Standards for laser safety: origins, level, perspectives
Protection against open laser radiation is a part of a major problem which roots deeply to commercial nature of industrial use of laser technologies. Their implementation has led to emergence of new sources of threats to health of the person, unusual for classical methods of safety [1–7]. This article discusses a number of problems connected with conditions of safe operation of laser equipment and forming corresponding regulatory base.
There are some inconsistent normative documents in the territory of Russia and Union States . On top of everything, the edition of the updated Sanitary Standards and the Rules SanPiN 184.108.40.20659–16  planned for 2017 does not normalize situation whatsoever. The source of contradictions is introduction of 2009 of the Russian language version of the European standard of the International Electrotechnical Commission IEC 60825–1–2007 Safety of laser products – Part 1: Equipment classification. Requirements and user guide. It was positioned as GOST R IEC 60825–1–2009. The latest version of this standard with small improvement of translation quality is GOST IEC 60825–1–2013 (hereinafter referred to as GOST IEC). GOST IEC in many parameters is inadequate to "Sanitary standards and rules of the device and operation of lasers No. 5804–91" existing in the CIS during 1991–2016 (hereinafter referred to as SanPiN-91). The MPE (Maximal Permissible Exposure) values of radiation for eyes determined by the specified documents in some cases differ by some orders (!). There is a mass of questions of legal status of SanPiN and expediency of use of GOST IEC in the suggested form.
It is obvious that the detailed analysis of differences of the mentioned documents and the reasons which have caused these differences had to be preceded by the introduction of the new standard connected with health care. Such analysis with involvement of specialists of the corresponding profile was not carried out according to my data. Thus, the question of how we pay for unification of our standards with the Western European analogs has been ignored. The problem of comparison and the subsequent forecast of potential effects of the confusion brought by joint introduction of the specified documents is a rather volume task. Within the article it is possible to try to allocate a part of the most critical moments and roughly plan possible ways of improvement of this situation.
The references provided herein on the discussed perspective are not full. The extensive information content about available publications can be found, for example, in the bulky monograph . It represents the ideology and scientific base both the first American, and later Western European normative documents defining safe conditions during the work with lasers in detail. Rather extensive bibliography is provided in the published report . From the last monographs, apparently, it is possible to recommend the book  including the section devoted to safety when operating laser equipment. The history and the principles of creation of SanPiN-91 are briefly stated in article . The current adjustments of the European and American standards are, as a rule, discussed in the Health Physics magazine and on the web-site of ICNIRP (International Commission on Non-Ionizing Radiation Protection) http://www.icnirp.org/.
This article draws attention to disagreements in assessment of maximum permissible values of the power radiation characteristic between SanPiN-91 and of IEC standards, as well as (as far as possible) the sources of these disagreements. "Pathogenesis’ of disagreements is closely connected with history of creation of the discussed documents. Therefore duplication of some materials from article  was inevitable. We will focus on the analysis of problems of safety at single direct irradiation of eyes with collimated flows of monochromatic radiation and we will use the following terms and abbreviations: PLI (permissible level of irradiation) is a value of the power radiation characteristic which impacts (in this case) the eyes of the person creating risk of potential irreversible damage of native structure of tissues (and other changes in human organism) with the probability not exceeding 0.1%. For designation of the corresponding characteristics, MPL index is used: EPLI (W/m2) – irradiance, HPLI (J/m2) – power exposure, QPLI (J) – radiation energy. MPE (maximum permissible exposure) is the term used in the IEC standard, having the same meaning as PLI. Hereinafter, in order to facilitate comparison of the values recommended by the considered documents we will denominate MPE as the values relating to the IEC standard (to EMPE, HMPE, QMPE) and, respectively, PLI shall relate to SanPiN-91 (EPLI, HPLI, QPLI).
Threshold values of power characteristics of laser radiation shall mean the values of power characteristics of radiation flow which impact on tissues of eyes causing minimum (as will be specified below) ophthalmoscopically detected irreversible changes of tissues with probability of 50%. Initial characteristics of luminous flux are measured in the eye cornea plane. Injuries of retina or cornea are determined through certain interval of time (normally in 1 hour). These values are designated as ED50 (irradiance), HD50 (power exposure), QD50 (radiation energy).
General sequence of creation of the discussed normative documents shall be reminded. The first step, obviously, is the pilot study of dependency of threshold power characteristics of laser radiation on conditions of influence and properties of the irradiated object. In relation to single laser exposure of eyes it is the dependence of ED50 (HD50) on exposure duration τ in the range of, about, 10–13 < τ < 104 s, wavelengths λ (180 < λ < 105 nm) and typical dimension of the irradiated area (diameter of laser spot) d (about 10–5 < d < 10–2 m). At the first stage the eyes of laboratory animals are used as objects of radiation in vivo. In our case it is rabbits of pigmental breeds (usually chinchilla gray) and then monkeys in testing experiment (most often rhesus monkey).
