Luminous efficiency of the sodium vapour lamp

Q.  The practical luminous efficiency of the sodium vapour lamp is of the order of
- Published on 07 Sep 15

a. 25 to 40 lumens per watt
b. 40 to 45 lumens per watt
c. 45 to 50 lumens per watt
d. 60 to 67 lumens per watt

ANSWER: 45 to 50 lumens per watt

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