Frequently Asked Questions
Q. I know you can use black and white film and a filter to take reflected UV images with film cameras. How is this done?
A. Standard black and white film (Kodak T-Max 400, for example) is sensitive to UV light. One can load it in a standard 35mm SLR camera and place a Wratten 18A filter or equivalent on the front of the lens to block visible light and pass UV to the film.
Q. How is the UVCorder™ better than film for UV imaging?
A. Even under laboratory conditions, film-based UV photography is very inefficient. When the film camera has the UV pass filter on it, the photographer cannot compose the shot, since the eye sees nothing through that filter. So the shot must be made using a tripod. The filter is removed for composition, and then is replaced before the exposure is made. The exposure value must be guessed, although filters like the UG-1 have a filter factor of about 6 stops with sunlight illumination, and using 6 stops of compensation often works. Still, this technique requires bracketing, which consumes a lot of film. Black and white film is getting harder to find, and fewer and fewer labs process it. The UVCorder™ eliminates every one of these problems and adds video capability as well.
Q. What about the Fuji S-3 UVIR camera? Is that a good near-UV camera?
A. We have experimented extensively with this camera, which is designed to image from 380nm to 1100nm. The near-infrared performance is excellent, but the near-UV response is much lower than the UVCorder™. We believe that this is because the Bayer RBG mosaic filter is on the sensor, which modulates and attenuates the UV or NIR signal. We end up using just the color channel that gives the highest contrast when post-processing the RAW images from the Fuji in Photoshop. The reduced response means that one often has to use very bright UV lights to get appreciable signal. The Fuji does have a live preview feature that makes it possible to retract the SLR mirror and get a live near-UV or near-IR image on the viewfinder in the back. However, doing that kills the batteries very fast, it is hard to see the dim UV image on the viewfinder and the live preview only runs for 30 seconds before one has to navigate through various menus to start it again. The camera is very high resolution, though. It is very convenient to use the UVCorder™ to find UV features of interest, then photograph these features or phenomena with the Fuji UVIR for high resolution.
Q. What about resolution?
A. There is no question that 35mm film has higher resolution that most digital camera sensors, although even that is starting to change. The UVCorder™ images at a relatively small image size, about 0.3 megapixels. For many applications, this is perfectly adequate. Multiple images of a scene can be acquired in rapid succession and stitched together using software like Adobe Photoshop if higher numbers of pixels are required.
Q. There are other electronic devices for imaging in the UV out there. Why not use them?
A. Image-intensifier based UV systems suffer from radial distortion and an inability to record images without adding cumbersome cameras and relay lenses on the back. They also do not have lenses with adjustable apertures, which is inconvenient and leads to non-optimal exposure values under bright lighting conditions (like sunlight), as well as low lighting.
Q: Does the UVCorder™ image UV fluorescence?
A: There is a very common misperception that UV imaging is the same thing as UV Fluorescence imaging. The UVCorder™ records UV light reflected or emitted from scenes or objects. The UV channel on the UVCorder™ detects UV light only, not visible or infrared light. Fluorescence is a phenomenon whereby light at one wavelength (often UV) strikes a material that re-emits light at a different, longer wavelength. The classic case is a fluorescent highlighter pen. UV light will cause the pigment in the marker to emit yellow light, which is easily detected by the eye.
If one images a fluorescent object or material with the UV channel of the UVCorder™, it will nearly always look dark, since UV light tends to be absorbed by the fluorescent material. Any visible fluorescence emitted by the material is not detected by the UVCorder™, since it is designed to see only near-UV light.
The visible channel on the UVCorder™ can be used to image fluorescence phenomena when it is equipped with a special filter that blocks any UV light from reaching the sensor. With the filter in place, the image formed is visible light only. If the target is illuminated with a pure UV light in a darkened room, the visible fluorescence signal will be very apparent. Various filters are available upon request. We can advise which filter is appropriate for the application. The filters are mounted in 30mm rings which screw onto the front of the Sony camcorder.
Q: Can the UVCorder™ take images in both visible and UV at the same time?
A: No, you have to choose one or the other. Simultaneous imaging will require a second camcorder that will record in the visible band. The UVCorder™ and this second camcorder can be mounted on a common mounting plate.
