Authors: Honglei Ji1,2*, Zelong Li 2 , Huaishu Xu2
1. University of Chinese Academy of Sciences, Zhejiang, Ningbo,China
2.TCL Electronics Holdings Limited., Shenzhen, China
*Corresponding author, jihl@tcl.com
Abstract: For the current mainstream low-blue light standards implemented internationally and domestically, including the low-blue-light certification standards of the TUV Rheinland certification body from German, the T/CVIA-02-2017 standard for low-blue display devices proposed by the China Electronics Video Industry Association, and the blue light hazard standards related to lamps and lamp systems in IEC/EN 62471, the compliance of various indicators and blue light hazard standards have been studied, which indicators include different parameters such as brightness and color point etc of TV products. Some results such as each standard above has certain limitations for TV products have been obtained, and some suggestions for improvement of
standards have been given.
Key words: Low blue light, standard, display device, eye hazard, evaluation index
Biohazard of visible light mainly include blue light hazard and non-visual biological effect. Blue light hazard refers to the potential hazard of retinal caused by photochemical reaction in the human eye between the radiation of 400~500nm[ i – ii ]. Non-visual biological effects usually refer to the blue light component in visible light by inhibiting the release of melatonin from the pineal gland, by stimulating the secretion of cortisol from the adrenal gland, thereby changing the circadian rhythm, regulating the body’s alertness and the biological clock, i.e., the rhythmic effect[iii].
In 1966, Noell et al.[iv] first publicly reported that blue light can cause hazard of rod cell. Subsequently, large numbers of studies have confirmed that the blue light band can cause photochemical hazard to the retina［v-vi］. In 2001, Dawson et al.［vii］conducted a test of blue light hazard on rhesus monkeys using light-emitting diode (LED), confirming that blue light can cause hazard to the primate retina. In 2002, Berson et al.［viii］ discovered a specialized rhythm-sensitive photoreceptor ganglion cell (ipRGC) associated with the rhythm, which is neural connected to the suprachiasmatic nucleus (SCN) and the pineal gland, and which regulates the body’s biological clock, i.e., non-visual biological effects. Branard et al.［ix］first determined the non-visual bio-spectral response curve, i.e., the rhythm function, and used it to characterize the intensity of the effects of different wavelengths of light on human rhythms. In 2011, Youssef et al.［x］ analyzed the principle of light hazard to the retina and the self-protection mechanism of the human eye. In 2013, Lu Yuhong et al.［xi］ reported the effects of different wavelengths of blue LED on working ability and working speed of human brain. In 2013, the National Electric Light Source Quality Supervision and Inspection Center (Shanghai) conducted a blue light hazard test on 27 kinds of LED lamps and lamp systems according to GB/T20145-2006, and obtained the assessment of retinal blue light hazard［xii］ . In 2016, Zhou Xiaoming et al.［xiii］ studied the change of the body’s pulse rate and the contraction rate of pupil caused by LED illumination with different dimming methods.
Currently, TV mainly uses blue LED chips to excite the phosphor as a backlight source. As is well known, FWHM of blue light emitted by the LED is narrow and the intensity of peak is high. Therefore, it is necessary to pay attention to the hazard of eye caused by the blue light. This paper details three domestic and international mainstream evaluation standards of blue light hazard to human eye, and the three standards are used to evaluate the spectrum of TV products, and the problems and the future directions of improvement of the three standards are pointed out as follows:
1. Photobiological safety standard IEC62471-2006[xiv]gives the weighting function of the blue light hazard and the equation to calculate the efficiency factor of blue light hazard in visible light band. In this paper, simplified algorithm equation is used to calculate the hazard level of spectrum from TV products, and relevant suggestions are put forward.
2. The low blue light standard proposed by TÜV Rheinland from German is detailed introduced in this paper. By using its low blue light calculation equation of the standard, the hazard level of mainstream TV spectrum is calculated, and relevant suggestions are put forward.
3. The T/CVIA-02-2017 standard for low-blue display devices proposed by the China Electronics Video Industry Association is detailed introduced in this paper. By using its low blue light calculation equation of the standard, the hazard level of mainstream TV spectrum is calculated, and relevant suggestions are put forward.
2 Relevant standards of blue light hazard
2.1 IEC62471-2006
Photobiological safety standard IEC62471-2006(equivalent to CIE S009/E2002, and the corresponding Chinese national standard is GB/T 20145-2006) provides guidance for photobiological safety evaluation of all non-laser electric lamps and lamp systems with emission wavelengths ranging from 200 to 3000 nm.
In order to understand the relationship between blue light hazard and photometrical and colorimetrical concepts such as brightness, illuminance, spectral distribution and correlated color temperature better, the concept of blue light hazard efficacy of luminous radiation (KB,V) from IEC /TR 62778 is utilized. KB,V is defined as follows:
(1)
Where: the unit of KB,V is lm/ W; the value of constant Km is 6831m / W ; ∅ 𝜆(𝜆) can be replaced by 𝐸 𝜆(𝜆); B(λ) is a weighting function representing the hazard of blue light.𝐾𝐵,𝑉 can be replaced by the simplified equation as follows[xv]
(2)
The equation above shows that KB,V and (1-x-y) /y are in a proportional relationship.The blue-light weighted radiance LB can be calculated by the following equation:
LB = 𝐾B,V ∙ L
（3）
Where L is the spectral radiance. Equation (3) shows that the magnitude of the blue light hazard (blue-light weighted radiance LB) is proportional to the two quantities: one is the blue hazard performance KB,V, which is only related to the spectrum, and the other is the brightness L. According to IEC/TR 62778, for large angle applications such as lamps and lamp systems, the maximum viewing time without causing retinal hazard is as follows:
(4）
According to the definition of IEC 62471, the hazard classification of the tmax of the product is shown in Table 1.
