confirmation of spectral jitter - c ash, g town, m clement
DESCRIPTION
This document quantifies the effect of the temporal pulse has on the spectrum of light. This is the first document to show scientifically the rate of change in the spectrum of light due to change in current density within a gas discharge lamp. This work originated from an idea of monitoring the individual wavelengths of light from the broadband spectrometer. This analysis and manuscript was written on the flight from London to Chicago in 2008. I presented the work at ASLMS, Washington in 2009TRANSCRIPT
Review
Confirmation of spectral jitter: a measured shift in the spectraldistribution of intense pulsed light systems using a time-resolved
spectrometer during exposure and increased fluence
C. ASH*{, G. TOWN{ and M. CLEMENT{
{School of Medicine, Swansea University, Swansea, SA2 8PP, UK{GCG Healthcare, Haywards Heath, RH16 2LT, UK
(Received 16 June 2009; revised 19 September 2009; accepted 9 October 2009)
High quality intense pulsed light (IPL) systems can offer simple, safe and effective
treatments for long-term hair reduction, skin rejuvenation and removal of benign
vascular and pigmented lesions. Considerable differences in clinical efficacy and adverse
effects have been recorded amongst different IPL systems despite comparable display
settings. This study examines the variation in pulse structures exhibited by several
popular professional IPL systems that can cause a spectral change within the broadband
output depending on the pulse structure chosen by the system designers. A fast
spectrometer was used to capture IPL spectral outputs. A spectral distribution shift that
occurs both within a pulse and between pulses is clearly demonstrated and is more
prominent with uncontrolled free discharge systems than with square pulsed technology,
which provides a constant spectral distribution throughout the pulse duration.
Keywords: Pulse duration; Spectral distribution; IPL; Time-resolved spectroscopy
1. Introduction
Typically, intense pulsed light (IPL) systems discharge
radiation between 510 nm and 1100 nm, with wavelengths
filtered as required, depending upon the condition being
treated [1]. These visible and near-infrared wavelengths
penetrate skin and are absorbed by target chromophores
via selective photothermolysis [2] for treatments such as
hair reduction [3–5] and skin rejuvenation [6–9].
Transient post-treatment erythema, peri-follicular oede-
ma and hyperpigmentation [10] are common treatment
endpoints in IPL treatments. There are safety issues
regarding such intense light sources on human skin but
they are generally considered safer than laser systems [11].
However, cases of permanent side effects [12] and ocular
damage [13] have been reported. As the technology is
relatively new, long-term effects of IPL treatment are still
unknown, with many physicians and scientists recommend-
ing further investigation into biological changes and
malignant lesions [14,15].
Knowledge of the optical dosimetry characteristics of an
IPL device is essential to establish a scientific basis for
applications involving light–tissue interaction using an IPL
device. There is only limited published literature on the
measurement of IPL devices [16–22]. Town et al. [23]
identified five key IPL measurement parameters: pulse
duration, radiant exposure (fluence), spatial profile, spec-
tral output and time-resolved spectral output. Standardiza-
tion of measurement introduces consistency into a system,
lowering the risk of adverse reactions from device
malfunction and improving treatment efficacy and relia-
bility [22].
Time-resolved spectral measurement is significant in
optimizing treatment parameters and assessing clinical
*Corresponding author. Email: [email protected]
Journal of Medical Engineering & Technology, Vol. 34, No. 2, February 2010, 97–107
Journal of Medical Engineering & TechnologyISSN 0309-1902 print/ISSN 1464-522X online ª 2010 Informa UK Ltd.
http://www.informaworld.com/journalsDOI: 10.3109/03091900903402089
and ocular hazards [20–22]. Different absorption charac-
teristics of chromophore targets in skin (figure 1) require
that sufficient energy be delivered in the wavelength range
that is most likely to be biologically effective, whilst
minimizing adverse reactions by filtering potentially dama-
ging wavelengths being delivered to the tissue matrix.
