Different Infrared Wavelengths used in Night Vision Technology|
Article- November 5, 2013 By Larson Electronics.com
Larson Electronics LED IR Illuminator Light
Since first seeing practical development in the 1940’s, night vision technology has progressed greatly. Starting out with rudimentary active night vision scopes with a range of only 100 yards and requiring infrared lamps so large they had to be transported on trucks, night vision devices have evolved into equipment small enough to be worn on the head and capable of operating without the help of infrared light sources. Despite this advancement, much of the night vision equipment available today still relies on a separate source of infrared illumination to enhance its performance, and it is these infrared light sources which we’ll be looking at here.
Most modern night vision devices in use by the military and law enforcement are known as passive NVD’s, which basically means they do not require an external source of infrared light to operate. These devices are image intensifiers which take in ambient light and through a complex series of mechanisms, increase its intensity before feeding it to a screen to be viewed. These passive systems tend to be the most expensive and only the latest generation of NVD’s provide high enough performance to be truly effective in modern military applications. Since these systems are passive, there is no external source of infrared light radiated which could betray the NVD user’s position to a potential enemy who also possesses NVDs.
Active NVDs rely on an external source of infrared light to produce an effective image in low or no light conditions and are far less expensive than passive devices. In these versions of night vision, an infrared sensitive camera is paired with an external infrared lamp which provides the infrared light needed to produce an image. These systems are also quite effective, and due to the ability of others to see the infrared light source with the use of their own infrared imaging devices, very limited in their effectiveness for military applications. Active infrared NVD’s however remain quite popular and are widely used in both military and non military uses such as security, hunting, law enforcement, marine use and similar applications where total invisibility is not critical.
Regardless of the type, passive or active, all image intensifying night vision devices are sensitive to light in the infrared end of the light spectrum. Because of this, most night vision devices can benefit from an external source of infrared illumination. Except in cases where an external IR source must be excluded to avoid detection by other operators of NVD systems, an external IR source can improve image resolution, enhance detail, and provide enhanced performance in extremely low light conditions where moonlight and starlight is obscured by cloud cover or where ambient light levels are very low. There are caveats however, and you cannot use just any source of infrared illumination with any particular type of NVD.
Most of the latest generation of military and law enforcement night vision devices have their IR sensitivity tuned towards the higher end of the infrared wavelengths. The more common forms of NVDs such as security cameras and handheld camcorders are sensitive to infrared in the lower ranges. The most common wavelengths used with NVDs are 850 nanometers for the common security camera or hunters trail camera, and 940nm for military 3rd Gen NVDs. So, since a common security camera is typically more sensitive to the 850nm wavelength, using an IR light source in the 940nm end of the IR spectrum will yield poor results, and vice versa. As a result, anyone considering augmenting their existing NVDs or cameras with an external IR light source should first determine which wavelength their device is most sensitive to in order to avoid poor results.
Another interesting thing to note is that although for most intents and purposes light in the infrared range is invisible to the naked eye, most light sources such as lamps and LED emitters designed to emit light in the 850Nm range and lower still radiate a slight reddish glow that is indeed visible at the emitter. For example, take a look at the images below. In the first image we can see an LED light bar designed to radiate light in the 750Nm range of the light spectrum. As can be seen, there is a solid and very visible orange/red glow around the LED emitters. An IR emitter such as this in the 750nm range has few useful applications as far as night vision devices are concerned, and is instead more suited to specialty applications and NVDs where covert operation isn’t important and devices respond well to wavelengths below 800nm.
In the next photo we see an LED light bar designed to radiate higher in the IR spectrum at 850nm. Although we can still see a definite glow, note the glow is now a light pink/red color in appearance. To get an idea of why this happens as you shift higher into the IR spectrum, consider if you heated a nail with a torch. At first the nail will get hot and not glow, but after longer exposure to heat it begins to give off a yellow glow, then orange, and then red. If we heat it enough, it will eventually become hot enough to appear white in color. The same phenomena is at work within the infrared end of the light spectrum, and eventually as you go higher in wavelength, the energy radiated will pass out of the visible range.
In the final photo we see an LED light bar configured to emit light in the 940Nm range. We can clearly see in this case that there is almost no visible light produced and no reddish glow. This demonstrates how much difference a slight shift in wavelengths can make, and how important it can be to use the correct IR wavelength if a system is to remain entirely covert. For example, if installing exterior security cameras and you wish them to remain totally hidden to observers, it is best to utilize cameras sensitive to the 940 nm range and IR lights with a matching output wavelength. Otherwise, the slight reddish glow produced by the IR light source can identify the systems location, leading to avoidance or even damage from vandalism or deliberate destruction.
Although all night vision devices are sensitive to light energy in the infrared range, not all are attuned to the same PART of the infrared range. IR lights with different wavelengths can affect how well a particular NVD works, and in order to get the best performance it is important to use an IR light source that emits light energy within the same wavelength range the device is sensitive to.