Figure 4 - An ultraviolet light bulb also looks like clear glass to the video camera
Figure 4 - An ultraviolet light bulb also looks like clear glass to the video camera

The bulb shown in Figure 4 is called a "black light" bulb and will emit mostly ultraviolet radiation in the 340 nanometer to 400 nanometer range. These bulbs are often used for special effect lighting, where a certain phosphorescent color is made to look as though it is glowing in the dark. These bulbs are also used for medical purposes, as well as to identify the anti-counterfeiting security strip on money. In this project, the ultraviolet light will be used as an illumination source, giving the security camera the ability to see the entire room light with light that human eyes cannot perceive.

As you probably already know, ultraviolet radiation in high doses is not healthy for the skin or eyes, and although a black light is designed for "human safe" operation, it is certainly not a good idea to expose your eyes to the light for extended periods of time. If you are going to purchase a black light bulb, follow the warnings on the package, and remember that just because you can't see much light coming from the bulb, it doesn't mean that there isn't light. In reality, the most dangerous effect of the black light might be the intense heat that is generated by the bulb due to having most of the visible light blocked by the glass. These things get extremely hot in seconds, unlike "standard" white light bulbs.

The black light is shown in the monitor in Figure 4, and becomes so transparent that you can even see the tungsten filament and connecting wires inside the bulb. To my eyes and the digital camera used to take this photo, the black light bulb looks completely dark. It is strange to see the camera see right through it without any effort at all. This means that the black light bulb will be an amazing source of illumination for the video camera, which is obviously sensitive to the ultraviolet light. The downside is that the bulb also passes some light in the visible violet end of the spectrum, so the area being illuminated will have a dull purple glow that can be seen by human eyes. Of course, there are ways to block the violet light, as well other techniques shown in the next few steps.

Figure 5 - Developed film and floppy disk material can be made into infrared pass filters
Figure 5 - Developed film and floppy disk material can be made into infrared pass filters

Once you have a low lux black and white security camera connected, you can test all kinds of dark looking materials for their ability to perform as infrared pass filters. Some of the materials I learned about that pass infrared light are shown in Figure 5 - exposed camera film and floppy disk material. Having a massive collection of retro junk on hand, I was able to find some old 5.25inch floppy discs to hack up, but the smaller 3.5 inch types will also give the same results. Oh, and don't worry, I didn't hack up the Commodore Dos 3.3 disks - those are classics!

Exposed film is also a good material that will block most visible light and pass infrared radiation, but I found that the film would also pass some deep red light as well. The film used is just common 35mm film that has been exposed and then developed. In other words, you load a new roll of film into your camera, point it at an evenly lit surface, and then snap away until you have as many frames as you need to cut up and make your filter. Even the film from a disposable camera will work fine, and remember to tell the film processor that you purposely made blank photos when you bring it in for developing, or they may toss out your negatives.

As for the light source, a standard halogen bulb hand held flashlight will be used, as these light sources also include a large infrared component. Because the film and the floppy disk material cannot withstand a great deal of heat, the flashlight is a better choice than a larger incandescent light source. A 60 watt white light bulb might offer more illumination, but the thin film would certainly melt within seconds of being exposed to such intense heat. Remember that for a filter to block light, it must also absorb the light, which means a great deal of heat must be contained as well.

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