Polygraph Lie Detector Equipment
The equipment used by polygraph examiners has been around for over 100 years, and has evolved substantially during that period. Modern polygraph instruments incorporate several different required components. The following is a primer about lie detector instruments. Each instrument must have these components in order to meet APA and ASTM standards for testing. See our video to the right for a demonstration.
These are the required components of any polygraph instrument certified for use under APA and ASTM standards. If any of the required sensors are inoperative or malfunctioning, the instrument is not approved for testing.
- Computer Interface Module
- Computer containing appropriate software
- Electrodermal sensors
- Blood Pressure Cuff and Sphygmomanometer with Bulb
- Pneumograph Sensors
- Motion Sensors
- Plethysmograph Sensor (optional)
- Blood Pressure Thumb Cuff (optional)
COMPUTER INTERFACE MODULE
This is the “magic box” that all the polygraph sensors are connected to. Within this box, the Sensor data is converted from analog to digital before it is sent to the computer for processing.
COMPUTER
The Computer Interface Module (see above) must connect to a computer so that that Examiner can monitor and adjust the inputs from the Sensors. Each manufacturer of polygraph equipment has proprietary software the examiner must install so that the data can collected and analyzed.
ELECTRODERMAL (EDA) SENSORS
These Sensors, typically attached to the hands or fingers, measure changes in skin conductivity or skin resistance, which is displayed in the polygraph software. These Sensors can be metal plates (held on with velcro) or adhesive pads that can be attached to the Examinee’s skin at almost any location. These are sometimes referred to as Electrodermal Response (EDR) or Galvanic Skin Response (GSR) Sensors.
BLOOD PRESSURE CUFF
This cuff is used to measure changes in blood pressure, and can be placed on the Examinee’s upper arm, forearm, or calf, depending on availability. An inflation bulb is used to pressurize the cuff, and a Sphygmomanometer (gauge) is used to monitor the amount of pressure in the cuff.
PNEUMOGRAPH (RESPIRATION) SENSORS
These hollow rubber tubes, known as pneumograph tubes, are placed on the Examinee’s torso to measure changes in breathing and oxygen intake. Two separate pneumograph tubes (upper and lower) are required to achieve adequate detail.
MOTION SENSORS
Since movements can cause distortion of polygraph data, Examiners must use motion sensors to determine whether an observed reaction is caused by a movement instead of a psychological response. These sensors are normally placed on the Examinee’s seat, under the arms, or under the feet.
PLETHYSMOGRAPH
This Sensor measures the amount of blood flow in and out of the fingers. This is an optional Sensor.
BLOOD PRESSURE THUMB CUFF
In some cases, the standard blood pressure cuff can not be used. An alternative is the thumb cuff, which is simply a much smaller version of the arm cuff, but it attaches to an Examinee’s thumb instead. This is an optional Sensor.
WHICH INSTRUMENT WILL BE USED BY MY EXAMINER?
There are four major manufacturers of polygraph equipment. Each manufacturer typically has a few different models available. In the hands of a qualified and experienced examiner, all polygraph instruments are equally reliable. Each model has specialized features that may be preferred by some examiners, but they all record the exact same thing and present the data in similar ways. To see which instrument is being used by a particular examiner, visit the individual Testing Location page, where the equipment type is provided.