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Extended sources—concepts and potential hazards
Extended potential
sources-concepts hazards
and
E. SUTTER The hazard for the eye produced by extended sources depends on the size of the source and the imaging devices used between the source and the eye. Formerly, the maximum permissible exposure was described in international standards such as IEC 825, by giving a limit value for the radiance. As a result of biological data, which show that the permissible irradiance on the retina decreases in inverse proportion to the size of the image on the retina, this international standard has recently been changed. Ocular hazards associated with extended sources are discussed from these aspects and typical examples are given of how these hazards should be assessed. It is shown that, in some cases, the new rules for the classification of extended sources are too restrictive. This could seriously limit the use of light-emitting diodes (LEDs), which are now included in the scope of the standard. KEYWORDS:
extended
sources, hazards, laser safety, standards
Introduction The concept of extended sources was introduced very early into laser safety standards. These standards’ m4 therefore contained two sets of maximum permissible exposure (MPE) values, one for in&abeam viewing and another for the viewing of extended sources and diffuse reflections. Both designations for the viewing conditions are not very appropriate. The expression ‘in&abeam viewing’ only means that the eye of the observer is within the laser beam, which could also be the case for extended source viewing. Viewing of point sources would be a much more appropriate expression. In the case of laser radiation, most people think of a parallel beam of light, i.e. a point source at an infinite distance from the eye. However, viewing a light emitting diode (LED), or the end of an optical fibre, could also well be described as viewing a point source--both are so small in their linear dimensions that, without optical aids, they would form a diffraction limited image on the retina, even though they emit not a parallel but a divergent beam of light. To speak of ‘viewing a laser beam by diffuse reflections’ as a designation of the second viewing condition is equally misleading. A diffuse reflection of a laser beam of typically 1 to 2 mm diameter -like most HeNe and argon laser beams-- with the eye at a The author IS in the Physikalisch-Technische Bundesanstalt, Braunschwelg, Bundesallee 100, D-381 16 Braunschweig, Germany. Received 5 January 1994. Revised 14 June 1994 Accepted 16 August 1994.
0030-3992/95/$10.00 Optics & Laser Technology Vol 27 No 1 1995
distance of 1 m would subtend an angle of about 1 mrad, an angle for which the point source MPEs would apply. As a matter of fact, the discussions in the standardization bodies did not take much account of the extended source values because their applications were very limited. The viewing of laser diode arrays was the only example always mentioned besides diffuse reflections. These imprecise names for the two sets of MPE values have created many misunderstandings among the users of the standards. For example, the angle of acceptance defined in the paragraph on measurement conditions for extended sources’ was very often understood as the divergence angle of radiation from fibre ends or laser diodes. Owing to the growing market for laser diodes and light-emitting diodes (LEDs), the importance of the MPEs for extended sources has increased over the last few years. Initially, LEDs were not included within the scope of the laser standard. However, since the eye hazard does not depend on the coherence of the laser radiation, it was quite logical to include such hazards in the scope of the IEC standard5, especially since, from a physical point of view, there is no clear dividing line between LEDs and laser diodes. The typical dimensions of these devices are a few pm by a few hundred pm. To the naked eye, both would appear as point sources, but, due to the very large divergence of these diodes they are usually fitted with a lens of a short focal length. This lens acts as a magnifying glass and the size of the image of such a
@ 1995 Elsevier Science.
All rights reserved 5
Extended
sources-concepts
and potential
hazards:
diode on the retina would no longer be diffraction limited. The various problems associated with the change of the MPE values and of the classification rules for extended sources, as well as the inclusion of LEDs in the scope of the laser safety standard, are discussed in the following.
Basic concepts Due to the heat flow patterns in the retina it is necessary to make a distinction between the MPE values for point sources and those for extended sources. A diffraction limited image of a point source on the retina would have a diameter of less than 10 pm. If radiation is absorbed within such a tiny spot, heat flows not only forward and backward parallel to the direction of propagation of the radiation, but a substantial amount of heat also flows in radial directions in the plane perpendicular to the direction of propagation (see Figs l(c) and (d)). The other extreme would be encountered if a very large area on the retina were irradiated. In this case, most of the heat would flow from the centre of this area in the two axial directions perpendicular to the irradiated area, and heat flow in radial directions would be negligible. The temperature rise in the central part of the area would be proportional to the irradiance (see Fig. l(a)). In the case of a thermal damage mechanism, where the temperature determines if damage occurs, the hazard would be proportional to the irradiance. Due to the laws of geometric optics, the irradiance on the retina depends only on the radiance of the source. The radiance will not be changed by optical elements in the beam path as long as these do not absorb or reflect a part of the radiation. These circumstances lead quite logically to MPE values which are expressed in terms of radiance (SI unit W m ~’ sr ‘) or time-integrated radiance (SI unit J m * sr~ ‘) for short exposures. All MPE values for extended sources are therefore expressed this way in the 1984 edition of the IEC standard’ and also in the 1993 edition of the standard’ if large areas are irradiated. The transition point would be case (b) in Fig. 1 where. only in the centre of the
E. Sutter
irradiated area, would the maximum be encountered.
temperature
rise
Since the 1970s it has been well known’ ’ that almost independent of the exposure durations the retinal lesion threshold (exposure) is roughly inversely proportional to the diameter d of the irradiated spot on the retina. This relationship has been shown to be valid for retinal image diameters between 30 pm and 1 mm and has recently been confirmed by Courant et ~1.‘~. In their paper they plotted the retinal exposure necessary to produce ophthalmoscopically visible damage (damage threshold) versus the retinal image diameter for wavelengths between 400 nm and 1064 nm. The maximum permissible exposure (MPE) values vary over the whole wavelength range. A ‘safety factor’ is therefore defined in the present paper as the ratio of the damage threshold divided by the applicable MPE value (see Fig. 2). The l/d dependence of the lesion threshold on the spot size would also be expected from thermal models of the heat flow in the retina, where the diameter of the irradiated spot on the retina is not larger than a few /lrn (see Ref. 11). However, some models have suggested that no such dependence should exist for exposure durations shorter than approximately 1 ms, i.e. for all Q-switched lasers. As can be seen from Fig. 2, the l/d relationship holds even for pulses as short as IS ns. These biological data have led to the conclusion that the MPE values in the laser safety standards are not safe for extended sources’“. In Fig. 2 these data are
loo0 t-
1
\o \
2 ‘x,
\
\\
loops
‘b,
515nm
\
s\‘\, \
9
l&s/
4
+ 50Onm ’ 15ns 1064nm 1
Z
.600ns -694nm
a 633nm 0.1 1
1s 4 00- 700nm
100 Retinal
C Fig. 1 function
6
d Schematic dragram showing the temperature of the illuminated soot srze on the retina
rise as a
4
1OOOpm Image
Diameter
Fig. 2 The dependence of the safety factor for a retinal lesion on the Image size on the retina. Dashed lines correspond to damage thresholds determined by fluorescence angiography
Optics
& Laser Technology Vol 27 No 1 1995
Extended
replotted as the ratio of the retinal lesion threshold divided by the corresponding MPE value’,3 for point sources. This ratio gives the safety factor in the MPE values. A safety factor of about 10 is usually used to define the MPE values from experimentally determined, ophthalmoscopically visible, damage thresholds. This safety factor is used in order to take into account that other methods of damage detection, such as fluorescence angiography, microscopy and electron microscopy, are much more sensitive (see Fig. 2). Furthermore, the damage threshold is usually defined by a 50% probability of damage, a level much too high to provide adequate protection. This safety factor of 10 is therefore absolutely necessary. In the case of extended sources the safety factor would be reduced to 1 for an image size of 100 pm on the retina” and to less than 0.1 for larger images (see Fig. 2). This knowledge has led to the decision to change the standards and to reduce the point source limit by a factor of C, = ri/amin, where x is the angle subtended by the source at the eye and z,,,~” is a limiting angle below which a source is considered to be a point source. According to the Draft Amendment 2 to IEC 825 (Ref. 5), the l/d relationship applies for angles between c(,,,~”= 1.5 mrad and z,,,~,, = 100 mrad, i.e. for retinal spot sizes between 25.5 pm and 1.7 mm (using a focal length of the eye of 17 mm). Due to eye movements, the lower limit is time dependent and varies between 1.5 mrad for times < 0.7 s to 11 mrad for times > 10 s. In Fig. 2, a spot size of 25.5 itm was used in all cases for the comparison of the retinal damage thresholds with the MPE values. In the case of oblong sources where one dimension is larger than the perpendicular one, the shortest dimension” must be used for the calculation of the extension of the source if the MPE values are expressed in irradiance or integrated irradiance”. This has been derived from linear superposition of steady-state solutions of the heat equation for the retina for circular sources’2. For angles cz > !&ax = 0.1 rad the MPE values are still expressed as radiance’, due to the fact that for large irradiated areas, radial heat flow finally becomes negligible. The value of this limiting angle M,,, is not experimentally confirmed. No experiments exist in this range, only a theoretical model” giving values up to 2 mm spot size. These calculations, and other similar calculations13, show no change of the slope or the l/d dependence up to 2 mm. This leads to the conclusion that the application of the concept of radiance to the MPE values is not yet valid just above 1.7 mm.
