This is the first of a series of papers devoted to a global study of the photometric properties of the red stellar sequences in a complete sample of the Large Magellanic Cloud clusters , by means of near infrared array photometry .
Deep J , H , Ks photometry and accurate Color Magnitude Diagrams down to K \approx 18.5 , i.e . \approx 1.5 Â mag below the red He-clump , for six intermediate age clusters ( namely NGCÂ 1987 , NGCÂ 2108 , NGCÂ 2190 , NGCÂ 2209 , NGCÂ 2231 , NGCÂ 2249 ) are presented .
A quantitative estimate of the population ratios ( by number and luminosity ) between Red Giant Branch and He-clump stars for each target cluster is provided and discussed in the framework of probing the so-called Red Giant Branch phase transition ( RGB Ph-T ) .
By using the Elson & Fall s-parameter as an age indicator , the observed RGB population shows a sharp enhancement ( both in number and luminosity ) at s = 36 .
Obviously , the corresponding absolute age strictly depends on the details of theoretical models adopted to calibrate the s -parameter .
Curiously , the currently available calibrations of the s -parameter in term of age based on canonical ( by Elson & Fall 1988 ) and overshooting ( Girardi et al .
1995 ) models provide ages that well agree within 10 \% , suggesting that the full development of the Red Giant Branch occurs at t \approx 700 Â Myr and be a relatively fast event ( \delta t \approx 300 Â Myr ) . However , the RGB Ph-T epoch derived from the overshooting calibration of the s -parameter turns out to be significantly earlier than the epoch provided by the recent evolutionary tracks by Girardi et al .
( 2000 ) .
A new calibration of the s -parameter based on high quality Color Magnitude Diagrams and updated models is urged to address the origin of this discrepancy and finally establish the epoch of the RGB Ph-T .