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~ an Advanced Laboratory Imaging and Analysis System ~



Sample Size


ALIAS uses an optical process to measure particle dimensions. The granularity of the optical pixels that ALIAS uses (about 0.05 mm) limits the accuracy which ALIAS can achieve, as do other characterisitcs of the optical scanner, especially the two-dimensional viewing geometry of three-dimensional particles, the roughness of particles which have jagged boundaries, and the displacement of particles from the centerline of the scanner.

Accordingly, ALIAS makes use of certain assumptions to maximize accuracy. One of those assumptions is that there will be several particles present when it conducts a scan, and that they will be scattered somewhat randomly over the scan area. Consequently, ALIAS accuracy is specified for sample averages, not for individual particles.

ALIAS can be used to scan a single particle, but ALIAS accuracy is greater when computing a sample average for many particles. If you wish to scan a single particle, accuracy will generally be greatest if you align the long axis of the particle parallel to the long axis of the scanner glass, and if you place the particle close to the centerline of the scanner. (You may place the particle near the "top" of the scanner, near the lid hinge, if you wish; it is only the left-to-right--that is, the side-to-side--displacement from the centerline that matters for this purpose.)

ALIAS expects to find at least 40 particles in a sample. You can of course measure smaller samples, but ALIAS will warn you of that fact. (You can set this limit to another value--see Options | Messages | Minimum Sample Size.) ALIAS will also alert you if the sample is not randomly placed. (You can turn off this warning or change the limits; see Options | Messages | Random Placement.) ALIAS will also warn you when the standard error recommends a larger sample size than was present during the scan.

As a rule of thumb, Cascade Data Systems recommends sample sizes of about 200 particles. With practice, an experienced ALIAS user can distribute and separate 200 particles over the "large" scan area in a minute or so.

Before operating ALIAS outside its intended operating limits, it is strongly recommended that you thoroughly test ALIAS to ensure that it provides the level of results that you desire.