Luminous transmittance & HazeClarityPhoto elasticity

Amal Raj R B

1st M.sc BPSCbpst, kochi

LUMINOUS TRANSMITTANCE & HAZE Luminous transmittance is defined as the ratio of transmitted light to the

incident light.

It is expressed as % of light transmitted.

Luminous transmittance measures the amount of light passes through a sample.

Luminous transmittance = transmitted light

incident light Haze is defined as the cloudy appearance comes on the surface of

specimen.

The amount of transmitted light is deviated more than 2.5°c from the incident beam .

Haze is caused by surface imperfections ,density change .

Significance

Light that is scattered, up on passing through a film or sheet of material can produce a hazy or smoky field when objects are viewed though the material.

Haze and luminous transmittance values are especially useful for quality control and specification purpose and are primarily used in products or uses where in true colour and visibilities are prerequisites.

Window glazing requires low haze and high light transmittance where as housing material for the light fixture requires high transmittance and haze.

Test method ASTM D 1003-07 : Standard test method for Haze and Luminous

transmittance of transparent plastics.

Specimen and conditioning

Commonly disc of 50mm diameter is used.

Test specimen is conditioned at 23 ± 2ºc and 50 ± 5% relative humidity for not less than 40 hours prior to test.

Equipment

Procedure Test sample is placed between the specimen holder and the lens. Light from the light source is allowed to fall over the sample. Test is conducted by taking four different consecutive readings

and measuring the photocell out put as follows:-

T1 = specimen and light trap out of position T2 = specimen and reflectance standard in positionT3 = light trap in position specimen and reflectanceT4 = specimen and light trap in position

Result

Total transmittance Tt=T2/ T1

Diffuse transmitted Td=[T4-T3(T2 /T1) ] / T1

Haze percent=Td/Tt ×100

Factors affecting

Defects in specimen

Effect of incorporation of additives

Type of material

CLARITY

Clarity refers to the property of transmitting light which as minimal scattering when passing through the object.

If the transmitted light deviates less than 2.5°C from the incident angle, the material is said to be clear.

Significance

Clarity, which may also be prescribed as quality of image formation through a sheet ends upon the linearity of the passage of light ways through material.

Haze measurement depend upon wide angle scattering while clarity is determined by small angle scattering.

Test specimen and conditioning Specimen may be any sheet or film.

Specimen is conditioned at 23 ± 2ºc and 50 ± 5% relative humidity.

Procedure

The optical alignment of photometric unit is checked. The image of source aperture is centred precisely. Centring is controlled by micro adjust receiving aperture assembly, which is

standardised until a reading of 100 is obtained. The beam is completely blocked with apiece of black folt or paper and the knob is

adjusted to zero. The sample is placed and the receptor position is re adjusted. If slit optics are used , the specimen may be rotated its own axis to a maximum

value of transmitted light. The reading at different positions or orientations may be taken and results are

taken.

Factors affecting

Humidity Wavelength of light Particle size.

PHOTO ELASTICITY

It is the property exhibited by some transparent solids whereby the become doubly refractive, under stress.

Most of the plastics exhibit stress-Optical properties when subjected to internal or external stress and viewed through polarised light.

Significance This properties of the transparent materials are useful in stress

analysis.

Used to determine residual stress and degree of orientation in molded parts.

It is a non-contact optical method for determining stress and load paths in structures or components.

Specimen and conditioning

The test specimen can be a sheet or moulded part.

Specimen is conditioned at 23 ± 2ºc and 50 ± 5% relative humidity.

Equipment

The instrument designed to observe objects under polarised light are called polariscopes.

Two types of polariscopes are there:- The transmission polariscope and The reflection polariscope.

Procedure

The specimen is placed in the center of the polarizing medium viewed from the opposite side of the light source.

The number fringes or rings formed due to the polarized light can be related to the extent of stress. Higher order of fringes represents high stress level and whereas unstressed areas is represented by

low-order of fringes. The close spacing of fringe represents high stress gradient and uniform colour indicates uniform

stress in the material. Several additional techniques are used in conjunction with the basic photo elastic concept, Which is

used to analyse complex problems :-

Strobo elasticity Thermo photo elasticity Photo-visco elasticity Scattered light photo elasticity

Factors affecting

Residual stress:-

The presence of residual stress, which in turn depends on mould design, gate size and location, weld line etc. changes the photo elasticity of the material.