Calibrated Uv Reflectance Photography of Hebomoia Glaucippe Sulphurea
Total Page：16
File Type：pdf, Size：1020Kb
CALIBRATED UV REFLECTANCE PHOTOGRAPHY OF HEBOMOIA GLAUCIPPE SULPHUREA EVELYN AYRE1 AND GEORGE BEVAN2 1Conservator, Ottawa, Ontario, Canada [email protected] 2Department of Classics, 49 Bader Lane, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Downloaded from http://meridian.allenpress.com/collection-forum/article-pdf/30/1-2/34/1505769/0831-4985-30_1_34.pdf by guest on 27 September 2021 [email protected] Abstract.—Ultraviolet (UV) reflective and absorbent markings on wings of male Hebomoia glaucippe sulphurea butterflies are important visual markers used in mating to differentiate them from other species. The objective of our study was to determine whether these markings deteriorate in museum collections over time. We first characterized quantitatively the UV reflective and UV absorbent wing markings from fresh and naturally aged male H. glaucippe sulphurea using UV reflectance microphotography, which was calibrated with handmade reflectance standards. The results of calibrated UV reflectance photography were then compared qualitatively with the same markings using visible light photography, transmitted and reflected visible light microscopy, and scanning electron microscopy (SEM). A UV-converted Nikon D200 with a Baader Ultraviolet Venus lens filter was used to record UV reflective and UV absorbent wing markings of the specimens. The handmade reflectance standards were prepared using magnesium oxide, plaster, and carbon, photographed alongside the specimens, and used to calibrate the photographs. The easily and affordably produced handmade reflectance standards were effective in calibrating the UV reflectance digital photographs, which allowed for each pixel of the digital photographs to be used for optical densitometry measurements. Quantitative data from calibrated UV reflectance photography demonstrated little evidence of deterioration in the UV reflective markings, although there was clear deterioration in the UV absorbent markings. This quantitative data, along with the calibrated UV photographs themselves, offered complementary documentation to visible light microscopy and SEM images. Results show that both visible-spectrum and UV markings fade in naturally aging museum specimens. We conclude that by using calibrated UV reflectance photography, a relatively inexpensive technique, a baseline and eventual degradation of Lepidoptera wing markings may be quantified and may provide valuable data to clarify the mechanisms behind this degradation. With the rate of change quantified, and the mechanisms of fading understood, it is hoped that preventative measures can be taken in the future to remedy this loss of valuable data in collections. Key words.—Lepidoptera, markings, photography, standards, ultraviolet Associate Editor.—Christine Johnson INTRODUCTION Photo documentation allows for the preservation of unique visual information contained in delicate Lepidoptera specimens, which may otherwise change or disappear as a specimen stored or displayed in a natural history museum ages. Photographs can capture and preserve wing patterns, anatomical details, and a sense of surface texture that then serve as a complementary record of the specimen, an archive of the specimen, or a surrogate should the specimen be lost or degrade to the point at which the original appearance has been irretrievably altered. Many researchers, conservators, and collections managers recognize the need to photograph individual specimens in collections for posterity through conventional photography. With the advent of high-resolution UV-sensitive digital cameras, additional information can be obtained from the photographic record, such as UV markings on butterfly wings, some that are visible only in the UV range of the electromagnetic spectrum. These important characteristics can now be captured relatively inexpensively and can, consequently, be used to record the spatial distribution of UV wing markings across a much greater number of samples than was possible using UV-sensitive film photography. Collection Forum 2016; 30(1):34–50 E 2016 Society for the Preservation of Natural History Collections 2016 AYRE AND BEVAN—CALIBRATED UV REFLECTANCE PHOTOGRAPHY 35 Ultraviolet light is electromagnetic radiation with wavelengths between 100 and 400 nm (ISO 2007). Ultraviolet reflectance photography (UVR) is a photographic technique that captures ultraviolet wavelengths reflected from objects and produces a monochrome representation of the image that we can interpret visually. The human eye is unable to detect near-UV light waves due to ocular filters that remove wavelengths shorter than 400 nm (Dyer et al. 2004). Similarly, digital cameras have internal cutoff filters (ICFs) in front of the