Measuring Gel Coupled IPL Sources

Author: 
Dr. Ephraim Greenfield

Background

Intense Pulsed Light sources are widely used in skin treatment. These pulsed sources are used either coupled through the air to the skin or coupled directly to the skin with gel between the source and the skin to eliminate reflections of light.

How does one measure the output of such a source when gel coupled to the skin of the patient?
Ophir has defined the output of such a device as follows: the output will be defined as the amount of energy that will be coupled through index matching gel to an ideally absorbing (totally black) skin. This does not exist in practice but this defines the ideal coupling of energy through gel. When IPL energy is coupled to the skin through the air, less reaches the skin because some of the light is reflected back into the machine from the front window (and some is reflected from the skin). In general, the difference in energy coupled through the air and through gel is on the order of 10-15% .

Measuring Gel Coupled IPL Sources

L40(150)A-IPL sensor for gel coupled sources

The Ophir L40(150)A type energy meter is designed to measure the energy output of IPL type sources operated with optical index matching gel between the light source and the skin of the patient. In order to simulate this situation, the DB type meter has a glass pyramid that is coupled with gel to the source. The pyramid allows almost all of the widely divergent light from the source to reach the thermal disc detector as shown in the illustration on the right. The thermal sensor disc has the efficient Ophir LP2 coating that absorbs >96% of the light over a wide range of wavelengths and angles. If the IPL source is larger than the input face of the pyramid, then some of the light will not get into the pyramid and the sensor will only measure the light that gets into the pyramid front face.

Measuring Gel Coupled IPL Sources

 
In order to calculate the efficiency of the L40 type sensor, we have to know the angular distribution of the light. We have measured the output of several IPL sources gel coupled into a glass hemisphere to allow light emitted at large angles to pass. (In an ordinary flat glass, any light at angle over 42 degrees will be totally reflected). The angular distribution of the light output is shown on the right for several IPL sources

Measuring Gel Coupled IPL Sources

Efficiency and calibration of L40 type IPL sensor

After we know the angular distribution of light from the IPL source we can calculate the efficiency of the sensor for measuring IPL light. Doing ray tracing analysis, we found that the L40 type sensor is very efficient at capturing light from highly divergent IPL sources. Of the light that gets into the pyramid, depending on the angular distribution, 93 to 95% gets out the other end of the pyramid. Taking into account the mounting points of the pyramid and other small losses we find that 90% of the light input into the pyramid gets through to the sensor disc We calibrate the L40 sensor is by passing a straight 1070nm laser beam through the pyramid and onto the sensor disc. In this case, 92% of the light gets through the pyramid (taking into account of reflection from the top and bottom of the pyramid). So taking into account these factors we calculate: Calibration factor = (efficiency with IPL source 90%)/(efficiency with laser 92%) So we calibrate the sensor to read 2% higher than the laser energy applied.

Measuring Gel Coupled IPL Sources

L50(300)A-IPL sensor for gel and air coupled sources

The L40 pyramid type IPL meter is very accurate since it captures almost all of the IPL light and does not depend much on the angular distribution. However, it has a small 22x22mm aperture and will only capture a part of IPL sources with larger cross section. We therefore have the L50(300)A-IPL with a front glass that can accept much larger size sources as well as air coupled sources. The front glass window is uncoated on the front surface and antireflection coated on the rear surface. For gel coupled operation, the IPL source is gel coupled to the front glass of the L50 type. The light goes through the glass and is absorbed by the LP2 coated sensor disc. In this case, any light diverging at more than 42 degrees from the normal will be back reflected so the dependence on the angular distribution of the light is greater. A ray tracing analysis of the performance of the L50 type shows efficiency of light through the glass to the sensor disc varies between 84% and 90% depending on light angular distribution. Similar to the L40 type, the L50 sensors are calibrated by a 1070nm laser straight through the front glass. Since most IPL sources have an angular distribution similar to type C in the illustration above, we take the efficiency to be in the range above but closer to type C. Calibration factor = (efficiency with IPL source 88%)/(efficiency with laser 96%) So we calibrate the sensor to read 8% higher than the laser energy applied.

Measuring Gel Coupled IPL Sources

Experimental verification

To verify these calculations we measured pulses from an Energist i Pulse IPL source into the L40 type IPL sensor and the L50 type IPL sensor. The i Pulse has a type C angular distribution. Both of these sensors were calibrated using the calculated calibration factors as derived above. The results of the irradiation are given below:
 
Reading of L40 type: 34.9J
Reading of L50 type: 64.0J
 
After correction for the amount absorbed by the L40 type (22x22mm input aperture out of the 27x33mm source aperture) and assuming uniform output from the source, we obtain the following:
 
Energy density of L40 type: 34.9J/4.8cm² = 7.27J/cm²
Energy density of L50 type: 64.0J//8.91cm² = 7.18J/cm²
 
There is excellent agreement between the two methods of measurement – a discrepancy of the order of 1%
 
We have thus shown that within variations of angular dependence, both the L40 and L50 type IPL sensors can be reliably used to measure gel coupled IPL sources.

Measuring Gel Coupled IPL Sources

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