Detailed pilot studies in all specified intervals of parameters are tremendously bulky concerning their volume. Therefore the technique consists of search of some basic dependency, for example, ED50 (τ) at fixed λ, d in interval of τ, available to the existing laser technology. Then based on the theoretical models and the fragmentary researches, ED50 (λ, d) at the fixed τ creates overall picture of ED50 (τ, λ, d). Further, having entered certain coefficients of hygienic coefficients η (omitting details), PLI (MPE) is calculated as EPLI = ED50/η (EMPE = ED50/η).
Both in the Western countries and here it was common to develop similar normative documents in accessible form, being guided by the user with very moderate analytical skills. Therefore the specified functional dependencies both for EPLI and for EMPE have been approximated to simple functions. These functions, in effect, have made standard basis. Where such similar approximations can lead will be demonstrated later.
Apparently, American standard ANSI Z-136.1–1976 American National Standard for the Safe Use of Lasers was the first detailed document of the state level devoted to the discussed problems. When developing this document, the results of the researches conducted not only in the USA, but also in countries of Western Europe (Great Britain, Germany, France, and Sweden) have been widely used. Experimental practices, principal and biological physics of the American standard are practically fully used when creating corresponding early IEC standards and their subsequent modifications. Therefore the data provided here with reference to ANSI Z-136 as the primary source, fully relate to the European standards (unless there is a corresponding note) and, naturally, to their Russian counterparts.
VISUAL SYSTEM TRANSMISSION SPECTRAL COEFFICIENTS
Tissues of anatomic elements of visual system of eyes contain about 90% of water with small variations. It defines spectral dependence of optical transmission of normal eye of the person (fig. 1) at the site from cornea to pigmental layer of retina (retinal pigmental epithelium – RPE). RPE contains melano-protein granules absorbing the main part of the visible radiation entering the eye and is damaged first of all if irradiance of retina exceeds the threshold value.
Radiation in the intervals of λ < 350 nm and λ > 1400 nm is absorbed mainly by cornea. Range of about 450–900 nm is the most dangerous from the point of view of potential photodamage of tissues of eyes. Here the luminous flow with small losses is focused on retina. In the focus of laser beam when pupil is dilated, the increase of power density at the site of cornea – retina can reach (3–5) . 104. Spectral ranges of 350–450 nm and 900–1400 nm are intermediate. There is final probability of photodamage of both retina (with weakened radiation flow) and elements of structure of eyeball (iris, crystalline lens, vitreous body).
ANSI Z-136 standard contains simplified differentiation of degree of danger of laser radiation on spectral ranges. In the intervals of 180 < λ < 400 nm and λ > 1400 nm were defined during researches only by ED50 (QD50) of photodamage of cornea, and then adequate values of MAE of radiation of eyes were established. In the range of 400 < λ < 1400 nm (visible and near Infrared range), the MAE values were defined similarly, but, proceeding from conditions of photodamage of retina and adjacent structures, considering focusing of radiation flow by optical system of eye.
UV-RADIATION IMPACT ON THE EYES
Impact on cornea of powerful UV-radiation in the range from 180 to 300 nm (very approximately) causes deep changes of structure of tissues at the molecular level. These changes are close to those arising after ionizing radiation. Time of natural reparation is great therefore at repeated single or repeated radiation the effect of accumulation (summing) of the specified destructive changes is implemented. As a result (when energy (power) of radiation falling on cornea is less than MPE, single radiation by laser impulse of the considered duration of τ), the ANSI standard, and later SanPiN-91, establishes maximum permissible daily radiation doses irrespective of τ and quantity of impulses at their repetition. Strictly the term "exposition dose of photon radiation" (J/kg) means total energy of the photons absorbed by substance unit of mass for the fixed time interval. For simplification of metrological provision in the ANSI standard the total maximum permissible QMPE radiation energy passing through limited aperture is normalized whereas SanPiN-91 normalizes total power exposure on cornea HPLI. Basic distinctions are not present here. The choice of units of measurement has been caused by the nomenclature and characteristics of the equipment for measurement of characteristics of laser radiation, widespread in the USSR during that period.
Action of UV-radiation on bio-tissue, including eye cornea, has been studied in the Soviet Union in rather wide scales. The results of these researches do not contradict to the standards established by the ANSI and IEC in spectral interval from 180 to ≈350 nm. Distinctions of the limit values of HPLI recommended by the "Rules" and the corresponding QMPE values set by GOST IEC 60825–1–2013 (IEC 60825–1–2007) are minimal.
