Laser Safety- Hazard Identification Methodology & Engineering Control Measures

Transcription

1 Abstract: Laser Safety- Hazard Identification Methodology & Engineering Control Measures S. Thalapathi Raj Scientist C, Jai prakash Sc E, Raj Kumar Sc E & Manpreet Singh Sc F Terminal Ballistics Research Laboratory, Ministry of Defence (DRDO), Sector- 30, Chandigarh. thalaps@yahoo.com In recent years, Lasers have found their way from the Research laboratory to the Industrial, Defence as well as Medical and Communications field. In many applications such as Optical Fibre communications the laser s radiant energy output is enclosed. Therefore, the user faces no health risk, and the presence of a laser embedded in the product may not be obvious to the user. However in Research, Industrial or Medical applications the laser s emitted radiant energy is accessible and may pose a potential hazard to the eye and skin. Because the laser process can produce a highly collimated beam of optical radiation, a laser can pose a hazard at a considerable distance -quite unlike most hazards encountered in the workplace. This review paper is a part of guidelines being prepared to handle various high power lasers for detonics and explosive research in Terminal Ballistics Research Laboratory, Chandigarh. This paper describes the laser display hazard identification methodology which provides a structured approach to identify hazards associated with the use of lasers based on various modules of laser display. This approach ensures that hazard assessment is carried out in an efficient and systematic manner, thus minimizing the possibility of any areas being overlooked. This paper also describes the hazard classifications of the lasers with respect to the hazard potential based on their output power and potential hazard during normal use of the laser and explains the nature of hazards and various engineering control measures, administrative & procedure controls and relevant safety standards to reduce the risk level so as to ensure the safe working environment. 1.0 Introduction Electromagnetic radiation (EM) consists of electrical energy and magnetic energy that travel together through space as waves. In studying laser light, we are most concerned with the optical spectrum region of the EM spectrum. This includes ultraviolet, visible and infrared light. Laser beams can be extremely hazardous if not understood and properly controlled. 2.0 Properties of Laser that affect Safety The most important parameters that are concerned in evaluating safety aspects are 2.1 Wavelength: The operating wavelength of a visible laser corresponds to the color of the lasers output beam. Some lasers are invisible because their wavelength is outside the range of visible light. Light composed of wavelengths longer than visible light is called infrared light and shorter than visible light is called ultraviolet light. The wavelengths determines the actual site where damage occurs because certain parts of the eye and skin are more easily damaged by visible light and others are damaged by longer or shorter wavelengths. 2.2 Duration and nature of exposure: The duration of the exposure is important aspect in determining hazards. Longer the duration of exposure higher is the risk. Lasers may operate continuously or be pulsed. 2.3 Output Power:

2 The output wavelength defines the portion of the Ultraviolet: 100nm - 400nm, Visible: 400nm - optical spectrum in which the laser operates. 3.0 Classification of Laser, Nature of Hazards and their Precautions: 700nm, Infrared: 700nm nm The American National Standards Institute (ANSI 2000) has developed four categories of hazard potential based on their output power and potential hazard during normal use of the laser product. The higher the classifications number the greater the hazard potential. Class I (Radiant power less than 0.39 mw) Class II (Radiant power less than 1.0 mw) Class IIIA Class IIIB Class IV 3.1 New Classification of laser: (Radiant power 1-5 mw) (Radiant power mw) (Radiant power C.W > 500 mw The classification of a laser is based on the concept of Accessible Emission Limits (AEL) that is defined for each laser class. This is usually a maximum power (in W) or energy that can be emitted in a specified wavelength range and exposure time. Class I <(0.39 mw) Class II (<1 mw) Class III A (1-5 mw) Class III B (5-500 mw) Class IV The natures of hazards for each classification have been identified using Hazard identification methodology and relevant precautionary measures are given in this paper. 4.0 Laser Display Hazard Identification Methodology This methodology provides a structured approach to identifying of hazards associated with the use of lasers based on various modules of laser display. This approach ensures that the hazard assessment is carried out in an efficient and systematic manner, thus minimizing the possibility of any areas being overlooked. The methodology is as under Laser Links between compartments Primary Optics Beam path Secondary Optics Control Systems Equipment Associated with the laser

