Swedish Laser-Medical Society

Transcription

1 What lasers can make you blind by Lars Hode Swedish Laser-Medical Society

2 Background: Since 40 years, there are special rules and restrictions i for the use of certain lasers. The reason is that some laser types, sometimes are able to cause eye injury, also at distance.

3 What can happen if you by mistake look into a strong laser beam? If you can cut in steel, then you can destroy an eye. How dangerous are actually different lasers?

4 The classification of lasers is almost the same everywhere in the world, but the rules concerning use of lasers and rules for permission and registration of instruments is very different. E.g. in Sweden class 4 lasers for hair removal, for tattoo removal, for wrinkle treatment etc, are completely pe ey free since five eyeas years. In most countries you need a doctor behind you and in other countries the doctor has to be the therapist.

5 Many people think that looking into a laser beam can make them blind. Is that correct? Yes there are such lasers! I will describe Yes, there are such lasers! I will describe some of them and show examples.

6 10 meter Weight 20 ton 6 MW output Output power 6mW This laser, a laser diode, often used in a 10 mm therapeutic laser instrument. This laser can not harm you. Weight It can possibly heal 2 gram an injured eye! This laser was never produced. But it is possible to make it, if enough money is available. It was constructed theoretically under Richard M. Nixons administration in USA and about 20 units was thought to circulate in satellites around the earth. The intention was to burn Russian nuclear missiles when they came up through the atmosphere. This laser could make you blind.

7 Before One week later Two months later This fibroma was removed with a 15 watt surgical CO2 laser, 50 mm focal length. Can that laser make you blind? NO. It might burn the cornea in or near the focal area, but not make someone blind!

8 An Alexandrite laser for hair removal. Can this laser make you blind? NO

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10 These lasers can make you blind But they are very expensive very heavy But they are very expensive, very heavy and takes a lot of kilowatts to run

11 This laser can make you blind Price: 380,000

12 A Q-switched Nd:YAG-laser can it make you blind if someone shoots right into your eyes? NO

13 Dart arrows... Price: can make an eye blind

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15 E t ti Ear protection must be worn too!

16 A slingshot can make you blind. It can be used by anyone older than 5. It is very cheap. You don t need a responsible doctor...

17 No comments!!! Bullet proof vest must be worn. It would be much safer if it was a class 4 laser; they might cause an injury, but they can t make you blind.

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19 Other laser risks High voltage Warning! High voltage capacitors are ridiculously dangerous things. They can store enough energy to kill you instantly.

20 Retina injured by strong laser pulses. Is the person blind? No

21 Now, a few words about the risk factors. A laser is a lamp. There are many types of lamps. The light from a laser has always two properties: It has one wavelength very narrow bandwidth It is highly coherent (well organized, very pure, in the same way as the tone from a flute is more pure than the sound of a jet plane). Are these two factors making laser light dangerous? No

22 Further, much stronger lasers are used in the following eye treatments: Laser trabeculoplasty for open angle glaucoma Laser iridotomy in pigment dispersion syndrome Laser photocoagulation for neovascular age-related macular degeneration Photodynamic Therapy of wet macular degeneration Laser assisted in situ keratomileusis i (LASIK) for myopia correction Laser treatment of diabetic retinopathy Laser treatment of solar retinopathy Laser treatment of age related macular degeneration Lasering drusen Treatment of keratoconjunctivitis with LLLT Correction of myopia (PRK) and astigmatic keratotomy (AK) Laser in transpupillary thermotherapy (TTT) Laser photolysis of cataract Laser welding of the cornea Laser posterior capsulotomy. and some other treatments

23 Also, it is not "laser" light that may be dangerous. It is any light of certain power, and exposure time, in certain wavelength interval, with certain pulse energy (if pulsed), with certain geometric configuration (divergence, solid angle) etc that may cause such irradiation in the retina that the MPE (maximum permissible exposure) is exceeded much enough. E l f h li ht IPL t fl h Examples of such light sources are IPL, strong flash lamps, certain xenon arc lamps.

24 What is the best protection? ti Knowledge. Safety glasses can be hazardous: If you use the wrong type, you may think that you are protected, but instead...

25 Coherency Coherency has nothing to do with parallelism. It is often said that t in laser light, the waves are in phase with each other, but that is only true inside the laser cavity. After the light has left the laser, this statement is wrong. This picture is showing light of long coherence length laser light and this picture shows light with shorter coherence length - monochromatic ordinary light.

26 Other properties, p like parallel beam, high power may be at hand with some lasers. Most lasers are not strong. Most lasers emit the light in a divergent Most lasers emit the light in a divergent beam.

