Determination of IR-LED Emission Spectrum and Radiated Power
Prepared by Markus Raschke, Ph.D., Associate Professor of Chemistry, Adjunct Associate
Professor of Physics, University of Washington, Seattle, Washington
This study was undertaken on behalf of Eng3 Corporation. For more information contact Eng3 directly
at:
Phone: 206.525.0227
Email: [email protected]
www.eng3corp.com
© Copyright 2011 Eng3 Corporation. All rights reserved. M087-rev02
877.571.9206 | www.eng3corp.com | [email protected]
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Determination of IR-LED emission spectrum and radiated power
The primary objective of this study is to compare a specific near-infrared (1200 – 1300 nm) light
emission from two different technical approaches used in three different devices and their radiated
power on exposed water molecules.
The specific near-infrared light wavelength of 1270 nm (non-visible) is emitted by singlet oxygen
molecules when they undergo radiative relaxation into their triplet state (S0 -> T). Singlet oxygen
molecules are also known as ROS (Reactive Oxygen Specie). The process of emitting relaxation
energy is also called phosphorescence and known to trigger oxidative response.
One technical approach, used in two devices (Airnergy and ActiveAir) relies on so-called catalyst
based/singlet oxygen generating activation cells (chambers). First, a catalyst is activated with a
wavelength of the visible red light spectrum. Second, the activated catalyst excites ambient oxygen
and shifts it into singlet oxygen. Third, the singlet oxygen emits its relaxation energy to exposed water
molecules in an air stream flowing through the activation cells (chambers).
A second technical approach is used by one device (NanoVi), which uses special electroluminescence
cells equipped with non-visible near-infrared emitting diodes (NIR LED), which, in a single step, emit a
specific wavelength to water molecules within an air stream. The technical approach of using nearinfrared emitting diodes (NIR LED) in the activation cells in the NanoVi device also allows constant and
pulse modes for the direct energy emission to the water molecules in the exposed air stream.
The spectral emission, power, and water vapor exposure for both the catalyst based/singlet oxygen
generating activation cell devices and the device with NIR LED activation cells were measured and
compared.
Power:
In order to characterize the power emitted in the near infrared spectrum of 1200nm, an Ophir Orion
power meter was used to characterize several diodes positioned at fixed distance of ca. 1 cm, i.e.,
reproducibly detection a defined solid angle of emission.
Spectral emission:
Measurement of the spectral emission of the NIR LED was performed with an HP71951 optical
spectrum analyzer. The emission spectra, recorded with an optical fiber (62 um entrance aperture) 1
cm from the NIR LED, are displayed below along with the angular dependence of the emission
measure using a goniometer setup.
Results
Device type: Near-infrared emitting diodes (NIR LED) activation cell (NanoVi)
Figure 1 shows that the peak emission of the NIR LED in the activation cells occurs at 1200nm with a
spectral full width half maximum of approximately 80 nm. The two primary modes of interest were
when the NIR LED was powered with a constant current power supply and with a pulsed power supply.
The average recorded power for four sample diodes with constant power supply was found to be 9.8
mW with a standard deviation of 1.7 mW. Under a pulsed power supply the same four near-infrared
photo diodes exhibited an average power of 2.6 mW with a standard deviation of 0.8mW.
© Copyright 2011 Eng3 Corporation. All rights reserved. M087-rev02
877.571.9206 | www.eng3corp.com | [email protected]
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Figure 1:
Power
Figure 1: The detected power spectrum emitted by two examples NIR LEDs with an HP HP71951 optical spectrum
analyzer with a 60um entrance aperture
Device type: Near-infrared emitting diodes (NIR LED) activation cell (NanoVi)
Figure 2 shows the angular dependence of recorded wavelength to the receiver. Using the goniometer,
the half angle of emission of the diodes was determined to approximately 15 degrees. From the known
solid angle of detection and diode distance to the power meter head we conclude that about 90% of
the radiated power is measured by the power meter head.
Figure 2
Figure 2: Angular dependence of diode emission of the two sample NIR diodes.
