Safety assessment of near infrared light emitting diodes for diffuse optical measurements

Alper Bozkurt, Banu Onaral, Alper Bozkurt, Banu Onaral

Abstract

Background: Near infrared (NIR) light has been used widely to monitor important hemodynamic parameters in tissue non-invasively. Pulse oximetry, near infrared spectroscopy, and diffuse optical tomography are examples of such NIR light-based applications. These and other similar applications employ either lasers or light emitting diodes (LED) as the source of the NIR light. Although the hazards of laser sources have been addressed in regulations, the risk of LED sources in such applications is still unknown.

Methods: Temperature increase of the human skin caused by near infrared LED has been measured by means of in-vivo and in-vitro experiments. Effects of the conducted and radiated heat in the temperature increase have been analyzed separately.

Results: Elevations in skin temperature up to 10 degrees C have been observed. The effect of radiated heat due to NIR absorption is low--less than 0.5 degrees C--since emitted light power is comparable to the NIR part of sunlight. The conducted heat due to semiconductor junction of the LED can cause temperature increases up to 9 degrees C. It has been shown that adjusting operational parameters by amplitude modulating or time multiplexing the LED decreases the temperature increase of the skin significantly.

Conclusion: In this study, we demonstrate that the major risk source of the LED in direct contact with skin is the conducted heat of the LED semiconductor junction, which may cause serious skin burns. Adjusting operational parameters by amplitude modulating or time multiplexing the LED can keep the LED within safe temperature ranges.

Figures

Figure 1
Figure 1
The optical window used in diffuse optical measurements
Figure 2
Figure 2
Setup to measure the heating effect of the semiconductor in vitro
Figure 3
Figure 3
Description of the in-vivo experimental setup
Figure 4
Figure 4
(in vitro) Sample temperature increase result for an experiment
Figure 5
Figure 5
(in vitro) Temperature increase with varying peak irradiance levels for DC, pulsating and modulated waveforms
Figure 6
Figure 6
(in vitro) Temperature increase with varying duty cycles and pulse durations
Figure 7
Figure 7
(one wavelength vs. two wavelength) Temperature increase for pulsating and phase modulated waveforms
Figure 8
Figure 8
(in vivo vs. in vitro) Temperature increase for set up in fig 3 and for brain-imaging device

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Source: PubMed

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