Comparison and calibration of LEP bunch length measurements

Measurements have been made on short bunches (2 - 5 mm) at 22 GeV and 45 GeV with all wiggler magnets ramped down or off. The bunch lengths obtained from the streak camera and button monitor have be en compared with theoretical predictions. The variation of the streak camera bunch lengths with Q$_S$ agrees with the predictions, although the measured difference between the length of 95 $\mu$A and 25 $\mu$A bunches is much less than expected from 1997 button monitor measurements. After an offline correction, the button monitor analysis still indicates substantially less than the expecte d variation of bunch length with Q$_S$. However, after the button monitor algorithm was modified on 28$^th$ July, the two measurements agreed within 0.2 mm in the range of bunch lengths 9 - 11 mm . In addition, the response ratio (button / streak) to bunch length variation observed in the range 9 - 13 mm improved from 0.55 to 0.80.