Q: Does the UVcorder™ need any special software?
A: The UVCorder™ records JPEG still images and AVI movies to the Memory Stick Pro Duo. It also records digital video (when recording to mini-DV tape). The JPEG images can be loaded into your PC via a USB card reader and opened in innumerable software applications including Adobe Photoshop. The AVI movies can be played in Windows Media Player, among others. The digital video can be streamed via FireWire from the camera to a host computer running Final Cut Pro or Adobe Premier. One can also output NTSC video from the audio/video jack to a TV monitor or VHS deck. It should be noted that no audio signal is recorded when the UVCorder is recording UV video, but audio is present when the camera is recoding color or near-IR video.
Q: Can the UVCorder™ actually measure the signal strength of a UV source?
A:
The UVCorder™ will not output a measurement in watts/square cm or other engineering units, but it can qualitatively tell you about the relative brightness between multiple sources. The UVCorder is an imaging device. It does not have a radiometric calibration in it like many hand-held thermal imaging cameras designed to measure temperature, for example. The automatic gain control in the UVCorder™ adjusts the shutter speed of the camera to match the scene brightness present. In low light conditions, the camera become more sensitive as needed. We can recommend vendors that sell calibrated photodiode systems for measuring the brightness of a UV laser beam or other UV sources.
Q: What is the spectrum of the 395nm LED light source that comes with my UVCorder?
A: The five LEDs used in the LED flashlight are spectrally very pure, with virtually no energy at wavelengths more than 20nm from the peak in either direction. The spectrum is shown below:
Q: What is the spectrum of the 365nm LED made by Nichia? Do you sell an illuminator with this LED?
A: The 100 mW Nichia i-LED is very bright and also spectrally very pure, with virtually no energy at wavelengths more than 15nm from the peak in either direction. The spectrum is shown below. We don't have a standard flashlight with this LED in it. We can make a custom illuminator with it, however.
Q: Why is there a brass spacer behind the lens interface?
A: The UVCorder™ is designed to use conventional C-mount TV lenses that are much lower cost than quartz UV lenses, and in some cases, provide superior image quality. The C-mount back working distance of these lenses assumes that visible light is being imaged. UV light focuses slightly longer than visible light, requiring a thin brass shim spacer so that the UVCorder™ will properly form an image for targets at infinity.
Q: I notice that if the target is moving, still UV images show some roughness on the edge of the moving target in the scene. What causes this?
A: The UVCorder™ uses a frame grabber inside the camcorder to acquire a video image from the EIA (NTSC) video stream from the UV Module. Since EIA video uses interlaced fields, one can get artifacts along the edges of moving objects, since alternating lines of the final still frame were acquired 1/60 second apart.
Q: Is the UVCorder™ qualified for a CE Rating?
A: We have not qualified it for CE. The main test is EMI, or electromagnetic interference. The UVCorder™ is not likely to be a significant source of EMI.
Q: Will the UVCorder™ work in sunlight?
A: Yes, the solar spectrum at sea level contains plenty of UV light in the 330-420nm band. At higher altitudes, there is even more ambient UV light to image. We have designed the UVCorder™ to have excellent spectral purity when used in sunlight. There is less than 1% out-of-band contamination of the signal.
Q: Can I see corona discharge in sunlight with the UVcorder™?
A: No, and you cannot see corona any better than the dark adapted eye, even in the dark. The UVCorder is not suitable for detection of corona. The correct product for this application is the DayCor camera, which uses a solar-blind UV image intensifier that is fused to a visible-light image of the scene. The UVCorder can image electrical discharges such as those from natural lightning and Tesla Coils, but the discharges generally appear no brighter than they would on standard color video.
Q: Can I record audio with the UVcorder™?
A: You can record ambient audio using the built-in microphone on the camcorder only when recording visible or near-infrared Nightshot video. When you are recording UV video, you are in the camcorder's Play/Edit mode, and the microphone is disabled. You would have to connect a microphone with an externally powered preamplifier to the audio input of the camcorder to simultaneously record audio. We have not made a provision for this in our standard UVCorder™ cabling. A specially wired version can be done at additional cost if this is a requirement for your application.
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