Photometry and illuminance
Answers are for parents and teachers.
1
A point source has total flux 628 lm. What is its luminous intensity approximately in cd? (4π ≈ 12.56)
50
2
I = 150 cd and R = 5 m. What is E in lx?
6
3
I = 96 cd, R = 2 m, φ = 60°. What is E in lx? (cos60° = 0.5)
12
4
Two lamps (I₁ = 40 cd, I₂ = 90 cd) are 2 m and 3 m from a surface, rays perpendicular. What is the total illuminance in lx?
20
5
Why is the equal-illuminance comparison with a second lamp used when testing how E depends on lamp intensity?
Intensity cannot be read directly from the luxmeter; with a standard lamp I₁ can be calculated from the ratio of distances.