We present an empirical calibration of the Mixing-Length free parameter { \alpha } based on a homogeneous Infrared database of 28 Galactic globular clusters spanning a wide metallicity range ( -2.15 < [ Fe/H ] < -0.2 ) .
Empirical estimates of the red giant effective temperatures have been obtained from infrared colors .
Suitable relations linking these temperatures to the cluster metallicity have been obtained and compared to theoretical predictions .
An appropriate set of models for the Sun and Population II giants have been computed by using both the standard solar metallicity [ Z / X ] _ { \odot } = 0.0275 and the most recently proposed value [ Z / X ] _ { \odot } = 0.0177 .
We find that when the standard solar metallicity is adopted , a unique value of \alpha =2.17 can be used to reproduce both the solar radius and the population II red giant temperature .
Conversely , when the new solar metallicity is adopted , two different values of \alpha are required : \alpha = 1.86 to fit the solar radius and \alpha \approx 2.0 to fit the red giant temperatures .
However , it must be noted that , regardless the adopted solar reference , the \alpha parameter does not show any significant dependence on metallicity .