Active sensory systems are sensory receptors that are activated by probing the environment with self-generated energy. Examples include echolocation of bats and dolphins and insect antennae. Using self-generated energy allows more control over signal intensity, direction, timing and spectral characteristics. By contrast, passive sensory systems involve activation by ambient energy (that is, energy that is preexisting in the environment, rather than generated by the user). For example, human vision relies on using light from the environment.