Light Hazard Measurement on an Ophthalmic Instrument
David Melo
1
, Pedro Vieira
2
and Filipe Soares
1
1
Fraunhofer Portugal AICOS, Rua Alfredo Allen, 455/461, 4200-135, Porto, Portugal
2
Department of Physics, Faculdade de Ci
ˆ
encias e Tecnologia, Universidade Nova de Lisboa,
Quinta da Torre, 2829-516, Caparica, Portugal
Keywords:
Light Hazard Measurement, Ophthalmic Instruments, Fundus Camera.
Abstract:
When the acquisition of images is performed on human body structures, the safety of the person on which
the images are being obtained acquires fundamental importance. This condition gathers even more relevance
when the goal of the image acquisition is related with health examination. To ensure that every physician, when
imaging human structures is undoubtedly secure that it will not harm or cause any non-tolerable discomfort
to the subject undergoing the exam, medical devices have to surpass a set of specific norms depending on the
nature of the structure being examined. More specifically related with instruments for eye related diseases
diagnostic and treatment, the ISO-15004-2 norm is the currently accepted. As some types of eye exams
rely on highly concentrated radiation, its power has to be limited in order to prevent irreversible damage.
In this paper, a Light Hazard Measurement for a specific prototype will be addressed, on which the power
calculation method for the emitted radiation will be described and the compliance of the limiting norms will
be verified. The main conclusion from the work here described is that the prototype can be classified as a
Group 1 Ophthalmic Device according to the ISO-15004-2 norm.
1 INTRODUCTION
In the work presented in this paper, a fundus cam-
era prototype named EyeFundusScope, currently un-
der investigation by Fraunhofer Portugal AICOS, is
submitted to a light hazard measurement. The goal
of this procedure is to understand if the prototype can
be used in screening and diagnostics situations, with-
out provoking any harm to the agents participating on
them, from patients to examiners.
The prototype uses light sources emitting at differ-
ent wavelengths, with one of them emitting on a nar-
row region of the near infrared the electromagnetic
spectrum and the other emitting broadband white
light. The light sources used were Light Emission
Diodes (LEDs) and the measurement of its power was
performed with a photodiode power sensor. The emis-
sion spectrum of each light source was obtained using
a spectrometer.
In this section the crucial principles and tools
needed for the measurements are introduced and in
the next ones, the procedure, the consequent results
and the conclusions that may be obtained from them
are described.
1.1 Eye Examination
The eyes along with the brain, the nervous system and
its information transfer channels ensure one of the ve
ways human beings can acquire and understand in-
formation advent from its surrounding environment.
Taking this into consideration, a regular eye examina-
tion is of unaccountable importance.
According to the American Academy of Ophthal-
mology every adult should perform an ocular evalua-
tion, at least, every 5 years (Feder et al., 2016). The
frequency of the examinations for patients with, one
or more risk factors, such as diabetes or with an al-
ready diagnosed ophthalmological pathology should
naturally be increased (Feder et al., 2016).
During screening actions and regular medical ap-
pointments, several regions of the eye are observed.
Depending on the type of exam and the eye region
the physician wants to observe, radiation may have
to be directed towards the eye. For instance consid-
ering retinal examination, taking into account fundus
low reflectance, the retina can only be observed when
a considerable amount of light passes the pupil, en-
ters the ocular globe and reaches the fundus of the
eye (DeHoog and Schwiegerling, 2009; Hammer and
Schweitzer, 2002; Preece and Claridge, 2002). For
Melo, D., Vieira, P. and Soares, F.
Light Hazard Measurement on an Ophthalmic Instrument.
DOI: 10.5220/0007575800870098
In Proceedings of the 12th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2019), pages 87-98
ISBN: 978-989-758-353-7
Copyright
c
2019 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
87
the observation of more anterior structures the use of
light is not so imperative for illumination but can also
be used to understand several physiological mecha-
nisms as occurs in pupillometry, on which the pupil-
lary response to light of specific wavelengths is stud-
ied (Kelbsch et al., 2016; Rukmini et al., 2015).
1.2 Light Emission Diodes(LED)
Nowadays, Light Emission Diodes are available
in a wide-range of wavelengths, existing an al-
most continuum spectrum of wavelength availabil-
ity from near ultraviolet to near infrared existing
also monochromatic emitters at deeper infrared wave-
lengths (Capit
´
an-Vallvey and Palma, 2011).
