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An infrared flashlight, sometimes also known simply as an IR flashlight, is a device designed to emit light that falls outside of the normal visible spectrum. It usually looks and feels a lot like a standard flashlight, but unless people are using special eyewear or viewing the light through certain recording devices, pushing the “on” switch will seem to have no effect at all. Light is actually being emitted, but in the infrared range — a range that humans can’t see on their own. This sort of flashlight is most commonly used in the context of active night vision, in which the users are also wearing or using other night vision technology that allows them to see previously imperceptible lights. Most infrared flashlights are created with infrared technology from the start, usually with special bulbs or light emitting diodes (LEDs) that only cast light within certain spectrum ranges. In some cases it is also possible to fit a normal flashlight with an infrared filter, but this can be less precise and often requires a lot of knowledge about wavelengths and their relative strengths. Wavelength synchronization is also really important when trying to use these sorts of flashlights in tandem with other night vision technologies.
One of the most important things about IR flashlights is that they emit light that is entirely beyond the visible range. The human eye is only capable of seeing a very narrow band of the electromagnetic spectrum, which is typically referred to as visible light. Wavelength and frequency are both used to describe the various colors of light found within this spectrum, from purple at one end to red at the other.
In most cases the lowest frequency the human eye can see is about 480 terahertz (THz) and the longest wavelength is 750 nanometers (nm), both of which correspond to the color red. Light that falls immediately beyond that range is referred to as infrared, while microwaves and radio waves have substantially longer wavelengths and lower frequencies. The light emitted by infrared devices is very real, but can typically only be detected with other sight-enhancing technologies.
Since infrared light is invisible to the naked eye, it is possible to shine this sort of flashlight in the darkness without being detected — and this is actually one of the biggest draws of the device. Without the aid of a special recorder or some other night vision device (NVD), the flashlight will appear to be off.
The light will almost always show up on camera, thought. Many modern TV remote controls use infrared technology to do things like change channels, and one of the easiest ways to simulate how an infrared flashlight works is to aim one of these remotes at a camera or camcorder that is actively recording. Unless the camera is equipped with an IR filter, the LED will appear to flash brightly in the recording when the remote is activated. Importantly, though, bservers watching in real time won’t notice anything; they won’t see any light or any flashes. In most cases the light when it is visible will appear blue or purple, which is usually the same way the light from an infrared flashlight will look on camera.
There are a number of reasons why people might want to see in the dark with an all but invisible flashlight, but police activity, military operations, and hunting are three of the most common. Some lights are handheld, but they might also be mounted on buildings or vehicles; smaller models can also sometimes be attached to tactical clothing. They illuminate a space in undetectable way, which is beneficial for many covert and nighttime activities that involve movement through unknown terrains.
In order to use an IR flashlight with an NVD, it is important to make sure they operate on the same wavelengths. All IR flashlights emit infrared light, but they do not all work on the exact same frequencies. The general range that most of these flashlights fall into is from about 715nm to 1,000nm, but many NVDs cannot see wavelengths longer than about 880nm. An infrared flashlight that emits light at the low end of the range can sometimes be somewhat visible to the naked eye, while those that only cast light in the 1,000nm range tend to be invisible to certain NVDs and significantly dimmer even on specially designed night vision cameras.