A BE1900D IN CLB AT FL210 DECLARED AN EMER AND DIVERTED DUE TO THE CAPT'S WINDSHIELD SHATTERING.

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PRIOR TO BEGINNING THE RUN-UP FOR ORIGINATING FLT XXY OUT OF ZZZ2; I CONSULTED A MECH (OUR AIRLINE) IN PERSON REGARDING APPARENT DELAMINATION OF THE CAPT'S FORWARD WINDSHIELD. THE UNUSUALLY LARGE DELAMINATION WAS IN THE BOTTOM R CORNER; MEASURING APPROX 4 INCHES HIGH AND 1 1/2 INCHES WIDE. IT DID NOT INTERFERE WITH MY VISION OUT THE WINDOW; HOWEVER; AND THE MECH ADVISED ME THAT BECAUSE OF THAT FACT A WRITE-UP IN THE MAINT LOG WAS NOT REQUIRED (SINCE THIS WAS NOT CONSIDERED A DISCREPANCY). THE MECH STUDIED THE SUSPECT AREA BOTH IN THE INTERIOR AND EXTERIOR BEFORE GIVING ME HIS RECOMMENDATION; HOWEVER; HIS EVALUATION MAY HAVE BEEN MADE MORE DIFFICULT DUE TO THE POOR LIGHTING CONDITIONS ON THE DARK RAMP. LATER; DURING CLBOUT ON THE FIRST LEG OF THE MORNING; AS WE WERE APCHING FL210 AND THE CABIN PRESSURE DIFFERENTIAL REACHED CLOSE TO 5 PSI; WE HEARD A LOUD POP AS 1 PLY OF THE WINDSHIELD SHATTERED COMPLETELY; WITH SPIDER WEBBING SO EXTENSIVE I COULD NOT SEE OUTSIDE IT VERY WELL. THE FO WAS FLYING; SO I DIRECTED HIM TO STOP CLBING AND BEGIN A DSCNT TO 10000 FT IN ORDER TO REDUCE THE CABIN PRESSURE DIFFERENTIAL. I THEN INFORMED CTR OF OUR SIT AND REQUESTED AN IMMEDIATE DSCNT TO 10000 FT AND A RETURN TO OUR DEP ARPT. A DSCNT WAS IMMEDIATELY CLRED TO SOMETHING LIKE 14000 FT OR SO; WITH SUBSEQUENT LOWER ALTS BEFORE LEVELOFF.; SINCE OUR REQUEST FOR DSCNT WAS IMMEDIATELY GRANTED; I FELT THE USE OF ANY EMER AUTH WAS NOT IMMEDIATELY REQUIRED. THE ACFT WAS MAINTAINING CABIN PRESSURE AS WELL; SO OXYGEN USE WAS NOT REQUIRED. DURING OUR DSCNT; I CONTACTED DISPATCH AND INFORMED THEM OF OUR SIT. UP UNTIL THIS POINT I HAD NOT DECLARED AN EMER WITH ATC. HOWEVER; THE DISPATCHER EXPRESSED CONCERN OVER MY NOT BEING ABLE TO SEE OUTSIDE FOR TFC AND POINT OUT ANY DEVS AS THE FO LANDED THE AIRPLANE. HE SAID HE WOULD DECLARE AN EMER AND HAVE CFR STANDING BY IN ZZZ2. I AGREED AND DECLARED AN EMER AS WELL WITH ATC FOR THE SAME REASONS. AN UNEVENTFUL LNDG IN ZZZ2 WAS MADE; AND I ELECTED TO HAVE THE FO TAXI TO THE GATE. I DECIDED NOT TO TERMINATE THE EMER UNTIL WE WERE PARKED; SINCE I COULD NOT SEE FORWARD VERY WELL AS THE FO TAXIED. GND CTL WAS NOTIFIED OF THE TERMINATION OF OUR EMER WHEN WE ARRIVED AT THE GATE.