Certain disagreements have arisen when assessing the degree of danger of radiation impact on eyes in the range of 350–450 nm (see fig. 1). They will be considered further in section 5.
In the last decades, a number of photodestructive processes (in particular, mutagenicity) specific to impact of UV-radiation on bio-tissues has been investigated, not taken into account when developing the discussed documents. Therefore both the IEC standards, and equally SanPiN-91, in the part defining MPE (PLI) impact of UV-radiation on eyes need to be essentially perfected.
ACTION OF VISIBLE AND NEAR INFRARED RANGE RADIATION
ANSI, IEC standards
Fig. 2 shows basic dependence of threshold energy of laser radiation of visible and near Infrared range on exposure duration used in the first edition of the ANSI standard. Experimental animals were rhesus monkeys. The pupil is expanded medicamentally (mydriasis). It is supposed that diameter of laser beam is less than the diameter of eye pupil. Pulse energy Q (J) of the radiation entering pupil was measured. The technique of measurement of threshold power characteristics of laser radiation is suggested by E. S. Beatriss & G. D. Frish in 1973 . It has been further developed, improved and used by many researchers, including in our country.
During experiment, the chosen site of eye bottom of animal has been irradiated with a series (usually a line from 7–10 impulses) of single impulses with step-by-step increase (reduction) in radiation energy between series. Through the set time interval (10 minutes and/or 1 hour) availability or absence of visible violations of structure is recorded in the alternative form "yes – no". The changes, for example, coagulate, is manifested in the form of gray spot on a rather dark background of retinal pigmental layer (RPE). For the ophthalmoscopy technique used at that period (fundus-camera, Retina Phot, non-reflex ophthalmoscope, slit lamp plus Goldman’s lens) the spot with the diameter of 20–25 µm was a spot at least distinguishable against not uniformity of pigmentation of eye bottom. A range of radiation energies is selected so that a pointed series of radiations included both 100 percent absence of visible violations of structure of tissues of eye bottom and 100 percent availability. The received results were subjected to statistical analysis used for calculation of value of radiation energy QD50 causing ophthalmogically visible photodamage of tissues of eye bottom with confidence level of 50%. To receive one experimental point on the schedule presented on fig. 2, about 40–50 applications are required.
Naturally, in the following years these basic dependencies were replenished with new experimental data, including in picosecond interval τ . Sources of errors of the measurements which have caused the seeming rise in threshold energy in the nanosecond range of exposure duration have been understood. Thus the approximations shown in fig. 2 for MPE have remained invariable till now (in the given interval τ) and have been successfully used in the IEC standards.
With all due respect for creators of experimental base and particularly ANSI Z136 standard, it is necessary to point some restrictions of use of the results presented in fig. 2. It shall be reminded that diameter of waist of the Gaussian beam focused by optical system of eye lies in the range from units to ≈10 µm (R. Gubisch, 1966; M. A. Ostrovskaya, 1969; G. I. Zheltov, 1989 et al ). The situation when extremely sharp focusing of laser radiation on retina is implemented is rather trivial. In this case an eye has to be accommodated to an object located (adjusted for chromatic aberration) slightly closer (farther) than the waist of initial Gaussian laser beam. In practice of operation of laser equipment in most cases it is an accommodation on the object located in the working room.
In the 70-80th years of the last century neither we, nor our colleagues in the Western Europe and in the USA had the technical means providing operating control of local fluctuations of pigmentation of tissues and at the same time space characteristics of laser beam within units of micrometers. The partial compensation of changes of refraction of eyes of laboratory animals under mydriasis (medicamentous dilation of pupil), as a rule, was used so that diameter of d of laser beam at the eye bottom had order of 50–80 µm. In the specified interval of the sizes of the irradiated area local fluctuations of optical density of retinal pigmental epithelium were automatically averaged, and the zone of destruction of tissues (above-mentioned gray spot with a diameter of 20–25 µm) with small error corresponded to maxim of the power density (energy) of laser beam in the axial area.
Later we have shown (fig. 4) that basic experimental data about the dependency of threshold energy of laser photodestruction of tissues of eye bottom due to exposure duration recorded in fig. 2 are adequate to diameter of the laser beam focused on retina in the interval (50–80 µm) stated above.
