Prevention of Overexposure by Means of Active Protective Reactions and Magnitude of Temporary Blinding from Visible Laser Radiation H.-D. Reidenbach1 1
Cologne University of Applied Sciences, Institute of Applied Optics and Electronics, Research Laboratory on Medical Technology and Non-Ionizing Radiation, Betzdorfer Str 2, D-50679 Koeln, Germany
Abstract— It has been shown in a field trial with 205 subjects who got an instruction, that active protective reactions could protect up to 80 % of the exposed volunteers against laser radiation during a period of 1.4 seconds. This is a considerable improvement compared with the unreliable blink reflex, which works only in every 5th case. Therefore adequate instruction to perform active protective reactions, i. e. moving the head or closing the eyes, might be a valuable contribution to prevent any potentially hazardous laser radiation and increases the safety against laser radiation arising from wrong labeled class 2 laser products and true class 3R lasers emitting in the visible spectrum. Temporary blinding as the result of a dazzling light in the visual field arising from class-1 and class-2 lasers at wavelength of 632.8 nm and 532 nm has been investigated. It was found that already at output powers below 30 PW of a He-Ne-laser the subjects reported intense glare effects and felt uncomfortable due to the high brightness. The durations of afterimages took up to 300 s and have been measured as a function of the angle between the line of sight and the laser beam direction for exposure durations up to 10 s. A dose relationship has been found which determines the afterimage duration. The inability to read due to the disturbance produced by afterimages lasts for about 20 s even if the exposure is not more than 0.25 s from a laser with about 0.8 mW. Keywords— Active protective reactions; laser; class 2; afterimage; temporary blinding.
16.7 % of all cases if an irradiation is performed with a class-2 laser product. In addition it has been shown with 829 volunteers that aversion responses in terms of head movements and eye closure occur only in 5.9 % [2]. Therefore the possibility of an overexposure exists due to the lack of natural aversion responses. For workers it is claimed for example in the European Directive 2006/25/EC that they shall not be exposed above the exposure limit values [3]. While minimum health and safety requirements regarding the exposure of workers to risks arising from laser radiation can be stated in such a directive, the general public might be adequately protected if products with potentially hazardous optical emission are either not allowed on the market or sufficient safety instructions are given in the accompanying user information. Due to the fact that not only deterministic effects like thermal damage to the retina are important in order to work in a safe manner with bright light sources like laser emitting in the visible spectrum, indirect effects arising from temporary blinding should be regarded in a risk assessment. Since indirect effects have not been included in most safety analysis up to now and there was not enough information available, investigations have been performed in order to improve the current knowledge. II. METHODS AND MATERIAL
I. INTRODUCTION The most effective approach toward laser safety was the introduction of an appropriate laser classification system according to the degree of potential optical radiation hazard. The accessible emission limit (AEL) for class 2 is derived by multiplying the maximum permissible exposure (MPE) for 0.25 s with the area of a 7-mm aperture, which is equivalent to the pupil diameter of the dark adapted eye [1]. In the case of low power lasers the safety philosophy is based on natural aversion responses including the blink reflex since many years. On the other hand it was found in a total of 2,020 volunteers that the blink reflex occurred in not more than
A. Active protective reactions Laser belonging to class 2 according to IEC 60825-1 [4] are based on the safety philosophy that aversion responses prevent that the maximum permissible exposure values are exceeded. Although it has been convincingly shown that these natural behavior does not really exist in any case [5], the proposal to reduce the currently allowed output power from 1 mW to 0.6 mW or 0.7 mW, which would be equivalent to an upper limit in compliance with the guidelines of the International Commission on Non-Ionising Radiation Protection (ICNIRP) [6, 7] for exposure durations of 2 s or 1 s, respectively, was not seriously agreed.
