618 BELL SYSTEM TECHNICAL JOURNAL 



Substituting T = 11,400 degrees, 



X,„ = 2535 (10)-« cm (19) 



Tliis is indeed an interesting result, since the mercury vapor discharge in 

 the fluorescent lamp radiates most of its energy at X = 2536.52 (10)~* cm. 

 The design of the lamp was guided by the effort to accentuate the radiation 

 at this wavelength, and the manufacturers state that this has been achieved 

 so that no other spectral line is excited to radiate more than two percent of 

 the input power. ^ The conversion loss from dc to 2536 (10)~^ cm is only 

 2 or 3 db. 



The striking similarity between the black body and the mercury vapor 

 discharge at these two wavelengths, 7.6 cm and 2536 (10)~^ cm, suggests 

 the following hypothesis: 



Hypothesis: In a gaseous discharge which is radiating light energy sub- 

 stantially monochromatically at a particular wavelength, X^ , the micro- 

 wave noise energy is the same as that available from a black body which 

 radiates its maximum energy at that wavelength. 



Applying this hypothesis to the case in hand, where X^ is 2536.52 (10)~^ 

 cm, and using Wien's displacement law (eq. 18) we calculate the tempera- 

 ture to be 



^ = 2W2 = ''•'''' ^''^ 



(Jo - = ''•'' *''' 



10 log (25^) = 15.84 db (23) 



Since this calculated value is so close to the measured values of 15.8 db 

 and 15.86 db, it will be assumed to be correct until proved otherwise. 



Conclusions 



A commercial fluorescent lamp is a reliable source of microwave noise 

 energy. At 4000 mc its effective temperature is 11,394 degrees Kelvin which 

 is convenient for measuring noise figures of 20 db or less. The noise power is 

 practically independent of the fluorescent coating, the current density and 

 only slightly affected by the room temperature. The lamp lends itself 

 readily to a broad-band impedance match in the waveguide. 



8 G. E. Tnman and R. N. Thaver, A. I. E. E. Transactions, Vol. 57, pp. 723-726, Dec. 

 1938. 



