Patent ID: 6875973

Claim:
A focusing system configured to focus on an object in a field of view, while there is a relative motion between the object and the focusing system, comprising: (a) a primary optical element disposed to direct light traveling from an object passing through the field of view along a collection path, the primary optical element being selectively positionable; (b) at least one beam splitter disposed to direct light that has traveled from an object and through the primary optical element along both a first optical path and a second optical path; (c) a first optical grating disposed in the first optical path, the first optical grating modulating the light traveling along the first optical path to produce first modulated light having a modulation frequency proportional to a velocity of the object passing through the field of view; (d) a first light sensitive detector on which the first modulated light is incident, the first light sensitive detector producing a first electrical signal responsive to the first modulated light; (e) a second optical grating disposed in the second optical path, the second optical grating modulating the light traveling along the second optical path to produce second modulated light having a modulation frequency proportional to the velocity of the object passing through the field of view, the second optical grating being disposed such that light traveling from the primary optical element to the second optical grating must travel farther than light traveling from the primary optical element to the first optical grating; (f) a second light sensitive detector on which the second modulated light is incident, the second light sensitive detector producing a second electrical signal responsive to the second modulated light; and (g) a processor for processing the first electrical signal and the second electrical signal to determine a direction along which the primary optical element should be moved to improve a coincidence between a focal point of the primary optical element and the object.