They present several advantages when compared
with the traditional incandescent lamps namely, a
longer lifetime, higher brightness, lower power con-
sumption and the possibility of electrical modulation
at higher rates (Malinen et al., 1998; Patrick and
Fardo, 2000). Its usage in Healthcare Applications
is widespread not only as part of diagnosis instru-
ments, but also for disease treatment, more impor-
tantly within the scopes of dermatology, ophthalmol-
ogy and recently, neurology (Rahman et al., 2013;
Saltmarche et al., 2017; Eells et al., 2004). The tech-
nique is named photobiomodulation and can be de-
fined as the irradiation of tissue with low-intensity
light in the infrared band, provoking alterations on the
cellular functions (Desmet et al., 2006).
LED principle of operation is based on electrolu-
minescence, the production of light by flow of elec-
trons (Schubert, 2018). More specifically for the pro-
duction of white light, a phosphor is usually added to
a blue light LED chip. The phosphor is excited by the
blue light coming from the chip and absorbs part of
it. Due to the excitation, it emits light of other visi-
ble wavelengths, usually in the green and red regions
of the electromagnetic spectrum that together with the
blue light partially transmitted, generates white light
(Chen, 2005; Schubert, 2018).
1.3 Photodiode Power Sensors
Photodiode Power Sensors are constituted by a semi-
conductor that produces a flow of electrons (current)
dependent on the amount and wavelength of photons
(light intensity) that reach its working surface (Senior,
1986). From the current produced the power can be
obtained.
The power output of the photodiode power sensor
reveals a high dependence on the wavelength of the
radiation being measured. So, they can only be used,
as a direct power meter, when the light source being
measured has a narrow emission band. With this in
consideration, the photodiode operator can select the
expected wavelength of the radiation (hereinafter de-
scribed as λ
0
). Although, as the suppliers normally
provide information on the Sensor Spectral Response
(Figure 1) methods for the measurement of broader
emission bands situations can be conceived.
Figure 1: Spectral Response of the PD300-3W with the re-
placeable filter in. R(λ) is the relative sensitivity of the pho-
todiode for the wavelength λ.
The power measured by the photodiode is usually
performed on the highest peak of the spectrum and
is defined by the following equation, according to the
White Paper by the Doctor Efi Rothem ”Measuring
LED Power and Irradiance with Calibrated Photodi-
odes” available at Ophir website (Rothem).
P
Meas
=
1
R[λ
0
]
×
Z
R(λ) × I(λ) dλ (1)
In which P
Meas
is the power read from the photo-
diode output display, R(λ) is the relative sensitivity of
the photodiode for radiation of wavelength λ, I(λ) is
the spectral intensity of the radiation being measured
and λ
0
is the wavelength selected on the photodiode.
Photododiodes are considered valuable and ac-
curate tools when the light source to be studied
presents an extremely narrow wavelength such as
lasers (Ready, 1971).
To validate Equation 1 it is possible to test if for
an ideal single-wavelength light source, accordingly
to the manufacturers the optimal situation for use of
photodiodes, the power measured by the photodiode
is equal to the real power.
The Equation 1 for the ideal light source presented
in Figure 2 (example of the spectrum of a laser emit-
ting at 550 nm ) becomes:
P
Meas
=
1
R[555]
×
Z
R(λ) × I(λ) dλ (2)
And the real power for any type of light source is
given by the Equation 3.
P
Real
=
Z
I(λ) dλ (3)
BIODEVICES 2019 - 12th International Conference on Biomedical Electronics and Devices
88
Figure 2: Spectral distribution of an ideal single wavelength
light source.
By the spectrum is easily deductible that I(λ) = 0
for every wavelength except when λ = 555nm. So
as shown in Equation 4 it is possible to prove that
P
Meas
= P
Real
. For validation of the equality, the value
of
R
I(λ) is not replaced by the intensity value from
the spectrum (Figure 2).
P
Meas
=
1
R[555]
×
Z
R[555] × I(λ) dλ
P
Meas
=
R[555]
R[555]
×
Z
I(λ) dλ
P
Meas
=
Z
I(λ) dλ = P
Real
(4)
This way, it is shown that the Equation 1 for the
power measured by the photodiode, is congruent with
the fact that photodiodes are accurate direct power
meters for narrow band sources.
