An introduction into using infrared radiation to create stealthy night vision spy gadgets
Infrared radiation can illuminate a scene for night vision operations
Night vision is one of the most important factors when considering any kind of video operated spy gadget as this technology allows the viewer to see in complete darkness while the subject is completely unaware. Because infrared light (radiation) falls just below red on the visible light spectrum, making up the wavelengths from about 750 nanometers to about 1500 nanometers, this light cannot be seen by human eyes, but it can easily be seen by many video cameras, making it useful as a covert lighting method in night vision systems.
A common example of infrared light is the medium for communication between your remote control and television set. The LED on the end of your remote sends out pulses of infrared light which is received by the infrared detector on the TV and demodulated back into data. Of course, you cannot see the pulses because they are out of our visual range, but the infrared receiver in the television can see the pulses perfectly.
Security cameras and mini spy cams can also see infrared radiation very well. They are easy to connect, inexpensive and can be easily hidden. There are many good quality security cameras available on the market that include a low lux video camera in a weather proof housing along with an array of infrared LEDs for night vision applications. Black and white security cameras and small board cameras are particularly sensitive to infrared light. These ultra low lux cameras can usually be purchased for about $100 or less, especially from online sellers. Add 10 or more infrared LEDs, and you now have a night vision system that is better than those that sold for thousands of dollars in the 1980s.
Figure 1 - The light spectrum, showing the small segment we call visible light
The light spectrum shown in lower half of Figure 1 covers light from ultraviolet right to infrared and shows the small portion of the light spectrum that can be seen by human eyes. Light is visible to our eyes from approximately 400 nanometers (violet) to approximately 700 nanometers (red), and has green (550 nanometers) at the midpoint. What is interesting is that the imaging system in a video camera can see the light extending past both ends of the visible light scale, which includes both infrared and ultraviolet. Since the infrared portion of the light spectrum is not visible to our eyes, the correct term for this portion of the spectrum is infrared radiation, not infrared light, but often both terms are used.
The infrared portion of the light spectrum that can be seen by most video cameras covers the range of 700 nanometers well into 1500 nanometers, with exceptional sensitivity around 800 nanometers. Video cameras can also see ultraviolet radiation, but infrared radiation is easier to generate and at higher output levels will not cause any biological dangers like ultraviolet will. Not all video cameras will be able to see infrared light though, especially those designed for high quality color imaging. Camcorders and digital still cameras contain a glass filter that essentially blocks out all infrared light, leaving only the visible portion of the light spectrum so that the image quality is maintained. For this reason, camcorders will not be able to see the infrared light form an illumination system unless you are willing to open up the case and remove the tiny glass filter that has been affixed to the CCD imager.