1. Introduction

As the global optical medical device market has grown in size, the use of light-emitting diode (LED) lighting products has steadily increased in both household and hospital applications ⁽¹⁾. However, due to the absence of test standards specifically designed to evaluate such medical technologies, conventional LED test standards for illumination have been used instead. A significant challenge with this approach is that conventional standards were not designed to provide traceability; thus, it is difficult to relate various research data sets. For this reason, it is essential that studies be conducted on the test standards used to evaluate medical LED light sources to develop methods to ensure traceability. Conventional LED test standards, such as CIE 127, KS C 7104, which specifies a standard method for evaluating the performance of LEDs, and KS C 7652, which specifies the requirements for Non-ballasted LED lamps, primarily focus on evaluating the performance of the light source itself and detail methods for evaluating the illuminance of light sources by finding a beam angle at which 50% of the light intensity is reached at the center point or by measuring the illuminance at a specific distance or point. Such conventional standards are specifically designed to assess visible light factors, such as the intensity, luminance, illuminance, and total luminous flux [2-4]. In contrast, for medical light sources, it is necessary to accurately analyze the amount of irradiation absorbed by cells and skin tissues instead of measuring the irradiation required to ensure visibility. Studies on medical light sources require measurement of the irradiance [mW/㎠] in the development stage in terms of the radiant energy density [J/㎠] to ensure the optical properties of sufficiently high quality.

As shown in Fig. 1, LED light sources in medical devices are typically tested based on the amount of light irradiating the cultured cells on the bottom surface of the cell plate. Thus, an analysis of the optical properties of the light source is required based on the irradiance that is directly involved in the survival of the cell.

Fig. 1. Mimetic diagram of a cell irradiation experiment

2. Materials and methods

The purpose of the present study is to develop methods to measure the optical properties of medical LED light sources used in cell testing when their light is directly irradiated onto a cell plate and to assess the associated impact on cell viability.

Tests were conducted to compare and analyzed the irradiance values calculated for each measurement method to the results obtained from conventional LED measurement methods.

All optical measurements were performed in a non-ventilated test environment at a temperature of (25 ± 3)°C and a relative humidity of (30 ± 10)%, as specified in Annex A of KS C 7652: 2014 (Method for Measuring Optical Properties of LED Lamps) ⁽⁵⁾.

The optical equipment used for the measurements was calibrated by a certified calibration laboratory within one year of the test date.

2.1 LED light source

The LED light source used in the samples was of a type commonly used in therapeutic applications, such as skin irradiation, and had peak wavelengths between 390 and 698nm and a light irradiation distance of 1.4cm. It was configured to represent a near-light source. To verify the reliability of the method used to measure the various properties of LED light sources, such as the array pitch, light emitting shape, and mixed state, four types of LED modules were fabricated.

1. The first module consisted of five comb-shaped LED modules with a peak wavelength of 630nm and a pitch of 2.2cm (Fig. 2).

2. The second consisted of an LED module with two 390nm LED packages (Fig. 3).

3. The third comprised four identical 630nm LED packages (Fig. 4).

4. The fourth consisted of an LED module with one 630nm LED package (Fig. 5).

Fig. 2. Comb-shaped 630nm LED module

Fig. 3. Mixed module with a peak wavelength of 390nm

Fig. 4. LED module with a peak wavelength of 630nm

Fig. 5. 630nm LED module

Fig. 6. 698nm LED module

5. The fifth consisted of an LED module with one 698nm LED package (Fig. 6).

2.2 Measurement system and methods

In this study, comparisons were made between the results obtained using the commonly used conventional LED test standards ^(2-7) and those obtained using the four measurement methods evaluated in this study.

Fig. 7. Measurement system: portable optical wave meter(A) and goniophotometer(B, C)

The equipment used in the testing employed a portable optical wave meter and goniophotometer, both of which are depicted in Fig. 7. The cell plate used for calculating the irradiance was circular with a radius of 2.64cm.

