Polygraph Test Instrumentation
Instrumentation used in polygraph examinations have developed and kept abreast of technological changes in modern society.
Although the old traditional analogue instrument is still
used by a few examiners in South Africa, most of the polygraphists are making
use of new state of the art computerised instruments.
Both the analogue and computerised instruments makes use of
the same four sensors which register the following:
- Abdominal and thoracic breathing (pneumograph tubes)
- Cardio-vascular changes (blood pressure cuff)
- Galvanic skin resistance (finger plates)
- Although an analogue in good working condition is very reliable there are certain advantages to using the computerised instrument:
Data Quality
With the analogue, traditional polygraph data characteristics
are typical of technology 30-40 years ago. As ink pens pivot, they distort data
position by their curved motion. Trace data is permanently lost at pen stops.
In contrast, the computerised instrument has a high sensitivity to
physiological responses and the ability to preserve the integrity of the
original examination.
Display And Print Characteristics
The computerised instrument provides clarity of tracings,
flexibility of review and ease of analysis and a rich set of display options.
Polygraph Examiners can adjust chart width, height and colour, line thickness
and gridding. Charts can be stacked for comparison purposes, etc. Data can be
adjusted during replay without altering the original results. With the analogue
instrument, chart tracings are limited to one original set per examination.
Chart quality is restricted by analogue technology limitations such as fixed
times, fixed gridding and fixed trace centering.
Calibration
Any polygraph instrument requires calibration when it has
been moved. When using the computerised instrument calibration is a very simple
procedure which takes very little time, whereas the calibration of the analogue
instrument is a lengthy and complicated procedure taking about 15 minutes.
Storage and Retrieval
The computerised instrument provides the facility of storage
and retrieval of charts, reports, question lists and other material. This
simplifies the transfer of information to supervisors, other examiners,
attorneys, etc. Examinations can be stored on diskettes or on a hard drive. All
data including charts, reports, questions, etc are immediately accessible and
can be transferred instantaneously, using fax/modem or similar technology. An
unlimited number of original charts can be printed.
Examination information may be viewed at any time using the
computer monitor. Details of examinees and times of tests are recorded
automatically on each chart. The Analogue is limited by a total reliance upon
the original paper charts or photocopies of them. Original charts must be
stored at whatever store facility is available and sent for when needed. Other
information such as question lists; reports and other material may be separate
from charts, requiring more trips, coordination, etc.
Power Portability
The computerised instrument has the ability to operate for
periods of time without external source of electric power. Sensor Box power
requirements are supplied through the serial cable and are so insignificant
that the computer’s normal battery operating time is virtually unaffected by
the Sensor Box requirements. The analogue instrument generally requires access
to an external electric power supply.
Source: Axciton Computerised Polygraph Instruments
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