2.2 Low blue light standard of TÜV Rheinland
For display devices such as TV, TÜV Rheinland proposes two types of low-light certification. Method 1 is a software-based approach: the first condition is that In
a band with a bandwidth of ±20nm next to the blue peak,a maximum of 20% of power of the whole spectrum is allowed, and the second condition is that the Blue peak is not allowed to be more than double as high as the highest peak in other color range. Method 2 is for displays with hardware (and optional software) based approach, which is divided into two cases: when the color temperature CCT is between 5500K-7000K, the ratio of light in the range from 415nm-455nm compared to 400nm-500nm shall be less than 50%; when the color temperature CCT is above 7000K, the ratio of light in the range from 415nm-455nm compared to 400nm-500nm shall be less than 5%.
2.3 T/CVIA-02-2017
Part 2 of the “Health Display Devices’’ defined by China Electronic Video Industry Association, named “Technical Requirements and Test Methods for
Low-Blue Display Devices for Displays,” defines that the true or potential damage of the retina caused by the radiation of energy is mainly in the wavelength range of
400nm to 500 nm.
Equation for calculating the blue light weighted radiance LBB is as follows:
（6）
Where L is the brightness measured by a luminance meter, whose unit is cd/m².
Then, the blue-light-hazard level of display can be evaluated using the value of BR calculated by equation (6), and three levels are shown in Table 2. Value of BR greater than Table 2 will not be certificated as a low blue display.
3 Related experiments and data calculated
3.1 IEC62471-2006
Figure 1 shows a spectrum diagram of a mass production TV from TCL. LED of this TV uses blue
chips to excite the yellow phosphor to obtain white light spectrum.
The spectrum is calculated by equations (1) to (4), and the obtained data is shown in Table 3:
Table 3 Blue-light-hazard level
As can be seen from table 3, current hazard level of TV products is far less than the level of exemption.
In TV applications, color points are mainly used to characterize the color temperature of the machine. Influences of different color temperatures on tmax under the same brightness of machine are calculated by table 4.
It can be seen from Table 4 that as the color point x/y rises(which means CCT decreases), tmax increases(which means hazard decreases).
According to equations (1)~(4), when tmax equals 10000s under the color point of 0.313/0.329, value of brightness is 112000 nits, which means, the brightness level of 112000 nits is in exemption level. However, according to the actual viewing of TV products, it is extremely uncomfortable for human eyes to watch for a few minutes at a brightness of 5000 nits. Therefore, the standard applied to display evaluation of eye hazard needs to be redefined.
3.2 Low blue light standard of TÜV Rheinland
The compliance of TV products to the standard of TÜV Rheinland is analyzed by using the calculation formula described in 2.2, and its color temperature was adjusted to 6500K. The conventional 55P6 is a mass TV production machine, whose dominant wavelength of the LED chip is about 450nm. The New 55P6 moves dominant wavelength of the LED chip to about 460nm. The results calculated is shown in Table 4:
It can be seen from Table 4 that when short wavelength of blue light of the conventional 55P6 is too large, standard of TÜV Rheinland cannot be passed by conventional 55P6. Proportion of short-wave blue light of the new 55P6 can be reduced by shifting the dominant wavelength of the blue light to 460 nm, so it can pass method 2 of TÜV Rheinland. However, since the overall brightness of blue light(overall brightness of the machine) did not take into account by the standard, it is still not perfect.
3.3 T/CVIA-02-2017
Equations (5) to (6) were used to calculate the conventional 55P6 and new 55P6, respectively. At a color temperature of 10000 K, the respective values were 11×10-4and 12×10-4. The value is higher than the 9×10-4
of the third-level of the standard, which does not satisfy the low blue-light standard, and the parameter was caused to rise due to the reduction of short-wave blue-light. Further calculation and analysis found that when the color temperature of TV is lowered, its parameters are significantly reduced. It can be seen from the calculation that the standard pays more attention to the proportion of blue light in the overall spectrum, but the evaluation of hazard to human eye of the short-wave and total amount of blue light is not enough, so the standard is still needs to be defined more comprehensive in the future.
4 Conclusion
Several standards for low-blue light evaluation of display devices are introduced detailed by this article, including low blue light standard of IEC-62471, low blue light standard of TÜV Rheinland, and T/CVIA-02-2017 standard for low-blue display devices proposed by the China Electronics Video Industry Association. The detailed contents, calculation methods and evaluation indicators of these three standards are introduced. Some results were obtained by applying these standards to evaluate TV products. Through analysis, it is found that these standards are still not perfect: IEC-62471 evaluates the total amount of blue light (including duration), ratio of blue light (color temperature), and ratio of short-wave blue light. However, when the standard is applied to evaluation of TV display, it is too loose for the total amount of blue light hazard, so more reasonable indicators are necessary to be formulated. Ratio of blue light (color temperature) and short-wave blue light are
evaluated by TÜV Rheinland, but evaluation of the total amount of blue light (including duration) should be considered; T/CVIA-02-2017 focuses on the evaluation of blue light ratio (color emperature), but the evaluation of ratio of short-wave blue light’s hazard to human eye is not enough, and the evaluation of total amount of blue light (including duration) is lacking. In summary, this paper argues that the following three indicators should be included to evaluate blue light hazard to human eye: the total amount of blue light (including duration), ratio of blue light (color temperature), ratio of hort-wave blue light, and reasonable evaluation parameters for the three indicators should formulated.
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Symp Digest of Tech Papers – 2019 – Ji – P‐8 2 Research and Discussion on Current Low Blue Light Standards for TV Products
