Review of Scientific Instruments
Feedback | Help | Exit
Review of Scientific Instruments
   
 
 
 
Next Article
Design and development of a portable gamma radiation monitor
A portable gamma radiation monitor has been designed and developed. The monitor can be used effectively in the dose range from 0.07 to 500 mGy/h due to gamma rays of energy greater than 65 keV. The mo...

Invited Article: High resolution digital camera for infrared reflectography

Rev. Sci. Instrum. 80, 071301 (2009); doi:10.1063/1.3174431

Published 15 July 2009

You are logged in to this journal.

Charles M. Falco
College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, USA
This paper describes the characteristics of a high resolution infrared (IR) imaging system operating over the wavelength range of 830–1100 nm, based on a modified 8 Mpixels commercial digital camera, with which nonspecialists can obtain IR reflectograms of works of art in situ in a museum environment. The relevant imaging properties of sensitivity, resolution, noise, and contrast are characterized and the capabilities of this system are illustrated with an example that has revealed important new information about the working practices of a 16th century artist. ©2009 American Institute of Physics
History: Received 9 April 2009; accepted 14 June 2009; published 15 July 2009
Permalink: http://link.aip.org/link/?RSINAK/80/071301/1
FULL TEXT OPTIONS   (FREE)
Download PDF (1087 kB) View Cart

KEYWORDS and PACS

Keywords
PACS
  • 42.79.Pw
    Imaging detectors and sensors
  • 42.30.Va
    Image forming and processing
  • 07.57.Kp
    Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
  • YEAR: 2009

RELATED DATABASES

Web of Science

View this record in Web of Science

PUBLICATION DATA

ISSN:
0034-6748 (print)   1089-7623 (online)
Publisher:
AIP is a member of CrossRef AIP

