A CORPORATE FLC CONFUSED 2 NAV IDENTIFIERS; WHICH WERE BOTH PROGRAMMED INTO THE FMS. THE FLC PROCEEDED TO TRACK THE WRONG NAVAID UNTIL ARTCC INTERVENED.

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DEPARTED HOU; GIVEN A FULL RTE CLRNC RATHER THAN NORMAL SCHOLES 5 DEP (WE HAVE REGULAR TRIPS OUT OF HOU). THERE WERE STORM CELLS AROUND HOU AND ALONG DEP RTE. CLBOUT WAS IMC WITH RAIN; SOME LIGHTNING TO EACH SIDE. GIVEN VECTOR TO N JUST AFTER TKOF; THEN EVENTUALLY GIVEN R TURN DIRECT LFK. THOUGH NO DIRECT SUN; THERE WAS INTENSE GLARE IN COCKPIT. WE PROCEEDED IN R TURN; THE FMS NEEDLE POINTING TO R; WHERE IT MADE SENSE THAT LFK SHOULD BE. I DID LOOK AT THE CONSOLE CRT TO VERIFY LFK; AND RECALL BEING ABLE TO SEE 'LF' CLRLY; BUT LAST LETTER WAS OBSCURED BY GLARE. LESS THAN 2 MINS LATER; ZHU ASKED US WHERE WE WERE GOING; THEY SHOWED US HDG ESE. WE REPLIED DIRECT LFK; WHEN IN FACT WE WERE GOING DIRECT LFT. (LFT IS PART OF SHOLES 5 SID; VERY COMMON FOR US). AS OUR ORIGINAL TKOF VECTOR WAS N; BOTH LFK AND LFT WOULD BE TO OUR R. ATC GAVE US AN IMMEDIATE L TURN TO 340 DEGS; AND AMENDED ALT TO FL240 (HAD BEEN CLRED TO FL270). THERE WAS NO CONFLICT WITH OTHER ACFT; AND THE TCASII DID NOT SHOW ANYONE CLOSE. THE MAJOR CONTRIBUTING FACTOR TO THIS ERROR WAS THE GLARE IN THE CITATION COCKPIT. ADDITIONALLY; THE ONE LETTER DIFFERENCE IN 2 NAVAIDS BOTH LOCATED E OF HOU; AND BOTH USED IN TRANSITIONS; MADE FOR MORE PROBS.