2 POWER MEASUREMENT
In this section, light hazard measurements will be pre-
sented for a smartphone-based handheld fundus cam-
era prototype. This prototype uses two LEDs as light
sources, one with a narrow peak on the near infrared
(NIR) region of the electromagnetic spectrum and the
other emitting broadband white light. The method for
examination of the retina with this prototype is char-
acterized by firstly illuminating the fundus with NIR
light for guidance. When the examiner is satisfied
with the image being observed, a button is pressed
and white light is turned on, only for a few millisec-
onds (typically about 50 to 100 ms is sufficient). The
coloured image obtained is stored and becomes avail-
able for consultation on the smartphone. It was es-
tablished like this, to prevent miosis, an effect that
consists on the narrowing of the pupil by action of
the Autonomous Nervous System, when visible light
reaches the retina (Wang et al., 2016; Tran et al.,
2012). As miosis is not verified when infrared light
reaches the retina (Shen and Mukai, 2017), to limit
the use of white light only to the minimum to obtain
a color image of the fundus, this method was chosen
to allow a comfortable examination for the patients
and to avoid the need of mydriatic agents (drugs that
dilate the pupil) (Yuvacı et al., 2015).
The level of the emitted light intensity is con-
trolled by Pulse Width Modulation (PWM) (Dyble
et al., 2005). On the smartphone application the op-
erator can select this level on a range between 0 and
100 % of the maximum value allowed.
The emission spectrum of each LED was obtained
using a AvaSpec-ULS2048-USB2-VA-50 spectrome-
ter supplied by Avantes. The instrument used to ob-
tain the power values was a photodiode power sensor
PD300-3W. Photodiodes principle of operation was
already described in Section 1.3.
In order to collect all of the emitted light, in both,
spectrum and power measurements, the aperture of
the measurement device (spectrometer or photodiode)
had to be centered and placed at the focal distance of
the optical system covered by the prototype.
Its usable range goes from the near ultraviolet
wavelengths to the near infrared region (360-1100
nm) so it was able to recognize the full spectrum of
both LEDs.
The White LED is a broadband light source with
two peaks, being the first narrow and in the blue re-
gion and the other with a wider band and a peak on
the yellow\green regions of the spectrum. The NIR
LED only has one peak.
The several steps one must consider when per-
forming this type of measurement in such conditions
for each LED, may be resumed by the following:
1. Obtain the spectra with the spectrometer;
2. Normalize the spectrum according to the maxi-
mum counts;
3. Calculate the wavelength of the highest peak by
post-processment of the spectrum (in this example
python libraries such as NumPy and SciPy were
used);
4. In the photodiode select the wavelength obtained
in the step 3;
5. Select several levels for the intensity of light
through the smartphone application - in this setup
Light Hazard Measurement on an Ophthalmic Instrument
89
the selected values were 30, 50, 75 and 100 % of
the current allowed by the hardware;
6. As photodiodes are very wavelength sensitive, it
is needed to multiply the Spectral Response pre-
sented in Figure 1 by the Spectrum obtained on
step 2;
7. By the direct proportionality relationship between
the area beneath the spectral curve and the power,
find the real power emitted at each peak wave-
length.
The Step 6 is justified by the fact that the Power
measured by the Photodiode is given by the Equation
1 in Section 1.3, at the same time that the power emit-
ted by a light source is derived with the Equation 3.
To bring more clarity to the method described be-
fore, in the following subsections, not only the final
radiant power values obtained will be shown, but also
the set of calculations required for each LED.
2.1 White LED
The white LED is a OSLON
R
Black LUW H9GP
supplied by OSRAM. Chromatically it has a temper-
ature of 6500 K, which is considered a cool white
with a strong prevalence of lower wavelengths (Violet
and Blue regions of the spectrum). As referred previ-
ously four different levels for the intensity of the emit-
ted light were measured. The changes on the spec-
trum along the intensity levels were not representa-
tive, only a slight increase on the signal-to-noise ratio
with the increase of LED intensity. The normalized
spectrometer output for the White LED is presented
in Figure 3.
The power measurements obtained on the photo-
diode are presented in Table 1.
Figure 3: Spectrum of White LED at maximum intensity
(100%) obtained with the AvaSoft 2048.
Besides the calculations of these values it was re-
quired the multiplication of each spectrum obtained
by the spectral response. The curves for the LED
spectral intensity, spectral response, and result of its
multiplication are presented in Figure 4.
Figure 4: Top - In red is presented I(λ), the normalized spec-
tral intensity of the light source and in blue is presented
R(λ) the relative sensitivity of the photodiode. Bottom - Is
presented in black the result of the multiplication of the two
curves presented in the Top image - R(λ)×I(λ), hereinafter
referred as Weighted Spectrum.
To use the Equation 1, the sensitivity of the pho-
todiode at the specific wavelength is needed. For the
visible LED, λ
0
= 439 nm, so R[λ
0
] = R[439].
