Measurement of the Visibility of Objects. 97 



of greatest interest from a purely scientific point of view,, 

 namely, the theoretical analysis of the visibility problem, 

 the measurement and numerical specification of visibility,, 

 and the instrument designed for this purpose. 



Nomenclature and Definitions. 



The intensity factor of the sensation resulting from the 

 incidence of radiant energy upon the retina is expressed in 

 terms of brightness. The brightness, B, of an element of 

 luminous surface from any point of view may be expressed 

 in terms of the luminous intensity per unit area of that sur- 

 face projected on a plane perpendicular to the line of sight. 

 When expressed in this manner it is measured in candles 

 per unit area of the projected area. Brightness may also be 

 expressed, and perhaps more logically, in terms of the specific 

 luminous radiation of an ideal perfectly diffusing surface, 

 that is a surface obeying Lambert's law. The brightness 

 unit in this case is the lambert, which is defined as equal to 

 the brightness of an ideal surface radiating or reflecting one 

 lumen per square centimetre. In practice this unit is too 

 large for convenience and hence the millilambert or *001 

 lambert is used. An ideal, perfectly diffusing surface 

 emitting one lumen per square foot will have a brightness 

 of 1*076 millilamberts. Measurements of brightness are 

 made by means of a suitable form of photometer calibrated 

 to read directly in the desired brightness units. The bright- 

 ness of a surface depends upon two factors, the illumination 

 of that surface and its reflecting power. 



The illumination, E, of a surface at any point is the 

 luminous flux density incident on the surface at that point, 

 or the flux per unit of intercepting area. " Luminous Flux," 

 F, is the rate of flow of radiant energy evaluated with refe- 

 rence to the visual sensation, and is expressed in lumens : 

 the lumen being defined as equal to the flux emitted in a 

 unit solid angle (steradian) by a point source of unit candle- 

 power. The c.g.s. unit of illumination is the phot, which is 

 defined as one lumen per square centimetre. The practical 

 unit in most common use, however, is the foot candle, which 

 is defined as one lumen per square foot, and which is equal 

 to 1'0764 milliphots. For a uniformly illuminated surface, 



F 



E = ~ , S being the area. 



Illumination is measured by a special type of photometer, 

 Phil. Mag. S. 6. Vol. 39. No. 229. Jan. 1920. H 



