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In 1988, Toshiba produced the first commercially available visible-light laser diode, a breakthrough that was widely expected to improve the quality and lower the cost of construction lasers. Several companies were launched to manufacture inexpensive battery-operated laser levels, but only one, Laser Tools Co. (12221 Arch Street Pike, Little Rock, AR 72206; 501/888-8831), is still in the business. (Emerging Technologies, also of Little Rock, has discontinued making laser levels but continues to make laser gun sights, and T6 Tools, of Chapel Hill, N.C., has disappeared.) I've had two of Laser Tools' small, hightech levels for several months. Here's what I've learned. What's in a Laser? Before laser diodes, most of the big laser transits used expensive He-Ne (or "heenie") tube lasers. These have a fragile