B777 CREW RESPONDED TO WHAT APPEARED TO BE A CONDITIONAL CLRNC FROM AN ORD LCL CTLR.

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I WAS THE FO ON THE FOLLOWING FLT. IT WAS NIGHT AND OUR B777 WAS DEPARTING RWY 14L AT ORD WITH LNDG TFC ON RWY 22R. WE WERE CLRED INTO POS AND HOLD. TWR THEN ADVISED WE WERE GOING TO USE VISUAL SEPARATION; CLRED FOR TKOF BEHIND ACFT SHORT FINAL RWY 22R. AS THE ACFT WAS ABOUT TO TOUCH DOWN ON RWY 22R MOVING FROM 11 TO 12 O'CLOCK POS ACROSS OUR WINDSCREEN WE BEGAN THE TKOF ROLL. I COMMENTED THAT'S OUR TFC. THE CAPT TOOK THE THROTTLES AND I WAS FLYING. WE CONTINUED TO OBSERVE THE ACFT PASS IN FRONT AND TO THE R AS I ROTATED AND WE LIFTED OFF. I COULD STILL SEE THE ACFT OVER TO OUR R ABOUT 500-1000 FT COMPLETING ITS LNDG ROLL. I WANTED TO ADDRESS THIS ISSUE BECAUSE THE PROX OF THE 2 ACFT WAS CLOSER THAN I INITIALLY JUDGED IT TO BE. THE PROB FOR ME WAS ACCEPTING THIS TYPE OF CLRNC AT DUSK OR AT NIGHT WHEN ONE'S ABILITY TO DETERMINE THE OVERTAKE OF A DECELERATING ACFT; OR THE SEPARATION FROM ANOTHER ACFT; IS DEGRADED FROM WHAT IT MAY BE DURING DAYLIGHT OPS. THE SIT WAS A LITTLE TOO CLOSE FOR COMFORT. NON STANDARD PHRASEOLOGY WAS USED BY THE TWR CTLR AFTER WE PASSED BEHIND THE LNDG ACFT. THIS MAY NOT BE CORRECT BUT I RECALL; 'FLY RWY HEADING; YOU ARE ALL RIGHT.' THIS MAY MEAN HE WAS CONCERNED ABOUT THE PROX OF THE ACFT BUT NOTHING ELSE WAS SAID. I WOULD RECOMMEND TO ASSIST CAPT'S THAT FO'S SHOULD ADVISE THEM NOT TO ACCEPT THIS TYPE OF CLRNC AT NIGHT OR DAYTIME TO EXPEDITE BUSY OPS. OUR JUDGEMENTS ARE BASED ON VISUAL CUES AND JUST ARE NOT AS KEEN AS IN THE DAY. THIS OBSERVATION IS IT NOT NEWS TO ANYONE BUT IT WAS CERTAINLY REINFORCED IN ME ON THIS NIGHT. CONVERSATION WITH RPTR REVEALED THE FOLLOWED INFO: RPTR WAS NOT AWARE THAT THEY HAD RESPONDED TO A CONDITIONAL CLRNC. HE SAID THE CTLRS COMMENT; 'FLY RWY HEADING; YOU ARE ALL ARIGHT;' JUST AFTER THEY ROTATED WAS NOT COMPLETELY UNDERSTOOD BY ANY OF THE CREW. IN A DISCUSSION AFTER TKOF; ONE OF THE CREW STATED THAT HE BELIEVED IT WAS JUST A VISUAL TKOF CLRNC.