Avoiding Table scans in a lookup function

Hello
Suppose that I have the table BusinessWeek(BiWk char(7), InitialDate date, EndDate date). I want to create a function that given a date, it would return the BiWk value corresponding to that value. The following query uses a Table ScanECLARE @BiDay AS DATE
SET
@BiDay = '2008-05-17'SELECTBiWkFROM
BusinessWeekWHERE @BiDay BETWEEN InitialDate AND EndDate
How can I boost the performance of this query? What indexes should be created? My intention is to process a large table row by row and to get the Business Week corresponding to a Date Field of that table.
Thanks a lot.

I would add an index covering both columns - InitialDate and EndDate and INCLUDE BiWk via the INCLUDE option.

A single scalar value (char(7)), which would be the Business Week. I would use it like:
Select
GetBiWk(LT.MyDateField) as BiWk
from LargeTable as LT
Thanks.