2.2.1 Measurement method based on a portable optical wave meter

The first measurement method was performed as per standard KS C 7612: 1987 (Illuminance Measurement Method) ^(2,7) and employed a portable optical wave meter, the specifications of which are shown in Table 1. The comb-shaped 630nm LED module shown in Fig. 2 was used as the sample for this test, during which the distance between the detector of the portable optical wave meter and the center of the light source was maintained at 1.4cm. The irradiance was measured within one minute after power was applied to the sample (Fig. 8).

Table 1. Specifications of the portable optical wave meter

Equipment name (Model)	Portable optical wave meter (HD2102.2)
Manufacturer (Nation)	Delta Ohm (Italy)
Wavelength range	400 to 1050nm
Wavelength accuracy	±1nm
Radiant power range	0.1mW/㎡ to 2,000W/㎡
Test items	lux, cd, lux/s, cd/s, W/㎡, mW/㎡, J/㎡, $\mu$J/㎠, $\mu$mol(㎡․s), $\mu$mol/㎡, cd/㎡

Fig. 8. Irradiance test using Portable Optical Wave Meter

2.2.2 Measurement method based on the radiant intensity at the center of the light source

The second measurement method was as per CIE 84 1st Edition (The Measurement of Luminous Flux) and LM79:2008 (Electrical and Photometric Measurements of Solid-State Lighting Products) ^(3-4). The equipment used in this test was a goniophotometer, the specifications of which are shown in Table 2. The measurements were performed at a distance of 80cm so that the distance between the detector and the center point of the light source was at least 10 times the size of the light source. The light source sample was mounted at the center of the jig and the position of the detector was fixed at 0° on the γ plane and 0° on the C plane. The radiant intensity (Ic) was measured with respect to the center point of the light source. The irradiance (Ec) was calculated as follows for a light irradiation distance of 1.4cm :

(1)

Ec = Ic / Y2

where

Ec = Irradiance using the radiant intensity in the center [mW/㎠]

Ic = Radiant intensity in the center [mW/sr]

Y = Light irradiation distance [cm]

Table 2. Specification of goniophotometer

Equipment name (Model)	Goniophotometer (Neolight G500)
Manufacturer (Nation)	PIMACS (KOREA)
Wavelength range	220 to 1,020nm
Wavelength accuracy	±0.3nm
Radiant power range	2 $\times$ 10-8 to 200W/㎡
Test items	lux, cd, lm, W/㎡, cd/㎡, W, W/sr, nm, beam angle, spatial distribution, spectrum, light distribution graph, wavelength purity, Vf, If, W, FWHM, XYZ, CIE xy, CCT, CRI

2.2.3 Measurement method using the average radiant intensity of the half angle

The third measurement method and associated equipment was the same as that described in Section 2.2.2 ^(3-4). In this case, the radiant intensity (Iθ) and light distribution was measured through a fixed detector when the light source was mounted on a rotating jig in the goniophotometer. The sample rotated from 0° to 360° in the γ plane in 2° steps and from 0° to 90° in the C plane in 90° steps. After the measurement was completed, an equation for computing the half angle (θ) for the light source was derived by considering the radius and light irradiation distance of the cell plate, as shown in Fig. 9. The half angle is the angle at which light was irradiated into the cell plate in the cell irradiation test. This was computed to be approximately 62.1° using Eq. (2). However, as the sample in the goniophotometer rotated on the γ plane in 2° steps and the additional precision is therefore not pertinent, the value was rounded to 62°. After the half angle was identified, the mean value of the radiant intensity (Iθ) irradiated within 62° of the half angle was found to coincide with the irradiance computed as per Eq. (3).

