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Potential Laser Hazards to the Clinician During Photocoagulation
Potential Laser Hazards to the Clinician During Photocoagulation D a v i d H. S l i n e y , M . S . , and Martin A . Mainster, M . D .
We measured reflected laser beams from seven different contact lenses used during laser photocoagulation to evaluate the potential hazards to the eyes of the clinician and bystanders. We found that although collimated beam reflections from the aiming beam of an argon laser photocoagulator could produce disability glare and discomfort to the laser operator (or to an individual viewing through the auxiliary eyepiece), the levels are not hazardous, by current occupational safety limits. Reflected laser light may exceed occupational exposure limits for momentary viewing by bystanders if they are within 1 meter of the laser contact lens. QUESTIONS are often raised about the safety of viewing back reflections from contact lenses during argon or krypton laser photocoagulation. 1 The ophthalmologist using the laser is protected from the treatment beam reflexes by a protective filter. When the low power aiming beam is operating, however, brilliant flashback may be encountered from the contact lens. We investigated the sources of these back reflections, measured reflected optical power, and assessed potential hazards to the laser operator and bystanders.
Accepted for publication March 2, 1987. From the Laser Microwave Division, U.S. Army Environmental Hygiene Agency, Aberdeen Proving Ground, Maryland (Mr. Sliney) and the Department of Ophthalmology, University of Kansas Medical Center, Kansas City, Kansas (Dr. Mainster). This study was supported in part by Research to Prevent Blindness, Inc., and Kansas Lions Sight Foundation, Inc. The opinions or assertions herein are those of the authors and should not be construed as official positions of the Department of the Army or Department of Defense. Reprint requests to David H. Sliney, M.S., Laser Microwave Division, U.S. Army Environmental Hygiene Agency, ATTN: HSHB-MR-LL, Aberdeen Proving Ground, MD 21010-5422.
758
Material and Methods We used seven different types of contact lenses with a Coherent Medical Model 920 Argon/Dye Laser Photocoagulator, which employed a Zeiss Model 30-SL-M slit-lamp delivery system. These included a Panfunduscope lens (Rodenstock), a four-mirror Karickhoff fundus lens (Ocular Instruments), a Universal three-mirror lens (Ocular Instruments), a Ritch trabeculoplasty lens (Ocular Instruments), a Goldmann three-mirror lens (Haag-Streit), an Abraham iridectomy lens (Ocular Instruments), and a Yannuzzi lens (Ocular Instruments). Reflected beam power passing through the slit-lamp eyepieces was measured for worst-case orientations of each contact lens and compared to measurements of reflections from a planar mirror oriented perpendicular to the laser beam. Measurements were made at the argon blue (488 nm) and green (514.5 nm) wavelengths, and at four dye laser wavelengths (577, 595, 620, and 630 nm). A Scientech 1-inch calorimeter with a Fluke Model 8024B millivolt meter, and a Coherent Model 210 Laser Power Meter were both used as cross calibrations of measurement of the treatment beam power (generally 0.1 to 1.0 W settings). Measurement of aiming beam and reflected beam power and irradiance required the use of a more sensitive detector, and a United Detector Technology Model 40X Optometer with a silicon detector was employed for these measurements.
