Colour measurements

ARJUN K GOPI1ST M.SC BPS

CBPST, KOCHI

Specular Gloss

Colour measurements

What is colour ?

Colour is a property of object that results from the particular wavelength of light which they reflect or give out, producing a sensation in the eye.

The variable of appearance include glossiness or matness, opacity or transparency, smoothness or toughness, metallic reflection or diffusion surfaces. If these appearance characteristics are eliminated, what remains is absorption of light by the object in such a way that some colours are removed from the light and part is reflected or transmitted.

Terminologies

Following terminologies are useful to understand colour theory.

Value or lightness : Colour range from dark to light in which black being the darkest, grey in the middle and white being the lightest.

Hue : Color has basic difference appearance such as red differ form blue green or yellow these distinction as called Hue.

Chroma : Chroma is defined as the attribute of color perception that express the degree of departure from gray of the same lightness.

Significance

Colour measurement is required for quality control, developing product specification and colour matching.

Evaluation and measurement of colour in plastics are important for several diverse and interrelated reasons.

If colour is to be specified, it must be described in specific term and should also be measurable.

Test method

ASTM D 2244 – 09 : Standard practice for calculation of colour tolerances and colour differences from instrumentally measured colour coordinates.

ASTM D 1729 – 96 : Standard practice for visual appraisal of colours and colour differences of diffusely – illuminated opaque materials.

ASTM E 308 – 08 : Standard practice for computing the colours of objects by using the CIE system.

Specimen and conditioning

The test specimen is taken from representative material and is conditioned at 23± 2ºc and 50 ± 5% relative humidity for not less than 40hrs prior to test.

Equipment

1. Calorimeter2. spectrophotometer

Calorimeter

Spectrophotometer

Procedure of spectrophotometer

Procedure of spectrophotometer

- The monochromatic light is obtained by allowing the beam of light to pass through a prism or diffraction grating (monochrometer)

- The monochromatic light is directed through a cuvette containing the sample, and the light that penetrates hits the photoelectric cell.

- The current developed by the photoelectric cell is translated into percent transmission or absorbance through a galvanometer.

- Then you can read the absorbance on the galvanometer.

- Absorbance is also called extinction and optical density.

Factors affecting

Angle of illumination : The optimal position of the light source and the detector relative to the sample surface are 45ºand 0º or 0º and 45º

Type of material : opaque materials do not transmit light. Light is either directly reflected from the surface as white light or absorbed.

Particle size : decreasing particle size increases the light scattering, causing the sample to appear lighter in color.

Specular gloss

Specular gloss is defined as the relative luminous reflectance factor of a specimen at the specular direction. Gloss is a measure of light reflected by the surface of plastic film.

Significance Specular gloss measure the shiny appearance of films and surfaces. It is used to compare two or more materials based on their appearance. Gloss is associated with other visual aspects of surface appearance such as

reflected images, haze and texture.

Test method

ASTM D 23 – 08 : Standard test method for specular gloss.

ASTM D 2457 – 08 : Standard test method for specular gloss of plastic films and solid plastics.

Specimen and conditioning

Specimen surfaces must have good planarity and free warpage, waviness or curvature.

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

Equipment

Procedure

The instrument is turned on and placed on a black glass primary standard.

The control knob is adjusted so that the meter indicates the value assigned to the primary standard.

The sensor is placed on the specimen surface and gloss value is read directly from the analog or digital display.

The instrument is calibrated regular intervals to ensure that the instrument reading is particularly constant

The instrument fails to repeat readings of the standard within 2% of the prior setting, the intervening result should be rejected.

Factors affecting

Surface property : Specular gloss is primarily due to reflection at the surface, therefore, anything that changes the surface physically or chemically is likely to affect the gloss.

Surface of the material : It should be perfectly smooth. Smoothnes depends on the quality of the surface of mold or die used to make the part.

Incidence angle : As the incidence angle increases, gloss value for the surface also increases.