sources-concepts
Distance
t-
Source
Fig. 3
and potential
E. Sutter
100 mm
Solid Angle of Acceptance max. 0. I rad Measurement
hazards:
conditions
Circular Aperture Stop (50 mm) for the clawflcation
Detector
of lasers
set was identical with the MPE values. They could be measured with an instrument having an aperture of 7 mm diameter, the maximum diameter conceivable for the eye pupil. In contrast to this, the power and energy of lasers emitting essentially parallel beams were to be measured with an instrument having an aperture diameter of 50 mm. The latter corresponds to common binoculars, which could reduce a beam diameter to the 7 mm diameter mentioned if the magnification of the binocular is slightly larger than seven. When the radiance is used as a limiting value (and considered to be constant), the irradiance on the retina is constant too. However, since the maxlmum permissible irradiance that does not lead to a damage, must be proportional to l/d, this simple description is no longer possible. How the power and energy of the laser are to be measured in order to guarantee that the concept of Class 1 lasers is inherently safe, must be assessed. IEC 825 uses, for measurements, an instrument having a 50 mm aperture” at a distance of 100 mm from the source (see Fig. 3), the same instrument being used for the measurement of point sources and parallel laser beams. The solid angle of acceptance Q would correspond to the dimensions of the source up to a maximum value of 0.1 rad. Compared with the former measurement rules (7 mm aperture diameter), this change increases the power of divergent sources intercepted by the measurement by a factor of (50/7)2 z 50. Whether or not this approach is too conservative, must be analysed. In spite of the fact that, for angles c( > x,,, = 0.1 rad, the MPE values are still expressed in radiance, energy and power are also measured in this case with an instrument of 50 mm aperture. Hazard
assessment
Radiance
This change has considerable impact on the classification of lasers. According to the concept of Class 1, these lasers should be inherently safe, or safe by virtue of their engineering design, i.e. the maximum permissible exposure level cannot be exceeded under all reasonably foreseeable conditions of use. As long as the MPE values could be expressed in terms of radiance, or time-integrated radiance for short exposures, life was very simple because radiance and integrated radiance cannot be changed by optical instruments. Consequently, one set of accessible exposure limits (AEL) for the classification of lasers was expressed in W m ~’ sr- ’ or J m _ ’ sr ’ too, and this Optics & Laser Technology Vol 27 No 1 1995
FOJ hazard assessment, it is most appropriate and simple to use the concept of radiance. The radiance L. in a given direction at a given point is the quotient of the radiant power dP passing through that point and propagating within the solid angle do in a direction E divided by the product of the area of a section of that beam on a plane perpendicular to this direction containing the given point (cos E d.4) and the solid angle dR (see Fig. 4 and Ref. 14) L=
dP dR d A cos I-:
(1)
7
Extended sources-concepts
and potential
hazards: E. Sutter If d, is the linear dimension of the source and d, is the linear dimension of the retinal image, for reasons of geometry
dC2
(6a)
and consequently
(W L, = L, = L.
Therefore
Fig. 4
Defimtion
Retinal
of radiance
irradiance
The irradiance The same definition holds for the time-integrated radiance if in (1) the radiant power dP is replaced by the radiant energy dQ. In most practical cases, the angle E z 0 and consequently cos e z 1. In the following simplified discussions this approach is considered and the radiance L is independent of I:.
t‘”
=
p
F”
It is easily shown that the radiance is independent of optical imaging for optical elements without absorption. In the following, the derivation is demonstrated in the case of the eye (see Fig. 5). If the radiating source has a radiance L, . an area F’s and is at a distance (I, from the eye lens (eye pupil area FJ, the solid angle dR is dR = &/a:
(2)
According to (I), or i: = 0, the total power P entering the eye through the pupil is
=
E, on the retina would be (with u2 =.fi)
LFf= LQ ./‘.f
o
In the case of laser eye protection, the pupil diameter d, is considered constant (7 mm), therefore F, = (ri 7r/4 is equally constant, like the focal length fi of the eye. Consequently, the irradiance on the retina depends only on the radiance L of the source. In laser safety standards, the MPE for point sources is always expressed as an irradiance value on the cornea of the eye. The irradiance on the retina is higher by a factor (d,/d,)‘. The angle a subtended by a source at the eye and the minimum angle z,,,~” used in the standard can be expressed as the diameter of the retinal image using the relations I/,, =
r,f,
or
(co
d,i” = x,i,.f$
Taking into account that the power entering the eye is spread over a retinal area proportional to the square of the image diameter on the retina, and that the permissible irradiance on the retina is proportional to l/d (C, = cc/x,,,), the MPE at the cornea can be expressed as
This total power P would be focused on the retina to a surface area F,. The distance of the retina from the eye lens is u2 2 j, where .I:, is the focal length of the eye (17 mm) and the solid angle is equal to R, = F,/u: = 0.133 sr. Applying (2) to the part of the imaging within the eye (radiance LZ) yields
MPE = MPE,;,
4
= MPE”,,”
Lin P=
(7)
’
= MPE,,,C,
%rnin
L,F,F”
(9
.
(4)
uf
Since the power of the laser is the same in both cases.
L2 F,u: Ll = F,LZf
(5)
where MPE,,, is the MPE for a point source. This relation holds as long as d, 3 dmin (or c( 3 ~l,~,,). If d is smaller. the permissible irradiance becomes independent of the image size. Due to the increase of the irradiance on the retina compared with the value on the cornea by a factor of (d,/d,)2, (9) leads to the following relation for the permissible irradiance E,, on the retina F:,, = MPE,,,
d;
(10)
dn dmin
Fig. 5
lmagmg
by the eye
Since d, decreases with the the eye, E,, increases with sources this value must be of the irradiance according
distance of the source from this distance. For extended put in relation to the value to (7). With F, = diq’4, Optics
8
& Laser Technology Vol 27 No 1 1995
Extended
one obtains
the following
requirement
LF, d, dmin
En
E np = MPE,i,
.f,’ dz
XL d, dmin 4MPEmi”
<1
(11)
.f,’
With (6a), this formula shows that, for the standardized viewing conditions3 with a distance of 10 cm from the eye, the risk is 2.5 times higher than for the generally assumed distance of 25 cm, because d, is 2.5 times larger in this case. Magnifying
instruments
The use of magnifying instruments influences the irradiance on the retina in two ways: the image is enlarged, but usually, at the same time, more power is collected. This is true above all for divergent sources. Without the magnifying instrument the eye would not approach the emitting source by more than the clear visual distance. At this distance the angle of acceptance for the radiation would be much smaller than with the use of the magnifying instrument where the distance would be approximately equal to the focal length of the instrument. The linear magnification M of an optical instrument is defined as the ratio of the visual angles subtended by the object with and without the instrument. The clear visual distance without an optical instrument is considered to be 25 cm. This is shown in Fig. 6. How much more power would be collected due to the use of optical instruments depends on the emission characteristics of the radiator. In Fig. 6 this is demonstrated by the different emission cones of monomode and multimode fibres. In the case of an LED the emission would even be broader than with the multimode fibre. Since typical fibre diameters are between 5 lirn and 100 pm, using the smallest value for z ( = rmin) leads to values for M of between 7 and 50 for fibres to become extended sources, and typical distances of the objective lens of the optical instrument from the fibre end or the emitting diode would be between 5 mm and 30 mm. In the case of a wide emission characteristic, (250/5)2 = 2500 times more power would be collected, whereas due to the magnification by a factor of (250/5) = 50 = C,, only 50 times the power would be acceptable without increasing the risk.
