The measurement setup is illustrated in figure for in-vitro and figure for in-vivo experiments. An LED (L730-805-850-40B32 from Epitex Inc.), that could emit NIR light at three wavelengths of 730, 805, and 850 nm, was employed as a NIR light source. These wavelengths are in the middle of the optical window described in figure . The LED was inserted in a cushioning material that is attached to the surface of the absorbing medium via a medically approved double-sided sticky tape (Adchem Inc.) to ensure good optical contact. An optical phantom with an absorption coefficient of 0.08 cm-1 and reduced scattering coefficient of 12.5 cm-1 is used as the light-absorbing medium. A thermocouple needle with a 0.1 mm diameter was inserted between the source and absorbing medium to monitor the temperature change continuously. The needle was connected to the thermocouple device (Sper Scientific Ltd.). Data was sent to a PC by using RS-232 connection to be stored and plotted continuously. "Testlink 188.8.131.52" software provided by the thermocouple company is used to plot and store the data with sampling frequency of 1 Hz and temperature resolution of 0.1°C.
Setup to measure the heating effect of the semiconductor in vitro
Description of the in-vivo experimental setup
In order to compensate for the effect of the ambient temperature, the ambient temperature was also continuously recorded on the tissue phantom using a second thermocouple 6 cm away. The change in the baseline due to drifts in ambient temperature was corrected after the experiment.
The heating of the thermocouple needle itself due to NIR absorption was tested in parallel. To do this, NIR light was applied to the needle where the contact to LED was avoided by means of an optical filter which only allows NIR light (NIR-pass filter). It has been observed that the NIR radiation does not significantly heat the thermocouple.
As discussed earlier, the LED source produces two types of heating effects due to conducted and radiated energy. Conducted heat due to the semiconductor junction is measured in-vitro using a NIR-pass filter as shown in figure . In this setup, measurements are performed with and without NIR-pass filter. The NIR-pass filter does not heat up due to NIR light absorption since it is transparent to NIR light. When filter is used, the thermocouple measures the temperature increase due to NIR light absorption. When filter is not used, the temperature increase is caused by the combined heating effect of semiconductor junction and NIR light absorption. Therefore, the difference between two readings gives the temperature increase solely due to semiconductor junction heating.
The temperature increase of the semiconductor junction depends on the effective (root mean square) dissipated power of the LED. Different effective powers can be obtained by changing the emitted waveform. DC, pulsating waveform with 12.5 ms pulse duration and 33.3 % duty cycle and sinusoidal waveform with 1 kHz frequency were tested in-vitro to study the effect of different waveforms. A single wavelength of 730 nm was used. Peak irradiances between 25 and 50 mW/cm2 were employed. The crest factor for pulsating waveform and sinusoidal waveform is 1.73 and 1.41, respectively.
For the pulsating waveform, effective power can be controlled by varying the pulse duration, duty cycle, irradiance and using single or multiple wavelengths. A function generator coupled to the LED driver was used to generate pulsating LED light to study the effect on the temperature of the semiconductor junction. Single wavelength of 730 nm was used. Temperature increase for pulse durations of 12.5, 50, 100 and 150 ms, duty cycles of 25 %, 33.3%, 50% and 75% were tested in-vitro. For these duty ratios crest factors are 2, 1.73, 1.41 and 1.15 respectively. The peak value of the pulses was selected to be 37.5 mW/cm2. This is the power range generally used in diffuse optical measurement applications.
Temperature increase is also affected by the number of different wavelengths used. In continuous wave diffuse optical applications, generally two wavelengths are used to resolve the absorption by two chromophores. Therefore, we tested for the effect of using two wavelengths in-vitro. Wavelengths of 730 and 850 nm were time multiplexed in a way that only one wavelength was turned on at a time. Pulse duration was 12.5 ms and duty cycle was 33.3% which corresponds to a crest factor of 1.73. Peak irradiance was 37.5 mW/cm2.
In addition to square pulses, 730 nm and 850 nm wavelengths with 1 kHz sinusoidal waveform and 90° phase shift were used in-vitro to study the heating effect of the quadrature phase-modulated system. The peak irradiance of 37.5 mW/cm 2 with a crest factor of 1.73 and wavelengths of 730 and 850 nm were employed as in the case of square pulses.
The combined effect of the radiated and conductive heat was also tested with an in-vivo set up on three Caucasian adult subjects. All in-vivo studies were carried out under Institutional Review Board (IRB) approval. The LED was directly coupled to the surface of a human arm to observe the in-vivo effect as described in figure . The change in the surface temperature directly beneath the LED was recorded continuously for 12.5 ms pulses with 37.5 mW/cm2 irradiance and 33.3% duty cycle.
During in-vivo experiments, the LED was coupled to the skin using a cushioning material attached to the skin by a medically graded double-sided sticky tape. It is well known that heat increase of the skin is dissipated by sweating mechanism and by heat exchange with air as well as convection via blood flow [8
]. The cushioning material that is used to attach the LED to the skin causes pressure and isolation of skin tissue from ambient air. In addition, it blocks sweating by closing the pores and hence contributes to the temperature increase. This was monitored with a secondary thermocouple attached to the skin a few centimeters away from the LED source using the same kind of cushioning material.
In addition to the test setup, the device currently used in Drexel University and University of Pennsylvania with the purpose of functional optical brain imaging using NIR light [9
] was studied for the heating effect during actual operation with human subjects. The device employs the same kind of LED as used in in-vitro studies. In these experiments, a standard duty cycle of 8.3%, pulse duration of 12.5 ms and irradiance of 37.5 mW/cm2
were used. The crest factor is 3.47 for such a pulse shape. The temperature rise in the semiconductor junction was tested in-vitro. The combined heating effect on skin due to both NIR absorption and the heating of semiconductor junction was experimented in-vivo on the human arm and forehead.
In each experiment, data were collected for 35 minutes. The temperature increase after 30 minutes of operation was assumed as the steady state temperature. Data points corresponding to 5 minutes following the 30th minute of the operation were avera ged to determine the resulting temperature increase.
An alternative way to analyze the temperature increase due to absorption of LED emitted NIR light is to compare it with sun emitted NIR light. With the geographic conditions defined by United States Committee on Extension to the Standard Atmosphere, for the 48 contiguous states of the USA over a period of one year as an inclined plane at 37° tilt toward the equator, facing the sun; i.e., the surface normal points to the sun at an elevation of 48.81° above the horizon [11
], the American Society for Testing and Materials (ASTM) developed and defined a standard terrestrial solar spectral irradiance distribution called ASTM G159 [12
]. This standard can be used to calculate the average NIR irradiance emitted by the sun and absorbed by human skin. The average power delivered by the NIR part of the sunlight calculated using this standard is 50 mW/cm2