NIST has established an absolute aperture area measurement facility for circular and near-circular apertures use in radiometric instruments. The facility consists of the aperture area instrument, which is housed in the Advanced Measurement Laboratory (AML). The instrument is based on non-contact, video microscopy. Apertures with nominal diameters ranging from 0.5 mm to 50 mm, with both knife and flat edges, can be measured. The measurement is made by locating the edge points along the internal aperture circumference and using fitting routines to determine the geometric aperture area.
The instrument consists of a broadband light source (white light with green pass filter), a Kohler illuminator, an air bearing-supported open-frame XY stage with a heterodyne laser interferometer feedback system for XY-axis positioning, a Z-axis translation stage carrying a microscope with long working distance objectives, a digital CCD camera and a control computer. The optical and mechanical components are supported on a granite base and vertical bridge structure mounted on a vibration-isolated optical table. A 10-cm diameter circular opening at the center of the base allows the Kohler illuminator to illuminate the sample from below. The base carries the XY stage and the interferometer and the bridge structure carries the Z-axis stage.
Characterization of the whole system and all its components were initially performed and only minor periodic recalibration and alignment are necessary.
Below is a table of the sources of uncertainty in area measurements in this facility. Two examples, a 5 mm and 50 mm diameter aperture, and the typical values of each component encountered in measuring these aperture sizes, are shown.
| || ||Uncertainties|
| || ||2.5 mm radius||25 mm radius|
|Random + form||50 nm||5.00E-5||2.00E-5||4.00E-5||5.00E-5||2.00E-6||4.00E-6|
|Sys. imaging||4 nm||4.00E-6||1.60E-6||3.20E-6||4.00E-6||1.60E-7||3.20E-7|
|TOTAL (k=1)|| ||6.69E-5||2.67E-5||5.35E-5||4.40E-4||1.76E-5||3.52E-5|
|TOTAL (k=2)|| ||1.34E-4||5.35E-5||1.07E-4||8.79E-4||3.52E-5||7.03E-5|