The variables needed for the Equation 1:
R[439] - 0.62
P
Meas
- 0.60 mW
Replacing the variables for these values, the Equa-
tion 1 becomes:
0.60 =
1
0.62
×
Z
R(λ) × I(λ) dλ
0.372 =
Z
R(λ) × I(λ) dλ
(5)
With the last equality of the Equation 5, and con-
sidering that the expression
R
R(λ) × I(λ) dλ is given
by the area beneath the Weighted Spectrum curve,
R(λ) × I(λ) (Figure 4 - Bottom), a direct proportion-
ality relationship between power and area can be es-
tablished. The variable Spectrum Area is the area be-
neath the curve presented in Figure 3, corresponding
to the emission spectrum of the LED and named I(λ)
in Equation 3.
With the variables presented next, the real power
(P
Real
) emitted by the White LED can be calculated.
Weighted Spectrum Area (WS
area
) - 49.46
Spectrum Area (S
area
)- 62.63
Power with photodiode sensitivity influence
(P
Sens
) - 0.372 mW
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90
Table 1: White LED Measured Power at 439 nm, P
Meas
, and Real Power, P
Real
, for each peak and for both peaks.
Smartphone Level
Wavelength
P
Meas
- 439 nm P
Real
- 439 nm P
Real
-555 nm P
Real
- Total
30 0,18 mW 0,044 mW 0.096 mW 0.140 mW
50 0,31 mW 0,077 mW 0.165 mW 0.242 mW
75 0,45 mW 0,111 mW 0.240 mW 0.351 mW
100 0,60 mW 0,150 mW 0.321 mW 0.471 mW
Considering the relationship of proportionality pre-
viously established, the following Equation can be
used:
P
Real
=
P
Sens
× S
area
W S
area
P
Real
= 0.471 mW
(6)
To compare the values obtained with the ISO
Norms, it is needed to know the percentage of the to-
tal power corresponding to each peak. In order to have
an accurate separation of the two peaks a Gaussian Fit
function from SciPy library was used. Two Gaussian
Fits were considered, one for each peak, with the pa-
rameters:
Wavelength of the Center;
Amplitude of the Gaussian;
Width of the Gaussian;
The graphical results of the fitting are presented in
the Figure 5.
Figure 5: Top - In black is presented I(λ), the spectrum of
the light source and in red is presented the Gaussian Fit
function. Bottom - Peaks are individually presented, the
first in green and the second in blue.
From the area of each peak it is possible to calcu-
late each peak proportion on the total power, as pre-
sented by the following equations.
A
Peak1
(%) - 31.8%
A
Peak2
(%) - 68.2%
P
Peak1
= A
Peak1
× P
Real
P
Peak1
= 0.318 × 0.471
P
Peak1
= 0.150 mW
P
Peak2
= A
Peak2
× P
Real
P
Peak2
= 0.682 × 0.471
P
Peak2
= 0.321 mW
(7)
In Table 1 it is possible to verify the final values
for the power of the White LED, for both peak wave-
lengths.
2.2 NIR LED
The Near Infrared LED is an Infrared Emitter
MTE1081C, supplied by Marktech Optoelectronics.
The normalized spectrum of the light emitted by the
LED is presented in Figure 6. Likewise what occurred
with the White LED, the changes on the spectrum
along the four intensity levels were not representative.
The method is slightly different to the one previously
presented for the White LED due to the existence of
only one peak.
Figure 6: Spectrum of the NIR LED at maximum intensity,
(100%) obtained with the AvaSoft 2048.
The power values measured with the Photodiode
are presented in Table 2.
The result of the multiplication of the NIR Spec-
trum by the Spectral Response of the Photodiode is
presented in Figure 7.
Light Hazard Measurement on an Ophthalmic Instrument
91
Figure 7: Top - In red is presented I(λ), the spectrum of
the light source and in blue is presented R(λ) the relative
sensitivity of the photodiode. Bottom - Is presented in black
the result of the multiplication of the two curves presented
in the top image - R(λ) × I(λ) - Weighted Spectrum.
Similarly to what occurred with the multiplication
of the white spectrum by the spectral response (Fig-
ure 4), the effect of the wavelength sensitivity of the
photodiode is noticeable. The sensitivity of the pho-
todiode for NIR wavelengths is much higher than it
is for lower wavelengths, fact that can be confirmed
by comparison of the y-scale of the weighted spectral
response of both LEDs (Figures 4 and 7 - Bottom).
Calculations will be shown for the maximum in-
tensity level.
