Handbook of Visual Optics, Volume Two

Instrumentation and Vision Correction

Pablo Artal

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781315355719
• Edition: 1
• Topic: Medizin
• Subtopic: Chirurgie & chirurgische Medizin

Handbook of Visual Optics offers an authoritative overview of encyclopedic knowledge in the field of physiological optics. It builds from fundamental concepts to the science and technology of instruments and practical procedures of vision correction, integrating expert knowledge from physics, medicine, biology, psychology, and engineering. The chapters comprehensively cover all aspects of modern study and practice, from optical principles and optics of the eye and retina to novel ophthalmic tools for imaging and visual testing, devices and techniques for visual correction, and the relationship between ocular optics and visual perception.

Part I

Ophthalmic instrumentation

1 Light safety

Ken Barat

Contents

1.1 Introduction

1.2 Why all the concern over lasers?

1.3 Continuous wave

1.4 Pulsed lasers

1.5 Hazard classification

1.5.1 Class 1

1.5.2 Class 1M

1.5.3 Class 2

1.5.4 Class 2M

1.5.5 Class 3R (once known as 3A)

1.5.6 Class 3B

1.5.7 Class 4

1.5.8 Class 1 product (sometimes referred to as an embedded laser system)

1.6 Common medical laser wavelengths

1.6.1 Laser safety items

1.6.1.1 Appointment of a laser safety officer

1.6.1.2 Laser protective eyewear

1.6.1.3 Signage

1.6.1.4 Operating procedures

1.7 Biological effects

1.7.1 Maximum permissible exposure

1.7.2 Nominal ocular hazard zone

1.7.3 Damage mechanisms

1.7.3.1 Electromechanical/photodisruption/acoustic damage

1.7.3.2 Photoablation

1.7.3.3 Thermal damage

1.7.3.4 Photochemical damage

1.7.3.5 Off-axis laser damage

1.7.4 Ultrafast laser injury

1.7.5 Laser radiation effects on skin

1.7.5.1 Epidermis

1.7.5.2 Dermis

1.7.5.3 Subcutaneous tissue

1.7.5.4 Skin effect by wavelength

1.7.6 Optics

1.7.6.1 Increased hazards when using optics

1.8 Aided viewing from a fiber

1.9 Accidents and related events

1.10 MAUDE database

1.10.1 Report 1: MAUDE DB Iris medical slit lamp report number 2939653-1999-00002

1.10.2 Report 2: Alcon Research Ltd/Huntington Acrysof Restor intraocular lens

1.10.3 Report 3: Device problem, self activation or keying

1.10.4 Report 4: MDR Report Key 4121056 device operated differently than expected

1.10.5 Report 5: MDR Report Key 4105971 ophthalmic femtosecond laser, lensex laser system

1.10.6 Report 6: Report Key 4076806 wavelight EX500 excimer laser

1.10.7 Report 7: MDR Reporting Key 3735743 Wavelight FS200 femtosecond laser

1.10.8 Report 8: MDR Report Key 4124364 Catalys precision laser system ophthalmic femtosecond laser

1.10.9 Report 9: Laser fire

1.10.10 Report 10: Model No. Purepoint LIO nature of problem, malfunction

1.10.11 Report 11: Event, negligence

1.10.12 Report 12: Device problem, device inoperable

1.10.13 Report 13: LASIK eye surgery

1.10.14 Report 14: Medlite C6

1.10.14.1 Manufacturer response

1.10.15 Report 15: Electrical shock

1.10.15.1 Manufactures response

Bibliography

Worth reading references

1.1 INTRODUCTION

The goal of this chapter is to discuss the hazard, in particular laser light. We will cover a number of topics that the reader may think they are familiar with, but in the case of safety, repetition is not a bad thing. In addition, there is coverage of a number of laser incidents. The author of this chapter hopes that by the end the reader will have an appreciation for the potential harm they may be exposed to. A number of resource materials will be referenced, and the author wishes to thank those individuals who allow open access to their material.

An awareness of laser safety is extremely important, not only for the benefit of users and ancillary/support staff but also to answer patient questions and concerns. With the increase in an aging population comes an increase in eye diseases. Ophthalmology has responded with technology such as mydriatic and nonmydriatic fundus cameras, phacoemulsification devices, femtosecond lasers, supercontinuum light sources, and optical coherence tomography. These advances support the need for understanding an appreciation of laser safety.

1.2 WHY ALL THE CONCERN OVER LASERS?

With all the hazards one faces during their day, why do we feel laser safety deserves special attention? While laser light or radiation has properties that distinguish it from natural light, the answer is rather simple. Visible (400–700 nm) and near-infrared wavelengths (700–1400 nm) are focused by the lens of the eye to a spot size an order of magnitude smaller than that of natural or incoherent visible light, a 300–200 μm spot size to one of 10–20 μm. Thus producing an irradiance (power per square centimeter) much higher than incoherent light. The item that gets lost sometimes is while no one expects to look directly into a laser beam, with this magnification of 100,000 to the macula even a small reflection of a laser beam between 400 and 1400 nm has the potential to cause some level of retinal injury.