The minimum diameter of distribution of irradiance of retina on the exp (–1) level at direct radiation of pupil with the Gaussian beam of visible range has order of 10 microns . The size of threshold energy of photodamage of retina QMPL (QMAE) in the first approximation is proportional to square of the irradiated area of pigmental epithelium RPE, or, otherwise, ~ d 2. Thus, there are serious bases for the statement that the impulse of laser radiation, with the energy positioned as threshold energy in fig. 2 with sharp focusing (d ≈10 µm) will lead to serious injury of retina with probability of 100%.
These notes were repeatedly published, discussed both open and private at the different conferences. The main counterarguments were low probability of event of extremely sharp focusing of beam on retina and availability of large (about 10) hygienic coefficient η upon transition from threshold energy of radiation of QD50 to QMPE. There were no objections on the essence of problem. Actually the problem is live and is still acute.
First domestic standards
The researches of mechanisms of destructive action of powerful optical radiation on different bio-objects (primarily, tissue of eyes) were conducted in the USSR in wide scales practically after the birth of lasers. It is worth mentioning that the first-ever industrial medical device for laser ophthalmologic surgery (ophthalmologic coagulator) has been released in the USSR in 1962. One of first-ever laser operation, laid the foundation not only for ophthalmologic, but also for laser surgery in general, has been carried out at the scientific research institute of eye diseases and tissue therapy named after V. P. Filatov, Odessa (hereinafter referred to as Filatov Scientific Research Institute) in October, 1962. The doctor who has carried out it (later becoming professor) L. A. Linnik has been awarded the corresponding certificate of the American society of ophthalmologists.
To the middle of the 80th, efforts of number of the organizations of medical and biophysical profiles have accumulated rather extensive experimental material and theoretical models of both destructive and therapeutic action of laser radiation on live objects. The conditions of safe operation of laser technology were dictated by local rules, methodical recommendations and specifications.
The priority of development of the domestic normative document on laser safety of nation-wide level, certainly, belongs to the creative union of the State Optical Institute named after S. I. Vavilov (Russian abbreviation is GOI) and Department of Ophthalmology of Military-Medical Academy named after S. M. Kirov (MMA). During this work, the data of domestic and foreign works on the discussed problem have been generalized and own pilot studies on animals in rather large volume have been conducted.
The laser unique for that period developed and made in GOI has been used in the experiment providing control of exposure duration in the interval of 10–10–10–1 sec. During researches methodical recommendations have been created and programs of statistical processing of experimental data have been improved. A number of priority results has been received when studying mechanisms of destructive action on bio-tissue supershort (about 10–10 s) radiant pulses, and in some other areas close to the discussed subject. The development of the first domestic Sanitary rules and regulations for design and operation of lasers No. 2392–81 was completed in 1981 (hereinafter referred to as the SanPiN-81) which was one of the final achievements of great practical value
The basic dependence of QD50 (τ), used when developing the specified document, is shown in fig. 3. The results of experiments are certainly reliable. Here, unlike the data provided in fig. 2, all measurements are executed on one laser installation having stable space characteristics of radiation flow, not depending on exposure duration. The researches have been conducted by uniform technique of the same group of researchers (the subjective factor is excluded). Thus the animals of one genetic line kept in identical conditions have been used. These results have persistent value as a contribution to the general base of experimental data about destructive action of laser radiation on eye retina.
Unfortunately, similarly to the data of American and Western European colleagues (fig. 2), provided dependencies have restrictions of applicability for PLI assessment. These restrictions are caused by rather large magnitude of diameter of the retina area irradiated during experiment (d ≈ 150 µm). As apparent from comparison of dependency of QD50 (τ) presented in fig. 2 and 3, the increase (reduction) in d leads to changes not only of QD50 value itself, but also to serious "deformations’ of nature of dependency of QD50 on exposure duration τ.
It is obvious that with correct problem definition basic dependency of threshold energy (or other threshold power radiation characteristics) on exposure duration have to be adequate to the conditions extremely dangerous from positions of potential photodamage of the irradiated object. Neither ANSI and IEC standards, nor domestic SanPiN-81 meet this requirement, unfortunately.
Rules and regulations for design and operation of lasers No. 5804-91 (SanPiN-91)
Guidelines for laser safety of the second generation were developed in our country in the late nineties within the extensive and many-sided State program devoted to safety of the personnel when operating laser technology practically in all spheres of application (science, industry, medicine, army, etc.). The list of problems from biophysical bases of interaction of laser radiation with live objects to potential occupational diseases and technical (metrological) control in the workplaces has been considered. The program had no world analogs as for depth of study and the list in coordination the solved tasks until today, I believe.