27 Then, what makes some lasers dangerous?

28 There are certain risk factors, which? Summary of risk factors 1. High power 2. Long exposure time 3. Parallel beam 4. Not visible wavelength 5. Small beam diameter 6. Extreme pulsing (e.g. Q-switching) 1. Extended source / several sources decreases the risk.

29 Narrow beam - one of the risk factors A wide beam is less hazardous to an eye than a narrow beam because only a fraction passes through the pupil, the rest comes outside.

30 Classification of lasers laser classes Parameters: Laser type Wavelength Output t power Divergence Extended source Pulsing The classification of lasers is one of the procedures that are included in the so called CE-approval containing a lot of international standards. A classified laser will belong to one of the groups , 2, 3A, 3B, 4 In Sweden, lasers in these groups are free to use. Doctor or dentist

31 An eye is working like a digital camera where the retina is a detector matrix with millions of pixels in the form of light sensitive cells. When these cells are hit by visible light, they are producing a nerve signal. From this signal, our brain is building up an image. The eye eye lid muscle sclerotic coat iris choroid retina pupil cornea aqueous humour optic nerve fovea (yellow spot) sclera muscle A parallel l beam is depicted d at the retina as a small point. When looking against the sun, the image will be a picture of the sun with sun spots and all. In the case of an unpleasant intensity, our blink reflex is switching off, first by blinking then by turning your head.

32 Waveleng th as risk factor Different risk levels l in different parts of the spectrum.

33 Therapeutic lasers are practically risk free. Fear of lasers cause more problems than the lasers themselves do. Therapeutic lasers are usually in the power region mw. Nobody,... yes, nobody has become blind from a laser. Any laser. Therapeutic lasers can cure many eye diseases

34 Low-Level Laser Therapy Improves Vision in Patients with Age-Related Macular Degeneration Boris T. Ivandic, M.D., and Tomislav Ivandic, M.D. Results: LLLT significantly improved visual acuity (p versus baseline) in 162/182 (95%) of eyes with cataracts t and 142/146 (97%) of eyes without t cataracts. The prevalence of metamorphopsia, scotoma, and dyschromatopsia was reduced. In patients with wet AMD, edema and bleeding improved. The improved vision was maintained for months after treatment. Photomedicine and Laser Surgery Volume 26, Number 3, 2008 Mary Ann Liebert, Inc. Pp

35 Conclusion: Lasers are of extremely low risk compared to: Dart arrows. Air guns. Knives. Stones thrown. Slingshots. Practically all weapons. Broken branches on trees in the forest. Let us forbid all these things before putting strong Let us forbid all these things before putting strong restrictions on therapeutic (and surgical) lasers

36 Thanks for listening

37 About lasers and IPL

38 In many beauty salons, instruments for permanent removal of hair are used. They are: Ruby lasers [691 nm] Alexandrite lasers [755 nm] Diode lasers [808 nm] Nd:YAG lasers [1064 nm] and IPL instruments [ nm] In most countries there are no rules for IPL devices as they are regarded more or less harmless to eyes.

39 How to know if a light source is dangerous? According to IEC and ICNIRP, a limit, MPE (Maximum Permissible Exposure) is given, above which, permanent eye injuries can occur. I have calculated the irradiation levels, both from a laser and an IPL instrument in the retina of a human eye, in order to determine the MPE from light pulses directed against an open unprotected eye.

40 Physical data used: Parameter: Ruby laser IPL device Unit Wavelength 694 nm nm λ Pulse energy max 8 joule max 80 joule Q Pulse time 5-15 ms ca 25 ms t Aperture round 0. 8 mm in diam. rectangle mm - Aperture area m m 2 A Divergence *) ca sr (90%) ca 0.4 sr (90%) Ω *) The percentage specifies that 90 % of the energy is within the specified solid angle.

41 Radiance L The parameter that decides if a source is dangerous to the eyes is its radiance, L. dφ is the momentary power of the radiation Φ = dq/dt da is the surface in front of the pupil. dω is the solid angle in direction to the eye θ is the angle between the normal of the aperture and the direction to the eye. L dφ Φ = da cos θ d Ω normal of the aperture and

42 The expression for L is valid for every point in the aperture that is depicted on the retina. If the aperture has a certain size that exceeds the vision angular α min, in view from the eye, the light source is defined as an extended source. α min depends among other things on the exposure time t. If t < 0.7 seconds, α min =1.5 mrad. When looking into the laser or IPL aperture at 20 cm distance, the angle, in both cases, exceeds α min.

