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Calculate Luminance

A typical 60-watt standard frosted incandescent bulb outputs 890 lumens for an efficacy ratio of about 15 lumens per watt. As an A19, its shape is spherical. What is the its calculated luminance?

A. 25,115 cd/in²
B. 25.115 cd/ft²
C. 25,115 cd/m²
D. Indeterminate, missing variable.

AnswerIf 1 candela (cd) = 4 ∏ lumens (lm), 890 x 1cd/4∏ = 71 cd, A19 = 19/8 = 2.375 in or 60mm diameter (= 30mm radius), L = intensity / projected area, 71 cd / ∏ r² = 71/ 3.14 x (.03m)² = 25,115 cd/m²

ARE 4.0 exam prep: BS

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