204 BIOLOGICAL EFFECTS OF RADIATION 



ERYTHEMAL SYSTEM 



It has uniformly been the pohcy of the National Bureau of Standards 

 to evaluate radiation from ultra-violet sources in absolute units, and the 

 effort has been to secure as far as is experimentally possible a method of 

 observation which is nonselective over the region of importance, such as, 

 for example, the balanced-thermocouple method, with the barium-flint 



filter. 



The practical necessity of establishing some criterion for the com- 

 parative evaluation of different types of ultra-violet sources for medical 

 purposes cannot be escaped. This has led to the adoption of certain 

 special units. Homogeneous radiation of wave-length 2967 A and 

 radiant flux density 10 ^w./cm.^ has been adopted tentatively by the 

 Council on Physical Therapy (16) as a Finsen unit (F.U.). It has been 

 observed that 2 F.U. for 15 min. is a representative requirement for 

 minimum perceptible erythema. Thus it has been proposed that one 

 erythemal unit be defined as 2 F.U. or^20 MW./cm.^ for 15 min. of homo- 

 geneous radiation of wave-length 2967 A. On this basis, the equivalence 

 of the erythemal unit for various ultra-violet sources has been determined. 

 It should be pointed out that on such a basis the units are both definite 

 and reproducible, but that the equivalence is dependent upon a number 

 of vital considerations. Where a source produces heterogeneous radiation 

 in the region of erythemal effectiveness, the evaluation of equivalence can 

 be no more definite than the evaluation of erythemal effectiveness. Only 

 by the arbitrary adoption of a standard curve of erythemal effectiveness 

 can the erythemal unit be definitely evaluated for other sources. If such 

 a standard weighting curve is adopted, the danger arises that subsequent 

 observations will show that the adopted standard curve does not coincide 

 sufficiently well with the physiological basis to make the determinations 

 of great biological significance. Thus, we are led to the probability of 

 frequent modifications in the standard weighting curve, as has been the 

 experience in the case of the illuminating system. 



Furthermore, the difficulty of establishing a satisfactory erythemal 

 weighting curve is far greater than that presented by the determination of 

 visibihty. Granting, however, that such a standard weighting curve of 

 erythemal effectiveness is adopted, it must be borne in mind that even 

 small changes in operating conditions of a source will change the radiation 

 erythemal equivalents. These points have been frequently emphasized 

 by Coblentz and must be taken into consideration in using such a table of 

 equivalents as may be found in the publication of the Council on Physical 

 Therapy on The Acceptance of Sun Lamps (16). 



More important still, let us reiterate our previous contention that 

 there is no necessary relation between erythemal effectiveness and 