sources-concepts
and potential
hazards:
E. Sutter
such as excimer lasers, do not have circular beam cross-sections but rectangular ones. In this case, a beam width can be said to exist in both the horizontal and vertical directions. Several concepts are used for the definition of the beam diameter. In radiation safety applications the beam diameter is defined as the distance between diametrically opposed points where the irradiance is l/e times the peak irradiance. This definition is not appropriate where the beam profile is not Gaussian. The beam waist is an important point when describing the propagation of laser beams in free space. It is that point of a freely propagating laser beam where its diameter is smallest (see Fig. 7). In some cases, the beam waist may be within the laser resonator or at the output mirror. The latter is approximately valid for LEDs and fibres. The beam diameter increases behind the beam waist as a result of the beam divergence. Up to the point where the beam waist is reached, the beam diameter may decrease with increasing distance from the laser in the near field. The beam divergence 6 is the full plane vertex angle of the propagating beam. In the case of a Gaussian beam, the divergence is given by:15 ii = 2 b = ~J(Z d,)
(124
where i is the wavelength of the laser radiation and d, is the diameter of the beam waist. For non-Gaussian beams, it may be calculated (see Fig. 7) from the relation li = (d, - d,),i~
( 12b)
Strongly diverging beams are typical of many communication systems. In these cases the numerical aperture NA is very often used to describe the beam divergence. The numerical aperture NA of a laser beam is the sine of one half of the vertex angle of the cone formed by a
Examples Considering a freely propagating beam, the beam diameter as a function of the distance from the emitter must be calculated. Only circular laser bundles can be described as having a beam diameter. Some lasers,
eye
r visual distancFig. 6
View of a fibre end with and without
Optics & Laser Technology Vol 27 No 1 1995
an optical instrument
Fig. 7 Increase of the beam diameter to the beam divergence
behind
the beam waist due
9
Extended
sources-concepts
laser bundle
and potential
hazards:
E. Sutter
(see Fig. 7)
NA = sin /i
(13)
According to this definition, for a small divergence the numerical aperture NA is approximately one half of the divergence. Due to its divergence, the beam diameter d increases with the distance II measured from the beam waist according to the following formula (d, is the beam diameter at the beam waist) d = (df + (ii u)‘)’
’
(14) Ftg. 8
Far away from the beam waist the contribution negligible and the following holds rl = ci LI
However, the definition of the beam diameter, which differs for communication applications, must also be borne in mind; using the numerical aperture for the description of divergence, the beam diameter is usually defined by the points where the irradiance is 5% of the peak irradiance in the beam. Based on a Gaussian beam profile, this different approach can be corrected for by dividing the divergence by a factor of 1.73 2 2. In this case
1.
Beam
Emitter type Monomode fibre Multimode fibre Diode laser
diameters
(16)
of divergent
wlthout
beams
Fig
and
9
power
View
of a laser diode
entering
Beam diameter at 25 cm distance
Fraction of the power entering the eve
Beam diameter atlOcm distance
0.1
25 mm
0.08
0.2
50 mm 66 mm
the
fitted
with
a condenser
instrument
of
lens
eye Fraction of the power entering the eve
Total power enters the eye at a distance of
10 mm
0.49
70 mm
0.02
20 mm
0.12
35 mm
0.01
26 mm
0.07
27 mm
Optics
10
an optlcal
If such a device were observed from some distance with a telescope (see Fig. 10) having a magnification
Typical value of the numerical aperture
(0.7/0.1) =a 0.26
of a laser diode
As an example, we have a quadratic emitter measuring 300 jlrn by 300 pm in the focal plane of a lens having a focal length of 30 mm and a diameter of 25 mm, i.e. the numerical aperture of this lens is 0.38. If this device is observed with the naked eye without optical instruments, at the retina this structure subtends an angle of 300 pm/30 mm = 10 mrad (using the length of the diagonal would give an angle 1.4 times greater). If the eye is close to the lens, the fraction of the total power entering the eye would be (7/25)2 = 0.078.
For laser diodes and LEDs the divergence is different for the two directions parallel and perpendicular to the emitting structure. In this case, the numerical aperture of the device may be calculated as the square root of the product of the two numerical apertures. The emission profile of these devices is much broader than that of fibre ends. Typical values for the numerical aperture of monomode tibres are 0.1, for multimode fibres 0.2, and for laser diodes and LEDs 0.1, and 0.7 respectively for the two directions, giving a geometrical mean of 0.26. Using the above formulas leads to the beam diameters given in Table 1 for a clear viewing distance of 25 cm and a distance of IO cm. the distance which is considered by IEC 825 to be the minimum focusing distance of the human eye (see Fig. 8). Between 10% and 50% of the radiation would enter the eye at these distances. The total power within the defined beam diameter would enter the eye at the distance given in the last column of Table I. In practice, this could happen if a magnifying glass with a focal length corresponding to these values were used.
Table
view
Owing to their high divergence, laser diodes and LEDs are mostly used with a condenser lens of short focal length in front of them (see Fig. 9). Typical focal lengths of such lenses range from a few mm to about 50 mm. The numerical aperture of these lenses is made as high as possible, values up to 0.5 and more are not unusual. Sometimes several emitting structures are arranged in parallel, forming an almost quadratic structure. In this case the divergence is almost equal in both directions.
(15)
ii 2 arc sin NA 2 NA
Direct
of d, is
& Laser Technology Vol 27 No 1 1995
Extended
sources-concepts
and potential
hazards:
E. Sutter
Of course, these considerations can be formulated in a more general way. Again, the most simple approach is to use the radiance since the radiance cannot be changed by optical imaging. In our case the solid angle within which the radiation of the emitting diode is encompassed, would be *, = Fig. 10 telescope
View
of a laser diode
fitted
with
a condenser
lens with
IrfTI
(17)
uf 4
a
where d, is the diameter of the condenser lens and a, is the distance of the condenser lens from the emitter. Using the numbers of the example 8 and a diameter of the objective lens of at least 25 mm, the total power would be collected and enter the eye. The image on the retina would be larger by a factor of 8 than without the telescope. This reduces the risk of a lesion by a factor of 8. But the total risk would still be higher by a factor of l/(8 x 0.078) = 1.6. In many cases, the emitters are not placed exactly in the focal plane of the lens in front of them, but are displaced by a small amount in order to get a defined divergence. From the safety point of view, the most unfavourable case would be if the emitter were displaced from the lens in such a way that an image would be formed on the pupil of the eye with a magnification M (see Fig. 1I), so chosen that the image would be exactly as large as the pupil diameter, and the total power could therefore enter the eye. In our example, M would have to be 7 mm/300 itrn = 23.3 mm. In the example, the image would be formed at a distance of 608 mm from the condenser lens. The eye lens could now no longer focus the emitter on the retina, but only on the lens in front of it. The image on the retina would subtend an angle of 25 mm/608 mm = 41 mrad. This angle is 4.1 times greater than the angle in the first case, reducing the risk by the same factor. However, since the total power would enter the eye, this case would be more hazardous by a factor of l/(4.1 x 0.078) = 3.1, the risk would be twice as high as in the case of an observation with the telescope previously discussed. The worst case would be encountered if the parameters of the telescope were chosen so as to make the risk the highest. This would be the case with a magnification M = 25,J7 = 3.6. In this case, the total power would also enter the eye and the risk would be higher by a factor of l/(3.6 x 0.078) = 3.6 than in the first case. This factor is slightly larger than in the second, latter, case.
R, = 252 mm2 7r/4/302 mm’ = 0.55 sr The radiance L=
averaged
(18)
over this solid angle would be
1 P (19)
R, d,2
where d, is the linear dimension of a square source (dt = d, d,:width times breadth in the case of an oblong rectangular emitter) and P is the total power emitted within 0,. The second parameter to be considered is the magnification by the optics used, since this would influence the reduction factor C, which determines the risk to the retina Without an optical instrument between eye and emitter, the linear dimension of the retinal image d,, would be (clear visual distance d, = 25 cm)
(20) Using an optical instrument having a linear magnification M, the dimension of the retinal would be d:, = d, M
(21)
and the irradiated
area on the retina would therefore
F n =F,M’
(22)
P Per
=
MPE,,,
4 dh”
MF,
(23)
Using (4) (20) and (22) with uZ =.1,
LF, Fn ,r’,’
=
LF, F, d,Z
(24)
M2
The ratio of both powers would be a measure risk. Using (8) and (20) we have LM Risk = ~ MPE,i”
Optics & Laser Technology Vol 27 No 1 1995
where
the condenser
lens forms
an
be
The MPE values are usually given as power or energy density. If (9) and (21) are used. the permissible power would be
P=
Fig. 11 View of a laser diode image on the pup11 of the eye
image Irk
d, dmin d,.L
of the
LM ‘0
'min
(25)
MPLin
with CY,= d,/d,, the angle at which the source would appear without optical aids. This relation is valid as
11
Exrended sources-concepts
and porenrial
hazards: E. Surrer
long as the image on the retina is not greater than %ex * the angle up to which the permissible power is proportional to C, and the total power enters the eye due to the magnifying instrument. For larger angles, the risk would become independent of the magnification, and be given by L
Risk = MPF,,,
rirfnin
(26)
With these equations, devices using optics or viewed with optical instruments may easily be analysed. Classification Parallel to the change of the MPE values in the new edition of IEC 825 (Ref. 6), i.e. the dropping of the values expressed in radiance or integrated radiance for extended sources, the classification rules for extended sources have also been changed. Whereas before, the radiance had been measured with a detector having an aperture of 7 mm diameter at 100 mm distance from the source, the diameter of the detector aperture is now 50 mm; the distance is still kept at 100 mm. This means that the detecting instrument has a numerical aperture of 0.25. In many cases this will reduce the permissible power for, say, Class 1 by a factor of (50i7)2 = 51. Whether or not this reduction is necessary for safety reasons, or if it is over-restrictive, should therefore be examined. During this analysis it should be borne in mind that the concept of Class I lasers is that they should be inherently safe. i.e. that the maximum permissible exposure level cannot be exceeded under reasonably foreseeable conditions of use. These foreseeable conditions would also include the use of optical instruments. The accessible emission limits (AELs) for Class 1 are derived from the MPEs by multiplying them by the area of the applicable aperture stop, i.e. with 0.4 cm2 (corresponding to a diameter of 7 mm) in the visible and in the near infra-red. The use of the correction factor C, for extended sources is judged on the basis of the smallest possible distance from the apparent source, but this distance will never be less than 100 mm. Having stated these basic considerations. it is obvious that there are conditions conceivable where Class I lasers are not safe at all: for example, if laser diodes, LEDs, or diodes fitted with pigtails of a high numerical aperture NA = 0.6 is not unusual are observed with an optical instrument having a numerical aperture greater than 0.24, a quite realistic case for microscopes and magnifying glasses of modern design. This case is also critical insofar as it is common practice in communication techniques to observe fibres and emitters with optical instruments in order to check if their surface is damaged. One would not hesitate to use optical instruments in this case knowing that the system only contains radiation corresponding to Class 1. i.e. lasers that are safe under all reasonably foreseeable conditions of use. In many other cases the use of a measuring instrument with a numerical aperture of only 0.24 is not critical. The classification of typical lasers emitting a parallel beam is not affected by the numerical aperture of the
measuring instrument since their divergence not higher than about 1 mrad.