R[820] - 2.57
P
Meas
- 0.57 mW
Replacing the above values in Equation 1:
0.57 =
1
2.57
×
Z
R(λ) × I(λ) dλ
1.465 =
Z
R(λ) × I(λ) dλ
(8)
The variables needed for the real power (P
Real
)
calculation are:
Weighted Spectrum Area (WS
area
) - 106.0
Spectrum Area (S
area
)- 41.40
Power with photodiode sensitivity influence
(P
Sens
) - 1.465 mW
P
Real
=
P
Sens
× S
area
W S
area
P
Real
= 0.57 mW
(9)
The major difference between the used method for
each LED is related with the number of peaks. The
NIR LED only has one peak and because of that the
Gaussian Fit is not required, and P
Real
can be consid-
ered directly proportional to all the area beneath the
curve I(λ). With this approximation for the compar-
ison with the ISO norms, all the radiation emitted by
the NIR LED is centered at 820 nm.
The results for every level of intensity are pre-
sented in Table 2.
Table 2: NIR LED Measured Power at 820 nm - P
Meas
and
NIR LED Real Power at 820 nm - P
Real
. SL - Smartphone
Level; WL- Wavelength.
SL
Power
P
Meas
P
Real
30 0,17 mW 0,168 mW
50 0,30 mW 0,299 mW
75 0,44 mW 0,439 mW
100 0,57 mW 0,570 mW
As can be perceived by comparison of the P
Meas
and P
Real
in Table 2, the values are equal or very iden-
tical.
This can be explained by the fact that on the region
where the NIR LED emits radiation ( 758-850 nm -
Figure 6), the Photodiode Spectral Response (Figure
1) is considerably flat. Consequently, for the integral
R
R(λ) × I(λ) dλ, on the spectrum region of interest,
R(λ) will be nearly constant and similar to R[820].
3 ISO COEFFICIENTS
CALCULATION
In order to ensure the safety of the prototype, the val-
ues calculated for the power in the previous section
must be converted to quantities that can be compared
with the limits displayed on the ISO Norms. So in
this section, the calculations required for this conver-
sion will be presented along with the limits applica-
ble to the wavalength of the radiation in question, ac-
cording to the ISO Standards ISO 15004-2 and ISO
10940 (ISO 15004-2, 2007; ISO 10940, 2009). The
first is for light hazard protection of Ophthalmic In-
struments and the second deals with the features of
Fundus Cameras (including light hazard protection).
As the limits provided on the ISO Norms are pre-
sented in irradiance, the areas of the irradiated struc-
tures have to be calculated.
The Field of View obtainable with this prototype
is 45
so, one way to calculate the retinal area illu-
minated is by using the formulas on the Equations 10
and 11 (ISO 10940, 2009):
BIODEVICES 2019 - 12th International Conference on Biomedical Electronics and Devices
92
ω = 4πsin
2
(
α
4
)
(10)
A = (1.7cm)
2
× ω
(11)
Where ω is the illumination solid angle in stera-
dians, α is the full cone angle in degrees and A is the
retinal area illuminated in square centimeters. Con-
sidering a Field of View of 45
the solid angle is 0.478
sr and consequently the retinal area is 1.381 cm
2
.
Knowing the area it is possible to calculate the ir-
radiance with the formula:
E =
dΦ
dA
(12)
Where E is the irradiance given in mW/cm
2
, Φ is
the radiant power given in mW, referred as P
Real
in
the previous subsection, and A is the area illuminated
in square centimeters. The radiant power was consid-
ered to be constant all over the area, so the equation
becomes:
E =
Φ
A
(13)
Therefore, the values for the maximum level of
intensity for the peaks of both LEDs are:
Retinal Irradiance White - 439 nm Peak =
0.109 mW/cm
2
Retinal Irradiance White - 555 nm Peak =
0.232 mW/cm
2
Retinal Irradiance NIR - 820 nm = 0.413
mW/cm
2
For the other eye structures the utmost scenario
was considered in which the area for calculation of the
irradiance was the area of the smallest circle of illu-
mination possible with the prototype. In other words,
was considered the area of the circle of illumination
at the focal distance from the prototype. It was empir-
ically calculated to be of about 0.071 cm
2
, resulting
from a circle with 3 mm diameter.
Corneal and Anterior Segment Irradiance
White - 439 nm Peak = 2.113 mW/cm
2
Corneal and Anterior Segment Irradiance
White - 555 nm Peak = 4.521 mW/cm
2
Corneal and Anterior Segment Irradiance NIR
- 820 nm = 8.028 mW/cm
2
As for some hazards the irradiance is weighted
according to the wavelength of the radiation, the co-
efficients for both thermal, R(λ), and photochemical
aphakic hazard, A(λ) provided by the ISO standards,
have to be considered for each peak wavelength ex-
amined (ISO 15004-2, 2007):
White LED, 439 nm Peak: R(λ) - 1; A(λ) - 1;
White LED, 555 nm Peak: R(λ) - 1; A(λ) -
0.0078;
NIR LED, 820 nm Peak: R(λ) - 0.58; A(λ) - 0;
Considering the principle of operation of the pro-
totype described before, ISO norms compliance was
addressed for 4 different situations.
