Combine cracker crumbs with brown sugar. Lay a sheet of foil in a 8 x 8 square baking pan, with the sides overhanging the edges. Tuck sides under. Spray foil-lined base with butter-favored cooking spray.
Press crumb mixture into pan. Briefly spray cooking spray on top, and press again.
In a medium bowl, beat light cream cheese until softened. Add lime zest and condensed milk. Beat until smooth. Add lime juice and beat until well blended.
Pour lime mixture on to crumb base. Bake for 25-30 minutes, until center is just set and edges pull away a little. Allow to cool. Chill for at least 2 hours. Cut into 16 bars.