Fig. 9. Cell irradiation test

(2)

θ = arcsin X /√(X2 + Y2) × 180 / π

where

θ = Half angle [°]

X = Radius of the cell culture plate [cm]

Y = Light irradiation distance [cm]

Z = Distance between light source and bottom surface of cell plate [cm]

(3)

EIθ = Iθ / Y2

where

EIθ = Irradiance using the average radiant intensity [mW/㎠]

Iθ = Average radiant intensity in the half angle [mW/sr]

Y = Light irradiation distance [cm]

2.2.4 Measurement method using the radiant flux in the half angle

The fourth measurement method employed the method and equipment described in Section 2.2.3. However, the irradiance was calculated as per Eq. (4) using the radiant flux (Wθ) rather than the radiant intensity (Iθ) irradiated within the half angle.

(4)

EWθ = Wθ / A

where

EWθ = Irradiance using total radiant flux in the half angle [mW/㎠]

Wθ = Total radiant flux in the half angle [mW]

A = Total bottom area of the cell plate [㎠]

3. Results and Discussion

The measurement results obtained from each of the previously described measurement methods with a comb-shaped 630nm LED module having a lambertian light distribution graph (Fig. 10) as a sample are shown in Table 3. The results for the method that employed the radiant flux in the half angle was used as the baseline. The variation rate represents the variation rate calculated via each of the four methods with respect to the baseline value.

When the first measurement method was applied, the average irradiance was found to be 1.512mW/㎠, which was approximately 44% lower than the baseline value. As shown in Fig. 11, the comb-shaped 630nm LED module used as a sample emitted light with a pitch of 2.2cm, while the diameter of the light-receiving portion in the detector was relatively small at approximately 1.6cm. In addition, as the distance between the light source and detector was fixed at 1.4cm in the tests, a lower amount of radiant flux was received by the detector, which meant the corresponding irradiance was significantly lower than that of the other measurement methods.

Fig. 10. Light distribution graph for a comb-shaped 630nm LED module: Comb-shaped 630nm LED module A (Fig. 2)

Fig. 11. Light emission in a comb-shaped 630nm LED module

As shown in Fig. 12, the diameter of the cell plate was approximately 5.28cm while the diameter of the detector in the portable optical wave meter was approximately 1.6cm. A mismatch of this magnitude in the relative sizes is problematic and indicates the portable optical wave meter equipment is actually unsuitable for measuring the amount of radiant flux actually irradiated onto the cells cultured in the plate in this configuration.

Fig. 12. Size of the cell plate and detector: cell plate(A), and detector(B)

When the second measurement method, which used the radiant intensity (Ic) at the center of the light source, was evaluated, the average irradiance was found to be 11.666mW/㎠, which was approximately 7.7 times higher than the irradiance measured using the portable optical wave meter and approximately 333% higher than the baseline value. The detected irradiance was higher because the maximum radiant intensity was detected at the center of the light source due to the nature of the Lambertian LED. In addition, this method exhibited the highest measurement error. Furthermore, as observed in the measurement method based on a portable optical wave meter, as this method provided no optical data that reflected the beam angle irradiated onto the cell plate, it was deemed unsuitable for measuring the optical properties of a medical light source.

When the third measurement method, which was based on the average radiant intensity (Iθ) in the half angle, was evaluated and the half angle was applied to the irradiance calculation, the average irradiance was found to be 9.684mW/㎠. This level was approximately 74% below that of the measurement method based on the radiant intensity at the center of the light source. However, the irradiance of this third method was approximately 6.4 times higher than that of the measurement method based on a portable optical wave meter. The difficulty here is that the value of the radiant intensity of radiation irradiated onto the cell plate versus the angle was computed using the average radiant intensity to calculate the irradiance, which was obtained from the mixed values between the radiant intensity irradiated in the downward direction from the center of the light source and the radiant intensity irradiated according to the angle of incidence. As a result, the measurement error with respect to the baseline value was found to be 259%.