3 Support System Operators Staff other than Operators Audience Other people Venue External Factors Laser: There could be more than one laser and hazards from each have to be considered. Links between compartments: To assess if there is any connections between the different sections of laser display to have hazards associated with them and also links between the control systems, the optics and the laser. Primary Optics: The primary and secondary optics had different hazards associated with them and would put different people at risk. It is also prudent to encase the primary optics, for safety and to minimize the influence of environmental conditions on the optical components. Beam path: This is introduced in to the methodology to ensure that the beam paths are specifically considered including the paths between the primary optical system and any secondary optics. Secondary Optics: This considers the mounting of the secondary optics, including any actual or required blanking. The accessibility of both the beam and the optical component should be considered, both during alignment and cleaning. Control Systems: It is preferred to have central control system or some form of communication between operating staff. The integrity of the control system hardware should be considered. Equipment Associated with the laser: The hazards associated with the equipments like High voltage power supplies, etc have to be identified and clearly designated. Support System: Typical potential hazards connected with the support system are specular reflections from the support system. The other factors like operators training, hazards associated with the venues and external factors are also considered. Using this methodology various hazards related with laser have been identified and various control measures are being elaborated in this paper as table I &II. Table I: Types of Laser, Nature of Hazards and their precautions Hazard class & Examples Nature of Hazard Precautions to be taken Class I <(0.39 mw) Eg: Laser printers, CD players, Intrinsically safe low power systems. Cannot emit radiation Essentially safe and enclosed systems which do not pose a

4 Geological and laboratory levels greater than the maximum hazard. equipments. permissible exposure (MPE). Class II (<1 mw) Maximum rated power is 1 mw for CW systems and 2 mj for PW systems. Eg: Laser pointers, Aiming devices and range finding equipment. Low power visible laser systems and emit radiation in excess of MPE. Can cause retinal injury. These are not intrinsically safe but are not powerful enough to cause injury. Class III A (1-5 mw) Eg: Laser pointers, Laser scanners, Laser alignment devices. Class III B (5-500 mw) Eg: Spectrometry, Stereolithography and entertainment light shows Class IV c.w > 500 mw Pulsed >10J/cm 2 Eg. Radiant power exceeding 500 mw. Surgery, research, drilling, cutting, welding and micromachining These are medium power visible laser systems capable of causing eye damage with short duration (<0.25s) Medium power laser systems. Radiation can cause eye injuries in exposures <0.25s. High power laser systems. Can cause eye damage with short duration <0.25s). Capable of causing severe skin damage and igniting flammable and combustible materials or fire. Table II: Hazard Class, Nature of Hazards and their precautions Limited to visible lasers that are safe for momentary viewing but may present some potential hazard if stared continuously Not safe even for momentary viewing and procedural controls and protective equipment are required. Never stare into beam or view directly with optical instruments. Not safe even for momentary viewing Direct exposure must be avoided. Avoid eye or skin exposure to direct or scattered radiation. Hazard Class(Type) Nature of Hazard/ Precaution Warning Labels Class I (< 7µW visible) A continuous laser at 600 nm can emit up to 0.39 mw, but for shorter wavelengths, the maximum emission is lower. Class 1M The total output power is below class 3B but the power that can pass through the pupil of the eye is within Class 1. Class 2 (< 1mW visible) nm. Class 2M The amount of light passing through the pupil cannot exceed the limits for class 2. Class 3 R (< 5mW visible (and < 25 w/m2)) Class 3B (< 500mW visible) wavelength range from 315 nm to far infrared are limited to 0.5 W. For pulsed lasers between 400 and 700 nm, the limit is 30 mj. Class 4: (> 500mW visible) It includes all lasers with beam power greater than class 3B. It is safe under all conditions of normal use. Safe for all conditions except when passed through magnifying optics such as microscopes and telescopes. It is safe because the blink reflex will limit the exposure to no more than 0.25 s It is safe because of the blink reflex if not viewed through optical instruments. Safe with restricted beam viewing. MPE can be exceeded, but with a low risk of injury. It is hazardous if the eye is exposed directly. Protective eyewear is required where direct viewing Class 3B lasers must be equipped with a key switch and a safety interlock. In addition to eye hazards, able to cut or burn skin. May ignite combustible materials, and thus represent a fire risk. Must be Maximum permissible exposure (MPE) cannot be exceeded. LASER RADIATION. DO NOT VIEW DIRECTLY WITH OPTICAL INSTRUMENTS. LASER RADIATION DO NOT STARE INTO BEAM DO NOT STARE INTO BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS. LASER RADIATION AVOID DIRECT EYE EXPOSURE. LASER RADIATION AVOID EXPOSURE TO THE BEAM WITH CLASS 3B LASER PRODUCT LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR

