What lasers can make you
blind
by
Lars Hode
Swedish Laser-Medical Society
• Background:
Since 40 years, there are special rules and
restrictions
i i
for
f the
h use off certain
i lasers.
l
The reason is that some laser types,
sometimes are able to cause eye injury,
also at distance
distance.
What can happen if you by mistake
look into a strong laser beam?
If you can cut in steel,
steel then you can
destroy an eye. How dangerous
are actually
t ll different
diff
t llasers??
The classification of lasers is almost the same
everywhere in the world, but the rules concerning
use of lasers and rules for permission and
registration of instruments is very different.
E.g.
E
g in Sweden class 4 lasers for hair removal
removal, for
tattoo removal, for wrinkle treatment etc, are
completely
o p e e y free
ee ssince
e five
e yea
years.
s In most
os countries
ou es
you need a doctor behind you and in other
countries the doctor has to be the therapist.
Many people think that looking into a laser
beam can make them blind.
Is that correct?
Yes, there are such lasers! I will describe
Yes
some of them and show examples.
10 meter
6 MW
output
Weight
20 ton
Output
power
6 mW
10 mm
Weight
2 gram
This laser, a laser
diode, often used in a
therapeutic
p
laser
instrument. This laser
can not harm you.
It can possibly heal
an injured eye!
This laser was never produced.
But it is p
possible to make it, if
enough money is available.
It was constructed theoretically
under Richard M. Nixons
administration in USA and about
20 units was thought to circulate
in satellites around the earth.
The intention was to burn
Russian nuclear missiles when
they came up through the
atmosphere. This laser could
make you blind.
blind
Before
One week later
Two months later
This fibroma was removed with a 15 watt
surgical CO 2 laser, 50 mm focal length.
Can that laser make you blind? NO. It might burn the cornea
in or near the focal area, but not make someone blind!
An Alexandrite
laser for hair
removal.
Can this laser
make you blind?
NO
These lasers can make you blind
But they are very expensive
expensive, very heavy
and takes a lot of kilowatts to run
This laser can make yyou blind
Price: 380,000 €
A Q-switched Nd:YAG-laser – can it make you
bl d iff someone shoots
blind
h
right
h into your eyes?
NO
Dart arrows ...
Price: 2.50 €
... can make an eye blind
Ear protection
E
t ti
must be worn too!
A slingshot can make you blind. It can be used by
anyone older than 5. It is very cheap. You don’t need a
responsible doctor .....
No comments ! ! !
Bullet proof vest
must be worn.
It would be much safer
if it was a class 4 laser;
they might cause an
injury but they can’t
injury,
make you blind.
Other laser risks
High voltage
Warning! High voltage capacitors
are ridiculously dangerous things.
They can store enough energy to
kill you instantly.
Retina injured
j
byy
strong laser pulses.
Is the person blind?
No
Now, a few words about the risk factors.
A laser is a lamp. There are many types of lamps. The light
from a laser has always two properties:
•
It has one wavelength – very narrow bandwidth
•
It is highly coherent (well organized, very “pure”,
pure , in the
same way as the tone from a flute is more pure than the
sound of a jet plane).
Are these two factors making laser light dangerous?
No
Further,, much stronger
g lasers are used in the
following eye treatments:
Laser trabeculoplasty for open angle glaucoma
Laser iridotomy in pigment dispersion syndrome
Laser photocoagulation for neovascular age-related macular degeneration
Photodynamic Therapy of wet macular degeneration
L
Laser
assisted
i t d in
i situ
it keratomileusis
k t il
i (LASIK) for
f myopia
i correction
ti
Laser treatment of diabetic retinopathy
Laser treatment of solar retinopathy
Laser treatment of age related macular degeneration
Lasering drusen
Treatment of keratoconjunctivitis with LLLT
Correction of myopia (PRK) and astigmatic keratotomy (AK)
Laser in transpupillary thermotherapy (TTT)
Laser photolysis of cataract
Laser welding of the cornea
Laser posterior capsulotomy
…. and some other treatments
Also, it is not "laser" light that may be dangerous.
