When light is measured over a wide dynamic range, just like voltage measurement circuits, gain can be employed to improve the measurement resolution and accuracy. Methods of detecting and measuring light in optical circuits include pin diodes, photo detectors, photomultiplier tubes, which are vacuum tubes, and avalanche photo diodes or APDs. When light strikes the detector a small amount of current flows. A pin diode can be thought of as a variable resistor while a photomultiplier tube and an avalanche photo diode behave like amplifiers in functionality because they gain up the current depending on the light intensity. Generally speaking, APDs provide higher gain or sensitivity and wider bandwidth than other optical gain methods. Key application areas for APDs include laser range finders, which measure distance, and optical receivers in fiber optic communication systems.