See Figures 1.1 and 1.2 for a graphic display of this point. A 1 mW/cm² beam in is 100 W/cm² at ones macula.

1.3 CONTINUOUS WAVE

Laser systems that can produce uninterrupted laser energy are called continuous wave (CW) lasers. As long as they are turned on, a beam of laser light is emitted. By convention, any laser that emits light for longer than 0.25 s is called a CW laser. External shutters can be used to “chop” the beam so it has a strobe appearance, but the output power remains constant even in the chopped beam. The seemly pulse of the bar code scanner is an example of a chopped CW laser. The power output of CW lasers is measured in units of watts.

1.4 PULSED LASERS

Pulsed laser systems emit a beam that is less than 0.25 s in duration. The 0.25 s demarcation between CW and pulsed lasers is based on the approximate time for a human reflex to very intense light to work (aversion response). The pulsing in a pulsed laser system is achieved internal to the laser. Some employ an electrical “Q-switch” or are “mode locked” to achieve the shorter pulse widths. The pulse is defined by the pulse width (or emission duration) and the number of pulses per second known as pulse repetition frequency. Unlike the CW laser, all of the energy is emitted in short bursts. The total energy is compacted into a shorter time interval so the peak energy output can be very high. This makes shorter pulses more hazardous. Many pulsed lasers have pulse widths measured in nanoseconds (1 billionth of a second). Pulse rates to attosecond (10¹⁸ s) pulse widths have been routinely produced (but only in vacuum). A laser must output pulses at a rate faster than 1 Hz to be considered a repetitive pulsed laser. The output is measured in terms of Joules (Watts × seconds). Turnkey units are available for the generation of femtosecond laser pulses, 10¹⁵ s.

With the development and refinement of the laser diode, a highly reliable and convenient laser source became available for medical applications. A source that is robust and does not require an in-house service person to keep it running has few maintenance needs. Many laser systems combine the diode or a diode array with a fiber optic system. The fiber optics is used as a means to deliver the laser radiation without the use of open beam paths or beam enclosures.

Figure 1.1 Implications from environmental lighting.

1.5 HAZARD CLASSIFICATION

An internationally agreed to hazard classification system has been developed. Its simple goal is to allow one to understand the potential risk of a laser by just knowing its classification. Classes range from Class 1, 1M, 2, 2M, 3R, 3B, and 4, and one should include Class 1 product. The specific class definitions are as follows.

1.5.1 CLASS 1

Class 1 lasers and laser systems by definition are not an eye or skin hazard, their output is usually in the microwatt range.

1.5.2 CLASS 1M

Class 1M lasers do not exceed the Class 1 AEL for unaided viewing but do exceed the Class 1 AEL for optically aided viewing. Class 1M lasers do not exceed the Class 3B AEL for optically aided viewing. This does not, however, mean that the system is incapable of doing harm. ANSI Z136.1 requires that classification is based only on unaided and 5 cm aided viewing conditions. Therefore, hazards may still exist when using viewing optics with a greater optical gain than 7.14 (5 cm optics). They also have no unique safety requirements for their use (no extra engineering controls or safety devices). They also have no unique safety requirements for their use (no extra engineering controls or safety devices).

Figure 1.2 Extended and point source power density at the retina.

1.5.3 CLASS 2

Class 2 lasers are visible (400–700 nm wavelength) CW and repetitive-pulse lasers and laser systems that can emit accessible radiant energy exceeding the appropriate Class 1 AEL for the maximum duration inherent in the design or intended use of the laser or laser system, but not exceeding the Class 1 AEL for any applicable pulse (emission) duration <0.25 s and not exceeding an average radiant power of 1 mW. Laser systems with invisible beams cannot be Class 2.

1.5.4 CLASS 2M

Class 2M lasers are limited to visible wavelengths (400–700 nm) at accessible emission levels below the Class 2 AEL for the unaided eye. However, they can exceed the Class 2 AEL under optically aided viewing conditions. Class 2M lasers do not exceed the Class 3B AEL for optically aided viewing.

1.5.5 CLASS 3R (ONCE KNOWN AS 3A)

Class 3R lasers include lasers and laser systems that have an accessible output between one and five times the Class 2 AEL for visible lasers and the Class 1 AEL for all other lasers, based on the appropriate exposure duration. The output range is 1–5 mW.

1.5.6 CLASS 3B

Class 3B lasers and laser systems include the following:

UV (180–400 nm) and IR (1400 nm to 1 mm) lasers and laser systems that emit accessible radiant power in excess of the Class 3R AEL during any emission duration within the maximum duration inherent in the design of the laser or laser system, but that (a) cannot emit an average radiant power in excess of 0.5 W (5–500 mW CW) for greater than or equal to 0.25 s or (b) cannot produce a radiant energy greater than 125 mJ in 0.25 s.

Visible (4...

Table of contents

• Cover
• Half Title
• Dedication
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Editor
• Contributors
• Part I Ophthalmic Instrumentation
• Part II Vision Correction
• Part III Impact of the Eye’s Optics on Vision
• Index