43 When is it dangerous? The MPE related to the radiance of a source is named L MPE. For extended light sources, the following equation is valid: L MPE H MPE [sr J m ] where H is the exposure according to H = dq/da In the expression above H MPE refers to a point source with H 05 MPE = 0.05 [ J m -2 ]. This gives: L MPE = 425 [sr -1 J m -2 ]

44 If the radiance of a light source is fairly equal in all directions over the solid angle Ω, the integrated radiance L P is a function of the power density H A in the aperture: H A = Ld Ω P Ω which, in directions close to the normal, i.e. where cos θ 1 andω<1 sr can be simplified to: H A =Ω L P

45 The value of H that may cause damage (H=H MPE )can be hence be written: H =Ω L Jm MPE MPE 2 [ ] Provided that the radiance is relatively uniform over the area A of the aperture, the pulse energy Q MPE is calculated l from the following simple expression: and Q = HdA A Q = Ω L A [ J] MPE MPE

46 As can be seen, Q MPE is not dependent d on the distance between the eye and the aperture. Hence the irradiance in a point in the image on the retina remains constant if the distance between the eye and the aperture is changed. However the area of the image is reversely proportional to the distance squared.

47 An IPL instrument with the parameters in the example above, have the following value of Q MPE : Q MPE = 0.4 [sr] 425[ sr Jm ] [ m ] = 0.034[ J] This means that it is safe to look directly into its beam if the pulse energy is less than joules. With the pulse energy given in the example above, its pulse energy is more than 2000 times higher than this safe limit.

48 Results: The calculations show that, if a light pulse from an IPL is fired from a distance of 20 cm against an open eye, the energy level can be more than 2000 times higher than maximum permitted exposure. Consequently, either the IPL instruments can be rather dangerous to human eyes or the internationally used MPE limit needs to reconsider.

49 IPL vs. Laser L [J/m 2 ] Irradia ance 300,000 L MPE 2,000 L MPE L MPE Risk for permanent damage L MPE Image area on the retina A [mm 2 ]

50 The chosen values of energy density and aperture area in the example, 40 J/cm 2 and 2 cm 2, are not exaggerated. There are instruments with twice as high energy density, according to the manufacturers and then the MPE can be exceeded by more than a factor 4000.

51 Consequences of an injury There is no doubt that certain lasers can injure the eyes because of their high radiance in the aperture. However, it is the consequence of an injury that t is important rather than the fact that the eye is injured. Further calculations show that an injury in the retina, caused by a laser, has a small consequence by means of effects on the vision while, if caused by an IPL instrument, the injury can be of much larger consequence.

52 Optics of the eye Projection of the aperture on retina Aperture 2 cm 20 cm A small aperture (such as from a laser) is projected to a small image on the retina while a large aperture (such as from an IPL) is resulting in a large image.

53 The consequences of eye injury caused by a laser Eye injury by laser from fiber aperture at 20 cm distance. Fiber Φ 0.8 mm Visual effect from eye injury at 2 m distance: round spot, 8 mm diam Size of blind spot: 8 mm 20 cm Focal spot on retina, Φ 0.08 mm 2 m

54 The consequences of eye injury caused by an IPL Eye damage by an IPL shot. Aperture at 20 cm distance. Visual effect from eye injury at 2 m distance: cm IPL aperture 10 x 20 mm Size of blind area Projection on the retina, 1 x 2 mm spot 20 cm 2m

55 Reported incidents

56 Journal for Ms Xxxx A 41 year old woman was treated on her eyelid (!) with an IPL device. She got irido- cyclite and was treated by means of adrenaline pads on the Ullevål hospital in Oslo.

57 At a demonstration of an IPL instrument in an exhibition, a shot was fired by mistake because somebody happened to step on the foot pedal. The hand piece, with the 1 2 cm aperture, was placed in its holder on the side of the instrument, about 80 cm up from the floor. A hole was burnt in a persons jacket.

58 My face was destroyed d IPL treatment of face. Longitudinal burns. The doctors comments: the instrument gave more than the setting

59 Conclusion: Even though the irradiance level resulting from a laser can be much higher than from an IPL, the levels are, in both cases, high enough to permanently destroy the cells of vision. A beam strong enough to burn a hole in a A beam, strong enough to burn a hole in a jacket is naturally dangerous to eyes.

60 Other considerations: Even though strong lasers may cause eye injury, one should bear in mind that every day strong lasers are fired right into unprotected open eyes in the treatment of retinopathy, macula degeneration, cataract, near-sightness and of some other eye problems. And, this does not make the patients blind.

61 My recommendations: The IPL instruments can imply pya great risk and it is important that there are regulations for them as well as for lasers. Rules for all strong light sources ought to be considered.

62 First of all: Therapeutic lasers are practically risk free. The stronger types (typically class 4) can cause an injury, but they can not make you blind. They are usually in the power region mw. Fear of lasers cause more problems than the lasers themselves do.