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Biological tissue exhibits autoluminescence, i.e. a spontaneous ultra-weak photon emission (UPE) without photoexcitation. In this study, we report UPE measurements on human hands using an electron-multiplying CCD (EMCCD) camera. The images from left and right hands (2 subjects, 10 images in total) were taken on different dates. We observed the following spatio-temporal aspects of the hand UPE images: (i) all images show clearly that human hands exhibit autoluminescence; (ii) there was large variability in the UPE intensity with respect to the factors: time (time of day, date), anatomical location on the hand, and individual subject; (iii) images taken on the same date (but at different times of day) showed a stronger similarity than images of the same subject taken on different dates; (iv) the mean UPE intensity (averaged over 2 regions of interest at the right and left dorsum of the hands) correlated with the time of measurement (lower intensity at around noon). Our study documents the feasibility of UPE imaging of hands with an EMCCD camera and shows that the spatio-temporal variability of the UPE pattern harbours information that seems to be correlated to internal and external factors. Introduction Whereas autofluorescence of cells and biological tissue is fluorescent light emission due to stimulated light reactions with endogenous fluorophores [1], tissue exhibits also autoluminescence, i.e. spontaneous ultra-weak photon emission (UPE) in the ultra-violet to visible/near-infrared spectral range (approx. 200–1300 nm) without photoexcitation [2] [3] [4]. The photon emission associated with autoluminescence is magnitudes lower compared to the case of autofluorescence, challenging the technical detection of this phenomenon. The UPE intensity has generally only an intensity in the order of 101–104 photons/s cm2 [5]. The source of the UPE due to autoluminescence is mainly biochemical, i.e. the deexcitation of energetically excited species like atoms and/or molecules to a deeper energy level generating photons [3] [6] [7]. The excited states are generally caused by oxidative reactions involving radical or non-radical reactive oxygen (ROS) and nitrogen (RNS) species that react with several biomolecules like lipids, proteins and nucleic acids. Also, the self-recombination of organic radicals is relevant [3] [6]. For example, tripleexcited carbonyls are responsible for the emission in the wavelength range of 350–550 nm, singlet excited melanin with 360–560 nm, dimolar singlet oxygen with 634 nm and 703 nm, and monomolar singlet oxygen with 1270 nm [3]. Autoluminescence has been detected from several living biological organisms like mice [8], rats [9], humans [10], cell cultures [11] and plants [12]. Due to its technical challenges, spatially-resolved (two-dimensional, 2D) UPE measurements on human body parts are currently done only by a few research groups worldwide. Initial work on humanUPE imaging has been done since the 1990s by the Japanese research group of H. Inaba, demonstrating with a 2D photomultiplier setup that human hands show an inhomogeneous spontaneous UPE pattern with the highest intensity at the fingers and the lowest at the palm region [13] [14]. The group also found that subSpatio-temporal dynamics of spontaneous ultra-weak photon emission (autoluminescence) from human hands measured with an EMCCD camera: Dependence on time of day, date and individual subject DOI: 10.19185/matters.2018031 Matters (ISSN: 2297-8240) | 2 jects with hypothyroidism had lower UPE values of their hands compared to a healthy control group [13] [14]. It was also observed that the UPE intensity at the right index finger in humans is generally higher than at the left one [9]. To obtain the UPE images, an exposure duration of 1 h has been used, illustrating clearly the extreme low intensity of the UPE detected. In the late 1990s, it was demonstrated for the first time that UPE images can be obtained from plants and animal tissue using a charge-coupled device (CCD) camera [9] [15]. Subsequent UPE imaging of human body parts with cryogenically cooled CCD cameras has been performed since the 2000s. Measurements on multiple body parts showed that a high UPE emission is detectable at the face and neck and that the hands exhibit a higher UPE intensity compared to the rest of the arm [16]. The next step in improving the technical setups to image human UPE was the usage of electron-multiplying CCD (EMCCD) cameras; first results were published in 2013 by the research group of R. van Wijk, E.P.A. van Wijk and J. van der Greef [10]. The UPE images of the hands showed a “high variability in emission intensity and pattern”. This study is a follow-up report on these measurements. Only a few reports of UPE EMCCD imaging have been published so far. A new development in UPE imaging is the realization of whole-body UPE counting and imaging of humans [17] [18] as well as the correlation of UPE images with metabolomics [19] [8] [20]. Objective Our objective was to document and analyze a series of UPE images of hands obtained in 2011 by EMCCD imaging.

matters

Spatio-temporal dynamics of spontaneous ultra-weak photon emission (autoluminescence) from human hands measured with an EMCCD camera: Dependence on time of day, date and individual subject