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Found 5 results

  1. Perhaps of interest primarily to the forum readers in the U.S.: If only Thomas Jefferson could settle the issue Happy 4th of July!
  2. I've been looking for focusing aids for IR and UV multispectral imaging, and while browsing on a whim at our local salvage store, Urban Ore, in Berkeley, I happened across an item that might be useful. It's a vintage face tanning device with both a UV lamp and IR resistance-heating sources. I'm guessing it dates to the '50s or '60s. It's a German made device by "Dr. Kern & Sprenger, KG" and it runs on 220V AC. I picked it up for $10. I haven't checked, but I wouldn't be surprised if you could find one of these on eBay. The IR heaters would radiate mostly in the thermal-range (3,000 to 30,000 nm, or 3-30 microns), outside the range of the CMOS sensors on most cameras, but they certainly also radiate in the near IR (700 to 1,300 nm). However, I'm thinking of disconnecting them or rewiring the switches because they draw a lot of watts and produce mostly heat. They can't be switched off while the UV lamp is on, unfortunately (the switches offer only IR "Wärme," or IR+UV "Sonne + Wärme"). I also have an IR LED flashlight from maxmax.com. I'm trying to find out more about the UV lamp in this device, particularly about the spectrum it emits and whether there are modern lamps that I can replace it with to get specific UV wavebands. It appears to be a high-pressure mercury lamp, but could also be a low-pressure lamp. A label on the back of the device identifies it ("Typ UV Brenner") as a "UV800". I've attempted, but haven't yet found a way to remove the lamp to replace it. It emits quite a bit of visible light, and while testing it I've been wearing sunglasses with a good UV-blocking rating to protect my eyes, and I avoid looking at the lamp directly. I'd also like to find out if I can attach dichroic or other filters in front of the UV lamp to specifically select certain wavelengths or wavebands of UV light. For safety reasons, I'd like to filter out any UVB and UVC. If anyone has experience with filtering for specific UV bands on the light source, I'd be interested to hear of your experiences. Other UV light sources for use as focusing aids would be welcome. I know others in this forum have made good suggestions for UV sources for actual imaging. For UV imaging, here's another example of a tunable UV light source: "High Power UV LED Radiation System: 365nm 385nm 395nm 400nm 405nm" http://photographyoftheinvisibleworld.blogspot.de/2012/09/high-power-uv-led-radiation-system.html Dr. Schmitt's blog is my favorite resource for UV photography, and his inventory of macro lenses is another fantastic resource: The Macro Lens Collection Database http://www.macrolenses.de/ Thanks to Dr. Schmitt for permission to link to his sites here. Anyway, apart from the novelty appeal and low cost, the tanning device is relatively small and light, and it has a nice hinge that allows the light to be directed at various angles. It's an interesting addition to my inventory for multispectral RTI.
  3. There are many techniques one can use to date a painting, but it is usually best to start with non-invasive methods. A good place to begin is by looking at the painting's verso to look for canvas makers' stamps, gallery stamps, the construction of the stretcher, canvas weave and thread counts, and primings, among other features. In many cases, paintings have undergone past treatments, such as relining. In such instances, the original canvas and hence, canvas supplier's stamps or other stamps (e.g., duty stamps), if present, are not visible because they are covered by the relining. Non-invasive techniques such as multispectral imaging can be useful to reveal hidden features, but art works are unpredictable and unique situations arise where a combination of methods is needed. Transmitted infrared imaging can sometimes reveal canvas makers' stamps on a relined canvas where other techniques, such as x-ray imaging, might fail. This is sometimes true in the case of a painting with a lead white ground layer, where the lead absorbs x-radiation but also happens to be relatively transparent to transmitted infrared radiation (TIR). But what to do if the canvas stamps are partly obscured by the horizontal cross-brace of the stretcher? In this case study, an on-the-spot solution was devised, with the aid of 3D photogrammetry, to identify a previously unknown artists' colourman in mid-19th century London as the supplier of the canvas, and bracketed the range of dates when the canvas could have been supplied, probably between c. 1844 and c. 1860. Because the canvas supplier's stamps were mostly hidden behind the central horizontal cross-brace of the stretcher, thin shims were placed between the canvas and stretcher to create a narrow gap, allowing TIR images to be captured at an angle, revealing most of the text of the stamps that would otherwise have remained hidden. However, reconstructing accurately scaled images of the stamps from the angled TIR images was a challenge. This is where photogrammetry proved to be