Results Beam power measurements—Treatment beam power measurements were within 1% agreement with the calibrated disk calorimeter, the accuracy limit of such measurements. Aiming beam power increased linearly with treat-
©AMERICAN JOURNAL OF OPHTHALMOLOGY 103:758-760, JUNE, 1987
Laser Hazards to Clinician
Vol. 103, No. 6
ment beam power, ranging from 0.1 mW/W of treatment power at an aiming beam power setting " 1 " to 2 mW/W of treatment power at setting " 4 . " Reflections to operator from aiming beam— Aiming beam power did not exceed the potentially hazardous 1 mW level for a momentary exposure. 2,3 Back reflections observed through the eyepieces, however, could be annoying if the contact lens were oriented in such a way as to maximize the reflection. To determine the maximum possible reflected beam power returning through the slit-lamp ocular, a fully silvered reference mirror was placed at the operating distance of the slit lamp, and oriented perpendicular to the beam path. Power setting and spot size were varied (Table 1). The mirror was positioned by a micromanipulator to obtain these worst-case reflections. Under a worst-case condition, the reflected power could exceed several microwatts of power, which would be potentially hazardous after several minutes to an hour of exposure. These power levels produce such an intense glare, however, that one could not perform effective photocoagulation, and the operator would adjust the lens to eliminate this annoyance within seconds. Orienting the lens to minimize discomforting glare increases the margin of safety. Thus, these worst-case reflections do not pose a realistic hazard (Table 2). Reflections to operator from treatment beam —In all laser photocoagulators, the manufacturer is required to protect the operator from viewing hazardous reflections through the slit lamp. All clinical photocoagulators have an
759
TABLE 2 W O R S T CASE MIRROR REFLEX BEAM POWER VS WAVELENGTH
WAVELENGTH
TREATMENT POWER SETTING
(NM)
(w)
577 595 595 620 630
0.5 0.5 1.0 0.5
All lines
1.0
AIM BEAM REFLEX (^w)
TREATMENT BEAM REFLEX*
11 8 15 7 8 21
0.41
(*»w) 0.055 0.048 0.09 0.05 0.05 0.25
'Treatment beam reflex power may have been predominantly blue-white fluorescence from the protective filter in the viewing optics. This is the origin of the puzzling "white flash" occasionally reported by clinicians using commercial photocoagulators.
appropriate filter that is switched into place when the footswitch (or hand trigger) is activated. In a properly designed system, the laser beam cannot be activated until this attenuator is in place. Measurements of back reflections for the full treatment power were made with the reference mirror and each lens placed at the working distance of the laser photocoagulator (Table 3). Normally, the attenuating filter was sufficiently dense so that only filter fluorescence (for example, yellow-orange or white light) could be detected.
Discussion Applicable occupational exposure limits for viewing a continuous wave laser beam vary
TABLE 1 WORST CASE MIRROR REFLEX POWER OF AIMING BEAM AT 514.5 NM
POWER SETTING
AIM BEAM*
(w)
(1 or 4)
0.1 0.1 0.5 0.5 1.0 1.0
High (4) Low(1) High (4) Low(1) High (4) Low(1)
50-jim SPOT x5
3.30 0.01 13.00 0.04 28.00 0.08
x20
12.00 0.03 50.00
0.14 100.00 0.30
TABLE 3 W O R S T CASE REFLECTED BEAM POWER T H R O U G H VIEWING OPTICS FOR TREATMENT BEAM
100/500-Mm SPOT x5
3.90 0.01 15.00 0.05 35.00 0.10
x20
6.10 0.02
IRRADIANCE LENS MANUFACTURER
LENS DESCRIPTION
(jiW/CM 2 )
Rodenstock
Panfunduscope lens
7
30.00
Ocular Instruments
OAIA Abraham iridectomy lens
6
0.08
Ocular Instruments
Yannuzzi lens
1
Haag-Streit
Goldmann three-mirror lens (U)
7
Ocular Instruments
Universal three-mirror lens
100.00 0.30
Ocular Instruments Karickhoff four-mirror lens "The ratio of the reflected aiming beam power to the treatment
Ocular Instruments
Ritch trabeculoplasty lens
1.5 20" 7
power for settings of 50-jtim retinal spot size and x 5 magnification was: 0.08 /xWA/V at setting 1; 0.64 M W / W at setting 2; 4.0 M W / W at setting 3; and 30 /xW/W at setting 4.
:,
1.5 /iW/cm 2 was obtained in all measurements except for one
position that required critical alignment.
