Tech Article: Multispectral Imaging: UV-A experiments...

Full spectrum cameras are all the rage, but if you capture an anomaly, can you determine the light band it was captured in? UV sensitivity is often mentioned, but what does it really mean? In reality, UV sensitivity for most modern full spectrum cameras out there means a slight sensitivity to the UV-A spectrum of light. This is the spectrum of ultraviolet light that is least harmful to sight and tissue and is also responsible for making your t-shirt or other florescent materials glow in that groovy way. The deeper spectrums of UV (UV-B and UV-C) are generally hard to image without expensive systems, and I will explain why momentarily.

Back to anomaly identification by spectrum. I believe that by "narrowbanding" by filtering, you can get a better idea of where anomalies are showing up at. Everyone knows about IR, but few experiment with UV. I find UV to be very intriguing, so I have been researching a way to narrowband image UV on a shoestring budget. The sliver of UV-A spectrum that I can afford to concentrate on is the area between roughly 380nm and 400nm. Tiny section, I know. At any rate, I have been looking at various options, such as lifepixel conversions, and filtering. The trick to successful UV-A imaging is extended exposure. Most full spectrum cameras have imaging chips that are horribly inefficient at UV wavelengths. You can over come this by extended exposure. I then remembered I had a monochrome astronomy imager that had pretty decent efficiency (roughly 55-58%) sensitivity in the UV-A neighborhood. I decided to through a cheap computar lens on it and test it out... and I am pleased and excited for what it may show for paranormal imaging applications.

Let's talk for a second about why I selected UV-A as my sliver of the ultraviolet spectrum to image with. First off, UV-A illumination sources are plentiful and cheap. I am currently using a 51 LED UV-A (395nm) flashlight as a strong and narrowband illumination source. Secondly, image dimming from lower chip sensitivity, lens attenuation, and possible anti-UV coatings can be overcome by extending exposure. At UV-A wavelengths, glass is still mostly transparent, allowing most light transmission to reach the chip. This is not the case the deeper into the ultraviolet spectrum that you go. Glass becomes opaque in UV-B, and virtually no light is transmitted through in UV-C. It is for this reason that when you want to experiment in imaging these wavelengths, you must purchase specialty lenses made of quarts to retain some light transmission to the imaging chip. These special lenses are very cost prohibitive, and usually require a host of adapters to mate to your imaging platform of choice. Lastly, illumination sources in UV-B,C spectrums can cause eye and tissue damage, even with short exposures. This is why UV lights in these wavelengths are used for germicidal and antibacterial purposes. UV-A illumination is generally pretty safe as long as you do not stare at the light source. I believe there to be a treasure trove of UV paranormal anomalies residing in the UV-B,C bands, but at this point the cost of admission for good imaging there is definitely a roadblock.

Initial testing with this setup pictured above is proving good, I need to get a Hoya UV pass filter to cut or severely attenuate any visible or infrared components. Here is some results from my quick experimentation today. I draw your attention to the calendar on the wall. Each picture set has two pictures, one where I illuminate the calendar and one where I don't. The spot illumination source is a 51 LED UV-A flashlight with peak emission at 395 nanometers. False color was added during capture on some to accentuate the UV-A illumination. Results look promising, can't wait to try this in the field!