Several important quality indicators for purchase of sunglass lenses
Quality requirements for sunglasses. The current sunglass standard implemented by the spectacles industry of China is QB2457-1999 Sunglasses. On the basis of the differences in luminous transmittance, the standard divides sunglasses into light-color sunglasses, sunshade sunglasses and the sunglasses for special purpose (skiing, beach and mountain climbing). The main technical requirements (quality requirements) for sunglasses set out in the standard are as follows:
Surface quality and internal defects
This indicator means that the lenses of sunglasses should meet the requirements in 5.1.4 of the national standard GB10810-1996 Lenses of Spectacles. That is, no internal defects, such as霍光 and spiral (meaning the defects of concentric circles or spiral waves on lens surface) may exist in an area of 30mm in diameter with the datum mark of the lens (geometrical center) at the center. Moreover, the lens surface should be free of scratch and grinding crack and remain polished, transparent and clear.
Optical performance of lenses
Defocus deviation
Defocus deviation is what is usually called the degree of spectacles. It is one of the important test indicators for sunglass products and also one of the important elements that concern whether the eyes will feel comfortable under sunglasses. Sunglasses should be non-degree spectacles with consistent optical density. If the defocus deviation exceeds a certain range, the wearer may feel deformation of the things seen and be easy to have the symptoms of visual fatigue and loss of vision, which will bring unnecessary damage to eyes. Defocus deviation is classified into that of spheric lens and that of column lens. The former should be -0.12 D～＋0.12 D (D as the unit of spectacles degree) and the latter should be 在-0.09 D～＋0.09 D.
Prism deviation
Similar to defocus deviation, prism deviation is also one of the important elements that influence the quality of sunglasses. The scope of the national standard for the item is ≤0.25△ (△ as the unit of prism degree). If the item goes beyond the scope, the wearer may feel deformation of the things seen and the symptoms of discomfort, such as dizziness and nausea, which will bring certain damage to the eyes. According to QB2457-1999 Sunglasses, prism deviation should be ≤0.25△in the horizontal and vertical directions. Prism deviation would be best if it is “0.00”.
Luminous transmittance
This is light transmittance (in popular terms, luminous transmittance is the percentage of light transmittance, which is expressed with “τv”). It is an important indicator for determining the functions of sunglass products. If this item does not meet the requirements of the standard, it will increase the irritation to human eyes caused by harmful rays, resulting in the blurring of the things seen and triggering visual fatigue.
Standard：
- Luminous transmittance of ＞40% for shallow color sunglasses
- Luminous transmittance of 8%~40% for shading sunglasses
- Luminous transmittance of 3%~8% for sunglasses for special purpose
It is not hard to see that the color turns from shallow to deep for the sunglass lenses ranging from type A to type C, meaning that the light filtering capacity of the lenses increases gradually. The luminous transmittance of different types of sunglasses must be in strict compliance with the above quantitative requirements.
Average transmittance (UV spectrum area)
Average transmittance (UV spectrum area) means the UV-blocking capacity of sunglass lenses. If this item falls short of the standard, a lens cannot effectively absorb UV rays and damage will be caused to retina and comea by wearing such sunglasses.
This indicator means the transmissivity allowed by the UV spectrum area (280nm～380nm) for sunglasses. It is the most important functional indicator for sunglasses and relates directly to the physical and mental health of wearers. In addition to light filtration, sunshading and glare proof, the isolation and interception of UV rays are the most important functions of sunglasses. Otherwise, the fundamental sense of wearing sunglasses is lost (except for their ornamental effect). The standard further divides UV bands into UVA and UVB.
- Sunglasses of shallow color
For UVB, its UV transmittance is ≤0.5τv ≤30%.
Why have two different standards been set? They are based on the light transmittance values actually measured for a certain type of sunglass lenses, which are divided into two circumstances to adopt whichever specific value that is smaller and stricter through the calculation of ≤0.5τv and the comparison with ≤30%.
- For example, a certain lens has a τv of 50% and its 0.5τv is 25%, obviously smaller than ≤30%. Therefore, the average transmission indicator that should be adopted for the lens is ≤0.5τv.
For another example, a certain lens has a τv of 80% and its 0.5τv is 40%. Obviously, the indicator of ≤30% should be adopted, for it is stricter than ≤0.5τv.
For the UVB waveband of sunshading sunglasses, the transmittance standard is also based on two different values, which have the same meanings and are adopted as described above and will not be discussed in detail hereinafter.
The standard for UVA provides that its average transmittance is ≤τ, meaning that the average transmittance actually measured should be smaller than or equal to the light transmittance value actually measured for the lens.
- Sunshading sunglasses
The standard for UVB provides that the average transmittance of the wave band of such sunglasses is ≤0.5τv ≤5%, while the standard for UVA provides that the average transmittance of the wave band of such sunglasses is ≤τv. Than is, the actually measured value should be smaller than or equal to the light transmittance value actually measured for a lens.
- Sunglasses for special purpose
For UVB, its average transmittance is ≤1%.
For UVA, its average transmittance is ≤0.5τv.
The standard indicator set out above for average transmittance (UV transmittance) is the maximum transmission value (maximum penetration amount) of ultraviolet rays (UVA, UVB) allowed for the lenses of all types of sunglasses. Actually, better sunglasses can intercept nearly all ultraviolet rays and some infrared rays.
Transmittance of traffic signals
An an important safety indicator, the transmittance test of traffic signals is designed to determine the ability of human eyes to identify the colors of traffic signal lights after wearing sunglasses. If this item falls short of the standard, color identification misconception is likely to occur for wearers of sunglasses, who will lag in response and misjudge the color of a traffic light, or even cause a traffic accident in a serious case.
Standard values required for the transmittance of traffic sinals (τsig).
- Sunglasses of shallow color
Red signal: ≥8%
Yellow signal: ≥6%
Green signal: ≥6%
- For sunshading sunglasses, the requirement for the transmittance of traffic signals is as set out above.
- Sunglasses for special purposes
This indicator for the transmittance of traffic signals is unnecessary is unnecessary, for such sunglasses are the special spectacles for special environments and occasions, such as skiing, mountain climbing and beaches.
To sum up, a pair of sunglasses must meet the requirements of every technical indicator set out above to be judged or call as qualified sunglasses.
Relative deviation of light transmittance
The relative deviation of light transmittance means the difference in the light transmittance of the left and right lenses of a pair of sunglasses. If the deviation is excessive, it will lead to difference when human eyes see things from the left and right lenses of a pair of sunglasses, thus causing eye discomfort.
Relative deviation in the light transmittance of the left and right lenses of a pair of sunglasses: ≤15%。
Color limit
Color limit reflects whether there is any change in the ability to identify colors after wearing sunglasses and is also an important test indicator of sunglasses. It is divided into the traffic signals of yellow/green and average sunlight, which are expressed by the color coordinates x, y respectively. If this indicator falls short of the standard, deviation will be caused to the color identification by the wearer. In particular, the deviation in the identificatio of traffic signals is easy to cause miconception, thereby leading to traffic accident. In the simulated use of lenses to observe the yellow/green traffic signals and average sunlight, the corresponding color coordinates should be within the corresponding area of the chromatic diagram. If the measured color coordinates go beyond the prescribed area of color limit, that will cause confusion of traffic signals and thus constitute one of the hidden dangers for traffic accident.