To illustrate the problems, the classification conditions are applied to the examples of Figs 8 to 11. In any case, the angle r subtended by the source at the eye must be calculated at a distance of 100 mm from the source and would, in this example, be z = (300 pm/100 mm) = 3 mrad. This value is higher by a factor of 2 than N,,,~,,,i.e. C, would be 2. Since the measuring instrument would have an aperture stop of 50 mm diameter at a minimum distance of 100 mm from the apparent source, it would not collect the total power, but only that fraction of the power that would correspond to the square of the ratio of the numerical apertures of the collecting instrument divided by the numerical aperture of the emitter. Since the diameter of the pupil of the eye is only 7 mm, there would be a high degree of safety. In the cases of Figs 9 to 11, the angle z subtended by the source at the eye would still have to be calculated at a distance of 100 mm from the source, and again it would therefore be 3 mrad and C, would be 2. However, in Fig. 9 the eye could be brought immediately in front of the collimating lens, i.e. to a distance of only 30 mm. In this case the source would subtend an angle x = (300 Llm/30 mm) = 10 mrad and C,, would be 6.7. For the measurement of the power of the source, the standard requires interpretation. The standard states that in the wavelength region from 302.5 nm to 4000 nm, the minimum distance of the measurement aperture from the apparent source must not be less than 100 mm. If this rule is interpreted purely geometrically, the detector would be placed at a distance of 70 mm from the collimating lens, which has a focal length of 30 mm. However, since the beam is parallel behind the lens, optically speaking, the detector would only be at a distance of 30 mm from the source. In this case the total power transmitted by the collimating lens would be collected, since its diameter is smaller than 50 mm. If the rule for measurements is interpreted optically, the measuring instrument with an aperture stop of 50 mm diameter would have to be placed at an optical distance of 100 mm from the apparent source. and it would not therefore collect the total power, but only a fraction (50 x 30/100/25)2 = 0.36. The reciprocal of this value is 2.8, i.e the real risk is higher by a factor of 2.8 in all eases where the total power can enter the eye. Since class 1 lasers should also be safe when optical instruments are used, the standard should be interpreted in a way which leads to the collection of the total power. Conclusions
and
outlook
As the preceding discussions have in IEC 825-1 are not, in all cases, characterize the real risk. In some restrictive, in other cases they are the concept of Class 1 lasers being rrrrsonah~~
for~srruhl~~
conditions
shown, the appropriate cases they too lenient ,sqf& under
new rules to are too to fulfil cd/
of use. Optm
12
is usually
& Laser Technology Vol 27 No 1 1995
Extended sources-concepts Another point should be mentioned here. The basis for using the shortest dimension of a source in the case of rectangular sources is the paper by Sliney and Freasier”. According to this paper, for thermal damage the total permissible power for a retinal exposure would be reduced from about 7 mW for an irradiated area of 25 pm x 25 pm to less than 4 mW if this area were 25 pm x 2000 pm! Such a result cannot be correct: thermal conductivity would reduce the temperature in the centre of this much larger spot”. The problem with the new IEC 825-1 is that many changes to the old IEC 825 were made in haste at a very late stage, some even during the final editorial stage. So it is already very clear that the next amendment will be necessary very soon. There will then be an opportunity to clarify and correct the inconsistencies and errors. Definitions This section contains the definitions important terms used in this paper.
of the most
Extended source. Biologically speaking, an extended source is a source forming an image on the retina which is large enough to limit considerably the heat flow from the centre of the image to the surrounding biological tissue. In the standard6 the minimum size is about exposure times less than 0.7 s.
25 pm for
Integrated radiance. The integral of the radiance given exposure time. Unit: J mm2 sr- ‘.
over a
Irradiance. The ratio of the radiant power d@ incident on the element of a surface of area dA of that element: E = d@/dA. Unit: W rnm2. Magnification M. The linear magnification M of an optical instrument is the ratio of the visual angles subtended by the object with and without the instrument. Point source. Biologically speaking, this is a source forming an image on the retina which is so small that heat can easily flow in a radial direction from the centre of the image to the surrounding biological tissue. In the standard’ the maximum size of a point source is about 25 pm for exposure times less than 0.7 s. Radiance. This term is extensively explained at the beginning of the section on hazard assessment. Unit: W mm2 sr-‘.
Optics & Laser Technology Vol 27 No 1 1995
and potential
hazards: E. Sutter
Radiant exposure H. The integral of the irradiance over a given exposure time, i.e the ratio of the radiant energy dQ incident on an element of a surface of area dA of that element: H = dQ/dA. Unit: J m~~2. Solid angle. The solid angle Sz with its vertex in the centre of a sphere of radius Y, is the ratio of the area A cut off by this angle on the surface of the sphere divided by the square of the radius: fi = ,4/r’. Unit: sr (steradian). A full solid angle has 471sr. References ANSI Z 136. I : American National Standard for the Safe Use of Lasers, American National Standards Institute, New York 1986 2 IEC Publication 825: Radiation Safety of Laser Products, Equipment Classification, Requirements and User’s Guide, Bureau Central de la Commission Electrotechnique Internationale, Genkve 1984 3 Amendment 1 to Publication 825 (1984): Radiation safety of laser products, equipment classification, Requirements and User’s Guide, Bureau Central de la Commission Electrotechnique Internationale, Gentve 1990 4 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indice.r, American Conference of Governmental Industrial Hygienists (ACGIH), Cincinnati, USA 2 to Publication 825 5 IEC DIS 76(Central Office)28: Amendment (1984) Radiation Safety of Laser Products, Equipment Classification. Requirements and Users Guide, Bureau Central de la Commission Electrotechnique Internationale. Genkve 1992 6 IEC Publication 825-l: Safety of Laser Products, Equipment Classification, Requirements and User’s Guide, Bureau Central de la Commission Electrotechnique Internationale, Gentve 1993 I Frisch, G. D., Beatrice, E. S., Holsen, R. C. Comparative study of argon and ruby retinal damage thresholds, fnt.est Ophthuf, IO (1971) 911&919 8 Ham, W. T. et al Retinal burn thresholds for the helium-neon laser in the rhesus monkey, Arch Ophthal, 84 (1970) 797-808 9 Goldmann, A. I., Ham Jr, W. T., Mueller, H. A. Ocular damage thresholds for ultra-short pulses of both visible and infrared radiation in the rhesus monkey, E.yup Eye Res. 14 (1977) 45-56 10 Courant. D.. Court, L.. Slinev. D. H. Spot-size dependence of laser dosimetry. SPIE IS 5 (i989) IX-.165 II Farrell, R. A., McCally, R. L., Bargeron, C. B., Green, W. R. Structural alterations in the cornea from exposure to Infrared radiation. Technical Memorandum, The John Hopkinb University, Laurel, Maryland (1985) 12 Sliney, D. H., Freasier, B. C. Evaluation of optxal radiation hazards. Appl Opt, I2 (1973) I-24 13 Clarke, A. M., Geerets, W. J., Ham Jr, W. T. An equilibrium thermal model for retinal injury from optlcal sources, 4ppl Opt. 8 (1969) 1051-1054 14 CIE Publication Number 50, Internationul Electrotechnical Vocabulary (IEV), Chapter 845: Lighting. Bureau Central de la Commission Electrotechnique Internationale. Geneve Suisse 1987 to Laser Ph,:sicx, Sprmger-Verlag 15 Shimoda, K. [nrroduction Berlin (1984) 16 Sliney, D. H., Wolbarsht, M. Sujety with Lasers und Other Optical Sources, Plenum Press, New York (1980) have shown that the 17 Note added in proof: recent calculations mean of the two dimensions should be used. I
13
sources-concepts hazards
and
E. SUTTER The hazard for the eye produced by extended sources depends on the size of the source and the imaging devices used between the source and the eye. Formerly, the maximum permissible exposure was described in international standards such as IEC 825, by giving a limit value for the radiance. As a result of biological data, which show that the permissible irradiance on the retina decreases in inverse proportion to the size of the image on the retina, this international standard has recently been changed. Ocular hazards associated with extended sources are discussed from these aspects and typical examples are given of how these hazards should be assessed. It is shown that, in some cases, the new rules for the classification of extended sources are too restrictive. This could seriously limit the use of light-emitting diodes (LEDs), which are now included in the scope of the standard. KEYWORDS:
extended
sources, hazards, laser safety, standards
Introduction The concept of extended sources was introduced very early into laser safety standards. These standards’ m4 therefore contained two sets of maximum permissible exposure (MPE) values, one for in&abeam viewing and another for the viewing of extended sources and diffuse reflections. Both designations for the viewing conditions are not very appropriate. The expression ‘in&abeam viewing’ only means that the eye of the observer is within the laser beam, which could also be the case for extended source viewing. Viewing of point sources would be a much more appropriate expression. In the case of laser radiation, most people think of a parallel beam of light, i.e. a point source at an infinite distance from the eye. However, viewing a light emitting diode (LED), or the end of an optical fibre, could also well be described as viewing a point source--both are so small in their linear dimensions that, without optical aids, they would form a diffraction limited image on the retina, even though they emit not a parallel but a divergent beam of light. To speak of ‘viewing a laser beam by diffuse reflections’ as a designation of the second viewing condition is equally misleading. A diffuse reflection of a laser beam of typically 1 to 2 mm diameter -like most HeNe and argon laser beams-- with the eye at a The author IS in the Physikalisch-Technische Bundesanstalt, Braunschwelg, Bundesallee 100, D-381 16 Braunschweig, Germany. Received 5 January 1994. Revised 14 June 1994 Accepted 16 August 1994.
