SQL function to verify 'Check Digit' of given VISA/Master Card

Hi,
 
Normally, every Visa/Master card have the 16 digits number printed(embossed) on the card. Last 16th digit is called as check digit.
 
There is an algorithm called MOD10 to verify that card number is valid or not. Here I am providing a link for detailed description about check digits logic: Cards Check Digits[^]
 
I am providing a SQL Server function for 'Check Digit' for Visa/Master Cards(16 Digits).

-- About using function: Pass the first 15 digits of the card no to the function 
-- and function will retuns the last check digit. 
-- That returned value should match with user provided 16th digit.

CREATE FUNCTION [DBO].[MOD10CHECKDIGIT](@STR VARCHAR(15))
RETURNS VARCHAR(1)
WITH EXECUTE AS CALLER
AS
BEGIN
DECLARE @RETURNSTR VARCHAR(100);
DECLARE @REVERSESTR VARCHAR(15);
DECLARE @CNT INT;
DECLARE @SUMVAL INT;
DECLARE @TEMPVAL INT;
DECLARE @TEMPVAL1 INT;
IF(LEN(@STR)<>15)
   SET @RETURNSTR='NO';
ELSE
  BEGIN
    SET @REVERSESTR = REVERSE(@STR);
    SET @SUMVAL=0;
    SET @CNT=1;
     WHILE @CNT < 16
      BEGIN
       SET @TEMPVAL =0;
       SET @TEMPVAL1 =0;
       IF((@CNT%2)<>0)
           SET @TEMPVAL =  (CONVERT(INT,SUBSTRING(@REVERSESTR,@CNT,1) * 2));
       ELSE
           SET @TEMPVAL =  (CONVERT(INT,SUBSTRING(@REVERSESTR,@CNT,1)));
       IF(@TEMPVAL>9)
          BEGIN
                SET @TEMPVAL1 = CONVERT(INT,SUBSTRING(CONVERT(VARCHAR,@TEMPVAL),1,1)) +
                             CONVERT(INT,SUBSTRING(CONVERT(VARCHAR,@TEMPVAL),2,1));
                SET @TEMPVAL=@TEMPVAL1;
          END;
       SET @SUMVAL = @SUMVAL + @TEMPVAL;
       SET @CNT= @CNT + 1;
      END
     SET @RETURNSTR=CONVERT(VARCHAR,(10 - (@SUMVAL%10)));
     --SET @RETURNSTR=CONVERT(VARCHAR,@SUMVAL);
  END
RETURN @RETURNSTR;
END

 
Note: Every VISA card starts with digit "4" and Mastercard stats with "5".