Hi:
For LEDs of kinds :
<www.gilway.com>
<www.eLumexLEDs.com>
Title: About LED's,Candelas and Watts.
Although this is not strictly a CS subject I think it's a useful
adjuvant to it
If I am wrong please accept my apologies.
I shall make some statements, they are at the best of my knowledge,
today. Of course comments and corrections are most welcome.
Visible LED's are usually specified in Candela units. This units are
weighted to the human eye response.
When LED's are used for tissue irradiation the important Item is the
forward (Light going forward, not in all directions) radiant power,
expressed in Watts (or milliwatts) and since Watts x seconds = to
Joules, the dose should be expressed in Joules (watts-second).
To obtain the radiant power from LED manufacturers has proven rather
difficult. So here are some numbers: (Values are approximations)
MGMicro LED clusters (from used traffic signals)
Red probably 660nm,
At 1.8 volts, 20 mA, 600 mCd, 0.65 mW
2.0 45 1000 1.37
Approximates Gilway E168
Green probably 565 nm
At 2.2 Volts , 20mA, 300 mCd, 0.064 mW
2.5 45 600 0.17
Approximates Gilway E166
As can be seen from the above numbers although the red and the green
have the same "brightness" (600 mCd) the power emitted by the red is 10
times that of the green.
This relationship approx. holds for all reds Vs green LED's.
Therefore the use of the candela measure can be misleading.
Also LED's with wider view angles (2 Omega1/2) and the same Candela have
more power output than narrower ones.
Infrared LED's are usually specified in milliwatts per steradian .
A steradian is a solid angle (3D's) which having its vertex at the
center of the sphere, cuts off a spherical surface area equal to the
square of the sphere radius.
So in a 1 meter radius sphere an angle of 1 steradian will encircle an
area of one square meter.
Here care must be exercised, when the specs said an LED has a 20 mW/sr.
It does not mean that if the power is measured in its center steradian
angle it will be 20 mW's, The power density will be equal to 20 mW/sr
only in the center one hundred of a steradian.
So once again the thing that counts is: The total radiant power emitted
forward.
Example: Giway IR LED # E24 specified at 20 mw/sr (typical) has the
following total power:
At 1.4 Volts, 20 mA, 3.20 mW at zero distance from LED face.
2.54 mW at 1.0 cm (into a 1cm detector)
1.21 mW at 2.5 cm (into a 1cm detector)
However if the power in the center 0.01 sr is measured and then is
multiplied by 100 it will equal to 20mW/sr.
In my experience irradiation with IR(880) and Red (660nm) clusters seems
to help in a number of maladies.
To build a powerful IR (72 LED's) cluster a kit can be purchased from
the maker <www.dalewheat.com> or from BGMicro. <www.bgmicro.com>
Complete kit $25.95 part # LED1069, Assembled and tested $35.95 LED 1070 .
A weather resistant enclosed version for $59.95.
The PCB part # LED1070, $9.95 can be used to built a RED version using
Gilway #E184 LED's (660nm, 2000 mCd, min., 3500 mCd max.) Its power
output at the led face is:
At 1.85 V, 20 mA, 2.0 mW
2.00 V, 36 mA 3.5 mW
2.30 V, 60 mA 5.3 mW
For continuous wave Operation, 200 Ohms resistors can be used.
For pulsed operation the circuit provide by Walter Cook at
<www.seanet.com/~waltc/page43.htm> is very useful.
For extensive information on LED's and Laser Diodes, use and dosages see :
Therapeutic Lasers Theory and Practice, by G.David Baxter,.
Churchill Livingstone, Publisher.
Last Comment:, Remember that:
Power Density = Irradiance = Total Incident Power / Area. [Watts / cm^2]
Radiant Exposure = Energy Density = Total Incident Energy / Area.
[Joules / cm^2] = [Watts-sec/cm^2]
Radiant Exposure Also is equal to:
Total Power Density x Total Time of Irradiation [Watts/cm^2 x Seconds]
Walter's schematic produces a frequency of 7 Hz at 50% duty cycle.
Baxter states that a 4Hz frequency decreases sensitivity to pain
immediately, Walter's circuit can be easily altered by changing a couple
of components .
Sincerely
Peter R
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