Continuous NIR LED at maximum intensity;
Continuous White LED at maximum intensity;
Pulsed White LED at maximum intensity;
Real acquisition simulation with the NIR LED
at maximum intensity, and pulsed White LED at
maximum intensity.
The distinction between continuous and pulsed
occurs because in the ISO norms it is considered that
when an instrument emits a pulse or a set of pulses
that last less than 20 seconds the limits that must be
calculated are different from continuous wave instru-
ments (ISO 15004-2, 2007). For each configuration
different hazards must be calculated and are going to
be presented in the next subsections.
3.1 Continuous NIR
For this configuration the values that have to be cal-
culated are:
Unweighted Corneal and Lenticular Infrared
Radiation Irradiance, E
IR-CL
;
E
IR-CL
=
2500
770
E
λ
× ∆λ (14)
With,
E
λ
=
dΦ(λ)
dA × dλ
=
Φ(λ)
A × ∆λ
(15)
Where E
λ
is the spectral irradiance given in
mW/(cm
2
.nm), Φ is the radiant power of the ra-
diation, given in mW, A is the area illuminated in
square centimeters and ∆λ is the wavelength inter-
val on which the irradiance was measured. This
approximation will be considered on every haz-
ard and is only possible because the radiant power
was considered to be constant all over the area and
all the emission wavelengths near the peak, were
considered to be the peak itself. Due to this ap-
proximation it is mathematically possible to ig-
nore the dependence on the value of ∆λ, as can be
observed next.
Light Hazard Measurement on an Ophthalmic Instrument
93
E
IR-CL
=
2500
770
Φ(λ)
A ×
∆λ
×
∆λ
E
IR-CL
=
2500
770
Φ(λ)
A
E
IR-CL
=
2500
770
E
E
IR-CL
= 8.028 mW/cm
2
(16)
Unweighted Anterior Segment Visible and In-
frared Radiation Irradiance, E
VIR-AS
;
E
VIR-AS
=
1200
380
E
λ
× ∆λ (17)
There is only one emitted peak in the NIR spec-
trum, with 820 nm as center wavelength. So, the
result for this hazard is equal to E
IR-CL
, therefore
E
VIR-AS
= 8.028 mW/cm
2
.
Weighted Retinal Visible and Infrared Radia-
tion Thermal Irradiance, E
VIR-R
;
E
VIR-R
=
1400
380
E
λ
× R(λ) × ∆λ (18)
E
VIR-R
=
1400
380
E × R(λ)
E
VIR-R
= 0.413 × 0.58
E
VIR-R
= 0.240 mW/cm
2
(19)
3.2 Continuous White
For this configuration the values that have to be cal-
culated are:
Weighted Retinal Irradiance for Photochemi-
cal Aphakic Light Hazard, E
A-R
;
E
A-R
=
700
350
E
λ
× A(λ) × ∆λ (20)
E
A-R
=
700
350
E × A(λ)
E
A-R
= 0.109 × 1 +0.232 × 0.0078
E
A-R
= 0.109 + 0.00181
E
A-R
= 0.111 mW/cm
2
(21)
Unweighted Anterior Segment Visible and In-
frared Radiation Irradiance, E
VIR-AS
; The
method for the calculation of this hazard was al-
ready demonstrated for the NIR LED in the previ-
ous subsection, so only the result will be shown.
E
VIR-AS
=
1200
380
E
E
VIR-AS
= 2.113 + 4.521
E
VIR-AS
= 6.634mW/cm
2
(22)
Weighted Retinal Visible and Infrared Radia-
tion Thermal Irradiance, E
VIR-R
; The method
for the calculation of this hazard was already
demonstrated for the NIR LED in the previous
subsection so, only the result will be shown.
E
VIR-R
=
1400
380
E × R(λ)
E
VIR-R
= 0.109 × 1 +0.232 × 1
E
VIR-R
= 0.341mW/cm
2
(23)
3.3 Pulsed White
For the pulsed mode, information on the duration of
each pulse is necessary. In the current prototype, the
user can select the duration of flash. Was verified em-
pirically that 0.095 seconds produced fairly good re-
sults when imaging an eye model so, it will be con-
sidered the time of the pulse for comparison with the
ISO Norms comparison. The energy of the flash nor-
mally used is the maximum intensity possible for the
White LED, so irradiance values are the same as those
for the continuous white calculations (Section 3.2).
