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(C) David Prutchi, Ph.D diy Shortwave UV Image Converters for Solar-Blind and RUVIS Imaging David Prutchi, Ph.D. www.UVIRimaging.com Technical Note 2016-1, June 2016 Ph.D. Prutchi, David (c) Figure 1 – DIY shortwave-ultraviolet imaging converters. Top: Converter based on fluorescent coating. Bottom: Short-wave ultraviolet viewer based on a surplus RCA UV image converter tube. © 2016 David Prutchi, Ph.D. All rights reserved. Page 1 Imaging in the shortwave ultraviolet spectrum (wavelengths below 300 nm) enables some very exciting applications. Light in these wavelengths is completely invisible (but potentially very harmful) to the unaided eye. I am most interested in a band known as “Solar Blind UV” or SBUV. As shown in Figure 2, solar radiation in the 240 nm to 280 nm range is completely absorbed by the ozone in the atmosphere and cannot reach Earth’s surface, thus allowing ultraviolet-emitting phenomena (e.g. electrical discharges, hydrogen combustion, etc.) to be detectable in full daylight. Ph.D. Prutchi, David (c) Figure 2 – Certain ultraviolet-emitting phenomena, for example electrical discharges and breakdown in high-voltage power lines are almost undetectable in the visible range (a), but emit strongly in the UV-C region (b) which daylight lacks, allowing visualization in full daylight by superimposing the UV-C and visible images (c). d) The spectral content of sunlight is heavily modified by the Earth’s atmosphere, and shows various dips that correspond to different gases absorbing energy at different points in the spectrum. Ozone in the atmosphere absorbs very heavily in the UV-C band (100 nm to 280 nm), so virtually no © 2016 David Prutchi, Ph.D. All rights reserved. Page 2 sunlight in that wavelength range reaches Earth’s surface. The darkest part of that range – 240 nm to 280 nm is known as the “solar-blind” band. Pictures a-c courtesy of Eran Frisch, Ofil, Ltd. SBUV images don’t show the context in which the SBUV-emitting phenomenon occurs, so SBUV images are often superimposed onto a visible picture. Figure 3-a shows a simplified block diagram for a camera made by Ofil, Ltd. (www.ofilsystems.com) – an Israeli company that is the worldwide leader in the development of solar-blind UV-C imaging systems. In Ofil’s camera, light from the scene is split by a two-way mirror (a component that in optics is known as a beamsplitter) and directed simultaneously towards a visible color camera and to a SBUV camera. The latter comprises a lens that is transparent to shortwave-UV, one of Ofil’s specialized SBUV filters and an image intensifier with a SBUV-sensitive photocathode. The intensifier’s output is imaged by a monochrome CCD camera. The video streams from the visible and SBUV cameras are finally combined into one superimposed video channel that shows SBUV-emitting phenomena in context with the visible landscape in real time. A dramatic demonstration of the usefulness of such a system is shownPh.D. in Figure 3-b and -c. Figure 3-b shows a scene in which a rocket-propelled grenade is being launched at a distance of 500m from the camera, yet nothing unusual is seen in this daylight picture. However, it’s impossible to ignore the rocket’s plume when the SBUV image is superimposed onto the visible image as shown in Figure 3-c. Ofil has also developed a system for detecting shoulder-launched missiles to civilian aircraft from terrorist threats. In their system, a SBUV sensor detects the UV radiation emitted by an approaching missile and hands over its coordinates to a fine-tracking and jamming system that can release chaff and flares to deviate the missile.Prutchi, SBUV imaging is much better for this type of application than any other remote sensing technology because it allows unequivocal detection of imminent threats with very low false-alarm rate. SBUV imagers are widely used worldwide for more mundane tasks such as locating electrical discharges in bright daylight, hence saving power distribution and electric-train companies millions by allowing them to detect corona discharges before an insulation break turns into a catastrophic failure.David Chemical industries also use SBUV cameras to detect and locate fires caused by fuels such as hydrogen that don’t produce bright visible flames. Imaging in the shortwave UV also has significant uses in forensics – most importantly, “Reflected(c) UV Imaging Systems” (RUVIS) are used by CSI units to discover latent fingerprints under shortwave UV illumination (254 nm) without the use of powders or chemicals. Illumination is commonly provided by a portable SW-UV lamp, and an imager sensitive to this wavelength is used to produce a picture visible to the operator. Fingerprint residues containing oils and/or amino acids reflect and scatter shortwave UV, making them evident against most smooth, non-porous backgrounds that reflect shortwave UV poorly. Untreated sweaty prints © 2016 David Prutchi, Ph.D. All rights reserved. Page 3 show as bright reflective