Purpose: The appearance of any object is a combination of its chromatic attributes (color) and geometric attributes (such as gloss, shape, texture, shininess, haze, and translucency). Both types of attributes should be measured and accounted for when making visual or instrumental assessments of appearance.
Chromatic Attributes
Chromatic attributes are associated with color and are typically divided into three components:
- Lightness: Indicates whether the color is closer to black or white. Sometimes referred to as 'value.'
- Hue: The perceived color of an object, such as red, green, blue, yellow, or orange.
- Saturation: The degree of departure from gray, representing the vividness or purity of a color. For example, bright candy apple red is more saturated than dull brick red.
Chromatic attributes are generally measured using Hunter Lab spectrophotometers and colorimeters, although they cannot be completely separated from geometric attributes.
Geometric Attributes
Geometric attributes relate to the distribution of light from an object. For example, a flat cotton weave fabric differs geometrically from corduroy, and a glossy photo print looks different from a matte one. Examples of geometric attributes include:
Gloss: The property of a surface responsible for a shiny or lustrous appearance.
Haze: The scattering of light within the surface of a nearly clear sample, causing a cloudy appearance.
- Directionality: The characteristic of a sample that causes it to look different depending on its orientation.
Visual Evaluations of Appearance
Conditions of illumination and viewing, as well as other factors, can greatly affect the perceived appearance of an object. To accurately and consistently compare the appearance of objects, viewing conditions should be standardized.
Gloss Evaluation: View the object at an angle equal to but opposite the angle of incidence of the source light, where specular reflectance occurs.
Color Evaluation: Shine light directly on the object from overhead and examine the specimen from a 45° angle, or vice versa. Further recommendations for color viewing conditions are provided in ASTM Method D1729:
Light Source: Daylight, incandescent, or cool white fluorescent, specified.
Photometric Conditions: Light intensity should be 75-175 foot candles.
Geometric Conditions: Light source overhead at 0° and viewing at 45°, or light source at 45° and viewing at 0°.
Background: Neutral gray and uncluttered.
Sample Placement: Samples should be placed next to each other (abutting) without overlapping.
Instrumental Evaluation of Appearance
Instruments used to measure color and appearance mimic the human observing situation, with a light source, an object, and an observer (detector). The configuration of these elements can vary, and factors affecting instrument measurements include:
Instrument Geometry: Discussed in detail in previous Applications Notes.
Object Properties: Color, texture, and other properties of the object being measured affect the numbers obtained via instrumental measurement. Different object types (opaque nonmetals, opaque metals, transparent samples, and translucent samples) should be measured differently:
- Opaque Nonmetals: Measured in reflectance mode. Use a 45°/0° or 0°/45° instrument for visual changes due to surface texture or roughness, or a diffuse/8° (sphere) instrument to minimize surface differences.
- Metals: Smooth metal samples measured on a sphere instrument in reflectance - specular included (RSIN) mode. Roughened metal surfaces may use a 45°/0° instrument.
- Transparent Samples: Measured in transmittance mode using a sphere instrument. Choose diffuse, regular, or total transmittance. Transmittance haze and APHA are also possible parameters.
- Translucent Samples: Best measured in reflectance mode by fixing the sample thickness, adding layers to make it nearly opaque, backing with a white background, covering with a black cover to minimize ambient lighting interference, and using small area illumination with large area viewing to minimize light trapping.
Instrument Types
There are two main types of instruments for measuring color, distinct from the geometrical instrument categories previously discussed. Understanding these types is crucial when selecting an instrument for your application. The terms 'colorimeter' and 'spectrophotometer' often cause confusion, so their differences are detailed in the March 1995 Applications Note.
Basis of Instrumental Calibration
The calibration method and the primary standards from which readings are compared are significant when comparing measurements from different instruments. Details on perfect reflecting diffuser standards are provided in the March 1996 Applications Note.
Sample Preparation Method
The ideal sample for color measurement should be completely opaque, uniform, flat, smooth, non-directional, homogeneous, and slightly larger than the instrument measurement port. It should not be photochromic (changing color with light) or thermochromic (changing color with heat), and it should represent the entire lot from which it was obtained.
Most samples do not meet these criteria and must be prepared to be as ideal as possible.
For reflectance measurement, non-opaque samples can be layered or backed with a white tile. Non-opaque samples can also be measured in transmission mode. Non-uniform samples, such as powders and granules, may be crushed or ground into a uniform powder. Solid non-uniform samples, like patterned fabrics, can be divided into similar sections for individual measurement or measured as a whole with the color averaged over the sample area. Non-flat samples can be flattened by pressure at the measurement port or pressed into a pellet or plaque. Non-homogeneous samples may be stirred or multiple samples taken with replacement at the instrument port. For assistance with unusual sample preparation, contact HunterLab Customer Support.
Sample Presentation Method
There are various ways to present samples to the instrument. For example, a powder can be poured into a sample cup and measured through the glass, pressed into a plaque, or dissolved in water and measured in the transmittance compartment of a sphere instrument. Samples may be backed with a tile, covered with a light trap, rotated between measurements, or measured multiple times and averaged. The specific presentation procedure must be specified, including the orientation of the sensor and the size of the port plate used.
General measurement guidelines include:
- Prepare a checklist of steps for sample preparation and measurement.
- Be consistent with preparation and presentation parameters when measuring and comparing multiple samples.
- Conduct measurements in a clean, air-conditioned room, avoiding strong drafts, direct sunlight, fumes, and dust.
- Use the largest area of view possible for each sample.
- Take multiple measurements of each sample and average the readings.
- Avoid noticeable surface imperfections on samples.
- Use an instrument with circumferential illumination or viewing or a sphere instrument when sample directionality is obvious.
Reporting and Comparing Instrumental Measurements
Many factors affect perceptions and measurements of an object's color, including the light source, the object itself, the observer, the color scale or index, and the instrument type and geometry. Therefore, when reporting color measurements, the conditions of the measuring situation must be included.
Report the following information:
• The color scale or index being reported,
• The illuminant,
• The observer,
• The instrument type, geometry, and mode,
• The basis of the instrument calibration,
• The sample preparation method,
• The sample presentation method.
Comparing instrumental measurements is meaningless unless all these parameters are identical for the measurements being compared. Differences in these parameters are a common cause of disappointment when comparing measurements between plants or operators. If discrepancies persist after confirming the seven parameters are identical, only then should product or instrumental issues be suspected.