760
June, 1987
AMERICAN JOURNAL OF OPHTHALMOLOGY
with wavelength and exposure duration. 2,3 For momentary unintentional exposure to a bright laser beam, the limit is 2.5 mW/cm2 (equivalent to 1.0 mW entering a 7-mm pupil), which is calculated for the eye's aversion response time to visible light (0.25 second). For a 10-second exposure, the limit drops to 1.0 mW/cm 2 (400 u.W total power entering the eye). For lengthy periods of fixation on a bright visible point source of laser light (a condition generally considered unrealistic, as it can be quite uncomfortable), the exposure limit is wavelength dependent, and most stringent for short wavelength (blue) light. For example, the limit is 10 mj/cm2 between 400 nm and 550 nm. This corresponds to an irradiance of 100 |xW/cm2 (40 u.W total power) for 100 seconds, 10 (xW/cm (4 jiW total power) for 1,000 seconds (16.7 minutes), and 1 u.W/cm2 (0.4 u.W total power) for 10,000 seconds (2.8 hours) or longer. Examining the reflected power values given in the Tables, it becomes apparent that the operator would never be exposed to hazardous levels through the slit lamp. Exposures from the treatment beam are all well below the criteria for many hours of viewing. The worst case back reflections of the aiming beam are normally on the order of 10 u.W7cm2 (a 1,000-second exposure limit), and would last only momentarily. Even if we consider the additive effects of multiple flashbacks during a day, it would be unrealistic to assume that a person would exceed the cumulative daily limit of 10 mj/cm2. However, a hazard can exist for a bystander
at the side of the slit lamp who is exposed to unattenuated back reflections of the treatment beam from the contact lens. 4 Any irradiance exceeding 2.5 mW/cm 2 is hazardous, and such levels could be found to the side of the contact lens, within 1 m of its front surface. Although a dazzling aiming beam back reflection from the mirror of a laser contact lens may be distracting and annoying, it does not pose a real ocular hazard to the laser operator. Since the cumulative effects of such reflections are unknown, however, it is important to use the lowest effective aiming beam power setting and to minimize reflexes by proper use of laser contact lenses. Bystanders within 1 m of the lens should also wear appropriate protective laser goggles during any photocoagulation procedure..
References 1. Ward, B.: Mirror laser-treatment lenses. Possible risks associated with lens design. Arch. Ophthalmol. 104:1585, 1986. 2. Sliney, D. H., and Wolbarsht, M. L.: Safety With Lasers and Other Optical Sources. New York, Plenum Publishing Corp., 1980, pp. 217-283 and 581-583. 3. American National Standards Institute: Safe Use of Lasers. Toledo, Ohio, Laser Institute of America, 1986, Z-136.1. 4. Sliney, D. H.: Biomedical laser safety. In Goldman, L. (ed.): The Biomedical Laser. New York, Springer-Verlag, 1982, pp. 11-24.
We measured reflected laser beams from seven different contact lenses used during laser photocoagulation to evaluate the potential hazards to the eyes of the clinician and bystanders. We found that although collimated beam reflections from the aiming beam of an argon laser photocoagulator could produce disability glare and discomfort to the laser operator (or to an individual viewing through the auxiliary eyepiece), the levels are not hazardous, by current occupational safety limits. Reflected laser light may exceed occupational exposure limits for momentary viewing by bystanders if they are within 1 meter of the laser contact lens. QUESTIONS are often raised about the safety of viewing back reflections from contact lenses during argon or krypton laser photocoagulation. 1 The ophthalmologist using the laser is protected from the treatment beam reflexes by a protective filter. When the low power aiming beam is operating, however, brilliant flashback may be encountered from the contact lens. We investigated the sources of these back reflections, measured reflected optical power, and assessed potential hazards to the laser operator and bystanders.
Accepted for publication March 2, 1987. From the Laser Microwave Division, U.S. Army Environmental Hygiene Agency, Aberdeen Proving Ground, Maryland (Mr. Sliney) and the Department of Ophthalmology, University of Kansas Medical Center, Kansas City, Kansas (Dr. Mainster). This study was supported in part by Research to Prevent Blindness, Inc., and Kansas Lions Sight Foundation, Inc. The opinions or assertions herein are those of the authors and should not be construed as official positions of the Department of the Army or Department of Defense. Reprint requests to David H. Sliney, M.S., Laser Microwave Division, U.S. Army Environmental Hygiene Agency, ATTN: HSHB-MR-LL, Aberdeen Proving Ground, MD 21010-5422.