0030-3992/95/$10.00 Optics & Laser Technology Vol 27 No 1 1995
distance of 1 m would subtend an angle of about 1 mrad, an angle for which the point source MPEs would apply. As a matter of fact, the discussions in the standardization bodies did not take much account of the extended source values because their applications were very limited. The viewing of laser diode arrays was the only example always mentioned besides diffuse reflections. These imprecise names for the two sets of MPE values have created many misunderstandings among the users of the standards. For example, the angle of acceptance defined in the paragraph on measurement conditions for extended sources’ was very often understood as the divergence angle of radiation from fibre ends or laser diodes. Owing to the growing market for laser diodes and light-emitting diodes (LEDs), the importance of the MPEs for extended sources has increased over the last few years. Initially, LEDs were not included within the scope of the laser standard. However, since the eye hazard does not depend on the coherence of the laser radiation, it was quite logical to include such hazards in the scope of the IEC standard5, especially since, from a physical point of view, there is no clear dividing line between LEDs and laser diodes. The typical dimensions of these devices are a few pm by a few hundred pm. To the naked eye, both would appear as point sources, but, due to the very large divergence of these diodes they are usually fitted with a lens of a short focal length. This lens acts as a magnifying glass and the size of the image of such a
@ 1995 Elsevier Science.
All rights reserved 5
Extended
sources-concepts
and potential
hazards:
diode on the retina would no longer be diffraction limited. The various problems associated with the change of the MPE values and of the classification rules for extended sources, as well as the inclusion of LEDs in the scope of the laser safety standard, are discussed in the following.
Basic concepts Due to the heat flow patterns in the retina it is necessary to make a distinction between the MPE values for point sources and those for extended sources. A diffraction limited image of a point source on the retina would have a diameter of less than 10 pm. If radiation is absorbed within such a tiny spot, heat flows not only forward and backward parallel to the direction of propagation of the radiation, but a substantial amount of heat also flows in radial directions in the plane perpendicular to the direction of propagation (see Figs l(c) and (d)). The other extreme would be encountered if a very large area on the retina were irradiated. In this case, most of the heat would flow from the centre of this area in the two axial directions perpendicular to the irradiated area, and heat flow in radial directions would be negligible. The temperature rise in the central part of the area would be proportional to the irradiance (see Fig. l(a)). In the case of a thermal damage mechanism, where the temperature determines if damage occurs, the hazard would be proportional to the irradiance. Due to the laws of geometric optics, the irradiance on the retina depends only on the radiance of the source. The radiance will not be changed by optical elements in the beam path as long as these do not absorb or reflect a part of the radiation. These circumstances lead quite logically to MPE values which are expressed in terms of radiance (SI unit W m ~’ sr ‘) or time-integrated radiance (SI unit J m * sr~ ‘) for short exposures. All MPE values for extended sources are therefore expressed this way in the 1984 edition of the IEC standard’ and also in the 1993 edition of the standard’ if large areas are irradiated. The transition point would be case (b) in Fig. 1 where. only in the centre of the
E. Sutter
irradiated area, would the maximum be encountered.
temperature
rise
Since the 1970s it has been well known’ ’ that almost independent of the exposure durations the retinal lesion threshold (exposure) is roughly inversely proportional to the diameter d of the irradiated spot on the retina. This relationship has been shown to be valid for retinal image diameters between 30 pm and 1 mm and has recently been confirmed by Courant et ~1.‘~. In their paper they plotted the retinal exposure necessary to produce ophthalmoscopically visible damage (damage threshold) versus the retinal image diameter for wavelengths between 400 nm and 1064 nm. The maximum permissible exposure (MPE) values vary over the whole wavelength range. A ‘safety factor’ is therefore defined in the present paper as the ratio of the damage threshold divided by the applicable MPE value (see Fig. 2). The l/d dependence of the lesion threshold on the spot size would also be expected from thermal models of the heat flow in the retina, where the diameter of the irradiated spot on the retina is not larger than a few /lrn (see Ref. 11). However, some models have suggested that no such dependence should exist for exposure durations shorter than approximately 1 ms, i.e. for all Q-switched lasers. As can be seen from Fig. 2, the l/d relationship holds even for pulses as short as IS ns. These biological data have led to the conclusion that the MPE values in the laser safety standards are not safe for extended sources’“. In Fig. 2 these data are
loo0 t-
1
\o \
2 ‘x,
\
\\
loops
‘b,
515nm
\
s\‘\, \
9
l&s/
4
+ 50Onm ’ 15ns 1064nm 1
Z
.600ns -694nm
a 633nm 0.1 1
1s 4 00- 700nm
100 Retinal
C Fig. 1 function
6
d Schematic dragram showing the temperature of the illuminated soot srze on the retina
rise as a
4
1OOOpm Image
Diameter
Fig. 2 The dependence of the safety factor for a retinal lesion on the Image size on the retina. Dashed lines correspond to damage thresholds determined by fluorescence angiography
Optics
& Laser Technology Vol 27 No 1 1995
Extended
replotted as the ratio of the retinal lesion threshold divided by the corresponding MPE value’,3 for point sources. This ratio gives the safety factor in the MPE values. A safety factor of about 10 is usually used to define the MPE values from experimentally determined, ophthalmoscopically visible, damage thresholds. This safety factor is used in order to take into account that other methods of damage detection, such as fluorescence angiography, microscopy and electron microscopy, are much more sensitive (see Fig. 2). Furthermore, the damage threshold is usually defined by a 50% probability of damage, a level much too high to provide adequate protection. This safety factor of 10 is therefore absolutely necessary. In the case of extended sources the safety factor would be reduced to 1 for an image size of 100 pm on the retina” and to less than 0.1 for larger images (see Fig. 2). This knowledge has led to the decision to change the standards and to reduce the point source limit by a factor of C, = ri/amin, where x is the angle subtended by the source at the eye and z,,,~” is a limiting angle below which a source is considered to be a point source. According to the Draft Amendment 2 to IEC 825 (Ref. 5), the l/d relationship applies for angles between c(,,,~”= 1.5 mrad and z,,,~,, = 100 mrad, i.e. for retinal spot sizes between 25.5 pm and 1.7 mm (using a focal length of the eye of 17 mm). Due to eye movements, the lower limit is time dependent and varies between 1.5 mrad for times < 0.7 s to 11 mrad for times > 10 s. In Fig. 2, a spot size of 25.5 itm was used in all cases for the comparison of the retinal damage thresholds with the MPE values. In the case of oblong sources where one dimension is larger than the perpendicular one, the shortest dimension” must be used for the calculation of the extension of the source if the MPE values are expressed in irradiance or integrated irradiance”. This has been derived from linear superposition of steady-state solutions of the heat equation for the retina for circular sources’2. For angles cz > !&ax = 0.1 rad the MPE values are still expressed as radiance’, due to the fact that for large irradiated areas, radial heat flow finally becomes negligible. The value of this limiting angle M,,, is not experimentally confirmed. No experiments exist in this range, only a theoretical model” giving values up to 2 mm spot size. These calculations, and other similar calculations13, show no change of the slope or the l/d dependence up to 2 mm. This leads to the conclusion that the application of the concept of radiance to the MPE values is not yet valid just above 1.7 mm.
