Plastic optical characteristics include two categories: one for the transfer characteristics, including light transmission, reflection, scattering and refraction; the other for light conversion features, including light absorption, thermal, photochemical, optoelectronics and light photochromic and so on.Used to characterize light transmission characteristic indicators are transmittance, haze, refractive index, birefringence and the dispersion and so on.In the indicators, the light transmittance and haze two indicators main characterize translucent of the material,the refractive index, birefringence and scattering three main indicators used to characterize the quality of translucent.A good transparent material, requiring the excellent performance and balanced.
Transmittance (Tt) to characterize the transmittance level of resin is the most important performance indicators.The higher the transmittance of a resin, the better the transparency.Plastic products transparent need two conditions:one is products is non-crystalline;the other partially crystalline,however, small particles,smaller than visible wavelength range,without prejudice to the solar spectrum in the visible and near-infrared light through. Any transmittance of a transparent material are less than 100%.Even the best optical glass transparent transmission rate is generally difficult to over 95%.Resulting in the loss of incident light flux in the media causes the following aspects.
The reflection of the light transmitting the flux the incident light is returned into the polymer surface.Reflected flux accounting for the loss of light in most of the time through the media. Measure the degree of reflection of light characterization by reflectance, reflectance calculated of the index of refraction (n) ,relations between the two is as follows:for example, PMMA the refractive index n = 1.492,then its reflection has been calculated at 3.9%,Instructions the PMMA reflected light is relatively small, a large transmittance, good transparency.Light absorption excellent transparent plastic absorption of light small.Light absorbing depends on the polymer structure itself,mainly refers to the molecular chain groups of atoms and chemical properties.For example, a polymer having a double bond easy to absorb visible light to produce transfers to energy levels.Similarly to example PMMA, which is generally 93% transmittance, reflectance was 3.9%, the remaining 3.1% is the light absorption and scattering of light and of both.
1,Light scattering light scattering incident light to the polymer surface is neither nor reflection and absorption through part flux, which occupies relatively small proportion. Cause light scattering reasons: the product surface rough, uneven internal structures such as polymer molecular weight distribution is uneven, disordered phase and crystalline phase coexistence.
2, The haze.Haze is also known as turbidity,it can be measured without a clear transparent or translucent material or the degree of turbidity, which is characterized scattering index. Generation of haze due to internal or external surface of the material caused by light scattering cloudy or turbid appearance. Haze is defined as: the scattering material through the material flux and flux ratio of the percentage. The light transmittance and haze described above are measured of an important indicator of transparent material, the relationship between the following: In general, inverse relationship between transmittance and haze relationship, i.e. a material with high transmittance, which low haze; vice versa. However, the relationship between the two is not always so, and sometimes also the opposite result. Such as high transmittance of frosted glass, but the haze is relatively large. So transmittance and haze is both contradictory and interrelated two optical targets.
3, The refractive index
Refraction is the incident light and the transmitted light the two direction’s difference.
Measuring material refraction , can be characterization of the refractive index greater the refractive index, the more severe of the refractive material .The refractive index can used the ratio of the propagation velocity of light in air and in plastics to calculate.As the use of lens cap resin large refractive index is better.Greater the refractive index, the thickness can be reduced accordingly.
I.e. birefringent is the refractive index difference between material parallel direction and the vertical direction.Birefringence more larger, the more likely to cause phenomena such as image generation crooked shadow.
So that the birefringence reduces transmission quality of the optical material, should try to reduce the material birefringence.Material birefringence arises mainly determined by two aspects :the resin molecular structure and molecular orientation.
5, The dispersion
Material dispersion can be expressed with abbe number, abbe number of the material have a relation with refractive index of the material. Generally greater the refractive index, abbe number is smaller, the dispersion is stronger.
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