Thank you very much for the insight.
-----Original Message-----
From: Eric Cowdrey [mailto:ericc@cancercare.mb.ca]
Sent: Friday, June 23, 2000 3:12 PM
To: Multiple recipients of list
Subject: RE: Laser calculations
I think you are on the right track, but asking the wrong question.
I agree that the irradiance on a human eye would be 5.19 E-3 W/cm**2, where
E-3 means (10 to the minus three) and cm**2 means (centimeters squared), as
you have calculated.
The right question to ask now, it seems to me, is how long can one be
exposed to this irradiance without incurring eye damage.
I further agree that the appropriate MPE expression is 1.8 x t**.75 x E-3
J/cm**2, for viewing times in the range from 1.8 E-5 to 10 seconds.
We now set the equality: irradiance x t = MPE, to calculate the permissible
viewing time.
Therefore:
5.19 E-3 x t = 1.8 x t**.75 x E-3
from which t = .014 seconds. Since this falls in the range from 1.8 E-5 to
10 seconds, this is the appropriate MPE formula to use.
Clearly, the normal blink aversion time of .25 seconds would not protect you
in this case and this is not a Class II laser.
**************************************************************
Eric Cowdrey, M.Sc. (Physics)
Radiation Protection Officer
Department of Medical Physics
CancerCare Manitoba
100 Olivia Street
Winnipeg, Manitoba R3E 0V9
CANADA
Tel (204) 787-2166
Fax (204) 775-1684
e-mail ericc@cancercare.mb.ca
Industrial hygiene in Manitoba: http://aiha.cancercare.mb.ca
-----Original Message-----
From: radsafe@romulus.ehs.uiuc.edu
[mailto:radsafe@romulus.ehs.uiuc.edu]On Behalf Of George Rawls
Sent: Friday, June 23, 2000 12:02 PM
To: Multiple recipients of list
Subject: Laser calculations
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Perhaps some of the laser experts out there wouldn't mind checking these
calculations/assumptions. Your help will be greatly appreciated. Please
answer me directly unless you believe it is appropriate to answer to the
list.
The problem:
Given a 2 mW laser diode with a 651 nm wavelength and a 5mm aperture and a
human eye very near the aperture, I calculated the irradiance on the eye as
follows: .002 W divided by .385 cm squared, using 7mm as the area(human
dilated pupil as the limiting aperture). Answer equals 5.19x10 to the -3
W/square centimeter.
Also, calculating the MPE for this particular laser, using Table 5 from
Z136.1:
MPE=1.8(t to the 3/4)x 10 to the -3
t=0.25 seconds(aversion response)
MPE=6.36x10 to -4 J/square centimeter divided by 0.25
=2.5x10 to -3 W/square centimeter
Have I made some mistakes, or is the irradiance near the aperture really
more than twice the MPE?
Can I assume the power of the laser(thus the irradiance) will be less than 2
mW at, say, 8 feet from the aperture? Thanks in advance for all help.
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<META HTTP-EQUIV=3D"Content-Type" CONTENT=3D"text/html; =
charset=3Diso-8859-1">
<META NAME=3D"Generator" CONTENT=3D"MS Exchange Server version =
5.5.2651.75">
<TITLE>Laser calculations</TITLE>
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<BODY>
<P><FONT SIZE=3D2 FACE=3D"Arial">Perhaps some of the laser experts out =
there wouldn't mind checking these calculations/assumptions. Your =
help will be greatly appreciated. Please answer me directly =
unless you believe it is appropriate to answer to the list.</FONT></P>
<P><FONT SIZE=3D2 FACE=3D"Arial">The problem:</FONT>
</P>
<P><FONT SIZE=3D2 FACE=3D"Arial">Given a 2 mW laser diode with a 651 nm =
wavelength and a 5mm aperture and a human eye very near the aperture, I =
calculated the irradiance on the eye as follows: .002 W divided by .385 =
cm squared, using 7mm as the area(human dilated pupil as the limiting =
aperture). Answer equals 5.19x10 to the -3 W/square =
centimeter.</FONT></P>
<P><FONT SIZE=3D2 FACE=3D"Arial">Also, calculating the MPE for this =
particular laser, using Table 5 from Z136.1:</FONT>
</P>
<P><FONT SIZE=3D2 FACE=3D"Arial">MPE=3D1.8(t to the 3/4)x 10 to the =
-3</FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">t=3D0.25 seconds(aversion =
response)</FONT>
</P>
<P><FONT SIZE=3D2 FACE=3D"Arial">MPE=3D6.36x10 to -4 J/square =
centimeter divided by 0.25</FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial"> =
=3D2.5x10 to -3 W/square centimeter</FONT>
</P>
<P><FONT SIZE=3D2 FACE=3D"Arial">Have I made some mistakes, or is the =
irradiance near the aperture really more than twice the MPE?</FONT>
</P>
<P><FONT SIZE=3D2 FACE=3D"Arial">Can I assume the power of the =
laser(thus the irradiance) will be less than 2 mW at, say, 8 feet from =
the aperture? Thanks in advance for all help.</FONT></P>
</BODY>
</HTML>
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