The use of low-intensity LED lamps for luminescence dating laboratory safe lighting

IntroductionThe Institute of geophysics of Tehran University is establishing a luminescence sample preparation laboratory. The following investigations and experiments was performed to find the safe light for this lab. This paper is written in Farsi to provide answer to many questions that the first author has been asked locally about which light is suitable for both sample preparation lab and in the field during sampling in dark. To answer those questions we decided to publish our finding. Luminescence methods particularly optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) date the last time that quartz and feldspar has been exposed to light due to a manmade or natural event such as construction of bricks, building, dam, wall, earthquake, volcanic eruption, flood, tsunami and wind. Therefore, samples should not be exposed to natural light during sampling, transport and sample preparation in the lab. Otherwise, natural light bleach the natural luminescence signal and cause luminescence dating method to underestimate the real age of the event. Therefore, suitable light has a key role in establishing luminescence lab. In reviewing the excitation spectrometry of stimulated luminescence from quartz and feldspars and the various recommendations about laboratory lighting over the last three decades by different authors including Spooner and Prescott (1986), Smith (1988), Ditlefsen (1991), Galloway and Napier (1991), Bøtter-Jensen et al (1994), Lamothe, M. (1995), Spooner et al (2000), Huntley and Baril (2002), Mauz et al (2002), Berger, G.W., Kratt ,C (2008) it became clear that by changing understanding of the luminescence responses of quartz and feldspars, as well as with changes in the availability of affordable lighting technology, different light has been employed in different laboratories. However, the reddest visually comfortable illumination is preferred for preparing quartz, and wavelengths in the region ca. 530-630 nm has been preferable for feldspar. But usually luminescence labs has chosen either red or yellow orange light for preparing both quartz and feldspar. Therefore, we decided to find the suitable light available in Iran and analyses their suitability for our purpose.
Materials and methods The choice of laboratory illumination for use with optical dating samples involves a compromise between minimizing the unwanted bleaching of light-sensitive signals while providing sufficient visibility to allow efficient and safe workplace practice. We found that the light with wavelength between 575 to 625 nanometer satisfy both conditions. Since 1998 the first author has used Oxford luminescence lab and laboratory illumination has changed in different periods. Here, we summarize our own adoption of such lighting by presenting some spectra from representative LED lighting available in Iran and compare it with the spectra of LED lights that is used in Oxford. Due to the compatibility of both light, the effect of light on sample signal loess was measured in Oxford. We made 2 lamps each consist of 5 orange LED (612 nm) and 5 red LED (633 nm) from KB technology and investigated their spectrum and intensity to recognize if they can be used to produce safelight. Unfortunately, their spectra did not correspond with company specification.
Results and discussionThere was a 4% green tale in orange LED lamp spectrum. We also measured the spectra of a portable orange light which is made and used in Oxford luminescence laboratory. There was a 5.3% green tale in oxford light spectrum. We tested the effect of a 24 hours exposure to the orange light on a 90-150 µm feldspar sample (Sk1A collected from Iran) in Oxford. Three aliquots were exposed 2 meters below the orange LED lights with minimum intensity (the intensity at 2 meter distance was 0.1 Lx). The exposure reduced the signal 1±1% which was safe for sample preparation. However, when the aliquot were exposed 1 meter below the orange light with maximum intensity (the intensity at 1 meter distance was 9.3 Lx) exposure reduced the signal 30±3%. The red LED (633 nm) lamp produced no green tale and is safe for sample preparation but does not provide safe workplace practice.
ConclusionWe deduce that the exposures of quartz and feldspar either to orange LED lamp with minimum intensity or red LED lamp for up to a few hours would have no significant effect on luminescence signal accumulated in the grains. However, to use maximum intensity of orange LED lamp an optical orange filter is required and spectral and sample signal lost experiments should be performed.