President Reagan has signed a law sharply limiting the use of lie detector tests by most private employers.

The law, strongly backed by organized labor, bars the use of polygraph tests for job applicants or for random testing of current employees.Exemptions are provided for companies that provide security servcies and those manufacturing and distributing controlled drugs. Federal, state and local governments also are exempt from the ban, as are consultants to to federal national security agencies.

Private employers are allowed to use the so-called lie detectors in connection with investigations of specific criminal incidents if the employer has "reasonable suspicion" that an employee was involved. However, test results could not be used as the basis for action against an employee without supporting evidence.