Information about the true spectral footprint during IPL
exposure makes it possible for an ocular hazard assessment
to be completed. Ocular hazard assessment determines the
risk of exposure directed onto the human eye, which is
predominantly sensitive to wavelengths in the range 380–
1400 nm [28].
A light source that emits a spectrum that changes within
a pulse and from one pulse to another, is unlikely to
generate an efficient reproducible response with each
discharge in human tissue. At increased current density
within the flashlamp the spectral distribution is shifted
towards lower wavelengths. Vaynberg et al. [24] was the
first to state that the spectral distribution of an IPL system
is proportional to the input current. The ability to control
the flashlamp current facilitates more accurate management
of temperature differences between target and surrounding
tissue, thus improving selectivity and potentially improving
clinical outcomes.
IPLs can be categorized into two main types by the
method used to generate and deliver the energy required for
light-based treatments, i.e. free discharge and square pulse.
A free discharge system applies a large electrical charge to a
capacitor or a number of capacitors in parallel, then
discharges the entire stored energy directly though the
flashlamp; this discharge profile is characterized by a rising/
falling slope. It was theorized that as the energy supplied to
the flashlamp varies, the emission characteristics change
through a shift in the emitted wavelengths [25]. This effect
of ‘spectral jitter’ was first proposed by Clement et al. [25]
where the spectral output varies during a pulse or pulse
train and between energy levels. Thus, by the application of
a constant current, improved targeting of IPL treatments
based on selective photothermolysis is achieved. A constant
(square) current spectral emission over the duration of the
pulse controls the temperature difference between the
target and surrounding tissue more efficiently than can
be achieved with a conventional free-discharge IPL
treatment.
Whilst the role of measurement of pulse duration and
pulse profile for lasers and intense pulsed light sources has
been recognized recently [2,18,26], only a few studies to
date have attempted to document methods for measuring
IPL pulse durations or to examine in detail the time-
resolved spectral output of IPLs across each millisecond of
the pulse duration. The objective of this study is to show
the effect of spectral jitter with evidence of spectral energy
Figure 1. Absorption coefficient of melanin, oxyhaemoglobin, water and porphyrin with a typical IPL discharge emission
spectra overlaid for reference. Note: logarithmic axis.
98 C. Ash et al.
distribution and spectral stability during an IPL emission
using time resolved spectroscopy.
2. Material and methods
The authors performed measurements over a 6-month
period on three constant-current and 16 free-discharge
systems, which were all in daily use in private dermatology
clinics and salons in the UK. These included StarLux
(Palomar Medical Technologies, Burlington, MA), iPulse
(CyDen, Swansea, UK), NovaLight (Ultramed, Geneva,
Switzerland), Chromolite (Chromogenex, Llanelli, UK),
Crystal 512 (Active Optical Systems, Petach-Tikva, Israel),
EllipseFlex/EllipseLight (Ellipse, Hørsholm DK, Den-
mark), Harmony (Alma Lasers, Caesarea, Israel), ULTRA
(Energist, Swansea, UK), BBL (Sciton, Palo Alto, CA),
Lumina600 (Lynton Lasers, Cheshire, UK), GPFlash1
(General Project, Montespertoli, Florence, Italy), Plasma-
lite (Medical Biocare Sweden, Vastra Frolunda, Sweden),
Ecolite (Greenton London, London, UK), Quantum/Acu-
light (Lumenis, Santa Clara, CA), Freedom IPL (Freedom
Beauty, Leicester, UK), Trinity (Espansione Marketing
Spa, Bologna, Italy) and SkinStation (Radiancy (Israel),
Yavne, Israel). The authors previously grouped these
individual systems into four distinct categories by their
delivery pulse pattern namely, square pulse, free discharge,
close pulse stacking, and spaced pulse stacking [21]. One
example of each category was selected, analysed and
presented as part of this study.
Conventional spectrometers need a relatively long
sample time, rather like the exposure time on a camera.
This averaging effect dampens or eliminates the variations
in spectral peaks. Time-resolved spectral measurements
make it possible to assess variations in spectral composition
during the light pulse, and hence evaluate the quality and
consistency of sequential flashes of the IPL. These
assessments may then form the basis for hypotheses on
improvements that may be made to the pulse duration and
spectral pattern of an IPL’s output characteristics to
produce improved clinical outcomes.