O. Dössel and W.C. Schlegel (Eds.): WC 2009, IFMBE Proceedings 25/III, pp. 41–44, 2009. www.springerlink.com
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Due to the fact that a great many of class-2 laser are used without problems and that there are but a few reports concerning minor eye damage the standardization commissions did not change the previous classification. In order to find another solution a comparative study has been performed where part of the subjects became informed of the intended test procedure and a control group was uninformed. In addition the informed subjects have been instructed and asked to perform active protective reactions in the case of an unexpected laser beam in order to estimate the reaction times and frequencies of the various physiological behavioral patterns. Simultaneously the subjects were asked to carry out a special visual task on a monitor which was part of an especially developed eye-tracking system [8]. In detail, the subjects had to “catch” the reflection from their own eye and to “guide” this to a stationary cross on a monitor screen and at the time of coincidence of both spots a laser beam was released for a predetermined time duration, namely 0.25 s or 1 s. The respective laser output power was chosen to be 80 % of the allowed limit value in order to comply with feasible measurement uncertainties. A total of 205 subjects got an instruction, whereas 316 have been uninformed. The investigations have been done in field trials at two different locations.
exposure the subject turned his/her head to an especially developed reading test on a monitor for the determination of temporary blinding from laser irradiation. The respective time needed to gain the previous acuity – determined as reading capability – was registered. In this context it is important to assert that an afterimage was still present but no longer obstructed the test words. A total of 19 persons have been tested in this manner. III. RESULTS A. Active protective reactions In contrast to the results obtained with 316 uninformed subjects, where only about 7 % showed a blink reflex, we have found in field trials with 205 instructed persons, that active protective reactions, i. e. immediately closing the eyes and/or moving the head, are able to protect up to 34.4 % within 240 ms, up to 74.4 % within a second [9] and 80 % after 1.4 s against laser radiation. Active lid closure started already at 120 ms and showed a saturation after about 400 ms (Fig. 1, black triangle). This shows that a lid closure due to an instruction can be nearly as quick as a blink reflex, but happens more often.
B. Temporary blinding The fact that dazzle, flash-blindness and afterimages may be caused by bright visible optical radiation is well-known, but functional relationships are not yet available up to now. For this purpose a helium-neon laser (632.8 nm), and a frequency-doubled Nd:Yttrium Vanadate laser (532 nm) have been applied as a dazzling light source. Two set-ups were used in order to perform the investigations. In one case a helium-neon laser was mounted on a movable assembly where the respective beam position and direction could be adjusted on a semicircle between -40 degrees nasally and +60 degrees temporally. In detail the measurement of the afterimage duration has been done for exposure durations of 1 s, 5 s, and 10 s at power settings of 5 PW, 10 PW, 20 PW, and 30 PW. The measurement stop criterion was taken as the point in time when the afterimage disappeared and could not be retrieved not even by squinting. The time consuming trials have been done with a total of 10 volunteers in the laboratory. In a second set-up an alignment situation was designed where the subject was placed in a chin rest and fixated a point through which a laser beam from either a He-Ne(632.8 nm) or solid-state laser (532 nm) could be released during a preset exposure duration. This arrangement assured that the fovea was being hit definitely. Immediately after the
Fig. 1. Head movements, eye and eyelid movements (exposure duration: 250 ms; laser power: 0,8 mW, 125 subjects). Determination of active protective reactions was extended up to 1.4 s.
The results achieved in one part of this study on active protective reactions taken with 125 test persons have shown that squinting, which is regarded as a blink reflex below threshold, is the quickest reaction followed by the blink reflex (Fig. 1, open triangles), but limited to about 23 %. The exposure duration was 250 ms and the laser power 0.8 mW. Eye blinks and the blink reflex have been added in Fig. 1 since in both cases a reopening of the upper lid takes place within about 300 ms whereas in a voluntary lid closure the eyes stay closed over a much longer time duration due to the deliberate action. This clearly favors active lid closure as a protective means.
