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The following filters are recommended for use with the Target-UV™ and UV-Grey™
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Kodak Wratten 2e: filters UV radiation and a small amount of the blue light commonly emitted by UV lamps
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Peca 918: used for IR blocking
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Internal camera filtration: Most digital cameras are equipped with internal filtration that blocks most IR and also a small amount of red light, which helps match human perception. This filter is also known as a "hot mirror." A common example is the blue-green BG-38 filter (see below for transmittance spectrum). If you are using a DSLR that has been modified for use with reflected UV and reflected IR photography, or a camera without IR filtration (such as a view camera with a digital scanning back), you may need additional filtration to use the Target-UV™.
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Used together, these filters block UV and IR radiation commonly emitted by UVA sources. Any combination of filters that provides similar transmission between ~420nm to 700nm can be used.
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An overview of filters used with the Target-UV™
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Why don't my results match my visual perception?
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First, ensure that the correct filters are in place and that post-processing has been executed properly. You may also want to check your radiation source for visible light leakage. If you are still encountering issues with perception, continue reading.
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Radiation sources, subtleties of human vision, and variation in cameras pose significant challenges for the standardization of UV/Visible documentation. Our eyes see light and color differently from the camera, so significant filtration and programming is required to reduce these differences within the camera. In addition, UVA radiation sources vary greatly in their emission and filtration. The Target-UV™ and UV-Gray™ card are designed to overcome these variables and be interpreted as neutral by most camera sensors under a variety of UVA radiation sources.
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The Target-UV™ is analogous to visible light targets, but there are some major differences. Most notably, fluorescence documentation captures images produced by the emission of fluorescent materials. In contrast, visible light images capture reflected light from the object being documented. For this reason, techniques used in visible light photography may not be appropriate for the documentation of fluorescence.
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Further Discussion:
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Purpose and theory of the UV-Gray™ card and Target-UV™ products:
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The UV-Gray™ and Target-UV™ have been calibrated to appear neutral to the camera when excited by UVA radiation with a single peak of approximately 368nm. Using the UV-Gray™ card and Target-UV™ as suggested will create consistent images that mimic these radiation source conditions.
Because of variability in radiation sources, the Target-UV™ and UV-Gray™ will not appear neutral under every circumstance. However, through the use of the recommended filters and the prescribed workflow, the camera can be programmed to see a neutral gray. Programming the camera in this way provides a repeatable workflow and meaningful comparisons among various users.
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Factors that can impact UV/Visible viewing and documentation conditions:
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1. Variations in camera sensors and software:
Digital camera sensors are sensitive to a broader range of the visible spectrum than the human eye. Proprietary internal filtration (often called a "hot mirror"), bayer filters and software are required to bring the camera's sensitivity closer to the human eye. Methods and materials vary greatly between manufacturers and even models. This is also the reason that modified cameras require additional filtration, since the IR filter ("hot mirror") has been removed to allow for reflected UV and reflected IR photography. For more discussion of imaging sensors, see the following resources:
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The AIC Guide to Digital Photography and Conservation Documentation
WP's SloMo CCD and CMOS Sensor Info
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2. Variations in emission of various UV sources:
UVA radiation sources vary greatly in their peak emission. For example, many low-pressure fluorescent tube radiation sources emit with a single peak around 370nm, while mercury arc lamps typically have two emission peaks, one at 365nm and and another at 334nm. LED technology can be programmed to emit at a variety of ranges, which are often reported by the manufacturer. These variations in emission will affect the fluorescence of the UV-Gray™ card, the Target-UV™ and also the fluorescent objects you are documenting.
Most UVA radiation sources emit some amount of blue light, which can vary greatly in intensity and affect the color rendering of the items you are documenting. This can lead to a small difference in the perceived color of fluorescent objects. Filters (Wratten 2e) are needed to remove the blue emission from your images.
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Differences in the peak emission, overall emission and filtration of radiation sources can cause changes in the fluorescent emission of the object being documented. Below is a visual comparison of several radiation sources as they appear with a consistent white balance setting. All images were captured by a Nikon D800E with white balance settings determined by the UV-Gray™ card with two SuperBright II-model 3368 lamps (low-pressure mercury, fluorescent tube) as the radiation source. These settings for this camera at the time of this test were 6050K -8. Accompanying spectra show representative emission of three types of lamps.
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Comparison of Common UVA Radiation Sources
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3. The Nature of Human Vision
The ability of the human eye to perceive color is reduced in very low light conditions, such as encountered during UV/visible documentation. Your vision shifts from the reliance on photopic vision (well-lit, peak 555nm) to scotopic vision (low light, peak 507nm). Scotopic vision relies entirely on rod cells in the eye, which are not very sensitive to red light (this is why many cars and ship instrumentation glow in red, so the eye does not shift back to photopic vision). Night vision also requires time to adapt to low light conditions, and can change significantly with age.