very useful. A high-resolution 3D model of the painting's verso was constructed by capturing overlapping reflected visible-wavelength images that were processed using Agisoft Photoscan software. Each of the four quadrants of the painting's verso (defined by the vertical and horizontal cross-braces of the stretcher) were also captured in both reflected visible light and TIR. The visible and TIR images were registered, therefore the TIR images could then be aligned with the 3D model by swapping them for visible reflected images, and an orthorectified TIR image of the painting's verso could be constructed. The TIR orthophoto contained accurate scale information, which could be used to measure the overall dimensions of the stamps. By overlaying the TIR angle images of the stamps, which contained more of the stamps' textual information, onto the TIR orthophoto, the perspective distortion resulting from the capture angle could be accurately removed, and nearly complete, stitched images of the hidden stamps were reconstructed. Besides creating a more accurate and measurable record of the previously unrecorded stamps, the reconstructions allowed the canvas supplier's name, address, and dates of his business operations to be determined through further research, including London trade and post office directories and geneological data. In addition to providing a date range for the stamps, the position of the stamps on the 3-ft x 4-ft landscape painting was significant, since they were rotated 90 degrees from horizontal and centered behind the horizontal cross-brace. This suggested that the canvas had been purchased separately and stretched by the artist (or perhaps by an assistant) on the original stretcher, since it was not a standard size that was widely commercially available during the period.
  4. Here's an interesting paper about the application of multiple techniques for the study of murals and graffiti: http://www.ijcs.uaic.ro/public/IJCS-15-03_Cosentino.pdf Although it's not mentioned in the paper, one of the authors (A. Cosentino) describes an Arduino distance meter to check the position of the speedlight while capturing RTIs of the murals: http://chsopensource.org/reflectance-transformation-imaging-rti-with-arduino/
  5. For capturing high-resolution multispectral images of a 19th century landscape painting (36 x 50 inches), I found it helpful to create a spreadsheet for planning purposes, and also to supplement the shooting notes as a record of what was done. To get complete coverage of the painting at the desired resolution for RTIs and photogrammetry (500 ppi for RTIs, and up to 2600 ppi for certain details), it was necessary to take a series of overlapping images. The spreadsheet takes basic information about the object (size, material, UV sensitivity), camera and lens (format, sensor and pixel dimensions, focal length, and settings), and calculates various parameters (working distances for the camera sensor and light source, minimum size of the reflective spheres, number of images, and storage requirements) for the project, given the desired target resolution and various wavebands (UV, visible, and IR) to be captured. This information is useful for estimating the space and time requirements for capturing RTIs and photogrammetry. Since the painting is on the east coast of the U.S. and I'm in California, it was important to have a good understanding of these parameters before shipping equipment across country and for arranging studio space in which to do the work. The spreadsheet was also helpful for selecting the macro lens for the project. The storage requirements are based on a RAW image file size of 20 Mb (a slight overestimate for my 16 Mp camera) and don't take into consideration the processed file sizes. For example, generating .dng files with embedded RAW images approximately doubles the RAW file size, and exporting .jpg images adds approximately 50 percent to the storage. The final processed .ptm and .rti files range from approximately 250 Mb to 350 Mb per RTI, so accordingly, additional storage will be needed to process the files. The spreadsheet only estimates the storage needed for RAW image acquisition. Another variable is the amount of overlap for the images. For general imaging and RTIs at a given resolution, the spreadsheet uses 10 percent overlap, and for photogrammetry, it uses 66 percent overlap for the camera oriented horizontally. The spreadsheet calculates the distance to shift the camera in horizontal and vertical directions to get complete coverage of the object. It assumes three images per position for photogrammetry (horizontal and two vertical orientations) and 36 images per position for RTIs. These parameters can be adjusted for particular project needs. The input parameters are entered into the spreadsheet using metric units. A companion worksheet mirrors the format of the metric spreadsheet and automatically converts all the distances from metric to English units, for convenience. An example of the spreadsheet is attached, showing the calculations for this project. [see update below.]
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