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The
technique of InfiniteFocus is based on focus variation. The optical system
has been designed, constructed, and calibrated to produce precise depth of
focus information from which accurate, true color, quantitative 3D
reconstructions are performed. This non-destructive method utilizes coaxial
white light which is provided by a light source delivered through a beam
splitter to a series of selectable, infinity-corrected, high-N.A. objectives
contained in a six-place nosepiece (diagram: green light path). The
specimen’s reflected light is projected through the beam splitter onto a
color digital sensor (yellow and red light paths). Each objective comprising
the InfiniteFocus system is calibrated using a certified calibration
standard for a variety of characteristics including specific magnification
values. A 3D surface data set is created by superimposing successive focal
planes comprising the specimen’s surface. Focus variation is achieved by an
elevating Z-stage. As the focal distance is varied, a constant series of
images are captured. For each focal plane position of the stage, different
regions of the specimen are imaged as sharp or blurred surface patches
corresponding to the specimen’s 3D structure. The specimen’s actual
sharpness and color information is extracted from the 3D surface data set.
Since the encoded stage position varies the sharpness, relative changes in
focus are observed as focus varies from blurred to sharper to sharpest
focus; the point at which further focal plane travel yield values that
become more blurred. The variation of focus is utilized to calculate depth
for each point in the image. Thus a dense and accurate 3D color model is
calculated for the specimen surface. The operating principle is similar to
confocal but has its major difference that makes it an own unique technique
that has been added to the new ISO draft for classifying surface texture
methods. At confocal the optics is designed to have almost infinite small
depth of focus. Thus not the variation of focus is analyzed, but only the
absolute maximum value. |
