Invisible Flash sheds new light on photography in the dark
By Paul Ridden
August 6, 2009
As technology becomes available to help those wishing to avoid the annoying flash photography of the paparazzi get some payback, researchers Dilip Krishnan and Rob Fergus from New York University have developed a system for taking dazzle-free photos in poor lighting conditions which could result in celebs not even knowing they're being photographed. Named dark light flash photography by its creators, the system uses light waves beyond our visible range and special software and algorithms to produce photos comparable in quality to a long exposure shot.
One of the primary design goals was to create a system using standard off-the-shelf hardware, with some modifications of course. The researchers started with a standard Fuji IS Pro DSLR camera, which is marketed for applications involving ultra-violet (UV) and infra-red (IR) work and then removed the IR-block filter. An IR-block filter reduces the frequency range of the camera's sensor so that it matches the range of our color perception (400-700nm). Without the filter the raw silicon sensor responds to a much wider spectral range (350-1200nm). A standard Nikon 50mm f/1.8 lens incorporating a MaxMax CC3 filter (in order to block IR above 850nm) was then attached to the camera body.
They next modified a Nikon SB-14 flash by removing the UV absorbent coating on the Xenon tube. This would allow the flash to emit light over a similarly larger spectral range to the sensor. Visible wavelengths would need to be filtered out using a Hoya U360 filter. This gives the camera the ability to add light to a scene whilst being almost invisible to those being photographed. It also means that a fast shutter speed can be used, which helps cut down visible signs of camera shake.
Night scope view
Photos taken using the dark flash produce images that don't match the colors of our visual experience, producing a slightly green-tinted monochrome effect similar to something seen using night vision. So two images are recorded in quick succession (limited only by the camera's 3 frames/sec rate), one recording the dark flash spectrum and the other ambient light frequencies in the visible range.
Fortunately the researchers didn't have to mess around with specifically programming the flash motor so that it didn't fire for the ambient shot. The modified flash is used at full power for all shots and the cycle time is sufficiently long that it does not fire for the second shot, giving an image with ambient illumination alone.
Of course the latter image is dim, blurry and full of noise so a specially developed algorithm is used to compare the two images and look for any correlation, of which there is a high degree. Noise in the ambient image can therefore be cleaned up and produce a photo almost equal in quality to a long exposure shot.
Is it dangerous?
As the flash system uses UV light any possible health issues resulting from use of the system were examined. Using threshold limit value tables provided by the American Conference of Governmental Industrial Hygienists (a government body which looks at occupational and environmental health issues), the maximum number of safe flashes per day was calculated at 130,000 for a distance of 1m from the flash. According to the researchers: "if we assume that 30 minutes outside in the sun results in the maximum permissible UV dose on a bright summer day, then each flash is equivalent to being outside for 1/100th second." They therefore concluded that the dark flash system posed no significant health risk to subjects.
Other methods and applications
In developing this system other solutions for non-invasive low light photography were examined, including heavy and expensive wide aperture lenses, anti-shake hardware that can help reduce image blur and numerous de-noising techniques. Expense, inconvenience, limited success and photos which contained lots of unwanted artifacts (artifacts refer to a range of undesirable changes to a digital image caused by the sensor, optics, and internal image processing algorithms of the camera) led the research towards using wider spectral wavelengths instead.
Typically cameras use just three spectral bands to compose an image: red, green and blue. The flash/no flash system captures images using five different spectral bands: ultra-violet (370–400nm), blue (400–500nm), green (500–600nm), red (600-700nm) and infra-red (700nm–800nm). As well as allowing low light, no flash photography this could also open up whole new areas for creative art photographers to explore. The conversion algorithm developed for this project can also be used with standard photographic processes to improve image quality.
Not quite perfect
Despite the impressive results, a few problems remain. Some materials or objects absorb both UV and IR light and so don't appear in the dark flash image or the reconstruction. For instance, the doll's lips are visible in the long exposure image, but not in the dark flash image. Although uncommon it's still something that will need looking at before the technology could be put to any serviceable use.
Some materials also fluoresce when exposed to UV light (those parties or nightclub outings just wouldn't be same without all that white material reacting to the UV lights). This occasionally results in some purple artifacts on the images but can also make the flash visible to bystanders.
One of the primary objectives of this research was to make this process of flash photography invisible to those being photographed. The dark flash system has a luminous exposure of just 1.6 lux seconds which, when compared to a normal visible flash luminous exposure of 362 lux seconds, is pretty impressive. Not absolutely invisible but pretty close, with subjects looking directly at the flash while being photographed having reported seeing a weak purple light. Much more comfortable than a blinding flash.
Recognized system improvements include a smaller flash which uses UV and IR LEDs. This would offer more control over the pulse duration and tighter spectral emission and processing modifications so that only one photo needs to be taken instead of two. It could also lead to the technology being made available to the mobile phone market.
The research paper is being presented at the Siggraph computer graphics and interactive technologies conference in New Orleans this week.
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