About 10 organizations have participated in performance of task according to the section of the Maximum Permissible Levels of Radiation of Eyes and Skin Program. Pilot studies on animals (rabbits) were conducted by tandems GOI-MMA, Institute of biophysics – Central Research Institute TOCHMASH, Institute of physics (IF) NAS of Belarus – V. P. Filatov’s Scientific Research Institute. Experiments on apes have been carried out by the latter organizations on the basis of Scientific Research Institute of Experimental Pathology and Therapy (Scientific Research Institute EPIT, Nursery for monkeys, Sukhumi) with the involvement of specialists from lead establishments of the USSR of medical and biophysical profiles. The amount of financing has provided not only carrying out pilot studies (including field), but also creation of the special equipment including lasers with control of exposure duration and spectral distribution of radiation, system of transport and targeting of radiation flows during the experiments on animals and some other.
The initial concept of new edition of SanPiN-91 was a product of collective discussion of the participating organizations, including, certainly, SanPiN-81 developers. The main point in this concept was the creation of basic dependence of ED50 (QD50) on exposure duration for conditions of the most adverse accommodation of eyes (extremely "sharp" focusing of radiation flow on eye bottom). As noted above, we had no the technical means to provide operating control of primary, small photodamages of retina directly when carrying out experiments. However, by the beginning of the project the experimental and analytical technique of measurement of dependence of ED50 (QD50) on diameter of laser spot at eye bottom has already been developed and approved . Diameter of spot of d varied in limits available within measurement limits (~40–500 µm). The size of the threshold power characteristic for d = 10 µm was calculated by extrapolation method.
In order to illustrate the method, the results of experiments on measurement of QD50 during radiation of eye bottom of monkeys and rabbits by the laser with the wavelength of 1,064 µm at pulse width of 2 . 10–3 s are given in fig. 4. By the results of all measurements given in fig. 4 only one (!) point on graphics of basic dependence of QD50 (τ), corresponding to λ = 1 064 nm, τ = 2 . 10–3 is received (QD50= 4 . 10–4 J). It shall be mentioned that 6 large rhesus monkeys from one litter were used in the experiment for obtaining information (fig. 4), weighing 6–8 kg. It is necessary to add, about, 10 rabbits. It shall be reminded that for obtaining similar information by the technique (fig. 2 and 3) common for that period, there were enough 40–50 laser applications in one eye of one animal. Thus, adoption of the concept about the most adverse accommodation demanded essential increase in volume of researches and, respectively, expenses. And, nevertheless, this concept has been accepted.
To end up with the discussion about expediency of adoption of the specified concept, there was information about recording by ophthalmologists of multiple dot scotomas (coagulates) in paramacular area of retina of eyes among the persons professionally involved in adjustment of the gas lasers using the collimator method increasing during that period. It was helium-neon laser which power was significantly lower than MPE established by the IEC standard.
When discussing the concept, the practical value of unification of domestic guidelines with foreign standards was considered for sure. The priority by degree of the importance has been given to care of human health. The results received in the course of pilot studies rather well fitted into the database which is available for that period created by domestic and foreign colleagues. Fig. 4 shows the results of QD50 measurements taken directly from schedules in fig. 2 and 3 for τ = 2 . 10–3 s. These data are adequate to values of diameters of laser spot at eye bottom in the first case to the predicted d = (5–8) . 10–5 m, in the second measured d = (15–16) . 10–5 m. The fact that the basic QD50 values received in the assumption of the most adverse condition of accommodation of eye were about 5 times lower than those used in the ANSI and IEC standards is prominent (fig. 4).
Fig. 5 shows the fragment of the resulting dependency HIPLI (HMPE) on exposure duration. As a result of difference of the approaches considered above, HMPL is by 3–10 times lower for different conditions of radiation of eyes than HMPE. These variations are partly caused by choice of size of coefficient of hygienic reserve η by the developers of the ANSI and IEC standards. This choice is not discussed in the comments to this standard known to me.
The main definitions of classes of danger of lasers in SanPiN-91 and IEC standards are close. However, the established maximum permissible power parameters for lasers of the 1, 2 and 3 classes differ adequately to the distinctions of PLI – MPE. Both considered documents, as it was noted, are oriented to simplification of their use by means of approximation of real, sufficiently "soft" dependency of ED50 (τ) (fig. 2, 3) on linear symmetric or power functions. Thus in places of "breaks’ of approximations (intervals of 10–5–10–3 and 10–10–10–8 s), the size of hygienic coefficient is artificially increased, approximately, by 3–5 times without physically and physiologically reasonable necessity.
