(FORTUNE Small Business) – It is a good time for small companies to break into the prison market. After all, last year the nation's prison population reached 2.1 million, having grown at its fastest pace in four years, according to the Justice Department's Bureau of Justice Statistics. That translates into more opportunity for the companies that supply federal and state prisons with everything from food to high-tech devices.

For an example of the latter, witness Engineered Control Systems (ECS), a $12 million company that is based in Spokane and has seen revenues rise at least 25% annually since first entering jails in 1995.

Back then ECS supplied temperature-control systems to local hospitals, schools, and small prisons—a dwindling market. But after developing software for one prison to keep track of inmates, ECS discovered a new business: replacing the mess of blinking switches and pilot lights (usually two per cell) with one touch-screen monitor. Inmates' information—dietary needs, history of deviant behavior, etc.—can be pulled up instantly by tapping on a graphic map of cell blocks, ridding guards of clipboards. Next, the company plans to work with distributors abroad. "The opportunity is there for business to double overnight if we want it to," says ECS president Bob Ellis. —MAGGIE OVERFELT