Differentiating close-range measurements of time of flight

An optical sensing device, comprising:

• a light source, which is configured to emit one or more beams of light pulses toward a scene, wherein the light source comprises at least one laser configured to generate coherent light, which forms a speckle pattern when scattered from objects;
• an array of single-photon detectors, which are configured to output electrical pulses in response to photons that are incident thereon;
• light collection optics configured to form an image of the scene on the array;
• and
• processing circuitry, which is configured to count the electrical pulses output by the single-photon detectors during multiple time intervals following each of the light pulses, to detect, responsively to the counted pulses, an object located less than 10 cm away from the array that reflects at least one of the beams onto a group of the single-photon detectors, to make a comparison between respective counts of the electrical pulses output by the single-photon detectors in the group during a specified time interval immediately following each of a plurality of the light pulses, and to ascertain, responsively to the comparison, whether the object reflecting the at least one of the beams is fixed to the device or separate from the device by detecting changes in the speckle pattern between successive light pulses to distinguish between objects fixed to the device and objects separate from the device,
• wherein the processing circuitry is configured to compute differences between respective first and second numbers of the electrical pulses output by each of the single-photon detectors in the group that were counted in the specified time interval following first and second sequences of the light pulses, and to make the comparison by comparing the differences to a predefined threshold and to identify the object as being separate from the device when at least one of the differences is greater than the predefined threshold, and otherwise to identify the object as being fixed to the device.