Example of solution of specific problem based on GOST IEC
As example of application of the IEC standard, we will consider a simple problem about choice of limit power of two laser pointers with lengths of waves of radiation of 534 and 640 nanometers. Let’s be guided by the possibility of natural protection of eyes at direct laser irradiation on the basis of unconditional blink reflex with characteristic delay time of t = 0,25 s (the 2nd class of danger in accordance with GOST IEC). According to this standard (Table A1), MAE of radiation in spectral interval of 500–700 nanometers with exposure from 10–3 to 10 s is defined by ratio of HMAE = 18 . t 0,75 J/m 2. Therefore (as is easily shown) the power of lasers, safe for the eyes used in pointers should not exceed, about, 1 mW (10–3 W). It belongs to radiation with lengths of waves both of 534 and 640 nm.
From positions of modern ideas of action of laser radiation on eyes, both the calculation procedure and the end result essentially have 3 serious mistakes. In particular, as it was noted above, the basic principle of the IEC standard is the assumption of focusing of flow of laser radiation on retina in a spot with a diameter of 50–70 µm. Such size of spot is characteristic for eyes of the person with violation of refraction of 0,5–1 dioptries who has forgotten to put on glasses and meditatively looking into the distance. Possibility of extremely sharp focusing of radiation, for example, when using laser pointer in rather small rooms is certainly real. In this case accidental entrance in the eye of a person with 1 mW of laser radiation is injury-causing.
Second factor is as follows. Laser radiation in green spectral range (here – 534 nanometers) is unambiguously more dangerous to eyes, than in red one. It is caused by that the absorption index of the melanin defining spectral properties of retinal pigment epithelium is, approximately, twice higher for λ = 534 nanometers, than for λ = 640 nanometers. With radiation absorption by tissues (with other things being equal) the speed of heating of the environment adequately increases and the speed of denaturation of proteins of cells of pigment epithelium increases significantly. In the absence of information on position of the developers of the IEC standard equally probable two mistakes are represented. If rated value of MPE was assumed safe for radiation in green area of range, the hygienic coefficient η around 600–700 nanometers is unfairly overestimated by several times. Otherwise the probability of photodamage of eyes of the persons by pointer radiation λ = 534 nanometers additionally increases.
And the third important circumstance which is also important. It is necessary to remember that time of protective blink reflex (0,15–0,25 s) when developing the ANSI standard has been defined under conditions of impact of the powerful, bright flash of white light adequate to nuclear explosion or analogs . Similar serious researches of reaction of eyes to specific laser influence are unknown to me. The main radical differences from the specified conditions are caused by laser (monochromatic) exposure of small (in the extreme case measured by tens of µm) retina sites. From experience of my colleagues (in rare instances negative), the influence of visible laser radiation in particular on peripheral departments of retina does not cause feeling of discomfort. Apparently, it is possible to explain spot paramacular injuries of retina mentioned above among adjusters of gas lasers by the absence of unconditioned blink reflex upon influence of collimated flow of laser radiation. Obviously, the considered problem deserves detailed studying, and the concept of "natural protective reaction" in relation to impact of the laser on eyes shall be specified.
Example of solution of specific problem based on SanPiN-91
Omitting calculation parts, we will provide final recommendations of SanPiN-91 for the limit power of lasers in the conditions considered above. They are as follows: PPLI (λ = 534 nm) = 10–4 W; PPLI (λ = 640 nm) = 2 . 10–4 W. They consider the possibility of "sharp" focusing of radiation flow on retina, and distinction in degree of danger of radiation for two spectral ranges is mitigated. Time of blink reflex (with heavy consciousness of own powerlessness in an attempt to offer something best) is left similar, as accepted in the ANSI (IEC).
SPECTRAL INTERVALS OF 350–450 AND 900–1400 NM
With the distribution of flow of laser radiation in the eyeball two processes are completed: on the one hand, increase in power density (irradiance of environment E, W/m2), caused by focusing, and on the other hand, the reduction of this value determined by spectral absorption index of k (λ), m-1. In the visible spectral range the k is not enough, and in essence the action of this factor is ignored. In near UV and Infrared ranges where values of k are about, 10–1–10–3 m-1, such ignoring is inadmissible. Depending on ratio of the specified factors, the action of laser radiation can be dangerous not only for cornea and/or retina of the eye, but also for intermediate intraocular structures, such as iris, crystalline lens, vitreous body. Pathogenic action of such radiation on eyes is, for example, an occupational disease common both in our country and in the western world , namely beam cataract, both among glass blowers (UV-background), and among the employees of hot workshops in metallurgy, forge masterful, etc. (IR-background).
In ANSI Z136 and IEC standards these factors are not considered. Apparently, even when developing the concept of the American standard, it was decided to define a border of visible and UV-range on a wavelength of λ = 400 nanometers. The subsequent aspiration to simplify the use of the standard has led to an assumption that laser radiation with wavelength λ > 400 nanometers are dangerous only to retina. Respectively, with λ <400 potential possibility of photodamage of cornea only is considered.