sources-concepts
Distance
t-
Source
Fig. 3
and potential
E. Sutter
100 mm
Solid Angle of Acceptance max. 0. I rad Measurement
hazards:
conditions
Circular Aperture Stop (50 mm) for the clawflcation
Detector
of lasers
set was identical with the MPE values. They could be measured with an instrument having an aperture of 7 mm diameter, the maximum diameter conceivable for the eye pupil. In contrast to this, the power and energy of lasers emitting essentially parallel beams were to be measured with an instrument having an aperture diameter of 50 mm. The latter corresponds to common binoculars, which could reduce a beam diameter to the 7 mm diameter mentioned if the magnification of the binocular is slightly larger than seven. When the radiance is used as a limiting value (and considered to be constant), the irradiance on the retina is constant too. However, since the maxlmum permissible irradiance that does not lead to a damage, must be proportional to l/d, this simple description is no longer possible. How the power and energy of the laser are to be measured in order to guarantee that the concept of Class 1 lasers is inherently safe, must be assessed. IEC 825 uses, for measurements, an instrument having a 50 mm aperture” at a distance of 100 mm from the source (see Fig. 3), the same instrument being used for the measurement of point sources and parallel laser beams. The solid angle of acceptance Q would correspond to the dimensions of the source up to a maximum value of 0.1 rad. Compared with the former measurement rules (7 mm aperture diameter), this change increases the power of divergent sources intercepted by the measurement by a factor of (50/7)2 z 50. Whether or not this approach is too conservative, must be analysed. In spite of the fact that, for angles c( > x,,, = 0.1 rad, the MPE values are still expressed in radiance, energy and power are also measured in this case with an instrument of 50 mm aperture. Hazard
assessment
Radiance
This change has considerable impact on the classification of lasers. According to the concept of Class 1, these lasers should be inherently safe, or safe by virtue of their engineering design, i.e. the maximum permissible exposure level cannot be exceeded under all reasonably foreseeable conditions of use. As long as the MPE values could be expressed in terms of radiance, or time-integrated radiance for short exposures, life was very simple because radiance and integrated radiance cannot be changed by optical instruments. Consequently, one set of accessible exposure limits (AEL) for the classification of lasers was expressed in W m ~’ sr- ’ or J m _ ’ sr ’ too, and this Optics & Laser Technology Vol 27 No 1 1995
FOJ hazard assessment, it is most appropriate and simple to use the concept of radiance. The radiance L. in a given direction at a given point is the quotient of the radiant power dP passing through that point and propagating within the solid angle do in a direction E divided by the product of the area of a section of that beam on a plane perpendicular to this direction containing the given point (cos E d.4) and the solid angle dR (see Fig. 4 and Ref. 14) L=
dP dR d A cos I-:
(1)
7
Extended sources-concepts
and potential
hazards: E. Sutter If d, is the linear dimension of the source and d, is the linear dimension of the retinal image, for reasons of geometry
dC2
(6a)
and consequently
(W L, = L, = L.
Therefore
Fig. 4
Defimtion
Retinal
of radiance
irradiance
The irradiance The same definition holds for the time-integrated radiance if in (1) the radiant power dP is replaced by the radiant energy dQ. In most practical cases, the angle E z 0 and consequently cos e z 1. In the following simplified discussions this approach is considered and the radiance L is independent of I:.
t‘”
=
p
F”
It is easily shown that the radiance is independent of optical imaging for optical elements without absorption. In the following, the derivation is demonstrated in the case of the eye (see Fig. 5). If the radiating source has a radiance L, . an area F’s and is at a distance (I, from the eye lens (eye pupil area FJ, the solid angle dR is dR = &/a:
(2)
According to (I), or i: = 0, the total power P entering the eye through the pupil is
=
E, on the retina would be (with u2 =.fi)
LFf= LQ ./‘.f
o
In the case of laser eye protection, the pupil diameter d, is considered constant (7 mm), therefore F, = (ri 7r/4 is equally constant, like the focal length fi of the eye. Consequently, the irradiance on the retina depends only on the radiance L of the source. In laser safety standards, the MPE for point sources is always expressed as an irradiance value on the cornea of the eye. The irradiance on the retina is higher by a factor (d,/d,)‘. The angle a subtended by a source at the eye and the minimum angle z,,,~” used in the standard can be expressed as the diameter of the retinal image using the relations I/,, =
r,f,
or
(co
d,i” = x,i,.f$
Taking into account that the power entering the eye is spread over a retinal area proportional to the square of the image diameter on the retina, and that the permissible irradiance on the retina is proportional to l/d (C, = cc/x,,,), the MPE at the cornea can be expressed as
This total power P would be focused on the retina to a surface area F,. The distance of the retina from the eye lens is u2 2 j, where .I:, is the focal length of the eye (17 mm) and the solid angle is equal to R, = F,/u: = 0.133 sr. Applying (2) to the part of the imaging within the eye (radiance LZ) yields
MPE = MPE,;,
4
= MPE”,,”
Lin P=
(7)
’
= MPE,,,C,
%rnin
L,F,F”
(9
.
(4)
uf
Since the power of the laser is the same in both cases.
L2 F,u: Ll = F,LZf
(5)
where MPE,,, is the MPE for a point source. This relation holds as long as d, 3 dmin (or c( 3 ~l,~,,). If d is smaller. the permissible irradiance becomes independent of the image size. Due to the increase of the irradiance on the retina compared with the value on the cornea by a factor of (d,/d,)2, (9) leads to the following relation for the permissible irradiance E,, on the retina F:,, = MPE,,,
d;
(10)
dn dmin
Fig. 5
lmagmg
by the eye
Since d, decreases with the the eye, E,, increases with sources this value must be of the irradiance according
distance of the source from this distance. For extended put in relation to the value to (7). With F, = diq’4, Optics
8
& Laser Technology Vol 27 No 1 1995
Extended
one obtains
the following
requirement
LF, d, dmin
En
E np = MPE,i,
.f,’ dz
XL d, dmin 4MPEmi”
<1
(11)
.f,’
With (6a), this formula shows that, for the standardized viewing conditions3 with a distance of 10 cm from the eye, the risk is 2.5 times higher than for the generally assumed distance of 25 cm, because d, is 2.5 times larger in this case. Magnifying
instruments
The use of magnifying instruments influences the irradiance on the retina in two ways: the image is enlarged, but usually, at the same time, more power is collected. This is true above all for divergent sources. Without the magnifying instrument the eye would not approach the emitting source by more than the clear visual distance. At this distance the angle of acceptance for the radiation would be much smaller than with the use of the magnifying instrument where the distance would be approximately equal to the focal length of the instrument. The linear magnification M of an optical instrument is defined as the ratio of the visual angles subtended by the object with and without the instrument. The clear visual distance without an optical instrument is considered to be 25 cm. This is shown in Fig. 6. How much more power would be collected due to the use of optical instruments depends on the emission characteristics of the radiator. In Fig. 6 this is demonstrated by the different emission cones of monomode and multimode fibres. In the case of an LED the emission would even be broader than with the multimode fibre. Since typical fibre diameters are between 5 lirn and 100 pm, using the smallest value for z ( = rmin) leads to values for M of between 7 and 50 for fibres to become extended sources, and typical distances of the objective lens of the optical instrument from the fibre end or the emitting diode would be between 5 mm and 30 mm. In the case of a wide emission characteristic, (250/5)2 = 2500 times more power would be collected, whereas due to the magnification by a factor of (250/5) = 50 = C,, only 50 times the power would be acceptable without increasing the risk.