REFERENCES (41)
View in Separate Window/Tab
  1. The term “reflectography” was introduced in the 1960s to distinguish between the process of capturing images of paintings using raster-scanned vidicon tubes with sensitivity to wavelengths longer than 900 nm, and capturing “photographs” on film emulsions with sensitivity limited to wavelengths shorter than 900 nm. However, since it is the content of the final images that is of interest, advances in technology have made this distinction artificial and increasingly anachronous.
  2. J. R. J. van Asperen de Boer, Appl. Opt. 7, 1711 (1968).
  3. Art in the Making: Underdrawings in Renaissance Paintings, edited by D. Bomford (National Gallery, London, 2002).
  4. M. Faries, in Scientific Examination of Art: Modern Techniques in Conservation and Analysis, edited by B. Berrie, E. Rene de la Rie, R. Hoffman, J. Tomlinson, T. Wiesel, and J. Winter (National Academy of Sciences - National Research Council, Washington, DC, 2002), pp. 87–104.
  5. G. Chiari and C. Scientist, personal communication (2008).
  6. Citing declining demand, in 2007 Kodak discontinued production of their High Speed Infrared HIE film, which had sensitivity to wavelengths up to 900 nm. At the time of this writing the only IR film available with sensitivity beyond 800 nm is Efke IR820.
  7. Kodak Infrared Films, N-17 (Eastman Kodak, 1976).
  8. M. Gargano, N. Ludwig, and G. Poldi, Infrared Phys. Technol. 49, 249 (2007).
  9. Osiris, Opus Instruments, Ltd., 50 High Street, Bassingbourn, Royston, Herts SG8 5IE, UK.
  10. At the time of this writing a Canon 30D camera body retails for less than $1000, a Canon 35 mm f/2 lens for under $250, and the cost of conversion $450 or less, for a total system price of under $2000.
  11. S. Youn, Y. Kim, J. Lee, and D. Har, Proceedings of the IASTED International Conference on Internet and Multimedia System and Applications to Visual Communications (IASTED, Calgary, 2008), p. 128.
  12. Most lenses suitable for this work lack distance markings between 3 m and [infinity], requiring the operator to estimate where to position the lens for a desired focus distance, and rely on depth of field to compensate for the inevitable inaccuracy of the setting.
  13. Canon, Inc., 30-2, Shimomaruko 3-chome, Ohta-ku, Tokyo 146-8501, Japan.
  14. A more recent model, the 40D, uses the same CMOS technology, but with 11% greater linear resolution. However, since operation in the IR depends on light being transmitted by the dyes used for the blue, green, and red filters, similar tests to those described in this paper would have to be conducted with this camera to be certain it actually would function for this purpose.
  15. Coastal Optical Systems makes a 60 mm focal length, f/4 lens that is apochromatic over the wavelength range of 315–1100 nm. However, aside from its $4500 cost, the relatively long focal length requires positioning the camera a distance from objects that can be prohibitively far in some museum environments.
  16. My camera was modified by Life Pixel, http://www.lifepixel.com. The modification consisted of replacing the manufacturer's low pass filter on the sensor with an 830 nm high-pass filter (Schott RG 830 equivalent), and offsetting the electronic autofocus system to compensate for the different focal plane of the IR.
  17. Because of the wide range of IR wavelengths captured, it is impossible even with this offset to produce a perfectly sharp image using a lens designed for high performance in the visible. However, determining the degree of image degradation due to this factor is outside the scope of this paper. Absent the availability of a lens designed specifically to bring this range of IR wavelengths to a common focus, the image degradation due to this factor becomes intrinsic to the measured overall performance of this imaging system.
  18. At the closest focus distance of 0.25 m for this lens, and at the maximum aperture of f/2, the measured depth of field extends approximately 4 mm on either side of the point of best focus.
  19. It is important to note that the amount of electronic focus offset is specific to a given lens. It happens that the same fixed offset works for both the Canon 35 mm f/2 and the Canon 50 mm f/1.8. However, at closest distance, the focus error for a Canon 100 mm f/2.8 macro is 2 mm, resulting in visible degradation of the IR image at maximum aperture for this lens.
  20. The poster for this test was a multicolor print from a museum shop, made using ink on paper.
  21. Although the sensors in these cameras are of different size and pixel density, these factors are not important here since we are only concerned with determining the total exposure time when they are set at the same sensitivity (ISO).
  22. H. Angus Macleod, Thin Film Optical Filters, 3rd ed. (Taylor & Francis, London, 2001).
  23. The low-spatial-frequency target consisted of 2 cm black bars spaced by 2 cm, consisting of photocopier toner fused on white photocopier paper, in turn placed on matte aluminum foil, and illuminated by direct sunlight. Measured with a Konica Minolta LS-110 (1/3)-degree luminance meter, within experimental uncertainty the brightness range of this target in the visible was exactly 9 stops for one set of experiments described in the text and 7(1/3) for the other.
  24. The lens used for this test consists of three elements in three groups and has a focal length and aperture I measured to be 120 mm f/2.4. Although it has no markings other than the manufacturer's name, James W. Queen & Company, Philadelphia, its age and appearance makes me believe it was intended for a 19th century magic lantern projector.
  25. Pigments vary in reflectivity from ~4% for a deep black to ~90% for a bright white. This range of less than 5f-stops for all paintings is far less than that of a scene containing an actual light source and shadows, where the brightness range can exceed 15f-stops.
  26. The International Standards Association (ISO) has standards for specifying the sensitivity of photographic film to light, based on the measured density of an exposed negative above the density of the unexposed film. This numerical system was carried over to digital cameras in analogy to the behavior of film. For film, small grains of silver are less sensitive to light, but result in higher quality images, whereas for digital sensors lower electronic gain results in less noise in the image. “ISO 100” is the lowest gain, highest quality setting for the Canon 30D.
  27. Canon places an antialiasing filter is above the sensor to reduce Moire fringes from subjects with fine detail. This filter degrades the resolution to slightly worse than the 1 pixel that otherwise would be possible.
  28. A common 40 W light bulb with a blackbody temperature of 2800 K has peak intensity at approximately 1000 nm, and a 100 W, 3200 K tungsten-halogen bulb at approximately 850 nm.
  29. When “custom function” setting 02 on this camera is enabled, for exposures of 1 s or longer a second exposure from the sensor is automatically acquired with the shutter closed. The camera's processor then does a pixel-by-pixel subtraction of this “dark noise” from the image, thereby reducing the noise from this source. Although this process doubles the time required to acquire an image, even if for some reason a 30 s exposure were required, it would not represent a significant restriction for this application to IR reflectography.
  30. Edmund Optics 1000 nm long pass filter, part number NT32-770, with threaded filter holder NT59-445. The 52 mm thread on the holder attaches directly to the Canon 35 mm, f/2 lens and is large enough not to cause vignetting of the image.
  31. If the camera's autofocus system has focused on a painting at a measured distance of 2.00 m from the plane of the sensor, the lens must be manually offset to 1.60±0.05  m to bring the object into focus with the 1000 nm filter. This is accomplished by rotating the focusing collar by 5° in the direction of closer distances (i.e., to 1.6 m), which can be repeatedly done to an accuracy of approximately 1° using the serrations on the focusing collar (the lens used for this study has 144 serrations on its circumference or 2.5°/serration). Although the measured distances for correct focus in the IR differ somewhat from the scale on the lens, it happens that this 5° offset is appropriate for the 1000 nm filter everywhere within the distance range of 1–3 m as indicated by the scale. For all indicated distances less than 1 m, an offset of 7.5° brings the image into focus within the depth of field of the lens at f/4.
  32. “The Man of Sorrows with Saints and Donors,” unknown French artist, c1525 (oil on oak panel, 47×55  cm2). The Samuel H. Kress Collection at the University of Arizona Museum of Art.
  33. With the focus offset as described in Ref. 31, the measured resolution with the 1000 nm filter was (3/4)  mm at ISO 100 for the same medium contrast target located 2 m from the image plane as was used for the other tests in this paper.
  34. PAL has 625 vertical lines and NTSC has 525. If a 1 m diagonal television set is viewed from a distance of 3 m, these correspond to angular resolutions of 0.018° and 0.022°, respectively. For comparison, when held at a viewing distance of 30 cm the pixel spacing of the LCD screen of the Canon 30D corresponds to an angular resolution of 0.016°.
  35. It is not necessary to use the same model camera and lens for this purpose, or to take the visible and IR images from the identical location, but it does make the later process of accurately overlapping the images in an editing program somewhat easier.
  36. D. Hockney, Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters (Viking, New York, 2001).
  37. See, for example, D. Hockney and C. M. Falco, in Human Vision and Electronic Imaging X, edited by B. E. Rogowitz, Proceedings of the IS&T-SPIE Electronic Imaging (SPIE, Bellingham, WA, 2005), p. 326, and references therein.
  38. This 96×116  cm2 oil on canvas painting, also referred to as “Husband and Wife” in a number of sources is in the collection of the State Hermitage Museum in St. Petersburg, Russia.
  39. D. Hockney and C. M. Falco (unpublished).
  40. After focusing in the visible, the microscope stage is lowered by an appropriate amount to bring the IR into focus on the focal plane of the camera. The necessary offset distance need only be determined once for each objective on a given microscope by making a series of exposures at different distances.
  41. By using an ISO of 1600 I was able to capture IR reflectograms of approximately 50 paintings in Tokyo's National Museum of Western Art in under an hour without use of a tripod. Although the noise due to the high ISO reduced the resolution to ~1  mm, the longest exposure was 1/30 s, resulting in minimal additional loss in resolution due to camera motion.






Copyright © American Institute of Physics