758
Material and Methods We used seven different types of contact lenses with a Coherent Medical Model 920 Argon/Dye Laser Photocoagulator, which employed a Zeiss Model 30-SL-M slit-lamp delivery system. These included a Panfunduscope lens (Rodenstock), a four-mirror Karickhoff fundus lens (Ocular Instruments), a Universal three-mirror lens (Ocular Instruments), a Ritch trabeculoplasty lens (Ocular Instruments), a Goldmann three-mirror lens (Haag-Streit), an Abraham iridectomy lens (Ocular Instruments), and a Yannuzzi lens (Ocular Instruments). Reflected beam power passing through the slit-lamp eyepieces was measured for worst-case orientations of each contact lens and compared to measurements of reflections from a planar mirror oriented perpendicular to the laser beam. Measurements were made at the argon blue (488 nm) and green (514.5 nm) wavelengths, and at four dye laser wavelengths (577, 595, 620, and 630 nm). A Scientech 1-inch calorimeter with a Fluke Model 8024B millivolt meter, and a Coherent Model 210 Laser Power Meter were both used as cross calibrations of measurement of the treatment beam power (generally 0.1 to 1.0 W settings). Measurement of aiming beam and reflected beam power and irradiance required the use of a more sensitive detector, and a United Detector Technology Model 40X Optometer with a silicon detector was employed for these measurements.
Results Beam power measurements—Treatment beam power measurements were within 1% agreement with the calibrated disk calorimeter, the accuracy limit of such measurements. Aiming beam power increased linearly with treat-
©AMERICAN JOURNAL OF OPHTHALMOLOGY 103:758-760, JUNE, 1987
Laser Hazards to Clinician
Vol. 103, No. 6
ment beam power, ranging from 0.1 mW/W of treatment power at an aiming beam power setting " 1 " to 2 mW/W of treatment power at setting " 4 . " Reflections to operator from aiming beam— Aiming beam power did not exceed the potentially hazardous 1 mW level for a momentary exposure. 2,3 Back reflections observed through the eyepieces, however, could be annoying if the contact lens were oriented in such a way as to maximize the reflection. To determine the maximum possible reflected beam power returning through the slit-lamp ocular, a fully silvered reference mirror was placed at the operating distance of the slit lamp, and oriented perpendicular to the beam path. Power setting and spot size were varied (Table 1). The mirror was positioned by a micromanipulator to obtain these worst-case reflections. Under a worst-case condition, the reflected power could exceed several microwatts of power, which would be potentially hazardous after several minutes to an hour of exposure. These power levels produce such an intense glare, however, that one could not perform effective photocoagulation, and the operator would adjust the lens to eliminate this annoyance within seconds. Orienting the lens to minimize discomforting glare increases the margin of safety. Thus, these worst-case reflections do not pose a realistic hazard (Table 2). Reflections to operator from treatment beam —In all laser photocoagulators, the manufacturer is required to protect the operator from viewing hazardous reflections through the slit lamp. All clinical photocoagulators have an
759
TABLE 2 W O R S T CASE MIRROR REFLEX BEAM POWER VS WAVELENGTH
WAVELENGTH
TREATMENT POWER SETTING
(NM)
(w)
577 595 595 620 630
0.5 0.5 1.0 0.5
All lines
1.0
AIM BEAM REFLEX (^w)
TREATMENT BEAM REFLEX*
11 8 15 7 8 21
0.41
(*»w) 0.055 0.048 0.09 0.05 0.05 0.25
'Treatment beam reflex power may have been predominantly blue-white fluorescence from the protective filter in the viewing optics. This is the origin of the puzzling "white flash" occasionally reported by clinicians using commercial photocoagulators.
appropriate filter that is switched into place when the footswitch (or hand trigger) is activated. In a properly designed system, the laser beam cannot be activated until this attenuator is in place. Measurements of back reflections for the full treatment power were made with the reference mirror and each lens placed at the working distance of the laser photocoagulator (Table 3). Normally, the attenuating filter was sufficiently dense so that only filter fluorescence (for example, yellow-orange or white light) could be detected.
