A grease-spot photometer compares an unknown lamp with a standard lamp of certified…
PHYSICS · Multi-part · Fermi University Physics · Scientific Practice & Experimental Methods · Uncertainty in measurement · Designing and running experiments
Measurement and Method, Ch Designing the measurement: noise, averaging, null and differential methods
Problem
A grease-spot photometer compares an unknown lamp with a standard lamp of certified luminous intensity Is=100cd (candela, the SI unit of luminous intensity). The two lamps stand on opposite sides of a translucent screen on an optical bench; one lamp is slid along the bench until the grease spot on the screen disappears, showing that both sides are equally illuminated. At that null the standard lamp is at ds=45.0cm from the screen and the unknown lamp is at dx=62.0cm.
((a)) Using the inverse-square law for illuminance, E=I/d2, write the null condition and find the unknown lamp's intensity Ix.
((b)) A photocell power meter of similar sensitivity could instead be placed at the screen and used to read the illuminance from each lamp in turn on a calibrated scale. List which rows of the resulting uncertainty budget vanish when the null (equal-brightness) method is used instead, and which remain.