Apparatus and method for nondestructive testing using additive-subtractive phase-modulated interferometry

Apparatus for nondestructively analyzing a test object comprising:

• a) means for directing a beam of light towards the object,
• b) optical head means for detecting light reflected from the object and for generating an interferometric image of the object using the reflected light, the detecting means including a video camera comprising means for detecting images during a sequence of video frames,
• c) means for stressing the object,
• d) means for shifting a phase of the beam of light directed towards the object,
• e) means for synchronizing operation of the directing means, the stressing means, and the phase shifting means, the synchronization means providing means for obtaining at least two interferometric images at different levels of stress on the object during a particular video frame, the synchronization means also comprising means for controlling the phase-shifting means,
• f) means for adding said at least two interferometric images, taken during said video frame, in an analog manner, to produce a composite image, and for storing the composite image in digital form, and
• g) means for subtracting the composite digital image taken during a given video frame from a composite digital image taken during a previous frame.

An apparatus for nondestructively testing an object, comprising:

• a) means for exposing an image sensor of a video camera to at least one pair of interferometric images of the object, such that the pair of interferometric images are added together in an analog manner to create a composite image, the interferometric images being formed with a light beam having a phase, and means for storing the composite image in digital form,
• b) means for stressing the object in a manner such that the interferometric images of each pair are obtained at different levels of stress,
• c) means for altering a phase of the interferometric images, and
• d) means for subtracting the composite image from another composite image obtained by altering the phase of the light used to form the image, the subtracting means comprising means for generating a second-order composite image which provides information on a condition of the object.