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Prevention of Overexposure by Means of Active Protective Reactions and Magnitude of Temporary Blinding
Small eye movements occur only during the very early time of the stimulation (Fig. 1, black diamonds) whereas distinct eye movements are elicited much later. Figure 2 shows a comparison between normal aversion responses including the blink reflex and active protective reactions as a result of different irradiations (0.8 mW and 250 ms or 0.55 mW and 1 s). Natural physiological protection, i. e. aversion responses including the blink reflex, has been found with a frequency of about 12 % within 250 ms and increases to about 26 % after 1.4 s. On the contrary active protective reactions increase relatively rapid to about 35 % within 250 ms and slowly reach saturation. A maximum protection of about 80 % was achieved (Fig. 2).
Fig. 2. Comparison between normal aversion responses including the blink reflex and active protective reactions for different exposure situations (0.8 mW, 250 ms or 0.55 mW, 1 s)
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Fig. 3. Afterimage duration as a function of the applied optical energy (irradiation 5° temporal to the fovea, diamonds: measurement mean values)
In the tests with subjects performing a procedure like an alignment it has been found that a green laser beam produces longer durations of disturbance compared with a red one as far as the reading capability is concerned. The delay times measured have been between 13 s and 19 s for 532 nm and between 4s and 14 s, with one exception at 22 s, for 632.8 nm, if the exposure duration was 0.25 s and the power limited to a maximum of 0.783 mW. The respective numbers were between 22 s and 35 s for a green laser beam (cf. Fig. 4) and between 2.5 s and 19 s for the red laser beam if the exposure duration was 0.5 s and the power cap at 0.66 mW.
The comparison of both situations clearly shows the improvement achieved with active protective reactions. B. Temporary blinding The investigations with a laser beam which irradiated the retina at various locations have shown a strong dependence on the angle between the line of sight and the beam direction. Afterimage durations up to 300 seconds were found if the fovea of the human retina is irradiated from a class-1 laser beam, whereas much lower values are valid in the parafoveal region and in the periphery [10]. In addition a dose-relationship has been found between the duration of an afterimage and the applied optical energy in the investigated time interval between 0.5 s and 10 s. Because of the psychological glare as a result of the laser irradiation more detailed investigations have been made only with an exposure located 5° temporal to the fovea. The results are shown in Fig. 3. Since the afterimage lasts about a factor of 2 longer in the fovea the maximum afterimage duration can be derived from the applied optical energy Ptexp according to the following relationship (eq. 1):
t afterimage, fv s
| 50.6 ln (
P t exp µJ
) 13.4
(1)
Fig. 4. Duration of disturbance as a function of the applied optical energy (laser wavelength: 532 nm, exposure duration: 0.5 s, 4 subjects)
A comparison of figs. 3 and 4 shows that about 10 % of the total afterimage duration can be expected as disturbance time in order to perform visual tasks unrestricted after an irradiation with a laser beam in the above given power and exposure duration range. IV. DISCUSSIONN Since the eyes are not in danger as far as accidental and short-term exposure is regarded, laser products of class 2 may be applied without any additional protective measures if it is ensured that neither a deliberate intrabeam viewing of
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H.-D. Reidenbach
more than 0,25 s nor a repeated intrabeam viewing into a specular reflected laser beam could happen. There was a strong belief in aversion responses and especially in the blink reflex as a reliable physiological reaction if a bright light is viewed. A laser source certainly represents a very bright light. Compliance with the exposure limit values will ensure that workers exposed to laser radiation are protected against all known adverse health effects, since the limits on exposure to optical radiation are based directly on established health effects and biological considerations. There are currently very few, if any, reported eye injuries due to lasers especially belonging to class 2 and it has not been convincingly shown that a true class 2 laser is capable of producing serious eye damage at all. As far as the MPE values are concerned an exposure duration of 0.5 s instead of 0.25 s is equivalent to exceeding the MPE by about 22 % and a duration of 1 s means that the MPE is exceeded by 44 %. But on the other if the output power is exceeded by a factor of 2, e. g. 2 mW instead of 1 mW, than an intrabeam viewing of only 0.5 s or 1.0 s would correspond to a 32-fold or 64-fold overexposure, respectively. This should be prevented if possible. Normally people can adapt to changing luminous conditions and perform well regardless of the illumination level, but glare might impair visual functions strongly and with a lasting effect, due to the fact that the adaptation system is overridden and an afterimage is formed. The situation where the pupil of a stabilized head was aligned to the beam satisfies worst case accommodation, but nonetheless disturbance of visual functions can result in distracting and even dangerous situations. Its value might be depicted from Fig. 3, where two not unusual situations of a class-2 and class-1 laser illustrate the interference from temporary blinding even after relatively short irradiations. V. CONCLUSIONS Our findings do not state that class 2 laser are no longer safe, but that users of such lasers should be instructed to perform active protective reactions, i. e. close the eyes actively and avert the head in the case of intrabeam viewing as soon as possible. To exceed the MPE is not necessarily dangerous, but active protective reactions can prevent a violation of the MPE values and thus fulfill regulatory issues and simultaneously can be regarded as “prudent precaution”, describing a situation, in which laser safety could be increased without restricting the normal use disproportionately. In addition the knowledge of some quantitative relationships between laser exposure with a certain value
and the expected disturbance of visual functions should improve the handling of low power laser products.
