48 



HOW WE SEE 



are related to each other. The phot and foot- 

 candle, for example, correspond to the lux with 

 the words "centimeter" and "foot" substituted 

 for "meter" in their respective definitions. 



Brightness. In the older terminology, bright- 

 ness was a confusing concept, not only because 

 of the diversity of units used to measure it (see 

 Table V), but also because the concept implied 

 two different types of measurements : one in terms 

 of luminous intensity per unit area of the surface 

 (luminance in the newer terminology) ; the other 

 in terms of the luminous flux emitted by or re- 

 flected from the surface (luminous emittance in 

 the newer terminology). In Table V, units in 

 candles per unit area (mm., cm., etc.) are of the 

 first type; units in lamberts (or any lambert 

 combination) are of the second type (75). In the 

 case of perfectly diffusing surfaces, i.e., those 



square centimeter. The luminous emittance of a 

 surface is the total luminous flux emitted in all 

 directions per unit area of the surface. The unit 

 is the lumen per square centimeter. For a perfect 

 diffuser, the luminous emittance (in lumens/cm. 2) 

 is w times the luminance in candles/cm.^ For 

 such a surface, the luminous emittance (in lumens/ 

 cm. 2) is also exactly equal to the luminance in 

 lamberts. 



Brilliance. In the report of the first OS A 

 Committee on Colorimetry, Troland (89) sug- 

 gested the term "brilliance" to refer to the 

 magnitude of the visual sensation as distinct from 

 its corresponding photometric quantity, bright- 

 ness. The term brilliance was never generally 

 adopted by psychologists even though one might 

 have anticipated that they would have welcomed 

 it. 



TABLE IV 



Conversion Factors for Illumination (Illuminance) Units 

 (Value in unit in left hand column times the conversion factor equals the value in unit shown at 



the top of the column.) 



Foot-candle 



Mile-candle 



Sea mile-candle 



Centimeter-candle (Phot) 



Milliphot 



Meter-candle (Lux) 



Kilometer-candle 



Sea mile- 

 candle 



3.697 X 10' 



1.326 



1 



3.435 X 101" 



3.435 X 10' ll X 10-3 



3.435 X 106 1 X 10-4 



3.435 1 X 10-10 



Centimeter- 

 candle 



1.076 X 10-3 



3.863 X 10-» 



2.911 X 10-11 

 1 



Milliphot 



1.076 



3.863 X 10-8 



2.911 X 10-3 



1 X 10' 



1 



1 X 10-1 



1 X 10-' 



Meter- 

 candle 



1.076 X 10 

 3.863 X 10-' 

 2.911 X 10-' 

 1 X W 

 1 X 10 

 1 



1 X io-« 



Kilometer- 

 candle 



1.076 X 10' 

 3.863 X 10-1 

 2.911 X 10-1 

 1 X 101° 

 1 X 10' 

 1 X 106 

 1 



which follow what is called the cosine law, con- 

 versions between the two kinds of measurements 

 are simple and straightforward. Actually, very 

 few surfaces are perfect diffusers and many do not 

 even approach this condition closely. In practice, 

 however, visual scientists have ignored any 

 implied difference between the two kinds of units 

 and have used them interchangeably by means 

 of the conversion factors shown in Table V. Few 

 tables of this kind have ever been published, and 

 this one should be useful in helping the reader to 

 convert the measurements of other visual experi- 

 ments into those used here. It should also 

 impress him with the unnecessary duplication of 

 units which has been common. 



It is clear now why the OSA Committee has 

 suggested two new terms, luminance and luminous 

 emittance, to replace brightness in the old sense. 

 Luminance is the luminous flux per unit solid 

 angle emitted per unit projected area of the 

 source. Although all the units in Table V can be 

 considered measures of luminance, the OSA 

 Committee recommends one, the candle per 



Still Other Units. Other photometric units 

 have been suggested by Moon (67) for inter- 

 national adoption. His units include pharos 

 (for luminous flux), pharosage (for illuminance), 

 helios (for luminance), etc. These terms are so 

 unlike those which have been in common use that 

 they will not be discussed here. In England the 

 units glim, scot and nox have been proposed for 

 low-luminance measurements (102). These are 

 equal to 10"' foot lamberts, 10~^ equivalent meter 

 candles or 1 milli-microlambert, and 10~^ meter- 

 candles, respectively. 



Standardization Needed. The foregoing discus- 

 sion has undoubtedly impressed upon the reader 

 the need for standardization and simplification of 

 photometric concepts and measurements. Lest 

 the reader get the impression that this is, however, 

 an internal affair and of no great concern to Naval 

 scientists and engineers, the writer would like to 

 express his opinion that the problem of standard- 

 ization is one of the most pressing problems 

 confronting visual scientists at the present time. 

 As a practical illustration of its importance, 