Photopic vision versus scotopic vision
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What type of UVA radiation sources should I use?
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The Target-UV™ is designed to create consistent images with radiation sources that have major radiation peaks between 360-370nm.
Ensure that your lighting systems provides an even field of illumination of the subject. For example, the Messier LLC Lab uses two SuperBright II UVA lamps (model # 3368) from UV Systems, Inc.
These lamps are mounted to our existing lights to provide ease of use, as well as an adjustable lighting set-up. See below for an illustration of our copy stand set up with the UV lamps highlighted in red.
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Illustration of Messier UV set-up captured in normal light
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For more information see our list of suppliers and products
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How do I test my radiation source or camera for leakage?
Many UVA lamps emit visible light in addition to UVA radiation. Too much visible light leakage can reduce the quality of your images and cause color shifts. Similarly, most digital cameras leak some UVA and/or IR radiation. You can test your radiation source and camera for this leakage easily.
Test your radiation source and camera by photographing the lamp, or a reflection of the lamp (such as in a mirror). First capture and image without the recommended filters (Wratten 2e and PECA 918) are in place on your camera lens. Then capture a second image with these filters in place. These two images will illustrate: (1) how much visible light is emitted by your radiation source; (2) the effectiveness of your internal camera filtration; and (3) if the recommended camera filters are sufficient to remove the visible light leakage from your lamp.
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UV or IR leakage in your camera will appear as a magenta or red color cast in your image when captured without recommended camera filtration in place, as in image 1a.
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Visible light leakage from your lamp will show as white or blue light visible in your image when captured with recommended camera filtration in place, as in image 2b. An appropriate radiation source should have very little visible light leakage (as illustrated in image 1b). If there is a significant amount of visible light leakage (as illustrated in image 2b), the radiation source may need additional filtration.
The examples below show this test on two camera/radiation source combinations. Each pair of images were collected with the same exposure settings. Neither camera has been modified.
1. Low pressure mercury lamp using a Canon 5D Mark III.
2. LED lamp using a Canon 5D Mark II.
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1a. IR and UV Leakage Test - low pressure mercury
No camera filters in place
Note magenta coloration from UVA leakage and visible light from the lamp
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1b. Visible Light Leakage Test - low pressure mercury
Wratten 2e and PECA 918 filters in place on camera
Note reduction in magenta coloration and visible light
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2a. IR and UV Leakage Test - LED
No camera filters in place
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2b. Visible Light Leakage Test - LED
Wratten 2e and PECA 918 filters in place on camera
Note visible light leakage
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Why are my images dark?
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Underexposure most commonly occurs when the intensity level on the Target-UV™ is too high for the object being documented. Other common reasons for underexposure are covered below. Proceed with troubleshooting steps below in order until you solve the issue, particularly if you are unsure what is causing the underexposure. If you are documenting a composite object (i.e. an object with both very high and very low fluorescence), you may need to capture two images to illustrate the fluorescence.
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Ensure that you have set your camera exposure (f-stop and shutter speed) to levels to the Target-UV™ . This value should be +/- 127 RGB on the brightest grey patch for the level you have chosen. See exposure steps (step 20) in workflow for more information.
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Check your Raw settings. Ensure curves, camera calibration and other settings are correct. See relevant workflow for instructions on setting your raw settings.
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Try re-shooting your object with a lower intensity level. For example, if you underexposed with the “Medium” level, try re-shooting with “Low”. If it is still to dark, try the "low-low" modification described in the relevant workflow.
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Check filtration. Ensure that you are using both the recommended filters, and that UV radiation cannot leak between the lens and your filters. See workflow for more information. Recommended filters are the Wratten 2e and PECA 918. Use the additional BG-38 ONLY if you are using a modified camera or one with increased sensitivity in IR.
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Underexposed Image
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Why Standardize?
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Implementation of a standardized approach to the documentation of UV-visible fluorescence allows the meaningful comparison of images made by different users, in different places, over the course of time. A standardized and routine workflow is also critical to efficient practice: without the need to reinvent the wheel, users can focus on the production of more, and more meaningful, images.
This principle is the exactly the same for visible light photography. Use of the Target-UV™ provides the identical benefits of visible light targets and routine workflows currently used in conservation, scientific, and commercial photography. The combination of best practices and simple tools empowers photographers to manage exposure and reproduce color regardless of camera type, light source, and other variables. Widespread use of reference targets not only improves image quality but also provides visible metadata in the form of known gray and color values. At a glance, viewers can evaluate a range of fundamental image properties by examining these patches. This subject is discussed at length elsewhere, and many ISO standards provide parameters for interoperable approaches for reflected light imaging and measurement. For more information, see especially The AIC Guide to Digital Photography and Conservation Documentation.
Why shoot in RAW?