The time-resolved spectra in this study were produced
using an Ocean Optics HR2000þ spectrometer and its
counterpart SpectraSuite software (OceanOptics, Dunedin,
FL). This software has the capability of sampling a
spectrum of light with a minimum integration time of
1 ms by generating 1000 full spectral scans per second.
Time-resolved spectral data of IPL outputs were captured
and stored with an optical resolution of a monochromatic
source measured as full width half maximum (FWHM)
resolution of 0.035 nm. This fast spectrometer uses a Sony
ILX511 2048-element linear silicon CCD-array detector to
capture data into memory every millisecond interfaced to a
PC via a USB 2.0 port for later analysis. The
HR2000þ spectrometer has the facility of stray light
correction to compensate for ambient light, which could
otherwise create a slight offset in the results. Every result
was recorded with this facility enabled. The spectrometer is
externally triggered using a breakout box and because of
the relatively short pulse duration of an IPL system, the
sampling was taken over an extended time period to ensure
capture of the data. The source of the intense light from the
IPL system and the spectrometer optical fibre was
separated by a distance of 150–180 cm to prevent satura-
tion of exposed light upon the CCD array within the
spectrometer. During testing, suitable broadband protec-
tive eyewear was worn by all persons present within the
enclosed room.
The data was used to provide the spectral contribution
of a number of wavelength points from 300 nm to
1000 nm in 50-nm intervals. The spectral distribution was
plotted across the pulse duration with a 1-ms resolution,
or in the case of multiple pulses the spectral distribution
at the peak of the pulse is taken and plotted with pulse
number.
3. Results
The results are discussed with reference to the category of
pulse delivery and the effects of increasing fluence.
3.1. Square pulse
Figures 2(a) and 2(b) show the corresponding sequence of
time-resolved spectral emission views for each millisecond
of exposure (iPulse i200þ, CyDen) exhibiting a sharp cut-
off filter at 530 nm employed to reduce epidermal absorp-
tion. The pulse profile measured is a single pulse of 25 ms
duration. Figure 2(b) records the spectral distribution of
key wavelengths (300–1000 nm) during the pulse duration
showing wavelengths proportional and consistent to their
neighbouring wavelengths with respect to time. A consis-
tent distribution of wavelength with time may predict more
consistent treatment outcomes.
3.2. Free discharge
Figures 3(a) and 3(b) present the spectral analysis of
Chromolight (Chromogenex) showing a varying output
with time that is poorly filtered with 17% of spectral energy
below 500 nm, potentially posing a risk to skin and ocular
safety. Figure 3(b) shows the spectral distribution of
wavelengths changes with respect to time. The 650 nm
wavelength increases by 84% whereas 500 nm, 550 nm,
600 nm and 700 nm all decrease by 42% on average from
start to finish.
3.3. Close pulse stacking
Figures 4(a) and 4(b) record spectral analysis of Ellipse
Light (Ellipse) showing a sharp cut-off filter at 600 nm
Spectral distribution shift in intense pulsed light systems 99
employed to reduce epidermal absorption. The pulse profile
on the Ellipse Light is of seven sub-pulses stacked closely,
decaying with increasing fluence. Figure 4(b) shows
changes of wavelength contribution during the different
sub-pulses with respect to time of key wavelengths between
300 nm and 1000 nm. The 950 nm wavelength noticeably
increases by 93% whereas 600 nm, 650 nm and 700 nm
wavelengths all decrease on average by 23% from first sub-
pulse to the last.
3.4. Spaced pulse stacking
Figures 5(a) and 5(b) show the spectral analysis of a
Lumina600 (Lynton Lasers) with a 650-nm cut-off filter
Figure 2. (a) Time-resolved square pulse—spectral analysis of a square pulse system with a 530-nm filter for each millisecond
of exposure (iPulse i200þ, CyDen). (b) The spectral distribution of key wavelengths (300–1000 nm) showing stability during
the pulse duration.