It iis any light
li ht off certain
t i power, and
d exposure time,
ti
in
i
certain wavelength interval, with certain pulse energy
(if pulsed)
pulsed), with certain geometric configuration
(divergence, solid angle) etc that may cause such
irradiation in the retina that the MPE (maximum
(
permissible exposure) is exceeded much enough.
Examples
E
l off such
h light
li ht sources are IPL,
IPL strong
t
flash
fl h
lamps, certain xenon arc lamps.
What
Wh t iis the
th best
b t protection?
t ti ?
Knowledge.
Safety glasses can be hazardous:
If you use the wrong type, you may think that
you are protected, but instead ...
Coherency
Coherency has nothing to do with parallelism. It is often said
th t iin laser
that
l
light,
li ht the
th waves are in
i phase
h
with
h each
h other,
h but
b that
h is only
l
true inside the laser cavity. After the light has left the laser, this statement is
wrong.
This picture is showing light of long
coherence length – laser light ...
... and ...
picture shows light
g with
... this p
shorter coherence length monochromatic ”ordinary light”.
Other p
properties,
p
, like parallel
p
beam,, high
g
power may be at hand with some lasers.
Most lasers are not strong.
Most lasers emit the light in a divergent
beam.
Then, what makes some
lasers dangerous?
There are certain risk factors, which?
Summary of risk factors
1. High
g p
power
2. Long exposure time
3. Parallel beam
4. Not visible wavelength
5. Small beam diameter
6 Extreme pulsing (e
6.
(e.g.
g Q
Q-switching)
switching)
1. Extended source / several sources
decreases the risk.
Narrow beam - one of the risk factors
A wide beam is less hazardous to an eye than a narrow beam because
only a fraction passes through the pupil, the rest comes outside.
Classification of lasers – laser
classes
Parameters:
Laser type
Wavelength
O t t power
Output
Divergence
Extended source
Pulsing
The classification of lasers is one of the procedures that are included in the so
called CE-approval containing a lot of international standards.
A classified
l
ifi d llaser will
ill b
belong
l
tto one off th
the groups 1 – 4.
4
1,
2,
3A,
3B,
In Sweden, lasers in these groups are free to use.
4
Doctor or
dentist
An eye is working like a digital camera where the retina is a detector matrix with millions of
pixels in the form of light sensitive cells. When these cells are hit by visible light, they are
producing a nerve signal.
signal From this signal
signal, our brain is building up an image
image.
eye lid
The eye
muscle
sclerotic coat
choroid
retina
iris
pupil
optic nerve
cornea
fovea
( ll
(yellow
spot)
aqueous
humour
sclera
muscle
A parallel
ll l b
beam iis d
depicted
i t d att th
the retina
ti as a smallll point.
i t Wh
When llooking
ki against
i t th
the sun,
the image will be a picture of the sun with sun spots and all. In the case of an unpleasant
intensity, our blink reflex is ”switching off”, first by blinking then by turning your head.
Waveleng
g
th as risk
factor
Different
risk
i k llevels
l
in different
parts of the
spectrum.
Therapeutic lasers ...
... are practically risk free. Fear
of lasers cause more problems
than the lasers themselves do.
Therapeutic lasers are usually in
the power region 1 - 500 mW.
Nobody,
... yes, nobody has become
blind from a laser. Anyy laser.
Therapeutic lasers can cure many
eye diseases
di
Low-Level Laser Therapy Improves Vision in Patients with
A R l t d Macular
Age-Related
M
l Degeneration
D
ti
Boris T. Ivandic, M.D., and Tomislav Ivandic, M.D.
Results: LLLT significantly improved visual acuity (p 0.00001 versus baseline)
iin 162/182 (95%) off eyes with
ith cataracts
t
t and
d 142/146 (97%) off eyes without
ith t
cataracts. The prevalence of metamorphopsia, scotoma, and dyschromatopsia
was reduced. In patients with wet AMD, edema and bleeding improved. The
improved vision was maintained for 3–36
3 36 months after treatment
treatment.
Photomedicine and Laser Surgery
Volume 26, Number 3, 2008
© Mary Ann Liebert, Inc.
Pp. 241–245
Conclusion:
Lasers are of extremely low risk compared to:
Dart arrows.
Air guns.
guns
Knives.
Stones thrown.
