% Transmission.  Light is lost (absorbed) as it passes through the transparent object. The Percentage is a comparison of light passing through air compared to light passing through the transparent object.

What causes a reduction in transmission?

Light that has been passing through air has to slow down as it passes through a transparent object.  As it slows, it also bends to enter the object.  There is loss in this process.  If the object has color, the colorant particles will also absorb light.  As light exits the object, it will bend again and cause more loss.  All of these factors combined will cause a reduction in transmission.  If for some reason there is a change in thickness (or path length) of the transparent product, it too will cause a reduction in transmission.

Abstract

The haze-gard plus is a versatile instrument used to measure transparent characteristics for the plastics industry.  From liquid raw materials to transparent films and sheet goods, the haze-gard plus is equipped with hardware to measure all.  This paper will discuss the capability of the haze-gard plus and many of its diverse applications.  Additionally, this paper will explore trends in data measured by the instrument, and what those data reveal about process.

Introduction

Transparency, transmission haze and clarity are three concerning appearance attributes in the manufacturing of plastic sheet goods and films.  Transmission haze and clarity are transmission deviations caused by forwardly scattered light that occur when a light beam is passed directly through (rectilinearly) a transparent material.   Light scattering occurs for a number of reasons that will be discussed in this paper.    Such products include transparent films, glass substitutes, sheet goods and even transparent raw materials such as resins and solvents. 

Appearance changes and causes

Percent Transmission

As shown in figure 1, when light passes through a transparent object, a percentage of light is absorbed by the object.  When light passing through the object is compared to light passing through air, a percent of transmission can be calculated and expressed as an attribute of that object.  Standards can be made for that type of product and production runs compared to the standard to provide a pass fail criterion.

Transmission haze 

When a beam of light is directed through a transparent body, it can do one of three things.  It can travel in a forward, regular (rectilinear) direction as shown in figure 2, it can be diffused and uniformly distributed in many forward directions as shown in figure 3, or it can pass through the object with some diffusion and some rectilinearity as shown in figure 4.  Any light that is scattered in a forward direction and away from the regular beam of light is called diffuse transmittance factor.

regular transmission

diffuse transmission

diffuse transmission, a perfect diffuser (translucency)

Transmission haze is expressed as a ratio of the diffuse transmittance factor divided by the total transmission of light passing through the body.  It should be noted that transmission haze, as the name implies, has to do with near transparent objects.   When one can no longer see an image through the film or sheet, it is no longer transparent, but instead has become translucent.  Light can pass through it but not in a rectilinear or near rectilinear manner.

Percent Transmission Haze  =

Diffuse transmittance factor  x 100
       Total transmittance factor
 

Haze can also be described as causing a loss of contrast.  When a transparent film is placed directly onto a printed page, one can see through the film and read the printing as if the film was not even there.  When the film has transmission haze, the person viewing the printed page loses contrast.  The black printing appears to become lighter (dark gray, not black), where the white page begins to darken (light gray, not white). The eye then experiences a “loss of contrast” between the printed matter and the background. 

What causes transmission haze?

Often in the manufacturing process, foreign bodies can become imbedded into the surface or found scattered throughout the substrate. For example, if the film is colored, pigments may not be ground finely enough and therefore will scatter light as the beam strikes them.  Similarly, finely ground pigment particles may agglomerate and scatter light.  Contaminates may be introduced and scatter light. Air bubbles may be suspended within the film and cause forward scattering.