For the Pulsed Intruments Hazards the results to
be compared will take into consideration the spectral
radiant exposure, H
λ
, which can be derived from the
Irradiance, E
λ
, according to the following equations.
H =
Z
t
E × dt = E × t (24)
And so,
H
λ
= E
λ
× t (25)
Where E
λ
is spectral irradiance, H
λ
is spectral ra-
diant power and t is the duration of the exposure to
the radiation also named exposure time. The value of
H
λ
is given in mJ/(cm
2
.nm).
For the same reason that for the continuous
sources, ∆λ is not relevant. For this configuration the
values that have to be calculated are:
Weighted Retinal Visible and Infrared Radia-
tion Radiant Exposure, H
VIR-R
;
H
VIR-R
=
1400
380
(E
λ
× t) ×R(λ) × ∆λ (26)
BIODEVICES 2019 - 12th International Conference on Biomedical Electronics and Devices
94
H
VIR-R
=
1400
380
Φ(λ)
A
× t
× R(λ)
H
VIR-R
=
1400
380
E × t × R(λ)
H
VIR-R
= 0.109 × 0.095 ×1 + 0.232 ×0.095 × 1
H
VIR-R
= 0.0324 mJ/cm
2
(27)
Unweighted Anterior Segment Visible and In-
frared Radiation Radiant Exposure, H
VIR-AS
;
H
VIR-AS
=
1400
380
H
λ
× ∆λ (28)
H
VIR-AS
=
1400
380
Φ(λ)
A
× t
H
VIR-AS
=
1400
380
E × t
H
VIR-AS
= 2.113 × 0.095 +4.521 × 0.095
H
VIR-AS
= 0.630 mJ/cm
2
(29)
3.4 Real Acquisition Simulation
Lastly, it is necessary to test the prototype under real
acquisition conditions in order to predict if it will ful-
fill all the safety requirements on real use cases. As
was previously referred, the prototype uses NIR light
when the operator is performing the retinal align-
ment so a good image of the retina without provok-
ing mydriasis can be obtained and only acquires the
image with white light when observes referable opti-
cal structures for the disease being diagnosed (optic
disk, vessels...). Consequently, a good simulation of
an intended use case can be to consider that the ex-
amination takes about 5 minutes, and 5 images of the
retina are acquired. According to the ISO Norms, in
case of consecutive use of different light sources, the
limit for radiation reaching each surface of the eye is
given by (ISO 15004-2, 2007):
(E,H,L)
1
Limit
1
+
(E,H, L)
2
Limit
2
+ ... +
(E,H, L)
i
Limit
i
1 (30)
Where E is the irradiance, H is the radiant expo-
sure, L is the radiance and i is the i
th
source. As the
irradiance and the radiance are analogous, measure-
ments on only one of them are required. In the work
here presented only irradiance was used.
For pulsed sources the results obtained must be
multiplied by the number of pulses considered so,
when five pulses are considered the spectral exposure
results become:
H
5 Pulses
= 5 H
Single Pulse
(31)
As a consequence :
Result H
VIR-R
- 0.162 mJ/cm
2
Result H
VIR-AS
- 3.15 mJ/cm
2
The ratio presented above must be verified for dif-
ferent groups of hazards accordingly to the structure
they refer to, so the following equations have to be
verified.
Retinal Hazards:
Result E
VIR-R
Limit E
VIR-R
+
Result H
VIR-R
Limit H
VIR-R
=
=
0.240
Limit E
VIR-R
+
0.162
Limit H
VIR-R
(32)
Anterior Segment Hazards:
Result E
VIR-AS
Limit E
VIR-AS
+
Result H
VIR-AS
Limit H
VIR-AS
=
=
8.028
Limit E
VIR-AS
+
3.15
Limit H
VIR-AS
(33)
Corneal and Lenticular Hazards:
Corneal and Lenticular Hazards only consider ra-
diation emitted between 770 and 2500 nm, so the
pulsed white light does not present an hazard for
these structures.
As for all the other configurations, the results will be
presented in the following section (Section 4).
4 RESULTS AND DISCUSSION
In this Section the comparison of the results obtained
for the prototype with the norms currently considered
for Ophthalmic Devices will be demonstrated. Haz-
ards results and limits for the modes of operation of
the prototype are presented in Table 3. The compar-
ison between the results of the measurements with
the limits was performed for the maximum intensity
(100%), even though lower levels of light can also al-
low the acquisition of fairly good images.