The final measurement method, which was based on the radiant flux (Wθ) in the half angle, was an improvement on the previously described three measurement methods. In this final method, the horizontal irradiance irradiated onto the actual cells was computed based on the bottom area of the cell plate. The final average irradiance was calculated to be 2.695mW/㎠ using the radiant flux (Wθ) within a half angle of 62°, which is the angle at which the light is irradiated onto cells, based on the 21.89㎠ bottom area of the cell plate used in the testing. In this particular test, a cell plate with a light irradiation distance of 1.4cm and a diameter of approximately 5.28cm was used.

The results of the irradiance test for the LED light sources shown in Figs. 3 to 6 are listed in Table 4. The variation rate in the table indicates the variation rate of the irradiance relative to the baseline value of each sample. In the measurement method that used the center of the light source, the variation rate of the irradiance was 343% on average, and the average variation rate in the measurement method using the average radiant intensity in the half angle was 263%. When the results in Tables 3 and 4 are compared, the difference in the variation rate between the measurement methods that used the radiant intensity of the light source center amounted to approximately 10%, and the difference in the variation rate between the measurement methods based on the average radiant intensity in the half angle was approximately 7%. Thus, the error range between the measurement methods increased as the measurement method approached the conventional test method used the radiant intensity at the center of the light source.

As shown in Table 5, the sample with the highest content of radiant flux in a half angle of 62° was the 630nm LED module, which also had the narrowest light emitting beam angle and the highest variation rate of irradiance among the samples. In Table 5, the 698nm LED module has the lowest radiant flux content within the beam angle of 62° and the widest light directivity angle among the light source samples. Therefore, it was found that the variation in irradiance according to the measuring method was the lowest light source. This is shown in Table 4.

The results of measurements based on the conventional LED measurement method diverged significantly from the values obtained with the measurement methods evaluated in this study. The maximum data deviation between the conventional and proposed methods reached 374% because the conventional method was not designed for cell testing applications. Based on the results, the measurement method based on the radiant flux (Wθ) in the half angle was found to be the most suitable for testing medical LED light sources as it was not affected by the beam angle of the light source or the light-emitting pattern. Furthermore, this method computes the actual irradiated horizontal irradiance for the cell or strain by considering the situation in which cell irradiation tests are performed concurrently. In this way, the measurement traceability of this measurement method can be ensured.

4. Conclusions

The medical LED light source used in this study is of a type commonly used to deliver irradiation to the human body and for cell testing prior to clinical tests ^(8-13). For such medical light sources, it is important to measure the irradiance and irradiation time in order to compute the actual amount of radiation delivered to cells. However, conventional measurement methods do not measure these properties as their objectives are to evaluate the performance of the light source itself. Thus, if conventional measurement methods are used to evaluate medical LED light sources, the resulting error ranges from 44% to a maximum of 374%. The error range was found to increase as the beam angle of the light source became narrower or the deviation in the light distribution became more pronounced. In addition, when the irradiance was calculated using the radiant intensity rather than the radiant flux irradiated in the cell plate, the deviation of the irradiance data increased by at least 259%. For this reason, the conventional measurement method was deemed unsuitable for evaluating medical LED light sources used in cell testing. Thus, an improved measurement method was required to calculate the irradiance directly affecting cell viability.

The measurement method based on the radiant flux within the half angle, which was proposed in this study, calculates the horizontal irradiance involved in cell survival by using the bottom area of the cell plate as well as the total radiant flux in the half angle. Considering the rotation condition of the light source in the optical measurement using the goniophotometer, the total amount of radiant flux irradiated on the cell plate can be determined by using the half angle. Thus, the traceability of this measurement method can be ensured regardless of the type of light source. This measurement method can produce better results in situations where cell culture test results vary for different medical LED light sources in several studies and prevent the studies from advancing beyond clinical observation due to the difficulty of validating certain effects.

Most existing test standards for LED light sources have been standardized for evaluating the performance of the light source itself ^(2-7). Thus, when such standards are used in the evaluation of medical LED light sources, the result is an extremely large error and lack of measurement traceability.

In the future, test standards should be developed for medical LED light sources that include specialized test standards that consider cell irradiation test conditions.