Device type: Catalyst based/singlet oxygen generating activation cell (Airnergy / ActiveAir)
The power of emission in the near-infrared spectrum of 1200nm using the singlet oxygen generating
catalyst that was activated by a wavelength of the visible red light spectrum (around 700nm) in the
activation cells (chambers) was undetectable by the optical spectrum analyzer for both device types
tested. The signal to noise limit of the analyzer is 1 pW.
Result:
Significantly more 1200nm power is produced by the technology using near-infrared emitting diodes
(NIR LED) activation cells (NanoVi) than by the catalyst based/singlet oxygen generating activation
cell (Airnergy/ActiveAir).
© Copyright 2011 Eng3 Corporation. All rights reserved. M087-rev02
877.571.9206 | www.eng3corp.com | [email protected]
Page 3 of 4
This yields a lower limit for the fluence ratio of the IR-diode to the catalyst surface of >103 (near
infrared diode fluence)/(catalyst surface fluence).
This result is further corroborated by a simple application of the law of energy conservation. More
1200nm power is produced by the NIR LED activation cells (NanoVi) than by the catalyst based/singlet
oxygen generating activation cells (Airnergy/ActiveAir) as the NIR LED on their own produce more
power than the diodes that emit the visible red light spectrum used to excite the catalyst to produce
singlet oxygen. The diodes for visible red light spectrum exhibit an average power of only
approximately 2 mW. Approximately 4-5 NIR LED would generate more power than all of the diodes
for visible red light spectrum three catalyst cells combined. Even if every visible photon were
converted to a near-infrared photon (quantum efficiency =1, which is not the case), the generated
near-IR power would be at most 1 mW per visible red light diode, i.e. a tenth of what can be achieved
by one NIR LED. Considering that the overall (i.e. including singlet oxygen formation) quantum
efficiencies for photosensitized phosphorescence of singlet oxygen are on the order of 10-3 or lower,
the corresponding emission is expected to be several orders of magnitude less than the NIR LED
radiation.
Water Vapor/Molecules Exposure: for the air stream
For the water molecules we can estimate the photo induction = exposure energy through the nearinfrared light. This quantity is the amount of energy of light to which a volume element of water vapor
is exposed as it moves through the activation cells. This can be quantified through the following
equation E= ⋅(V/f). Here
represents the incident flux of light, V the volume of exposure, and f the
flow rate. For NIR LED (NanoVi device) illumination
is much greater than what could be achieved for
activation via singlet oxygen phosphorescence (Airnergy/ActiveAir devices). For a NIR LED (10mW)
in a NanoVi device illuminating a 5x5x5mm area (on the order of a typical flow line) flowing at a rate
of 0.5m/s, an exposure of 0.1mJ is achieved.
Lastly, as the emission power at 1200nm via singlet oxygen is below the detection limit, we may only
make an estimate of the upper limit of the energy exposure via photosensitized phosphorescence. The
flow line cells for the air stream with the water vapor/molecules in Airnergy/ActiveAir devices have a
larger volume, so as a result would have a longer exposure time to 1200nm than the NIR LED
illuminating the flow line cell in a NanoVi device. For an example volume of 5cmx5cmx2cm, an
illumination of 4.4⋅10-8 J can be estimated. In conclusion, even with the larger cell volume and
considering the overall geometry, the weak energy emission of the singlet oxygen results in a at best
a factor of 10-4 weaker excitation than one single NIR LED shining on the water vapor.
Conclusion:
With the “NanoVi” approach and its emission range of 1100 – 1300 nm by the NIR LEDs this
corresponds to the same energy range compared to that emitted by the singlet oxygen based NIR
generation, yet with several orders of magnitude higher power. The small difference in peak power
wavelength is not expected to have any physical effect on the efficiency on the excitation with any
molecular resonance. This approach eliminates the indirect multi-step singlet oxygen approach based
on first activating a catalyst that then generates singlet oxygen and subsequent emission of 1200 nm
NIR radiation.
© Copyright 2011 Eng3 Corporation. All rights reserved. M087-rev02
877.571.9206 | www.eng3corp.com | [email protected]
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