camera sensor that act in the same way as ocular filters in the eye. The ICFs remove light wavelengths shorter than 400 nm or longer than 700 nm to minimize Downloaded from http://meridian.allenpress.com/collection-forum/article-pdf/30/1-2/34/1505769/0831-4985-30_1_34.pdf by guest on 27 September 2021 improper light metering in the visible range. For the silicon sensor to record UV, or infrared (IR) wavelengths, the camera’s ICF must be removed, usually by a commercial service (Life Pixel 2012). Removal of the ICF still leaves the Bayer filter in place, and thus an array of red, green, and blue filters allows the sensor to record color information from the monochrome photosites. With the ICF removed and the camera sensitivity extended into the UV range, a lens made of quartz fluorite is used, which allows the transmission of UV wavelengths. This lens is fitted with an additional filter that removes visible and IR light to isolate UV wavelengths. Past UVR imaging was done with photographic film sensitive to both near-UV (320–400 nm) and visible light (Dyer et al. 2004). Filters placed in front of the lens removed infrared and visible light. Humans are unable to directly perceive UV light and may misinterpret photographs that capture wavelengths in the near-UV spectrum because the appearance of an object in the UV range does not necessarily correlate to the appearance of the same object in visible light. Markings observed under “normal” light may be perceived incorrectly because of poorly controlled contrast or artifacts of poor lighting. For this reason, calibrated UV reflectance gray-scale standards should be included in the photographs to ensure that the images are not falsely “enhanced” in post-processing (Dyer et al. 2004). UV reflectance (UVR) photography is distinct from, and must not be confused with, UV fluorescence (UVF) photography, also called ultraviolet-induced visible fluorescence. UVF photography records the fluorescence produced when electrons in an object are excited by a light source that emits UV wavelengths, which results in a visible light photon (fluorescence) being released. UV fluorescence is often used in conservation to provide qualitative visual information that can aid in identification of a variety of materials and was recently found to be effective in early detection of feathers fading due to light exposure (Pearlstein et al. 2015). By contrast, UV reflectance photography captures UV light reflected by an object through a filtering system that removes visible (including UV fluorescence photons) and infrared wavelengths and functionally allows the transmission of UV wavelengths only (Elen 2012; see Fig. 1) and the imaging of what is “visible” in the UV range. UVA wavelengths, 315–400 nm (ISO 2007), are important because many insects, as well as some birds and lizards, have ultraviolet visual receptors (Silberglied 1979) and use these perceived UVA wavelengths to detect markings invisible to humans (Kevan et al. 1973, Knuttel and Fieldler 2000). Many flowers have evolved “bulls-eye”-like markings in the UVA range to attract pollinator insects (Silberglied 1979). Lepidoptera can see in the UVA range and also have markings that reflect UVA light on their wings. These structures that reflect UVA are created by the laminar structures and pigments of wing scales. Many families of Lepidoptera, Papilionidae, Pieridae, and Nymphalidae (Bybee et al. 2011) and some moths and larvae (Silberglied 1979) produce wing markings or have other structures that are visible only in UV wavelengths. UVR photographs reveal the spatial distribution of these markings on Lepidoptera and therefore provide an important 36 COLLECTION FORUM Vol. 30(1) Downloaded from http://meridian.allenpress.com/collection-forum/article-pdf/30/1-2/34/1505769/0831-4985-30_1_34.pdf by guest on 27 September 2021 Figure 1. UV reflectance digital photography (adapted from Elen). complement to visible-light photography (Kevan et al. 2001). UVR documentation of butterflies has proven useful to researchers for several reasons. UVR photographs of butterfly wings, if assessed properly, can (1) serve as regular morphological characters in systematics if assessed properly (Knuttel and Fieldler 2000), (2) reveal differences in UV reflectance wing patterns in butterflies that are morphologically similar, (3) demonstrate intraspecific variability (Silberglied 1979), and (4) show variations in UVR wing patterns that have been correlated to diet differences in butterflies (Silberglied 1979). UVR markings play a role in interspecies and intraspecies communication, such as mate recognition and sexual selection (Robertson and Monteiro 2005). UVR in wing patterns has also been useful in butterfly taxonomy, and in detecting butterfly gynandromorphs, specimens that exhibit both male and female characteristics