The slump afflicting the computer industry is painfully apparent at this year's National Computer Conference -- a trade show usually brimming with people and exhibits.

NCC officials conceded that the number of exhibitors has dropped from more than 700 last year to slightly more than 600 this year, with several major firms -- including Apple Computer Co., Digital Equipment Corp. and Wang Laboratories Inc. -- skipping the show.

While the NCC expects this year's attendance to top last year's 80,000 visitors, the aisles in Chicago's giant McCormick Place center had more carpet than people.

"Usually it's packed on the first day," said Jerry Gossman, director of computer services at McDonald's Corp. "There are a lot fewer people here than at last year's show in Vegas."

"The energy level of the show ebbed after the first hour," said Tim Bajarin, an industry consultant with Creative Strategies in California.

NCC Chairman Karl E. Martersteck blamed "economic reasons" for the drop in the number of exhibitors, but added that "in the past, the industry has enjoyed 40 percent compounded growth -- now it's growing about 20 percent a year."

The industry slump, Martersteck and other computer observers insist, should be measured against expectations rather than a real decline in sales. However, though revenue from computer sales may be increasing, industry profits have continued to erode.

"Of course, we're trading margins for sales," said one computer company executive who asked not to be named. "That's better than accumulating inventory."

Other show attendees commented on the virtual absence of personal computer companies and software houses while components companies lined the aisles.

"I've never seen so many uninterruptible power supply companies in my life," complained Floyd Kvamme, a venture capitalist with Kleiner Perkins. "I don't see anybody using the show to talk about or do new things."

One of the new technologies that has emerged at this show is the optical disk as a high-density computer storage device. Traditionally, computers have relied on magnetic media such as tape to store computer data. Now, the cost performance of optical technology has made it the potential competitor in the multibillion-dollar memory-disk market.

The best known optical disks today are the compact audio disks that store and play music. A special light-sensitive material stores the digitally recorded sound, which can be encoded and "played" by a laser beam. Compact-disk players and records already have sold in the hundreds of thousands.

The same disk, however, also can store computer data. One side of a 5 1/2-inch optical disk can store up to 500 million bytes of information. It would take more than 700 floppy disks to store that much computer data.

"There's going to be a monster market for compact-disk memory," asserted John R. Robinson, vice president of marketing for Alcatel Thomson Gigadisc, a French optical disk company.

With their capacity, the optical disks allow the creation of very large databases that banks, insurance companies, government agencies and other volume users of historical data need. Optical-disk drives could be attached to personal computers and giant mainframe computers.

One problem, however, is that once information is stored on a compact disk, altering the memory is virtually impossible. Compact disks are "read only" memory devices.

Several companies at the show -- including Thomson and Optotech Inc. -- now offer "write once" memory. For example someone creating a mass database on a personal computer could store it -- write once -- on an optical disk in an optical disk drive.

The idea is proving popular.

"Our booth traffic is tremendous," said Nelson C. Yew, Optotech's president.