Like an itch, the impulse to buy something can pass very quickly. Most good salespeople know that the easiest way to lose a sale is to delay when a customer seems eager to put down money.

According to Bernard Riskin, that poses a problem for many companies that advertise products on network television or in national publications. An ad for personal computers, for example, might intrigue prospective buyers, but the company could lose the sale if the buyer does not know where to get a quick demonstration. Many companies now include a toll-free phone number for customers who want information, but these phone centers may also be far off.Riskin's invention, which received a patent this week, uses a computerized telephone system to give national advertising a local touch. The system, licensed to Applied Telematics, a start-up company in Wayne, Pa., answers phone calls from customers responding to an ad. The computer analyzes the first six digits of the customer's number and determines the location. It then selects the dealer nearest the location and connects the call to that dealer.

Applied Telematics began operating the system in January. Revenues from the system, which is sold as a service to national advertisers, are still negligible, but users include Minolta Business Equipment; Chrysler Financial, a subsidiary of the Chrysler Corp., and Yugo of America, which distributes the Yugoslav economy car.

Riskin received patent 4,757,267.

* Two inventions that imitate three-dimensional images on a flat screen received patents this week.

Daniel Wright, a doctoral candidate in physics at the University of Illinois in Urbana, designed a system that he thinks could help plastic surgeons reconstruct faces.

Wright's invention begins with a still camera that takes scores of pictures of every angle of the face. When combined, the pictures replicate the full face. A desktop computer, using software developed by Wright, then converts these images into digital data. The software offers an image of the face and what it would look like if certain features, like the nose, cheeks or chin, were modified. A plastic surgeon could then examine the effect of these changes from different perspectives.

A key element of his system, Wright said, is replicating the shadows caused by the contours in a face. Such shadows can make a lean person appear gaunt or a muscular person appear beefy. By incorporating a mathematical model representing shadows into his software, Wright's program tries to simulate their effect from all sides.

* In a separate development, inventors at a Dutch technical school took a different approach to 3-D. Trying to recreate the sense of depth in human vision, the inventors have taken advantage of a principle known as "motion parallax."

Motion parallax governs the way objects appear to move when the eyes or head are shifted. When a person looks at an object and then turns his or her head to the right, things in front of the object appear to move to the left. Things behind the object appear to move to the right. Though most people never realize it, this phenomenon supplies a guide to gauging the relative distance of objects.

A device developed in part for airplane simulation at Technische Hogeschool Delft, in the Netherlands, works on the same principle. The device trains a video camera on a fixed point in front of the scene or object to be watched. An observer watches the screen displaying this scene and is linked to the camera so that the camera mimics the person's head movement.

The result creates an impression of depth, as a person's brain decodes the movement of the scene in relation to the fixed reference point. According to the patent, the invention could have uses in engineering and surgery as well as in flight simulation.

Wright received patent 4,757,379. Technische Hogeschool Delft received patent 4,757,380.

* The Olympus Optical Co., the Japanese camera manufacturer, has received an American patent for spectacles made of electrically charged liquid crystals instead of glass or transparent plastic. According to the patent, the spectacles would be an improvement for wearers of bifocal lenses.

Liquid crystals are best known for their use in digital watches and calculators. When an electrical voltage is applied, molecules in the crystals line up, changing the display from transparent to opaque.

The Olympus lens consists of two layers of liquid crystals, separated by transparent plates. A miniature battery in the eye frame supplies electrical voltage, which changes the crystals' direction. Instead of turning the lens opaque, the crystals refract light like a lens.

Because the amount of refraction can be controlled by adjusting the voltage, a wearer of bifocal lenses could quickly adjust the lens depending on the desire to see near or far.

Olympus received patent 4,756,605.

To get a copy of a patent, send the number and $1.50 to the Patent and Trademark Office, Washington, D.C. 20231.