For the illustration of "operation" of this assumption we will consider the values of HMPE (J/m2) taken at a rough estimate from the last edition of the state standard specification IEC 60825–1–2013 for λ = 400 ± Δλ nm (Δλ is small) and duration of laser impulse of τ = 10–4 s, characteristic for semiconductor lasers. The calculated value of HMPE for λ = 400 + Δλ is equal 5 . 10–3 J/m2. At the same time for λ = 400 – Δλ the magnitude of HMPE grows up to 560 (!) (J/m2).
Apparently, comments are needless here. MPE relation for the next spectral ranges exceeds 5 orders here. With increase in exposure duration this relation only grows. Such "jump" is adequate to the repeated, also justified local increase (reduction) in hygienic coefficient in the borderline area.
Besides, it is necessary to take into account that the spectral transmission of optical system of the eye for radiation with the wavelength of 400 nanometers is 4–5% (see fig. 1). Therefore, with power exposure on cornea of HMPE = 560 J/m 2 beam load of retina is equivalent to influence of external radiation flow in visible range with the value of H of about 20 J/m 2. Maximum permissible power exposure for such radiation flow focused on retina is given above (5 . 10–3 J/m 2, that is by 3–4 orders of magnitude less). Thus, MPR established by the IEC (ANSI) standard for UV-range in the vicinity of 400 nanometers ensures safety of cornea of eye, but thus guarantees with probability 100% the heaviest injury of retina. Closer, but less pronounced roughnesses take place in intermediate Infrared range.
By the beginning of development SanPiN-91, the errors of the ANSI and IEC standards noted above, certainly, were revealed and published. It was unacceptable to adopt the American scenario in its original state. On the other hand, domestic developers were in conditions of number of the restrictions connected with terms of performance of task, the amounts of financing (researches of impact on radiation eyes in the range of 350–450 nanometers originally were not planned). Purely psychological orientation to unification of the document with the western standards and already mentioned intent to make the documen extremely available to the user was also an important factor.
By the results of discussion it was decided to shift demarcation of visible and UV-ranges from 400 to 380 nanometers. Thus the principles of rationing of PLI were unchanged, proceeding from conditions of photodamage of cornea by UV-radiation on the one hand, and destructive action of visible light on retina from the other hand.
As the transmission of optical system of eye in the range of 380 nanometers is close to zero , the problem of safety of retina has been solved. However, transfer of the specified demarcation has increased the area of unfairly overestimated hygienic coefficient by 20 nanometers due to approximation of real dependence HMPL (λ). A little softened sharp (contradicting to the laws of nature) change of MPL around 380 nanometers has remained, there were (and still are) unstudied and unaccounted conditions of photodamage of intraocular structures to front piece of eyeball, both in UV and near IR-radiation.
These and a number of similar problems have been supposed to solve in the course of subsequent completion of Construction Norms and Regulations. However, as we know, after 1991, the works in the discussed area have been stopped. Americans and later Europeans have not brought (or nearly have not made) the amendments in the standards offered by us. Discussion of the reasons is beyond this article. However we will note that in later development of the American standards of safety (e. g., ICNIRP, Guidelines on Limits of Exposure to Broad-band Incoherent Optical Radiation (0,38–3 µm), 1997) the border of UV-range is transferred to 380 nanometers.
DISCUSSION, CONCLUSIONS, RECOMMENDATIONS
In the author’s opinion both compared the document: ANSI-Z136 (later IEC) and the SanPiN-91 deserve extreme respect. They are based on profound and hard work in the field of knowledge, new to the mankind. Therefore some miscalculations and roughnesses are inevitable on the first steps. SanPiN-91 takes more advantageous position in this case as the standard of the second generation which is making use of experience, development and practical application of the ANSI standards and SanPiN-81. A number of the inaccuracies discussed above caused by the level of understanding of problem during this period has been eliminated. The new sections and standards based on the results of the unique researches of state of health of the persons which are professionally working with laser equipment, not having analogs in the West, have been added. These researches have been conducted, in particular, by Moscow Scientific Research Institute named after F. F. Erisman, Central Research Institute of Labor Protection of the All-Union Central Council of Trade Unions, Leningrad Scientific Research Institute of Occupational Health at the enterprises using lasers in technology processes (welding, cutting of materials), in the watch-making industry, in scientific and medical institutions and other areas. By the results of these researches for the first time in world practice SanPiN-91 determines safe working conditions of the specialists who are constantly using laser technology. In the Western Europe and in the USA these conditions are currently normalized by the special documents which often have lower legal status, than state standard.
