BE02 DEPARTS PAVED SURFACE DURING NIGHT TAXI AT POU.

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WHILE TAXIING FROM TERMINAL TO RWY 6; I TAXIED ONTO THE GRASS N OF THE TERMINAL. I IMMEDIATELY RETURNED THE ACFT TO THE RAMP PAVEMENT AND CONTINUED TO TAXI TO RWY 6. THE PROB AROSE BECAUSE: THE RAMP IS LIGHTED BY LOW LIGHTS ABOVE THE TERMINAL. 2) THERE IS NO WAY TO DETERMINE WHERE THE RAMP ENDED AND THE GRASS BEGAN. (NO LIGHTS; NO REFLECTIVE POSTS; NO DOUBLE YELLOW LINES; ETC.) 3) THERE ARE NO LINES (VISIBLE AT NIGHT) UNTIL ESTABLISHED ON THE W TXWY BTWN RAMP AND PARALLEL TXWY. 4) TAXI LIGHT WAS WEAK DUE TO OBSCURED LENS. IT WAS DISCOVERED BY LOOKING DOWN FROM MY WINDOW AND A ROUGH TAXI SURFACE. (FORTUNATELY; THE TERRAIN HAD BEEN FROZEN FOR THE PREVIOUS 24+ HRS.) CORRECTIVE ACTION SHOULD BE MADE TO IDENT EDGE OF PAVEMENT MORE DEFINITIVELY. HUMAN PERFORMANCE FACTORS ARE: 1) FO SHOULD HAVE BEEN MORE 'IN THE LOOP' AND MORE 'HEADS UP.' 2) I SHOULD HAVE HAD MORE COMMON SENSE AND USED MORE LIGHTS THAN JUST TAXI. 3) I SHOULD HAVE RETURNED TO THE GATE AND HAD THE ACFT INSPECTED FOR DAMAGE. 4) THE INITIAL WORRY OVER WHAT HAPPENED CAUSED A SMALL CHAIN OF SMALL MISTAKES AFTERWARD. SUGGEST; IF PUBLISHED; TO ADVISE CREWS TO STOP; TAKE A DEEP BREATH; THINK! DON'T LET ONE MISTAKE BREED MORE.