100 C. Ash et al.
employed to reduce epidermal absorption. This IPL system
uses five separate capacitors to discharge the energy in
short high-energy pulses. Figure 5(b) shows wavelengths
that are consistent with respect to the pulses of five
sequential sub-pulses. In this case, due to the relatively
short on-times and long dwell periods any changes to
the flashlamp characteristics reverse before the next sub-
pulse.
3.5. Increasing radiant exposure (fluence)
To explain why the spectral emission changes during
delivery of free discharge systems and not with square
pulse systems, a test was devised where we took time-
integrated spectral captures of the two respective systems
with set pulse duration and varied the fluence. A spectral
shift in some wavelengths occurs for both systems. Thus the
Figure 3. (a) Free discharge pulse—spectral analysis of a poorly filtered free discharge system (Chromolight, Chromogenex).
(b) The spectral distribution of key wavelengths (300–1000 nm) changing during the pulse duration.
Spectral distribution shift in intense pulsed light systems 101
spectral distribution is dependant on the current density
within the flashlamp. As the current density of free
discharge systems change with time, so does the spectral
emission. Figures 6(a) and 6(b) present the spectral analysis
of a square pulse system with various fluence values in the
range 6–18 J cm72 (iPulse i200þ, CyDen). The pulse
duration used during measurements was a single pulse of
25 ms in duration. The graph in figure 6(b) shows that the
spectral distribution of wavelengths changes with increas-
ing fluence. The 550 nm, 600 nm, 650 nm and 700 nm
wavelengths all increase, whereas other wavelengths remain
consistent with respect to fluence. Figures 7(a) and 7(b)
display the spectral analysis of a free discharge system with
various energy level values ranging from settings 1–10
Figure 4. (a) Close pulse stacking—spectral analysis of a close pulse stacking system with a 600 nm (Ellipse Light, Ellipse).
(b) The spectral distribution of key wavelengths (300–1000 nm) vary during the different pulses.
102 C. Ash et al.
(Chromolight, Chromogenex). The graph in figure 7(b)
shows the spectral distribution of key wavelengths 300–
1000 nm with increasing energy level. The spectral dis-
tribution of wavelengths changes with increasing fluence as
450 nm and 500 nm wavelengths increase whereas 650 nm
decreases with respect to increased energy level. For both
figures 6(b) and 7(b) the wavelengths most noticeable of
change are within the region of the plasma energy used to
create the broadband energy and are solely dependant on
the magnitude of fluence.
Figures 2(a), 3(a), 4(a) and 5(a) show the compromise
in spectral distribution systems made to provide pulses of
energy that are therapeutic and cautious. The spaced
stacking system has the highest cut-off filter at 650 nm,
Figure 5. (a) Spaced pulse stacking—spectral analysis of a spaced pulsed stacking system with a 650 nm filter (Lumina,
Lynton Lasers). (b) The spectral distribution of key wavelengths (300–1000 nm) is stable during the five sub-pulses.
Spectral distribution shift in intense pulsed light systems 103
set higher than the other systems to prevent adverse
reactions from the high energy interspaced pulses. As a
consequence the system is inefficient as the excluded
wavelengths result in excessive heat being extracted by
bulky and costly cooling systems. The example free
discharge system had the lowest cut-off filter, presumably
to maximize delivered fluence. The four very different
spectral distributions are important in assessing ocular
hazards as the spectral content is weighted against an eye
response function.
Figure 6. (a) Increasing fluence/square pulse—spectral analysis of a square pulse system with various fluence values ranging
6–18 J cm72 (iPulse i200þ, CyDen). (b) The spectral distribution of key wavelengths 300–1000 nm with increasing fluence
(J cm72).
104 C. Ash et al.
4. Discussion
This study has verified a spectral distribution shift
occurring both within a pulse and between pulses that is
more prominent with uncontrolled free discharge systems.
A change in spectral distribution is shown with increasing
fluence for both systems. However, square pulse technology
is shown to provide a more consistent spectral distribution
during the pulse duration compared with free discharge.