5 equipped with a key switch and a safety interlock. SCATTERED RADIATION. 5.0 Hazards of Laser Laser light is absorbed by body tissue. If the beam is powerful enough, the absorbed energy can cause injury. The skin and eyes are the most sensitive tissue to laser light. The amount of light absorbed depends on the wavelength of the beam. Hazards of lasers may be separated into two general categories Beam related hazards to eyes and skin and Non Beam hazards such as fire, electrical and chemical hazards, etc., 5.1 Beam related Hazards: Exposure to the laser beam could be either intrabeam or specular reflections or diffuse reflections. In intra beam exposure, eyes or skin or both is exposed directly. The specular reflections are from smooth or mirror surfaces and can be nearly as harmful as exposure to the direct beam. Diffuse reflections of Class IV lasers are capable of initiating fires. High power pulsed lasers particularly those generating higher harmonics in UV region can cause major damage to the eyes of the operator while aligning& optimizing laser performance Biological Effects: Laser induced biological effects are primarily dependent on wavelength, irradiance, exposure duration, target tissue and tissue condition. The probable biological harmful effects caused by lasers in human tissue are as follows a) Thermal Effects b) Acoustic Effects (Photomechanical) c) Photochemical Effects d) Chronic Effects a) Thermal Effects: Thermal effects or burning of tissues involve absorption of radiant energy by chromospheres (absorbing structure) such as melanin or hemoglobin. It is the major cause of laser damage to the eye or skin in exposure times from microseconds to seconds. Absorption increases the random molecular motion this is manifested in the tissues as heat, which produces damage by denaturing protein and inactivating enzymes. The degree of burning varies according to absorbency of the tissue and depends on the power output, irradiated area, duration of exposure and the pulse rate. b) Acoustic effects: It occurs when a brief mechanical shockwaves, may be generated Q switched or mode locked lasers are incident on tissues. The tissue vaporizes and explodes causing a shockwave to occur in surrounding tissue and may cause tissue ruptures. c) Chronic Effects: Chronic effects include premature aging of the skin, skin cancer and cataracts. They are due to frequent and regular exposure over a long period of time Effect of Eye absorption of laser radiation: The site of ocular damage for any given laser beam depends upon its output wavelength of the incident or reflected laser beam. Lasers in the visible and near infrared range of the spectrum have the greatest potential for

6 retinal injury, as the cornea and the lens are transparent to electromagnetic radiation of these wavelengths ( nm) and the lens can focus the laser energy onto the retina. The light entering the eye from a collimated beam in the retinal hazard region is concentrated by a factor of 100,000 times when it strikes the retina. Therefore a visible,10 mw/cm 2 laser beam would result in a 1000 Watt/cm 2 exposure to the retina, which is nm Affects the retina more than enough power density to cause damage. UV-C ( nm) Cornea surface UV-A ( nm) Affects the lens UV-B ( nm) Absorbed by the cornea Near IR (< 1400nm) Affects the retina Far IR Affects cornea and aqueous humor The effects of laser radiation on skin: Lasers can harm the skin via photochemical or thermal burns. Depending on the wavelength, the beam may penetrate both epidermis and dermis. Erythema, skin cancer and accelerated skin aging are produced from exposure to light with wavelengths of 280 to 400nm.

7 Table III. The probable biological effects-of eyes and skin with respect to wavelengths Photobiological spectral domain Ultraviolet C (200 nm 280 nm) Ultraviolet B (280 nm-315 nm) Ultraviolet A (315 nm 400 nm) Visible (400 nm 780 nm) Infrared A (780 nm-1400 nm) Infrared B (1400 nm-3000 nm) Infrared C (3000 nm nm) Photokeratitis (Cornea) Cornea Eye Photochemical cataract Photochemical and thermal retinal injury, cornea and lens Cataract and retinal burn Corneal burn, aqueous flare, cataract Corneal burn Skin Erythema(Sunburn) Skin cancer Accelerated skin aging Increased pigmentation Pigment darkening Skin burn Pigment darkening Skin burn Skin burn Skin burn Skin burn 5.2 Non-Beam Related Hazards : Fire Hazards: Laser beams deflected onto flammable materials can cause ignition and fires from high power continuous wave (CW) infrared lasers. High voltage pulse or flash lamps may cause ignition. Class 4 laser system represents a fire hazard. Enclosure of Class 3 laser beam can result in potential fire hazards if enclosure materials are exposed to irradiances exceeding 10 Watt/cm Electrical Hazards:

