ABSTRACT
One can measure thickness on many scales. œe galaxy is a spiral disk about 100 Em (1020 m) thick. œe solar system is pancake-like, about 1 Tm (1012 m) thick. œe rings of Saturn are about 10 km thick. Closer to home, the Earth’s atmosphere is a spherical shell about 40 km thick; the weather occurs in the troposphere, about 12 km thick.œe outermost shell of the solid Earth is the crust, about 35 km thick.œe ocean has a mean depth of 3.9 km.In the Antarctic, the recently discovered objects believed to be microfossils indicative of ancient Martian life are less than 100 nm thick. In terms of the man-made environment, industry must contend with thickness varying from meters, for construction projects, to millimeters on assembly lines and to micrometers and nanometers for the solid-state, optical, and coating industries. Perhaps the most familiar way of measuring thickness is by mechanical means, such as by ruler or caliper.Other means are sometimes called for, either because both sides of an object are not accessible; the dimension is either too big or too small for calipers; the object is too fragile, too hot, or too cold for direct contact; or the object is in motion on an assembly line-it may not even be a solid.œickness may also be a function of position, as either the object may have originally been made with nonuniform thickness, deliberately or not, or the thickness may have become nonuniform with time due to either corrosion, cracking, or some other deterioration.œe thickness may also be changing with time due to deliberate growth or etching, as example for thin Ÿlms. œus, it follows that, in more general terms, measuring thickness might require measuring the topography or height proŸle of two surfaces and taking the di¥erence. Alternatively, the measurement technique may produce a reading directly related to the di¥erence. Table 13.1 lists some of the many techniques suited to determining thickness, together with the range of usefulness and some comments on accuracy and/or precision.