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The RAW image contains the unprocessed information collected by your camera. Using the RAW mode is the best way to control the post processing of your image without adversely affecting image quality.
When your camera captures images in TIFF, JPEG or other non-raw formats, the manufacturer’s proprietary software will convert the raw information into the designated file format. This will set the saturation, contrast, sharpening and other parameters to those determined by the manufacturer. These settings can vary greatly by year, model and manufacturer, resulting in unacceptable differences in images captured with different cameras. In the interest of standardization, RAW capture and designated post processing steps in the UV Innovations Workflow are suggested to use the Target-UV™.
Designated post-processing in RAW is designed to give consistent results across users, cameras and processing platforms. Note the subtle differences in RAW post-processing of the image below.
Default RAW settings from Adobe Photoshop CS5
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Default RAW settings from Adobe Photoshop Creative Cloud 2014
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UV Innovations RAW Settings
Why change the color space?
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ProPhoto has a wider gamut than other commonly used colorspaces, like Adobe RGB. This helps keep highly fluorescent materials from reaching max RGB values in images.
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Many imaging programs are now using ProPhoto because this color space retains all the information from the camera. This color space works best in 16-bit mode.
How do I choose intensity?
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The Target-UV™ is designed with levels to match the relative intensity of the fluorescence emitted by materials. UV-Visible fluorescence is an emissive source, so the intensity of fluorescence will be different for each object and material. For this reason, users must choose the intensity level for each set of images. Do not worry about choosing the "wrong" intensity. The design of the Target-UV™ provides overlap of the grey patches in each intensity level, allowing for comparisons and flexibility in exposure.
Description of intensities:
Low/Medium: These levels are designed for materials with a relatively low level of fluorescence, such as pigments, varnishes and adhesives. Most applications in art conservation will use this side of the Target-UV™ .
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High/Ultra: These levels are designed for materials with a higher intensity of fluorescence. Some natural materials with thick coatings will require the “high” level. Additionally, older or degraded optically brightened materials may also fluoresce at this level. “Ultra” fluorescent materials are ones that have been designed to fluoresce, including most optical brightening agents (OBAs) and fluorescent pigments.
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Tips for choosing intensity:
Get a rough idea of which level you might need based on the material you are documenting. See the above descriptions, or those on page 2 of the workflow.
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Visually compare the Target-UV™ to the object you will be documenting under UVA radiation. Which level fluoresces at a similar intensity to your object?
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Take a test exposure with the level you have chosen. Make sure to follow the workflow and load the proper RAW settings. How does your image look?
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Is it too dark, with details that are difficult to see? Try a lower intensity level.
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Is it too bright, with overexposed areas? Try a higher intensity level.
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How do I attach all the filters to my camera?
The Target-UV™ is calibrated to work with two on-camera optical filters that provide consistent filtration of UV and IR radiation. Camera internal filtration varies considerably, but most DSLRs have some leakage in UV or IR that can cause color casts in images. The two filters recommended typically come in different formats, so it can be difficult to attach both filters to the camera lens.
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The Kodak Wratten 2e is available as a 3-inch square format. Larger formats are available.
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PECA 918 is stocked in 62mm circular format, but larger sizes are available at additional cost.
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Solutions:
The 2e (a gelatin filter) can be cut to fit a filter ring. This filter ring can then be attached directly to the PECA filter, reducing handling.
Use a filter holder. The Cokin P D-SLR system is one option. This system uses adapter rings to attach to your camera lens, and can hold up to 3 filters.
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Illustration of Cokin-P Filter Holder Use
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Why does my image have a magenta color cast?
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A color cast is an overall tint in an image that is unwanted. There are several sources of color casts in UV-visible images. In most cases, the incorrect filtration is the cause. If the incorrect filtration has also been used to set the white balance, it can be difficult to notice the color cast.
A magenta color cast is commonly caused by inadequate or incorrect filtration. The magenta color is an indication that UV or IR radiation is reaching the camera’s sensor. This color will be most visible in lighter areas and at the edges of the image, closest to the radiation source. If you have set your white balance with the correct filtration you will notice that RGB values on the grey patches of the Target-UV™ are not neutral.
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Check filtration. Check that both the Wratten 2e and the PECA 918 are correctly attached to your camera lens. Ensure that no emission from the lamp can enter through any gaps between the filter and the lens. If needed, use a lens hood, or create one out of black paper.
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A purple color cast is commonly caused by incorrect use of the filters for a normal (i.e. unmodified) camera. If you have set your white balance with the correct filtration you will notice that the RGB values on the grey patches of the Target-UV™ are not neutral. Note: if your white balance has been set with the correct filtration, the image may appear blue/green when captured with the BG-38.
Remove BG-38 filter. This filter should only be used on modified DSLR cameras, or models increased sensitivity.
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Illustration of Color Casts