Slingshots.
Practically all weapons.
B k b
Broken
branches
h on trees
t
iin the
th forest.
f
t
Let us forbid all these things before putting strong
restrictions on therapeutic (and surgical) lasers
Thanks for listening
About lasers and IPL
In many beauty salons,
salons instruments for
permanent removal of hair are used. They are:
•
•
•
•
Ruby lasers [691 nm]
Alexandrite lasers [[755 nm]]
Diode lasers [808 nm]
Nd:YAG lasers [[1064 nm]]
and
• IPL instruments [[600 – 1200 nm]]
In most countries there are no rules for IPL
devices as they are regarded more or less
harmless to eyes.
How to know if a light source is dangerous?
According to IEC and ICNIRP, a limit, MPE
(Maximum Permissible Exposure) is given, above
which, permanent eye injuries can occur.
I have calculated the irradiation levels, both
from a laser and an IPL instrument in the retina
of a human eye, in order to determine the MPE
from light
g pulses
p
directed against
g
an open
p
unprotected eye.
Ph i l data
Physical
d t used:
d
Parameter:
Ruby laser
IPL device
Unit
Wavelength
Pulse energy
Pulse time
Aperture
Aperture area
Divergence *)
694 nm
max 8 joule
5 - 15 ms
round 0.
0 8 mm in diam.
diam
0.5 × 10 -6 m 2
ca 0.125 sr (90%)
615 - 1500 nm
max 80 joule
ca 25 ms
rectangle 10 × 20 mm
2 × 10 -4 m 2
ca 0.4 sr (90%)
λ
Q
t
A
Ω
*) The percentage specifies that 90 % of the energy is within the
specified solid angle.
R di
Radiance
L
The parameter that decides if
a source is dangerous to the
eyes is its radiance,
radiance L.
L
• dΦ is the momentary
•
dΦ
dΦ
L=
dA ⋅ cos θ ⋅ d Ω
•
•
power of the radiation
Φ = dQ/dt
dA is the surface in front
of the pupil
pupil.
dΩ is the solid angle in
direction to the eye
θ is the angle between the
normal of the aperture and
the direction to the eye.
The expression for L is valid for every point in
the aperture that is depicted on the retina. If the
aperture has a certain size that exceeds the
vision angular αmin, in view from the eye, the
light source is defined as an extended source.
αmin depends among other things on the
exposure time t. If t < 0.7 seconds, αmin=1.5
mrad. When looking into the laser or IPL
aperture at 20 cm distance, the angle, in both
cases, exceeds αmin.
Wh is
When
i it
i dangerous?
d
?
The MPE related to the radiance of a source is named L MPE.
For extended light sources, the following equation is valid:
L MPE ≈ 8.5
85
×
10
3 ×
H MPE
[sr -1 J m -2 ]
where H is the exposure according to H = dQ/dA
In the expression above H M PE refers to a point source with
H MPE = 0.05
0 05 [ J m -22 ].
] This
Thi gives:
i
L MPE = 425 [[sr -11 J m -22]
If the radiance of a light source is fairly equal in
all directions over the solid angle Ω, the integrated radiance LP is a function of the power
density HA in the aperture:
HA =
∫
Ω
LP d Ω
which, in directions close to the normal, i.e.
where cos θ ≈1 and Ω<1 sr can be simplified to:
H A = Ω×
Ω LP
The value of H that may cause damage (H=HMPE) can
be hence be written:
H MPE = Ω× LMPE
[ Jm −2 ]
Provided that the radiance is relatively uniform over
the area A of the aperture, the pulse energy QMPE is
calculated
l l d from
f
the
h following
f ll i simple
i l expression:
i
Q = ∫ HdA
A
and
QMPE = Ω × LMPE × A [ J ]
As can be
A
b seen, Q MPE is
i not dependent
d
d
on the
h
y and the aperture.
p
distance between the eye
Hence the irradiance in a point in the image
on the retina remains constant if the distance
between the eye and the aperture is changed.
However the area of the image is reversely
proportional to the distance squared.
An IPL instrument with the parameters in the
example above, have the following value of Q MPE :
QMPE = 0.4 [sr]×425[sr−1Jm−2]×0.0002[m−2] = 0.034[J]
This means that it is safe to look directly into its
beam if the pulse energy is less than 0.
