for the Measurement of 1 r isibility oj Objects. 



133 



have been very small. Hence the curve plotted from the 

 observed values should agree quite closely with the theo- 

 retical curve V=/(W) for brightness contrast alone. It will 

 be noted that the curves agree fairly well in the region 



Fiff. 13. 



v =y(w) 



.3 fi .5 6 .7 . 



WEATHLK-COEFFICILNTIWJ 

 Comparison of Observed with Computed Values. 



W = '45 to W = '90, but that more marked differences exist fol- 

 lower values of W. This is probably due to the presence of 

 quality contrast which existed to a greater extent under those 

 weather conditions resulting in low values of W. Such values 

 of W usually denote clear shy and bright sunlight, and 

 •although the colour of the background may be constant, the 

 apparent colour of the object will be changed, due to the dif- 

 ference in quality of the incident illumination. 



Although the greater part of the work done with this 

 instrument relates to the visibility of ships as seen against a 

 sky background it is evident that the same method can be 

 applied to the evaluation of the visibility of other objects 

 under various conditions of background and illumination. 

 While it is considered that the general principles of the 

 method are applicable to all problems of this nature it is 



