Just three minutes before sitting down to write this story, I popped a film cassette into a slot and watched a movie, unfold on the screen. Ten minutes before that, I had popped a film cassette into a camera and was outside shooting. The same cassette.

After years of anticipation, Polaroid recently introduced Polavision - instant home movies - and it's taking the world by storm. The system is lightweight, portable and easy to operate; it consists of a camera, an optional light accessory, a player and the film cassette.

The Polavision Land Camera brings both speed and ease to the world of filming. It offers little that hasn't been seen in conventional movie cameras (except, of course, its voracious appetite for Polavision tapes), but it's thoughtfully engineered so that even a novice can pick it up and be filming within 10 minutes.

The camera boasts such error-free features as two-zone focusing, a manual zoom control that can be operated equally effectively by right and left-handed people, a battery check button, film level indicator, tripod-wrist strap socket and a viewfinder that shows both under-and over-exposure warnings, as well as focus setting. The finder also has a red indicator lamp for lead-in and end-of-film warnings.

On top of the camera are slots to accept the Twi Light accessory, a twin-socket reflector unit housing two quartz lamps good for illuminating to 12 feet. When the unit is inserted in the slots and screwed into place, a daylight conversion filter pops out of the way of the lens so the film will properly record colors under 3400 K illumination.

Of course, the star of the entire system is the film cassette, called Phototape. It provides 2 1/2 minutes of shooting time (compared to 3 1/3 minutes of conventional Super 8) on film rated at 25 ASA daylight and 40 ASA under 3400 K lights, the same as Kodachrome II.

Each Phototape contains its own processing fluid reservoir, through which exposed film passes and develops after insertion in the Polavision player.

Once put into the player (like stuffing a slice of bread into a toaster), the Phototape is whisked to its starting position and, within 90 seconds, the fully developed color image appears on the screen via light shining through a prism in the cassette. The player produces an amazingly bright image, easily viewed by several people in either daylight or a darkened room.

After it has played through, the Phototape pops up and the screen goes dark. It may then be popped back down (which begins the play-through again, without the 90-second delay) or removed and stored indefinitely with no loss of color.

The Polavision Player comes with a self-storing cord and pop-up handle for easy portability. It retails for a suggested list price of $465; Polaroid Polavision Land Camera with fl.8 12.5 - 25 mm zoom lens, four penlight (AA) batteries, extra battery holder, filter key and wrist strap, $210; TwiLight $39.50; Phototape $9.95.