Thus, there are all objective bases to give preference to SanPiN-91 when settling the controversial issues connected with assessment of degree of danger and working conditions during operation of specific laser equipment. Further it is offered to look at the problems of both Russian and European standards for laser safety under a bit different angle.
From the positions corresponding to modern achievements in the development of the metering (diagnostic) equipment and laser technology, the methods of measurement of ED50 and the subsequent calculation of MPL (MAE) which described in detail in the previous sections look primitive and archaic. Measurements were essentially made "at-glance"; statistical processing improved the situation, but did not change it fundamentally. The need to check, at least fragmentary, the received dependency with use, for example, of OCT (optical coherent tomography), ultrasonic methods of high resolution and modern systems of visualization and photoregistration of eye bottom seems to be obvious.
The general scientific, including physical, base (i. e. a set of ideas of mechanisms of interaction of laser radiation with bio-objects) in the discussed documents corresponds to the level of 70–80th. The last decades have presented many serious achievements and new opportunities of development in this scientific direction. These achievements are connected, in particular, with essential expansion of laser application in medicine. Here physical and physiological problems of influence of laser radiation are considered with the most different parameters on tissues and bodies, both locally and at the level of reactions of organism in general. The developed concepts and modern methods of the researches can be certainly used when updating standards for laser safety.
The research conducted in insufficient volume when developing of SanPiN-91 remains relevant owing to, as a rule, a lack of laser technology with necessary parameters during that period. It concerns mechanisms and conditions of photodamage of tissues by radiation in intermediate spectral intervals, near and far IR range, as well as, certainly, conditions of photodamage of tissues by laser impulses of picosecond and femtosecond duration. The data presentation form in the considered documents including the considered simplifications and approximations is absolutely inadequate to the level of the modern user mastering the computer practically from a five-year age.
The general conclusion is obvious. The considered normative documents both domestic, and Western European need radical reconstruction. Such reconstruction demands forming of the accompanying Program of researches and corresponding financing.
Creation of the normative document integrated with a simple, available for modifications computer program can be the acceptable decision, in my opinion. It is absolutely necessary to refuse all simplifying assumptions and current approximations. The program shall be based on real experimental and/or rated dependencies of threshold energy (or other threshold power characteristics) from radiation source parameters. In the far UV- and IR-ranges there shall be boundary conditions of photodamage of cornea, in the visible area – of retina, respectively, without any simplifications of function of optical transmission . In intermediate spectral intervals it is reasonable to use two or three dependencies defining the conditions of photodamage of cornea, retina or other intraocular structures and to choose the most potentially dangerous option. Thus the choice of conditional border of UV-visible range does not influence the end result and has purely directory character. The input of the program includes passport data of radiation source, the output includes the results of calculation of PLI, danger class, distance within which the source is dangerous to eyes (NOGR), and any other demanded information. Certainly the considerations stated above are only reference. There is suspicion that inclusion of the computer program in the structure of the normative document can contradict to any state standard specification from ancient times. It is necessary to be ready to resistance to pathological official bureaucracy.
Let’s note that the similar program oriented to the dependency set in SanPiN-91 has been created, approximately, in 1990 under the leadership of Professor B. N. Rakhmanov. It was a very useful development which has remained in limited circle of users, probably, only because there were no due measures to distribute it. Today the problem of creation of the normative document can be solved. In some years when we lose many SanPiN developers for the natural reasons (cherished memory of the passed away), it will be much more difficult to solve such problem. It is in many respects caused by the fact that in connection with reduction of volume of the discussed researches in this country (and abroad as well) the average link (age of 30–40 years old) of specialists in the field of interaction of radiation with bio-objects (laser-tissue interaction) has not increased. Training of new generation requires time.
Forming of the coordinate European-Russian Program for safety during the work with lasers including basic researches, amendment of the existing documents and the subsequent joint development of the updated international standards seems to be ideal (forgive me being naпve). Such decision includes both safety of the personnel and smooth harmonization of standards. Involvement of "arbitration judge", for example, China showing great interest to the discussed problems in recent time can be reasonable.
More feasible (and urgently necessary in essence) is stimulation of the researches aimed at providing safe operation of laser technology in the state scale. Argumentative evidence-based updating of hopelessly outdated standards in the field of laser safety has to become one of the most important practical results of such researches. It is worth mentioning that when developing State "Strategic program for photonics and its applications", accepted in 2014 in Russia and comprehensively covering the most important directions of development laser technology (including medical), apparently, have not considered the health protection of the people servicing such equipment.