This is a principal indication of the trensparency of a material, defined as the ratio of the amount of light transmitted through the material, to the amount of incident light.  Table 1 below lists the luminious transmittance values of several transparent materials.  Higher values indicate greater light transmittance or transparency.

Refractive Index

The refractive index of a material is another way of optically classifying clear materials.  it is the ratio of the velocity of light in the material under study.  Refractive index also can be defined as the ratio of the angle of incidence to the angle of refraction (i.e. sine incidence angle divided by sine refraction angle).  Like luminous transmittance, refractive index is an important property to be considered in the design of optical systems.

Table 2 gives the refractive indices of a number of transparent materials.  Lower values indicate that less refraction or distortion occurs as light passes through the material.

Haze

Haze is the percentage of transmitted light which when passing through a specimen, deviates from the incident beam by forward scattering.  Lower have values imply greater transparency.  It is an important property when designing for a transparent 'sight' application, in which observers must be able to see inside or through a part easily and clearly.  If a material has a high haze value, it will have decreased transparency - making it more difficult to see inside or behind the 'sight' part.  A part with a high haze value will transmit light, but images may appear foggy or blurred.

Typically, polycarbonate resins have haze value of about 0.5 to 2.0%.  Haze value ranges for other transparent materials are:

                    Polystyrene                                 0.1 ~ 3.0%

                    Styrene Acrylonitrile                     0.6 ~ 3.0%

                    Polymethylmethacrylate                1.0 ~ 3.0%

                    Cellulose Acetate                          0.5 ~ 5.0%

                    Glass                                           0 ~ 0.17%

Yellowness Index

Yellowness Index (YI) is a numerical representation of how yellow a material is in comparison to a 'clear' water-white standard.  Lower YI values indicate greater clarity.  The YI of polycarbonate resins is generally about 0.5 to 2.0.  This is slightly less yellow than are most other commercially-available transparent polymers, which typically have YI values of 1.0 to 3.0.

Polymerisation processes commonly induce a slight yellow or straw hue in the resins produced.  The presence of non-polymerised constituents or degraded material in a resin increases its YI.  The YI value thus also indicates the statistical 'cleanliness' of the final polymer.  Excessive heat or shear stress during the fabrication processes tend to increase the YI of a natural resin.  Thus, the normal injection moulding conditions for a natural resin tend to increase the YI of the material.

To avoid raising the yellowness of a part significantly, fabricators should be careful to:

1. Avoid excessive heat (caused by excessive melt temperatures and/or excessive length of exposure time at higher temperatures)
2. Limit the amount of incorporated regrind material to the recommended maximum level of 25%, because regrind (with its heat history) tends to increase the YI.