0 034 joules.
joules
With the pulse energy given in the example above
above,
its pulse energy is more than 2000 times higher
than this safe limit.
limit
• Results:
The calculations show that,
that if a light pulse
from an IPL is fired from a distance of 20
cm against an open eye,
eye the energy level
can be more than 2000 times higher than
maximum permitted exposure
exposure.
Consequently, either the IPL instruments
Consequently
can be rather dangerous to human eyes or
the internationally used MPE limit needs to
reconsider.
IPL vs. Laser
L
Irradia
ance
L [J/m2]
300,000 × L MPE
L MPE
Risk for permanent damage
2,000
0.005
L MPE
L MPE
2
Image area on the retina
×
A [mm2]
The chosen values of energy density and
aperture area in the example, 40 J/cm2
and 2 cm2, are not exaggerated.
exaggerated There
are instruments with twice as high energy
density, according to the manufacturers
and then the MPE can be exceeded byy
more than a factor 4000.
Consequences of an injury
There is no doubt that certain lasers can injure the
eyes because of their high radiance in the aperture.
However, it iis the
H
th consequence off an iinjury
j
that
th t iis
important rather than the fact that the eye is injured.
Further calculations show that an injury in the retina,
caused by a laser, has a small consequence by means
of effects on the vision while, if caused by an IPL
instrument,, the injury
j y can be of much larger
g
consequence.
Optics
p
of the eye
y
Projection of the
aperture
p
on retina
Aperture
2 cm
20 cm
A small aperture (such as from a laser) is projected
to a smallll image on the
h retina while
h l a large
l
aperture
(such as from an IPL) is resulting in a large image.
The consequences of eye injury
caused by a laser
Visual effect from eye
injury at 2 m distance:
round spot, 8 mm diam
Eye injury by
laser from fiber
aperture at 20
cm distance.
Size of blind
spot: 8 mm
Fiber Φ 0.8 mm
20 cm
Focal spot
on retina,
Φ 0.08 mm
2m
The consequences of eye injury
caused by an IPL
Visual effect from
eye injury at 2 m
distance: 10 × 20 cm
Eye damage by an
IPL shot. Aperture
at 20 cm distance.
Size of
blind area
IPL aperture
10 x 20 mm
20 cm
Projection on
the retina, 1 x 2
mm spot
2m
Reported incidents
Journal for
Ms Xxxx
A 41 year old woman
was treated on her
eyelid (!) with an IPL
device. She got iridoirido
cyclite and was treated
by means of adrenaline
pads on the Ullevål
hospital in Oslo.
At a demonstration of an IPL instrument in an
exhibition,, a shot was fired byy mistake because
somebody happened to step on the foot pedal. The
hand p
piece,, with the 1 × 2 cm aperture,
p
, was placed
p
in its holder on the side of the instrument, about 80
cm up
p from the floor.
A hole was burnt in a p
persons jacket.
j
”M face
”My
f
was d
destroyed”
t
d”
IPL treatment of face.
Longitudinal burns.
The doctors comments:
“the instrument gave
more than the setting”
• Conclusion:
Even though the irradiance level resulting from
a laser can be much higher than from an IPL,
the levels are
are, in both cases,
cases high enough to
permanently destroy the cells of vision.
A beam,
beam strong enough to burn a hole in a
jacket is naturally dangerous to eyes.
• Other considerations:
Even though strong lasers may cause eye
injury one should bear in mind that every day
injury,
strong lasers are “fired” right into unprotected
open eyes in the treatment of retinopathy,
retinopathy
macula degeneration, cataract, near-sightness
and of some other eye problems. And, this
patients blind.
does not make the p
My recommendations:
The IPL instruments can imply
p y a great
g
risk
and it is important that there are regulations
for them as well as for lasers
lasers. Rules for all
strong light sources ought to be considered.
First of all:
Therapeutic lasers are practically risk free. The
stronger types (typically class 4) can cause an
injury, but they can not make you blind.
They are usually in the power region 1 – 500 mW.
Fear of lasers cause more problems than the
lasers themselves do
do.