Haze – The amount of light that is subject to Wide Angle Scattering (At an angle greater than 2.5° from normal (ASTM D1003))
Clarity – The amount of light that is subject to Narrow Area Scattering (At an angle less than 2.5° from normal)
Measurement of these factors is defined in two International test standards-
ASTM D1003 
ASTM D1003 comprises two test methods:
Procedure A – using a Hazemeter
Procedure B – using a Spectrophotometer
BS EN ISO 13468 Parts 1 and 2 
Part 1 – Using a single beam Hazemeter
Part 2 – Using a dual beam Hazemeter
The test methods specify the use of a Hazemeter as shown below -
A collimated beam of light from a light source (ASTM D1003 - Illuminant C, BS EN ISO 13468 Parts 1 and 2 - Illuminant D65 ) passes through a sample mounted on the entrance port of an integrating sphere.
The light, which is uniformly distributed by a matte white highly reflective coating on the sphere walls, is measured by a photodetector positioned at 90° from the entrance port. A baffle mounted between the photodetector and the entrance port prevents direct exposure from the port.
The exit port immediately opposite the entrance port contains a light trap to absorb all light from the light source when no sample is present. A shutter in this exit port coated with the same coating as the sphere walls allows the port to be opened and closed as required.
Total transmittance is measured with the exit port closed.
Transmittance haze is measured with the exit port open.
Commercially available Hazemeters of this type perform both measurements automatically, the only operator interaction being the placement of the sample material on the measurement (entrance) port of the device.
- ASTM ASTM D1003 Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics
- BS EN ISO 13468 Plastics -- Determination of the total luminous transmittance of transparent materials
https://en.wikipedia.org/wiki/Haze (optics) was the original source. Read more.