring light beams reflected back from a second operating range (26.2) of the second SMI sensor (18.2) and for generating a second measuring signal from the reflected second measuring light beams, wherein the second SMI sensor (18.2) is aligned in such a way that the emitted second measuring light beams run at least partially counter to the direction of movement of the object, wherein the first working area (26.1) has an end (28.1) facing away from the first SMI sensor (18.1) and the second working area (26.2) has an end (28.2) facing away from the second SMI sensor (18.2), and the ends (28.1, 28.2) define a measuring distance with a measuring distance length (I M) parallel to the axis of movement; - a first self-mixing interference sensor (18.1), for emitting first measuring light beams along a first measuring axis (22.1), for receiving first measuring light beams reflected back from a first working area (26.1) of the first SMI sensor (18.1), and for generating a first measurement signal from the reflected-back first measurement light beams, wherein the first SMI sensor (18.1) is aligned in such a way that the emitted first measurement light beams run at least partially in the direction of movement of the object, - a second SMI sensor (18.2) for emitting second measuring light beams along a second measuring axis (22.2), for receiving second measuring light beams reflected back from a second operating range (26.2) of the second SMI sensor (18.2) and for generating a second measuring signal from the reflected second measuring light beams, wherein the second SMI sensor (18.2) is aligned in such a way that the emitted second measuring light beams run at least partially counter to the direction of movement of the object, wherein the first working area (26.1) has an end (28.1) facing away from the first SMI sensor (18.1) and the second working area (26.2) has an end (28.2) facing away from the second SMI sensor (18.2), and the ends (28.1, 28.2) define a measuring distance with a measuring distance length (I M) parallel to the axis of movement; - a control and evaluation unit for receiving the first measurement signal and the second measurement signal and for determining a velocity (v) of the object along the axis of movement from at least one of the measurement signals (44, 48), wherein the control and evaluation unit is designed to detect - a first characteristic change in the second measurement signal at a first time (T 1) - detect a first characteristic change in the first measurement signal at a second time (T 2) - detecting a second characteristic change of the first measurement signal at a third time (T 3), and