8 The use of laser systems can present an electric shock and electrocution hazard. This may occur from contact with exposed utility power utilization, device control, capacitors and power supply conductors operating at potentials of 50 volts or more during laser set-up or installation and maintenance. Protection against accidental contact with energized conductors by means of a barrier system is the primary methodology to prevent electrical shock. 6.0 STANDARDS OF EXPOSURE & CONTROL MEASURES 6.1 Maximum Permissible Exposure (MPE) It is the maximum level of laser radiation to which a person may be exposed without hazardous effects or adverse biological changes in the eye or skin. The MPE is expressed in terms of radiant exposure in J/cm 2 or irradiance in W/cm 2 depends on the laser parameters like Wavelength, Exposure duration, Exposure conditions and Nature of exposure. 6.2 Threshold Limit Values (TLVs): The TLVs are for exposure to laser radiation under conditions to which it is believed nearly all workers may be repeatedly exposed without adverse effects. Table IV : TLVs for Direct Ocular Exposure (Intra beam viewing) from a Laser Beam Spectral Region Wave Length (nm) Exposure t (s) TLV UVC 180 to to 3X mj/cm 2 UVB UVA to t ¼ J/cm to 3X mw/cm 2 Light nm to X10-8 J/cm 2 Table V: TLVs for Skin Exposure from a Laser Beam Spectral Region Wave Length (nm) Exposure t (s) TLV UVA 180 to to 3X mj/cm 2 Light nm 10-9 to C A X10-2 J/cm 2 IRA 10-7 to C A t 1/4 J/cm 2 IRB * C B µm to 3 X J/cm Engineering Controls They are design features applied to the laser or laser environment for overall effective controls. They restrict exposure or reduce irradiance to the eye and skin. Some engineering controls include Protective housing with interlock (Remote) System Master key control Service Access panels Viewing windows / portals Enclosed / Limited beam paths

9 Controlled areas and beam paths Warning buzzers / lights Remote firing Beam stop attenuators / enclosures Shutters Embedded lasers and enclosure concepts Guarding against specular reflections Equipment labels / warning indicators Emission delay system 6.4 Administrative and Procedural Controls: Administrative controls are procedures and information rather than devices. These are usually implemented by the Laser Safety Officer by the support of the management of the organization. These controls are include SOPs for start up/ shutdown procedures, maintenance schedule, emergency situations Education and training by qualified LSO Output emission limitations Designated laser operations Spectators restriction & Warning signs and Labels 6.5 Use of Personal Protective Equipment It is essential to use PPE when engineering controls do not provide adequate means to prevent access to direct or reflected beams at levels above the MPE. Protective eyewear is essential for Class 3 and 4 lasers. Eye protection may include goggles, face shields, spectacles or prescription eyewear using special filter materials or reflective coatings to reduce exposure below the MPE and it should be used only at the wavelength and energy /power for which it is intended. Selection criteria for eye protection should be considered as per the class of laser according to following criteria Wavelength Maximum anticipated viewing duration Maximum irradiance or radiant exposure to which the eye may be exposed Optical density needed for protective eyewear Type of eye protection 7.0 Conclusion: In defence research, industrial or medical applications the laser s emitted radiant energy is accessible and may pose a potential hazard to the eye and skin. This hazard identification methodology provides organizations with a valuable tool in their efforts to optimize productivity while maintaining the safe working environment for their employees. Equipped with these information and guidelines, organizations can ensure the safety of their employees and soundness of the structures in which they work. This review paper is a part of guidelines being prepared to handle various high power lasers for detonics and explosive research in Terminal Ballistics Research Laboratory, Chandigarh.

10 8.0 Bibliography: 1. IEC Standard 2. Paul R. Cooper, Explosives Engineering, 1996, VCH Publications, New York. 3. DOE Explosives Safety Manual, DOE M The ANSI Z136.1 Laser Safety Standard rd 5. American Institute of Physics Handbook, 3 edition. 6.