sources-concepts
and potential
hazards:
E. Sutter
such as excimer lasers, do not have circular beam cross-sections but rectangular ones. In this case, a beam width can be said to exist in both the horizontal and vertical directions. Several concepts are used for the definition of the beam diameter. In radiation safety applications the beam diameter is defined as the distance between diametrically opposed points where the irradiance is l/e times the peak irradiance. This definition is not appropriate where the beam profile is not Gaussian. The beam waist is an important point when describing the propagation of laser beams in free space. It is that point of a freely propagating laser beam where its diameter is smallest (see Fig. 7). In some cases, the beam waist may be within the laser resonator or at the output mirror. The latter is approximately valid for LEDs and fibres. The beam diameter increases behind the beam waist as a result of the beam divergence. Up to the point where the beam waist is reached, the beam diameter may decrease with increasing distance from the laser in the near field. The beam divergence 6 is the full plane vertex angle of the propagating beam. In the case of a Gaussian beam, the divergence is given by:15 ii = 2 b = ~J(Z d,)
(124
where i is the wavelength of the laser radiation and d, is the diameter of the beam waist. For non-Gaussian beams, it may be calculated (see Fig. 7) from the relation li = (d, - d,),i~
( 12b)
Strongly diverging beams are typical of many communication systems. In these cases the numerical aperture NA is very often used to describe the beam divergence. The numerical aperture NA of a laser beam is the sine of one half of the vertex angle of the cone formed by a
Examples Considering a freely propagating beam, the beam diameter as a function of the distance from the emitter must be calculated. Only circular laser bundles can be described as having a beam diameter. Some lasers,
eye
r visual distancFig. 6
View of a fibre end with and without
Optics & Laser Technology Vol 27 No 1 1995
an optical instrument
Fig. 7 Increase of the beam diameter to the beam divergence
behind
the beam waist due
9
Extended
sources-concepts
laser bundle
and potential
hazards:
E. Sutter
(see Fig. 7)
NA = sin /i
(13)
According to this definition, for a small divergence the numerical aperture NA is approximately one half of the divergence. Due to its divergence, the beam diameter d increases with the distance II measured from the beam waist according to the following formula (d, is the beam diameter at the beam waist) d = (df + (ii u)‘)’
’
(14) Ftg. 8
Far away from the beam waist the contribution negligible and the following holds rl = ci LI
However, the definition of the beam diameter, which differs for communication applications, must also be borne in mind; using the numerical aperture for the description of divergence, the beam diameter is usually defined by the points where the irradiance is 5% of the peak irradiance in the beam. Based on a Gaussian beam profile, this different approach can be corrected for by dividing the divergence by a factor of 1.73 2 2. In this case
1.
Beam
Emitter type Monomode fibre Multimode fibre Diode laser
diameters
(16)
of divergent
wlthout
beams
Fig
and
9
power
View
of a laser diode
entering
Beam diameter at 25 cm distance
Fraction of the power entering the eve
Beam diameter atlOcm distance
0.1
25 mm
0.08
0.2
50 mm 66 mm
the
fitted
with
a condenser
instrument
of
lens
eye Fraction of the power entering the eve
Total power enters the eye at a distance of
10 mm
0.49
70 mm
0.02
20 mm
0.12
35 mm
0.01
26 mm
0.07
27 mm
Optics
10
an optlcal
If such a device were observed from some distance with a telescope (see Fig. 10) having a magnification
Typical value of the numerical aperture
(0.7/0.1) =a 0.26
of a laser diode
As an example, we have a quadratic emitter measuring 300 jlrn by 300 pm in the focal plane of a lens having a focal length of 30 mm and a diameter of 25 mm, i.e. the numerical aperture of this lens is 0.38. If this device is observed with the naked eye without optical instruments, at the retina this structure subtends an angle of 300 pm/30 mm = 10 mrad (using the length of the diagonal would give an angle 1.4 times greater). If the eye is close to the lens, the fraction of the total power entering the eye would be (7/25)2 = 0.078.
For laser diodes and LEDs the divergence is different for the two directions parallel and perpendicular to the emitting structure. In this case, the numerical aperture of the device may be calculated as the square root of the product of the two numerical apertures. The emission profile of these devices is much broader than that of fibre ends. Typical values for the numerical aperture of monomode tibres are 0.1, for multimode fibres 0.2, and for laser diodes and LEDs 0.1, and 0.7 respectively for the two directions, giving a geometrical mean of 0.26. Using the above formulas leads to the beam diameters given in Table 1 for a clear viewing distance of 25 cm and a distance of IO cm. the distance which is considered by IEC 825 to be the minimum focusing distance of the human eye (see Fig. 8). Between 10% and 50% of the radiation would enter the eye at these distances. The total power within the defined beam diameter would enter the eye at the distance given in the last column of Table I. In practice, this could happen if a magnifying glass with a focal length corresponding to these values were used.
Table
view
Owing to their high divergence, laser diodes and LEDs are mostly used with a condenser lens of short focal length in front of them (see Fig. 9). Typical focal lengths of such lenses range from a few mm to about 50 mm. The numerical aperture of these lenses is made as high as possible, values up to 0.5 and more are not unusual. Sometimes several emitting structures are arranged in parallel, forming an almost quadratic structure. In this case the divergence is almost equal in both directions.
(15)
ii 2 arc sin NA 2 NA
Direct
of d, is
& Laser Technology Vol 27 No 1 1995
Extended
sources-concepts
and potential
hazards:
E. Sutter
Of course, these considerations can be formulated in a more general way. Again, the most simple approach is to use the radiance since the radiance cannot be changed by optical imaging. In our case the solid angle within which the radiation of the emitting diode is encompassed, would be *, = Fig. 10 telescope
View
of a laser diode
fitted
with
a condenser
lens with
IrfTI
(17)
uf 4
a
where d, is the diameter of the condenser lens and a, is the distance of the condenser lens from the emitter. Using the numbers of the example 8 and a diameter of the objective lens of at least 25 mm, the total power would be collected and enter the eye. The image on the retina would be larger by a factor of 8 than without the telescope. This reduces the risk of a lesion by a factor of 8. But the total risk would still be higher by a factor of l/(8 x 0.078) = 1.6. In many cases, the emitters are not placed exactly in the focal plane of the lens in front of them, but are displaced by a small amount in order to get a defined divergence. From the safety point of view, the most unfavourable case would be if the emitter were displaced from the lens in such a way that an image would be formed on the pupil of the eye with a magnification M (see Fig. 1I), so chosen that the image would be exactly as large as the pupil diameter, and the total power could therefore enter the eye. In our example, M would have to be 7 mm/300 itrn = 23.3 mm. In the example, the image would be formed at a distance of 608 mm from the condenser lens. The eye lens could now no longer focus the emitter on the retina, but only on the lens in front of it. The image on the retina would subtend an angle of 25 mm/608 mm = 41 mrad. This angle is 4.1 times greater than the angle in the first case, reducing the risk by the same factor. However, since the total power would enter the eye, this case would be more hazardous by a factor of l/(4.1 x 0.078) = 3.1, the risk would be twice as high as in the case of an observation with the telescope previously discussed. The worst case would be encountered if the parameters of the telescope were chosen so as to make the risk the highest. This would be the case with a magnification M = 25,J7 = 3.6. In this case, the total power would also enter the eye and the risk would be higher by a factor of l/(3.6 x 0.078) = 3.6 than in the first case. This factor is slightly larger than in the second, latter, case.
R, = 252 mm2 7r/4/302 mm’ = 0.55 sr The radiance L=
averaged
(18)
over this solid angle would be
1 P (19)
R, d,2
where d, is the linear dimension of a square source (dt = d, d,:width times breadth in the case of an oblong rectangular emitter) and P is the total power emitted within 0,. The second parameter to be considered is the magnification by the optics used, since this would influence the reduction factor C, which determines the risk to the retina Without an optical instrument between eye and emitter, the linear dimension of the retinal image d,, would be (clear visual distance d, = 25 cm)
(20) Using an optical instrument having a linear magnification M, the dimension of the retinal would be d:, = d, M
(21)
and the irradiated
area on the retina would therefore
F n =F,M’
(22)
P Per
=
MPE,,,
4 dh”
MF,
(23)
Using (4) (20) and (22) with uZ =.1,
LF, Fn ,r’,’
=
LF, F, d,Z
(24)
M2
The ratio of both powers would be a measure risk. Using (8) and (20) we have LM Risk = ~ MPE,i”
Optics & Laser Technology Vol 27 No 1 1995
where
the condenser
lens forms
an
be
The MPE values are usually given as power or energy density. If (9) and (21) are used. the permissible power would be
P=
Fig. 11 View of a laser diode image on the pup11 of the eye
image Irk
d, dmin d,.L
of the
LM ‘0
'min
(25)
MPLin
with CY,= d,/d,, the angle at which the source would appear without optical aids. This relation is valid as
11
Exrended sources-concepts
and porenrial
hazards: E. Surrer
long as the image on the retina is not greater than %ex * the angle up to which the permissible power is proportional to C, and the total power enters the eye due to the magnifying instrument. For larger angles, the risk would become independent of the magnification, and be given by L
Risk = MPF,,,
rirfnin
(26)
With these equations, devices using optics or viewed with optical instruments may easily be analysed. Classification Parallel to the change of the MPE values in the new edition of IEC 825 (Ref. 6), i.e. the dropping of the values expressed in radiance or integrated radiance for extended sources, the classification rules for extended sources have also been changed. Whereas before, the radiance had been measured with a detector having an aperture of 7 mm diameter at 100 mm distance from the source, the diameter of the detector aperture is now 50 mm; the distance is still kept at 100 mm. This means that the detecting instrument has a numerical aperture of 0.25. In many cases this will reduce the permissible power for, say, Class 1 by a factor of (50i7)2 = 51. Whether or not this reduction is necessary for safety reasons, or if it is over-restrictive, should therefore be examined. During this analysis it should be borne in mind that the concept of Class I lasers is that they should be inherently safe. i.e. that the maximum permissible exposure level cannot be exceeded under reasonably foreseeable conditions of use. These foreseeable conditions would also include the use of optical instruments. The accessible emission limits (AELs) for Class 1 are derived from the MPEs by multiplying them by the area of the applicable aperture stop, i.e. with 0.4 cm2 (corresponding to a diameter of 7 mm) in the visible and in the near infra-red. The use of the correction factor C, for extended sources is judged on the basis of the smallest possible distance from the apparent source, but this distance will never be less than 100 mm. Having stated these basic considerations. it is obvious that there are conditions conceivable where Class I lasers are not safe at all: for example, if laser diodes, LEDs, or diodes fitted with pigtails of a high numerical aperture NA = 0.6 is not unusual are observed with an optical instrument having a numerical aperture greater than 0.24, a quite realistic case for microscopes and magnifying glasses of modern design. This case is also critical insofar as it is common practice in communication techniques to observe fibres and emitters with optical instruments in order to check if their surface is damaged. One would not hesitate to use optical instruments in this case knowing that the system only contains radiation corresponding to Class 1. i.e. lasers that are safe under all reasonably foreseeable conditions of use. In many other cases the use of a measuring instrument with a numerical aperture of only 0.24 is not critical. The classification of typical lasers emitting a parallel beam is not affected by the numerical aperture of the
measuring instrument since their divergence not higher than about 1 mrad.