ACKNOWLEDGMENT The research has been funded by the Federal Institute for Occupational Safety and Health (BAuA) in Germany.
REFERENCES 1.
Reidenbach H-D (2007) Laser Safety. In: Handbook of Lasers and Optics (ed. Träger, F). Springer. New York, chap. 21 2. Reidenbach H-D, Dollinger K, Hofmann J (2005) Results from two research projects concerning aversion responses including the blink reflex. SPIE Proc. B5688, Ophthalmic Technologies XV, Manns F, Söderberg P G, Ho A, Stuck B E, Belkin M (eds.), pp 429-439 3. Directive 2006/25/EC of the European Parliament and the European Council of 5 April 2006 on the minimum health and safety requirements regarding the exposure of workers to risks arising from physical agents (artificial optical radiation) (19th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC), OJ L 114, 27.4.2006, p.38 4. International Electrotechnical Commission, IEC 60825-1: 2007-03 (2nd edition), Safety of laser products - Part 1: Equipment classification and requirements (former edition: IEC 60825-1: 1993 + A1:1997 + A2:2001, Safety of laser products – Part 1: Equipment classification, requirements and user’s guide.) 5. Reidenbach H-D (2006) Results of Investigations on the Blink Reflex as a Protective Means against Laser and LED Radiation: A Description Based on Fundamental Psychophysical Laws. Proceedings Second European IRPA Congress on Radiation Protection, (15– 19 May 2006, Paris, IRPA 2006), TA-68, pp 1-12 6. ICNIRP (1996) Guidelines on Limits of Exposure to Laser Radiation of Wavelengths between 180 nm and 1000 µm. Health Physics 71:804 – 819 7. ICNIRP (2000) Revision of guidelines on limits for laser radiation of wavelengths between 400 nm and 1,400 nm. Health Physics 79:431 – 440 8. Reidenbach H-D (2005) Aversion responses including the Blink reflex: Psychophysical behaviour and active protection reactions as an additional safety concept for the application of low power lasers in the visible spectrum. ILSC 2005, Conf. Proc., 67 – 76 9. Reidenbach H-D, Hofmann J, Dollinger K (2006) Active Physiological Protective Reactions should be used as a Prudent Precaution Safety Means in the Application of Low-Power Laser Radiation. Proceedings World Congress on Medical Physics and Biomedical Engineering 2006, (August 27 – September 1, COEX Seoul, Korea; IFMBE). Vol. 14, pp 2569-2572 10. Reidenbach H-D (2007) Local Susceptibility of the Retina, Formation and Duration of Afterimages in the Case of Class 1 Laser Products and Disability Glare Arising from HB-LEDs. ILSC 2007, Conference Proceedings pp 102 – 111 Address of corresponding author: Author: Hans-Dieter Reidenbach, Prof. Dr. Institute: Cologne University of Applied Sciences, Res Dept Medical Technology and Non-Ionizing Radiation Street: Betzdorfer Str 2 City: Koeln, D-50679 Country: Germany Email:
[email protected]
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