We present Strömgren and near-infrared ( NIR ) photometry of the bulge cluster NGC 6528 and its surrounding field in the Baade ’ s Window . uvby images have been collected with EFOSC2 on the New Technology Telescope ( NTT , La Silla , ESO ) .
The NIR catalogs are based on J,K -band VIRCAM @ VISTA ( Paranal , ESO ) and SOFI @ NTT photometry .
The matching of the aforementioned data sets with Hubble Space Telescope photometry allowed us to obtain proper-motion-cleaned samples of NGCÂ 6528 and bulge stars .
Furthermore , we were able to correct the Strömgren–NIR photometry for differential reddening .
The huge color sensitivity of the Strömgren–NIR Color–Magnitude–Diagrams ( CMDs ) helped us in disentangling age and metallicity effects .
The red-giant branch ( RGB ) of NGCÂ 6528 is well reproduced in all the CMDs by adopting scaled-solar isochrones with solar abundance , i.e . Z = 0.0198 , or \alpha -enhanced isochrones with the same iron content , i.e . Z = 0.04 , and an age range of t = 10 - 12 Gyr .
The same isochrones well reproduce most of the color spread of the Baade ’ s window RGB .
These findings support age estimates present in literature for NGCÂ 6528 .

We also performed a new theoretical visual–NIR metallicity calibration based on the Strömgren index m _ { 1 } and on visual–NIR colors for red-giant ( RG ) stars .
Scaled-solar and \alpha -enhanced models have been adopted and we validated the new Metallicity–Index–Color ( MIC ) relations by applying them to estimate the photometric metal abundance of a sample of field RGs and of a metal-poor ( MÂ 92 , [ Fe/H ] Â \sim - 2.3 ) and a metal-rich ( NGCÂ 6624 , [ Fe/H ] Â \sim - 0.7 ) globular cluster .
We applied the calibration to estimate the mean metal abundance of NGCÂ 6528 , finding [ Fe/H ] Â = [ M/H ] Â = -0.04 \pm 0.02 , with a mean intrinsic dispersion of \sigma = 0.27 dex , by averaging the metallicities obtained with the scaled-solar [ m ] , y - J and [ m ] , y - K MIC relations , and of -0.11 \pm 0.01 , with \sigma = 0.27 dex , by using the m _ { 1 } , y - J and m _ { 1 } , y - K relations .
These findings would support the results of Zoccali et al .
( 2004 ) based on high-resolution spectroscopy , which give [ Fe/H ] Â = -0.10 \pm 0.2 for NGCÂ 6528 , and a low \alpha -enhancement of [ \alpha /Fe ] Â = 0.1 , and of Carretta et al .
( 2001 ) , that find [ Fe/H ] Â = 0.07 \pm 0.01 , with a modest \alpha -enhancement , [ \alpha /Fe ] Â = 0.2 .
By applying the scaled-solar MIC relations to the sample of Baade ’ s window RGs , we find a metallicity distribution which extend approximately from [ Fe/H ] Â \sim - 1.0 up to [ Fe/H ] Â \sim 1 dex , with two metallicity peaks at [ Fe/H ] Â \approx - 0.2 and [ Fe/H ] Â \approx 0.55 ( [ m ] , y - J and [ m ] , y - K relations ) , and [ Fe/H ] Â \approx - 0.25 and [ Fe/H ] Â \approx 0.4 ( m _ { 1 } , y - J and m _ { 1 } , y - K relations ) .
These findings are in fairly good agreement with the spectroscopic studies of Zoccali et al. ( 2008 ) , and Hill et al .
( 2011 ) for the Baade ’ s window , of Uttenthaler et al .
( 2012 ) for a region centered at ( l,b ) = ( 0 ^ { \circ } , -10 ^ { \circ } ) , and with the recent results of the ARGOS surveys for the bulge ( Freeman et al. 2013 , Ness et al. 2013a ) .