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: caerwynash@yahoo.co.uk

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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