Abstract: The theoretical analyses of measurement error caused by non-standard viewing conditions, and the corresponding error-control methods through using a Dual Differential Luminance (DDL)algorithm in daytime meteorological visibility measurements are presented based on digital photography.It show s that :(i)the vertical relative gradients of sky luminance near due south and north horizons are mostly in the range of 4%/°～10%/°bigger than horizontal ones, which are mostly less than 2%/°, except at noon in sunny day or w hen the homogeneous cloud layer spreads over the w hole sky.The deviation angles between sight paths of different target-backg rounds can bring much bigger measurement errors and their effects should no t be neglected.The best solution is to set targets with same viewing elevation angle and to restrict the viewing azimuth angle deviations as small as possible during the operations of Digital Photography Visiometer System (DPVS).With the important advantage of matrix imaging technique, there is the possibility to further reduce their effects if the sky luminance along the same sight path of a given target is interpolated from the luminance distribution of the sky at both sides of the target.(ii)The nonuniform illumination along the sight path is also a measurement error source, which can' t be cont rolled manually.It is preferred to choose a site field with more uniform surface.And the error can also be reduced to a certain ex tent through averaging multi-samples.It is feasible to avoid result s with muchbigger error by monitoring the luminance distributions of site and sky and recognizing worse illumination conditions in real time.(iii)Observing some dark target s, such as trees, grasses, mountains, w alls and window s etc., can' t satisfy the demand of DDL measurements.I t is an optimal solution to adopt artificial black target s which have enough low reflectivity.