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Plastic optical characteristics

Oct. 31, 2017

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.

4, Birefringence

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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