B737 FLT CREW HAS LOW ALT ALERT DURING AN ILS TO HOU.

Narrative

I (THE FO) WAS FLYING A VISUAL APCH TO RWY 12R AT HOUSTON HOBBY. IT WAS A NIGHT VFR PATTERN WITH HAZY VISIBILITY. WE WERE ON A 90 DEG R BASE AT 4000 FT AT APPROX 12 MI WHEN THE CTLR BEGAN TO TURN US OUTBOUND TO GIVE US A LITTLE MORE ROOM TO INTERCEPT THE LOC. WE INFORMED THE CTLR THAT WE SHOULD BE ABLE TO FLY A VISUAL IF WE COULD GET A TURN TOWARD THE FIELD. THE CTLR GAVE A DSCNT TO 2000 FT AND A TURN INBOUND TO INTERCEPT THE LOC. I IMMEDIATELY BEGAN TO CONFIGURE THE ACFT WHILE IN A DSNDING R TURN TO FINAL AND THE CAPT CALLED THE FIELD IN SIGHT. I SLIGHTLY OVERSHOT THE LOC WHILE LOOKING FOR THE FIELD AND THE CTLR GAVE US A SLIGHT R TURN TO REINTERCEPT. I SAW A REDDISH WHITE LIGHT AHEAD WHICH I THOUGHT WAS THE VASI FOR RWY 12L;BUT COULD NOT SEE THE RWY LIGHTS. THE CTLR THEN ASKED US AGAIN IF WE HAD THE FIELD IN SIGHT AND THAT HE WAS GETTING A LOW ALT WARNING ON OUR FLT PATH. I IMMEDIATELY NOTICED THAT I WAS BELOW THE GS AND QUICKLY LEVELED OFF -- WE WERE AT 1000 FT AND 4 MI FROM THE ARPT. I THEN SAW THE RWY LIGHTS AND CONTINUED FOR AN UNEVENTFUL LNDG. LESSONS LEARNED: DON'T ATTEMPT TO FLY A VISUAL APCH UNLESS I (THE PF) HAS THE FIELD IN SIGHT. WHEN LOOKING FOR THE FIELD; KEEP THE AUTOPLT 'ON' WITH ALT HOLD AT THE MINIMUM VECTORING ALT FOR THE VISUAL APCH. MAINTAIN THE GS! I WAS SO 'TARGET FIXATED' ON CONFIGURING THE ACFT; LOOKING FOR THE FIELD; AND MAINTAINING THE LOC COURSE; THAT I DROPPED THE GS OUT OF MY XCHK. TO QUOTE A SAFETY ARTICLE POINT; THIS SIT WAS A 'RED FLAG' OF RISING COMPLEXITY AND THE FACTS WOULD NOT SUPPORT MY PLAN. MY MENTAL PICTURE OR 'VIDEO' WAS NOT CORRECT. I THOUGHT THE REDDISH WHITE LIGHT I SAW WAS THE RWY 12L VASI AND IT WAS NOT. I SHOULD HAVE CONCENTRATED ON FLYING THE ILS AND LET THE CAPT LOOK FOR THE FIELD INSTEAD OF TRYING TO DO IT ALL MYSELF.