An intervalometer is a device that counts intervals of time. Such devices are commonly used to signal, in accurate time intervals, the operation of some other device. For instance, an intervalometer might activate something every 30 seconds. Other names for such a device include interval meter and interval timer.
Examples of intervalometer use in aerial photography include delaying the start of picture taking by an unattended camera until some time after takeoff and separating multiple exposures in time, and thus distance as the vehicle containing the camera travels, to obtain the 3D effect (stereoscopy). To obtain the 3D effect each image should have about 60% of the surface in common with either the preceding or following image. The interval is calculated as a function of the altitude and speed of the vehicle; shorter intervals for low altitude and high speed.
Often the purpose of a photographic intervalometer is to reduce the resources required either to take the pictures or post-process them as similar images could be obtained by having the camera continuously take pictures as rapidly as possible. Using an intervalometer permits restricting the pictures taken to only those with the desired content. This reduces the requirements for resources such as power and storage media (e.g. film or memory card space).
Most digital single-lens reflex (DSLR) cameras are limited to 30 second or shorter exposures. An intervalometer can be used to take long (>30 seconds) or very long exposures (minutes or hours) using the "Bulb" setting. Long and very long exposures taken at night can be combined to create time-lapse animations, including star trails. Astrophotographers can use processing techniques with such exposures to create images of deep-sky objects in the night sky, like nebulae and galaxies.
Most modern cameras include the most basic intervalometer functionality, the "self-timer". This delays the shutter release for a short time, allowing the photographer to get into the picture, for example.
In the past, intervalometers were external devices which interfaced to a camera to trigger taking a picture, or series of pictures, at a set time. These sometimes used existing remote shutter features on cameras. Later, standalone products commonly referred to as intervalometers have added capabilities far beyond the basics of just measuring, and signaling, a time interval. One of the first features that was added to intervalometers was the ability to use an external event to signal the start of the time interval(s). The ability to sense an external event is such a common feature of intervalometer products that many people do not distinguish between the sensing of the event and the measuring of time intervals.
What is meant when someone refers to an "intervalometer" must be determined from context. Some possibilities are: time-lapse capability (strictly an intervalometer function), sensing of a remote event, a time delay longer than what most consider the "self-timer" range, etc. Strictly speaking, an intervalometer only measures, and/or signals, time intervals.
Almost all digital cameras have the basic hardware capability required for intervalometer functions: knowing the current and elapsed times. The implementation of more advanced functions is a matter of what the manufacturer chooses to implement in the camera's firmware. Functions beyond the self-timer are beginning to be seen in some digital cameras, and are used in some cases to distinguish models within a camera line.
The ALE-39 countermeasures system uses intervalometers manufactured by Ledex Inc. (now part of Johnson Electric) of Dayton, Ohio. The ALE-39 can fire flares in a synchronized pattern, very rapidly and with great reliability. The intervalometer used in the ALE-39 is essentially a solenoid-actuated rotary switch driven by a separate programmer which gives timing intervals and channel enabling to either of one or two channels. Intervalometers that contain internal interval clocks include the Lau-68, Suu-13 and similar electromechanically sequenced switches. Safety is provided to unfired outputs by maintaining a ground connection to all except the output being selected for firing; i.e., providing an electrical pulse to the firing squib.
Bomber aircraft can release all of their bombs at one time ("salvo") or drop individual bombs at intervals. If the bombardier selects the latter, he can program an intervalometer to control the pace at which the bombs are released. This, of course, determines how far apart they will land in the target area.
One of the more common types of intervalometers is the timer that is used to turn lights on and off at set times, or that of an automatic sprinkler system. These are commonly used by people when they leave their home for an extended period of time to make it appear the home is occupied. There are also a large number of commercial and industrial applications for even such basic intervalometers.