98 Mr. Loyd A. Jones on a Method and Instrument 



usually referred to as a lumeter or ilium mometer, these 

 being calibrated to read directly in some suitable illumina- 

 tion units. 



The coefficient of reflexion or total reflecting power of a 

 surface is defined as the ratio of the total reflected luminous 

 flux to the total incident luminous flux. In most practical 

 work this value is not of great importance, the value desired 

 being that of the reflecting power of the surface measured 

 under certain specified conditions, such as the angle of inci- 

 dence of the flux and the position from which the surface is 

 viewed. The term " Reflexion Factor," R, is used to indicate 

 this particular value and is denned as the ratio of the reflected 

 to the incident flux. Reflexion from a surface may be either 

 specular, diffuse, or a mixture of the two. In the case of 

 pure specular reflexion all of the incident flux is reflected in 

 such a way that the angle of reflexion is equal to the angle 

 of incidence ; while in the case of completely diffuse reflexion 

 the reflected flux is equal in all directions regardless of the 

 angle of incidence, the distribution being in accord with 

 Lambert's cosine law. Very few cases of pure specular or 

 diffuse reflexion are found in practice, there being generally 

 a superposition of the two. The reflexion factor is measured 

 by the use of a reflectometer, a photometer of special design, 

 care being taken that conditions of illumination and angle of 

 view are such as to give correct values for application in the 

 particular case under consideration. This value is purely 

 numeric and is usually expressed as a percentage value. If, 

 with a specified condition of illumination, the reflexion factor, 

 R, and the brightness, B, of a surface are measured from the 

 same position, then B = E . R, and hence the value of E may 

 be determined ; or in any case where two of these factors 

 are known the third can be computed. 



The quality factor of the luminous flux is that property 

 which depends upon the spectral distribution of that flux, 

 colour being defined as the subjective evaluation *as expressed 

 in terms of hue and purity or saturation. Hue is that pro- 

 perty of colour which depends upon the variation in the 

 ■sensation due to the variation of the wave-length of 

 the luminous flux, while saturation expresses the proximity 

 of the colour to a condition of monochromatism. Mono- 

 chromatic spectral light has a saturation of 100 per cent., 

 while pure white light has a saturation of zero. TVhite, 

 therefore, is a limiting colour having no hue and zero 

 saturation. In practice it has been found convenient in 

 many cases to express the saturation factor in the inverse 

 order, that is as impurity rather than purity. The term used 



