Context : An essential step to understanding protoplanetary evolution is the study of disks that contain gaps or inner holes .
The pre-transitional disk around the Herbig star HD 169142 exhibits multi-gap disk structure , differentiated gas and dust distribution , planet candidates , and near-infrared fading in the past decades , which make it a valuable target for a case study of disk evolution .

Aims : Using near-infrared interferometric observations with VLTI/PIONIER , we aim to study the dust properties in the inner sub- { \mathrm { au } } region of the disk in the years 2011-2013 , when the object is already in its near-infrared faint state .

Methods : We first performed simple geometric modeling to characterize the size and shape of the NIR-emitting region .
We then performed Monte-Carlo radiative transfer simulations on grids of models and compared the model predictions with the interferometric and photometric observations .

Results : We find that the observations are consistent with optically thin gray dust lying at R _ { \mathrm { in } } \sim 0.07 ~ { } { \mathrm { au } } , passively heated to T \sim 1500 ~ { } \mathrm { K } .
Models with sub-micron optically thin dust are excluded because such dust will be heated to much higher temperatures at similar distance .
The observations can also be reproduced with a model consisting of optically thick dust at R _ { \mathrm { in } } \sim 0.06 ~ { } { \mathrm { au } } , but this model is plausible only if refractory dust species enduring { \sim } 2400 ~ { } \mathrm { K } exist in the inner disk .

Conclusions :