452 Table 159 (concluded) 



RELATIVE SPECTRAL LUMINOSITY FOR THE HUMAN EYE AT VARIOUS 



LUMINANCE LEVELS 



Table 159 B. — Maximum luminous efficiency (K ma x) as a function of log luminance 

 (logioB) for a standard light source at 2360 °K. 



Note. — B is the luminance to which the observer's eyes are adapted. 



Table 160 

 HORIZONTAL VISIBILITY* 



General : 



The following tables pertaining to horizontal visibility may be considered to be sub- 

 divided into three classes: (1) tables for converting extinction coefficient 1 (<r) to trans- 

 missivity (7"), and tables for converting transmissivity to extinction coefficient; (2) 

 tables giving extinction coefficient as a function of daytime visual range of black 

 objects viewed against the horizon sky. [Angular dimensions of objects are assumed 

 to be in range 0.5° to 5°. Data are presented for two possible values of threshold of 

 luminance contrast, e, viz, e = 0.02 and « = 0.05, corresponding to the respective visual 

 ranges denoted by Vi and Vs.] (3) Tables applicable to the visual range of point 

 sources of light, and permitting computation of any one of the four variables involved 

 if the other three are known (see equations 11 and 12). 



Units: 



The kilometer is the unit used here for distance (x, D and R) and for visual range 

 (Vt and Vt) ; the unit of extinction coefficient (a) is the reciprocal of the kilometer 

 (km -1 ); the unit of illuminance or flux-density (E) is the lumen kilometer" 2 ; and the 

 unit of luminous intensity (h for a point source) is the candle (candela). The trans- 

 missivity, T, is defined as the base of natural logarithms (e) raised to the power (— <r). 

 T* and T D (that is, e~™ and e~" D ) must be dimensionless, hence the dimension of x and 

 D must be reciprocal to that of a. 



* Prepared especially for the Smithsonian Meteorological Tables by L. P. Harrison, U.S. Weather 

 Bureau. 



1 This entity is sometimes called the atmospheric attenuation coefficient and has been denoted as 

 ft by some authors (Duntley, reference 4). 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



