The results of the study on the threshold value of visual perception of Landolt's Ring visual targets within the visual field in a visual environment having uniform background luminance has already been reported. In the present paper are described the results of an experiment concerning the effect of a highly luminant object such as a light source or window existing in the visual field on the threshold value of visual perception. In the experiment, an apparatus as shown in Fig. 1 and a target consisting of a sheet of unglossed printing paper with its one half printed in black were used, and the observers were told to adjust the position at which the boundary of the white and blabk parts of the target became imperceptible as the target was brought near to the light source. Considering that at that point the contrast of the target and the background luminance were changed to the threshold value of visual perception by the veiling light produced by the highly luminant light source, the quantity of the veiling light was calculated. The results of the calculation obtained are summarized as follows : 1) The veiling light radically decreases in the inverse proportion to the light source deflection angle from the position at which the glare light source does not enter directly into the fovea centralis of the eye. 2) The quantity of the veiling light occurring in the human visual system increases approximately in direct proportion to the increase of the luminance of the glare light source. 3) When the size of the glare light source is increased, the quantity of the equivalent veiling luminance increases approximately in the direct proportion to the increased size. 4) For the quantity of the equivalent veiling light when the size of a light source is large, the large glare light source should be divided by the solid angle of 1°visual angle and the quantities of the equivalent veiling light generated by the respective units of the divided light source should be summed up, and thus the background luminance and the contrast can be obtained.