To illustrate the problems, the classification conditions are applied to the examples of Figs 8 to 11. In any case, the angle r subtended by the source at the eye must be calculated at a distance of 100 mm from the source and would, in this example, be z = (300 pm/100 mm) = 3 mrad. This value is higher by a factor of 2 than N,,,~,,,i.e. C, would be 2. Since the measuring instrument would have an aperture stop of 50 mm diameter at a minimum distance of 100 mm from the apparent source, it would not collect the total power, but only that fraction of the power that would correspond to the square of the ratio of the numerical apertures of the collecting instrument divided by the numerical aperture of the emitter. Since the diameter of the pupil of the eye is only 7 mm, there would be a high degree of safety. In the cases of Figs 9 to 11, the angle z subtended by the source at the eye would still have to be calculated at a distance of 100 mm from the source, and again it would therefore be 3 mrad and C, would be 2. However, in Fig. 9 the eye could be brought immediately in front of the collimating lens, i.e. to a distance of only 30 mm. In this case the source would subtend an angle x = (300 Llm/30 mm) = 10 mrad and C,, would be 6.7. For the measurement of the power of the source, the standard requires interpretation. The standard states that in the wavelength region from 302.5 nm to 4000 nm, the minimum distance of the measurement aperture from the apparent source must not be less than 100 mm. If this rule is interpreted purely geometrically, the detector would be placed at a distance of 70 mm from the collimating lens, which has a focal length of 30 mm. However, since the beam is parallel behind the lens, optically speaking, the detector would only be at a distance of 30 mm from the source. In this case the total power transmitted by the collimating lens would be collected, since its diameter is smaller than 50 mm. If the rule for measurements is interpreted optically, the measuring instrument with an aperture stop of 50 mm diameter would have to be placed at an optical distance of 100 mm from the apparent source. and it would not therefore collect the total power, but only a fraction (50 x 30/100/25)2 = 0.36. The reciprocal of this value is 2.8, i.e the real risk is higher by a factor of 2.8 in all eases where the total power can enter the eye. Since class 1 lasers should also be safe when optical instruments are used, the standard should be interpreted in a way which leads to the collection of the total power. Conclusions
and
outlook
As the preceding discussions have in IEC 825-1 are not, in all cases, characterize the real risk. In some restrictive, in other cases they are the concept of Class 1 lasers being rrrrsonah~~
for~srruhl~~
conditions
shown, the appropriate cases they too lenient ,sqf& under
new rules to are too to fulfil cd/
of use. Optm
12
is usually
& Laser Technology Vol 27 No 1 1995
Extended sources-concepts Another point should be mentioned here. The basis for using the shortest dimension of a source in the case of rectangular sources is the paper by Sliney and Freasier”. According to this paper, for thermal damage the total permissible power for a retinal exposure would be reduced from about 7 mW for an irradiated area of 25 pm x 25 pm to less than 4 mW if this area were 25 pm x 2000 pm! Such a result cannot be correct: thermal conductivity would reduce the temperature in the centre of this much larger spot”. The problem with the new IEC 825-1 is that many changes to the old IEC 825 were made in haste at a very late stage, some even during the final editorial stage. So it is already very clear that the next amendment will be necessary very soon. There will then be an opportunity to clarify and correct the inconsistencies and errors. Definitions This section contains the definitions important terms used in this paper.
of the most
Extended source. Biologically speaking, an extended source is a source forming an image on the retina which is large enough to limit considerably the heat flow from the centre of the image to the surrounding biological tissue. In the standard6 the minimum size is about exposure times less than 0.7 s.
25 pm for
Integrated radiance. The integral of the radiance given exposure time. Unit: J mm2 sr- ‘.
over a
Irradiance. The ratio of the radiant power d@ incident on the element of a surface of area dA of that element: E = d@/dA. Unit: W rnm2. Magnification M. The linear magnification M of an optical instrument is the ratio of the visual angles subtended by the object with and without the instrument. Point source. Biologically speaking, this is a source forming an image on the retina which is so small that heat can easily flow in a radial direction from the centre of the image to the surrounding biological tissue. In the standard’ the maximum size of a point source is about 25 pm for exposure times less than 0.7 s. Radiance. This term is extensively explained at the beginning of the section on hazard assessment. Unit: W mm2 sr-‘.
Optics & Laser Technology Vol 27 No 1 1995
and potential
hazards: E. Sutter
Radiant exposure H. The integral of the irradiance over a given exposure time, i.e the ratio of the radiant energy dQ incident on an element of a surface of area dA of that element: H = dQ/dA. Unit: J m~~2. Solid angle. The solid angle Sz with its vertex in the centre of a sphere of radius Y, is the ratio of the area A cut off by this angle on the surface of the sphere divided by the square of the radius: fi = ,4/r’. Unit: sr (steradian). A full solid angle has 471sr. References ANSI Z 136. I : American National Standard for the Safe Use of Lasers, American National Standards Institute, New York 1986 2 IEC Publication 825: Radiation Safety of Laser Products, Equipment Classification, Requirements and User’s Guide, Bureau Central de la Commission Electrotechnique Internationale, Genkve 1984 3 Amendment 1 to Publication 825 (1984): Radiation safety of laser products, equipment classification, Requirements and User’s Guide, Bureau Central de la Commission Electrotechnique Internationale, Gentve 1990 4 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indice.r, American Conference of Governmental Industrial Hygienists (ACGIH), Cincinnati, USA 2 to Publication 825 5 IEC DIS 76(Central Office)28: Amendment (1984) Radiation Safety of Laser Products, Equipment Classification. Requirements and Users Guide, Bureau Central de la Commission Electrotechnique Internationale. Genkve 1992 6 IEC Publication 825-l: Safety of Laser Products, Equipment Classification, Requirements and User’s Guide, Bureau Central de la Commission Electrotechnique Internationale, Gentve 1993 I Frisch, G. D., Beatrice, E. S., Holsen, R. C. Comparative study of argon and ruby retinal damage thresholds, fnt.est Ophthuf, IO (1971) 911&919 8 Ham, W. T. et al Retinal burn thresholds for the helium-neon laser in the rhesus monkey, Arch Ophthal, 84 (1970) 797-808 9 Goldmann, A. I., Ham Jr, W. T., Mueller, H. A. Ocular damage thresholds for ultra-short pulses of both visible and infrared radiation in the rhesus monkey, E.yup Eye Res. 14 (1977) 45-56 10 Courant. D.. Court, L.. Slinev. D. H. Spot-size dependence of laser dosimetry. SPIE IS 5 (i989) IX-.165 II Farrell, R. A., McCally, R. L., Bargeron, C. B., Green, W. R. Structural alterations in the cornea from exposure to Infrared radiation. Technical Memorandum, The John Hopkinb University, Laurel, Maryland (1985) 12 Sliney, D. H., Freasier, B. C. Evaluation of optxal radiation hazards. Appl Opt, I2 (1973) I-24 13 Clarke, A. M., Geerets, W. J., Ham Jr, W. T. An equilibrium thermal model for retinal injury from optlcal sources, 4ppl Opt. 8 (1969) 1051-1054 14 CIE Publication Number 50, Internationul Electrotechnical Vocabulary (IEV), Chapter 845: Lighting. Bureau Central de la Commission Electrotechnique Internationale. Geneve Suisse 1987 to Laser Ph,:sicx, Sprmger-Verlag 15 Shimoda, K. [nrroduction Berlin (1984) 16 Sliney, D. H., Wolbarsht, M. Sujety with Lasers und Other Optical Sources, Plenum Press, New York (1980) have shown that the 17 Note added in proof: recent calculations mean of the two dimensions should be used. I
13