What is haze?
As illustrated above, for the incident light (black color ray) transmitted through a transparent material, there are two components:
- Specular transmission (blue color ray): the component in the same direction of the incident light
- Diffuse transmission (red color ray): the component scattered in all directions
The fraction of the visible light transmitted through a transparent material is its luminous transmittance. There are total, diffuse and specular luminous transmittances:
- Specular luminous transmittance: the fraction of specularly transmitted visible light
- Diffuse luminous transmittance: the fraction of diffusely transmitted visible light
- Total luminous transmittance: the sum of the above two, total luminous transmittance = specular luminous transmittance + diffuse luminous transmittance
If the diffuse transmittance of a transparent material is large, the material appears hazy when viewing objects through it. The haze of a transparent material is defined as the ratio of the diffuse luminous transmittance to the total luminous transmittance:
Haze = 100 × diffuse luminous transmittance / total luminous transmittance
To have a clear view through a transparent material, it is preferred that the material is with high luminous transmittance and negligible haze.
How is haze measured?
- Total luminous transmittance measurement: in this mode, the reflectance port is covered and the specular component is included (SCI).
- Diffuse luminous transmittance measurement: in this mode, the reflectance port is uncovered and the specular component is excluded (SCE).
With the two measurement modes, the total and diffuse luminous transmittances can be measured and the haze can be calculated.
The haze measurement results are dependent on the instrument integrating sphere geometry. When the measurements are performed with the same instrument, most error sources are cancelled out and the haze measurement results are very accurate.