ridges on a black background, while oily prints appear as strong, dark ridges on a shiny background. This allows an investigator equipped with a sortwave camera or image converter to search, view, and capture latent prints not visible to the unaided eye. RUVIS is also used to enhance the contrast of faint or invisible latent prints that have been exposed to cyanoacrylate fumes. The microscopic fibers occurring with high humidity cyanoacrylate development (known as the cyanoacrylate bloom) reflect short-wave UV very strongly, so cyano-developed latent prints show very brightly under RUVIS against a jet-black substrate. Ph.D. Prutchi, David Figure 3 – Ofil,(c) Ltd.’s solar -blind imaging camera is able to combine images obtained from a solar-blind UV imager (operating in the 240 nm to 280 nm range) and a visible-light camera (a). b) It is next to impossible to see a rocket-propelled grenade (RPG) being launched at a distance of 500 m in broad daylight. However, the rocket plume is clearly visible in the solar-blind UV band, allowing for immediate detection and identification of a launch site. Images courtesy of Eran Frisch, Ofil, Ltd. © 2016 David Prutchi, Ph.D. All rights reserved. Page 4 Imaging in the Shortwave UV Unlike infrared and visible light, ultraviolet light has very little penetrating power into otherwise transparent or semitransparent materials. Because of its short wavelength, it is easily scattered by surface scratches and imperfections that are not apparent at longer wavelengths. These characteristics make ultraviolet imaging an ideal inspection tool in production lines. Unfortunately, these same characteristics make traditional camera sensors very inefficient at ultraviolet wavelengths because UV light can’t penetrate through the silicon bulk to the photosensitive sites on a camera sensor. Cameras specifically designed for ultraviolet or for very low light level imaging commonly use a “back-illuminated” sensor. Thinned-down back- illuminated sensors improve the sensitivity of a camera to UV light quite dramatically, but silicon imaging technology nevertheless reaches its sensing limit at around 300 nm. A simple, inexpensive and very effective trick used by camera companies to enhance a camera sensor’s UV response for shorter wavelengths is to coat the sensor Ph.D.with a substance that fluoresces under ultraviolet light. Think about the way in which fluorescent paints glow brightly under “black light” – which is simply long-wave UV. Actually, the ultraviolet light has much higher intensity than the paint’s glow, but our eyes are significantly more sensitive to the fluorescence wavelength (e.g. neon green, pink, or orange) than to the virtually invisible 365 nm “black light”. In the same way, a very thin fluorescent layer or “phosphor” (not to be confused with the element phosphorus) deposited on the sensor can be used to convert UV outside of the sensor’s range to a longer wavelength thatPrutchi, the sensor can easily detect. One fluorescent coating used in early UV astronomy experiments was sodium salicylate – a close relative to Aspirin - which glows around 400 nm when exposed to light with wavelengths below 350 nm. However, as shown in Figure 3, sodium salicylate’s glow is on the edge of an image sensor’s sensitivity. Later, coronene, also known as the aromatic hydrocarbon superbenzene, was used for UV astronomy because its fluorescence glow is more easily detected by a camera’s sensor. Modern cameras use composite phosphors made specifically for this purpose – the most common being MetachromeDavid and Lumogen to increase the sensitivity of cameras in the 120 nm to 430 nm range. Lumogen(c) glows in the 540 nm to 580 nm range when illuminated by violet/ultraviolet light with wavelengths shorter than 450 nm. At wavelengths longer than 460 nm, the very thin layer of Lumogen applied to the sensor becomes transparent, allowing it to work normally in the visible and infrared portions of the spectrum. A problem common to Lumogen coatings is a steady drop in sensitivity with accumulated ultraviolet exposure. However, new Lumogen-type coatings such as Photometrics’ Metachrome II (www.photometrics.com) are known to remain stable for long periods of time under constant UV exposure. © 2016 David Prutchi, Ph.D. All rights reserved. Page 5 Ph.D. Figure 4 – Fluorescence of a number of coating materials is used to extend the wavelength range of image sensors into the ultraviolet. These curves show the wavelength distribution of the glow that these materials output when illuminated by UV at wavelengths below 350 nm. Lumogen is the best because it glows at wavelengths that more closely match the sensor’s sensitivity. Prutchi, The dedicated phosphors are not easy to come by just for experimenting. That is why I decided to mention sodium salicylate and coronene, which can be easily purchased from chemical supply houses, allowing an enthusiast to experiment with DIY coating of sensors.
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