Discussion Applicable occupational exposure limits for viewing a continuous wave laser beam vary
TABLE 1 WORST CASE MIRROR REFLEX POWER OF AIMING BEAM AT 514.5 NM
POWER SETTING
AIM BEAM*
(w)
(1 or 4)
0.1 0.1 0.5 0.5 1.0 1.0
High (4) Low(1) High (4) Low(1) High (4) Low(1)
50-jim SPOT x5
3.30 0.01 13.00 0.04 28.00 0.08
x20
12.00 0.03 50.00
0.14 100.00 0.30
TABLE 3 W O R S T CASE REFLECTED BEAM POWER T H R O U G H VIEWING OPTICS FOR TREATMENT BEAM
100/500-Mm SPOT x5
3.90 0.01 15.00 0.05 35.00 0.10
x20
6.10 0.02
IRRADIANCE LENS MANUFACTURER
LENS DESCRIPTION
(jiW/CM 2 )
Rodenstock
Panfunduscope lens
7
30.00
Ocular Instruments
OAIA Abraham iridectomy lens
6
0.08
Ocular Instruments
Yannuzzi lens
1
Haag-Streit
Goldmann three-mirror lens (U)
7
Ocular Instruments
Universal three-mirror lens
100.00 0.30
Ocular Instruments Karickhoff four-mirror lens "The ratio of the reflected aiming beam power to the treatment
Ocular Instruments
Ritch trabeculoplasty lens
1.5 20" 7
power for settings of 50-jtim retinal spot size and x 5 magnification was: 0.08 /xWA/V at setting 1; 0.64 M W / W at setting 2; 4.0 M W / W at setting 3; and 30 /xW/W at setting 4.
:,
1.5 /iW/cm 2 was obtained in all measurements except for one
position that required critical alignment.
760
June, 1987
AMERICAN JOURNAL OF OPHTHALMOLOGY
with wavelength and exposure duration. 2,3 For momentary unintentional exposure to a bright laser beam, the limit is 2.5 mW/cm2 (equivalent to 1.0 mW entering a 7-mm pupil), which is calculated for the eye's aversion response time to visible light (0.25 second). For a 10-second exposure, the limit drops to 1.0 mW/cm 2 (400 u.W total power entering the eye). For lengthy periods of fixation on a bright visible point source of laser light (a condition generally considered unrealistic, as it can be quite uncomfortable), the exposure limit is wavelength dependent, and most stringent for short wavelength (blue) light. For example, the limit is 10 mj/cm2 between 400 nm and 550 nm. This corresponds to an irradiance of 100 |xW/cm2 (40 u.W total power) for 100 seconds, 10 (xW/cm (4 jiW total power) for 1,000 seconds (16.7 minutes), and 1 u.W/cm2 (0.4 u.W total power) for 10,000 seconds (2.8 hours) or longer. Examining the reflected power values given in the Tables, it becomes apparent that the operator would never be exposed to hazardous levels through the slit lamp. Exposures from the treatment beam are all well below the criteria for many hours of viewing. The worst case back reflections of the aiming beam are normally on the order of 10 u.W7cm2 (a 1,000-second exposure limit), and would last only momentarily. Even if we consider the additive effects of multiple flashbacks during a day, it would be unrealistic to assume that a person would exceed the cumulative daily limit of 10 mj/cm2. However, a hazard can exist for a bystander
at the side of the slit lamp who is exposed to unattenuated back reflections of the treatment beam from the contact lens. 4 Any irradiance exceeding 2.5 mW/cm 2 is hazardous, and such levels could be found to the side of the contact lens, within 1 m of its front surface. Although a dazzling aiming beam back reflection from the mirror of a laser contact lens may be distracting and annoying, it does not pose a real ocular hazard to the laser operator. Since the cumulative effects of such reflections are unknown, however, it is important to use the lowest effective aiming beam power setting and to minimize reflexes by proper use of laser contact lenses. Bystanders within 1 m of the lens should also wear appropriate protective laser goggles during any photocoagulation procedure..
References 1. Ward, B.: Mirror laser-treatment lenses. Possible risks associated with lens design. Arch. Ophthalmol. 104:1585, 1986. 2. Sliney, D. H., and Wolbarsht, M. L.: Safety With Lasers and Other Optical Sources. New York, Plenum Publishing Corp., 1980, pp. 217-283 and 581-583. 3. American National Standards Institute: Safe Use of Lasers. Toledo, Ohio, Laser Institute of America, 1986, Z-136.1. 4. Sliney, D. H.: Biomedical laser safety. In Goldman, L. (ed.): The Biomedical Laser. New York, Springer-Verlag, 1982, pp. 11-24.