Besides the certification that all the results are be-
low Group 1 limits it is necessary to perform the cal-
culations for the real acquisition conditions in order
to predict if the prototype will fulfill all the safety re-
quirements on real use cases. As previously stated in
subsection 3.4, in case of consecutive use of differ-
ent light sources, the following equations have to be
verified (ISO 15004-2, 2007):
Light Hazard Measurement on an Ophthalmic Instrument
95
Table 3: Group 1 ISO Norms Compliance. The cells filled in green represent the Hazards for which the results calculated
were below the Group 1 Limit.
Mode Hazard Result Group 1 Limit Compliance
Continuous NIR
E
IR-CL
8.028 mW/cm
2
20 mW/cm
2
E
VIR-AS
8.028 mW/cm
2
4000 mW/cm
2
E
VIR-R
0.240 mW/cm
2
700 mW/cm
2
Continuous White
E
A-R
0.111 mW/cm
2
0.220 mW/cm
2
E
VIR-AS
6.634 mW/cm
2
4000 mW/cm
2
E
VIR-R
0.341 mW/cm
2
700 mW/cm
2
Single Pulse White
H
VIR-R
0.0324 mJ/cm
2
6t
3
4
= 1.026 J/cm
2
=
1026 mJ/cm
2
H
VIR-AS
0.630 mJ/cm
2
25t
1
4
= 13.879 J/cm
2
=
13879 J/cm
2
Retinal Hazards:
0.240
Limit E
VIR-R
+
0.162
Limit H
VIR-R
=
=
0.240
700
+
0.162
3432
=
= 0,000390 1
(34)
Anterior Segment Hazards:
8.028
Limit E
VIR-AS
+
3.15
Limit H
VIR-AS
=
=
8.028
4000
+
3.15
20754
=
= 0.00216 1
(35)
Corneal and Lenticular Hazards:
For Corneal and Lenticular Hazards there is no
Pulsed ratio because on the acquisition mode the
prototype only emits white light.
The values for the radiant exposure limits were
obtained by replacing the value of t on the equations
that set the limits (H
VIR-R
- 6t
3
4
; H
VIR-AS
- 25t
1
4
) by
the product of the exposure time for a single pulse and
the number of pulses.
5 CONCLUSION
Considering the results presented on the previous sec-
tion, it is possible to conclude that the EyeFundusS-
cope prototype surpassed the Group 1 Ophthalmic
Device tests, according to the ISO 15004-2 Norm.
Group 1 Instruments can be defined as instruments for
which no potential light hazard exists (ISO 15004-2,
2007).
If the use case presented in Subsection 3.4 that
considers obtaining 5 images during 5 minutes, is not
applicable, and a different amount of flashes are di-
rected to the human eye, the calculations required to
attest the hazard can be easily performed, by replicat-
ing the ones presented in this paper. Considering the
differences of orders of magnitude between the result
and the limit (presented in the Results subsection 4),
it is not likely that a different setup would led to a
change of classification of the prototype.
By presenting a method for the measurement of
radiant power that uses devices considerably cost-
effective when compared to the regularly used equip-
ment (Spectroradiometers (Calandra et al., 2017;
Hong et al., 2017)), we believe that the work here pre-
sented may be beneficial for all the researchers that
want to perform similar tests.
To the best of the authors knowledge, in the litera-
ture there is not much information on power measure-
ments for broadband LEDs with photodiodes. Con-
sidering this, the work presented in this paper can be
of considerable importance in the field of Biomedi-
cal Metrology and provide valuable knowledge for the
designers of biological and biomedical instruments,
who regularly deal with radiation safety issues.
BIODEVICES 2019 - 12th International Conference on Biomedical Electronics and Devices
96
ACKNOWLEDGEMENTS
We would like to acknowledge the financial support
obtained from North Portugal Regional Operational
Programme (NORTE 2020), Portugal 2020 and the
European Regional Development Fund (ERDF) from
European Union through the project Symbiotic tech-
nology for societal efficiency gains: Deus ex Machina
(DEM), NORTE-01-0145-FEDER-000026.
For supporting with the necessary material and
guidance we would also like to acknowledge Lab-
orat
´
orio de
´
Optica, Lasers e Sistemas, a contract-
research laboratory within the Physics Department of
FCUL (University of Lisbon).
It is also important to acknowledge Ricardo
Peixoto, Jo
˜
ao Gonc¸alves, Sim
˜
ao Felgueiras and Jo
˜
ao
Oliveira, the Fraunhofer AICOS team members re-
sponsible for assembling and developing the proto-
type.
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