Thus the spectral distribution is dependant on the current
density within the flashlamp. Although this effect occurs, it
is unlikely to be clinically significant, as fluence and pulse
dosimetry are much greater dependant factors affecting
treatment outcome.
Figure 7. (a) Free discharge—spectral analysis of a free discharge system pulse with various energy level values ranging 1–10
(Chromolight, Chromogenex). (b) The spectral distribution of key wavelengths 300–1000 nm with increasing energy level.
Spectral distribution shift in intense pulsed light systems 105
Our results show that the spectral distribution of free
discharge and close stacking systems vary during pulses.
Figure 3(b) shows 650 nm increasing by 84% and four
other wavelengths decrease by 42% on average; figure
4(b) shows 950 nm increasing over the pulse duration by
93% and three other wavelengths decreasing by 23% on
average. Figures 2(b) and 5(b) show spectral stability
from square pulse and spaced pulse systems. From the
measurements taken, the spectral shift of wavelengths
during the period of exposure generally increases in the
region of 900–950 nm and decreases in the range 500–
700 nm. Many systems incorporate high cut-off filters to
prevent undesirable epidermal absorption, thus inadver-
tently concealing this decaying effect in shorter wave-
lengths.
4.1. Melanin
The effect on epidermal melanin during pigmentation
lesion treatment where shorter wavelengths greatly
absorb has a potential clinical impact as, during
exposure, the dissipated wavelengths shift to longer
wavelengths that are less well absorbed. The treatment
of hair removal is however unlikely to be affected by a
change in spectral distribution due to the broad absorp-
tion range of melanin.
4.2. Acne
Porphyrin absorption has five distinct peaks, the largest at
424 nm (Sorec band) and four smaller peaks in the Q-band
at 500 to 625 nm (figure 1). The decay of emitted
wavelengths in this region of the electromagnetic spectrum
may negatively impact the treatment of superficial acne
where the target bacterium is destroyed by photons
delivered in the shorter wavelength region where light
penetration is at its shallowest.
4.3. Haemoglobin
The largest absorption peak occurs around 578–585 nm,
this peak being around the pivot pointing spectral
distribution where shorter wavelengths decrease and the
longer wavelengths increase. Photons need to penetrate the
tissue matrix through the epidermal barrier where vessels
are located in the upper dermis.
Although the exact clinical extent of spectral jitter is
currently unknown, a deliberate swing in spectral
distribution during exposure could influence treatment
parameters of specific applications. There is one manu-
facturer of IPL systems advocating a spectral shift-
controlling property, however the effect of spectral
variation is clinically unmeasurable in this case as the
pulse duration is drastically altered with respect to
spectral shift [27].
5. Conclusions
It has been five years since Clement and colleagues [25]
first proposed a theoretical model involving the physics
of the time resolved pulse structure influencing the
output dosimetry of photo therapeutic treatments. This
paper presents quantitative results of spectral shift both
within pulses and between pulses, and with increasing
fluence. This spectral shift is more prominent with free
discharge systems. A square pulse IPL with a consistent
release of therapeutic wavelengths with time suggests a
more efficient and consistent treatment outcome with
fewer adverse reactions than with other pulse struc-
tures.
Optical spectral footprints provide valuable informa-
tion regarding IPL system performance, clinical effi-
ciency and patient safety. IPL manufacturers should
provide time resolved spectroscopy graphs to users.
These measurements could be helpful in determining
whether there is a potential impact on efficacy of
absorption of light by the primary skin chromophore
targets of interest.
Acknowledgements
The authors would like to thank CyDen Ltd, Swansea, UK
for part-funding and provision of equipment used in this
study. In addition, we thank Dr Susanna Town, University
of Calgary, Canada for review of the manuscript.
Declaration of interest: Godfrey Town receives consultancy
fees and travel grants from CyDen Ltd. Caerwyn Ash is a
PhD student at Swansea University and receives travel
grants from the university. He also receives salary from
CyDen Ltd.
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