Listed below are property definitions related to charts on our material properties pages.
Glass Transition Temperature
The temperature range in which a material transitions from a true solid to a very viscous liquid. This definition applies to non-crystalline solids.
Melting Point
The temperature at which a material turns suddenly from solid to liquid.
Softening Point
The temperature at which a glass deforms at a specific rate under its own weight.
Specific Heat
The amount of energy a body stores per unit mass for each degree increase in temperature (at constant pressure).
Strain Point
The temperature at which internal stresses in glass are substantially relieved in approximately 1 hour.
Thermal Conductivity
The rate at which heat flows through a unit area of homogeneous material for a given temperature difference.
Thermal Expansion
The change in length per unit length of a material corresponding to a unit change in temperature.
Thermal Shock
A parameter that characterizes the material cracking resulting from the temperature gradients caused by rapid change in temperature. A reduction in temperature is usually used for brittle materials.
Transmission Range
Wavelength range light will transmit through a unit length of optical material without significant optical attenuation.
Compressive Strength
The maximum compressive stress a material can withstand before failure.
Density
Mass per unit volume.
Elastic Limit
Greatest stress that can be applied to a material without causing permanent deformation.
Flexural Strength
Maximum stress developed in a specimen just before it cracks or breaks in a flexure test.
Poisson’s Ratio
The negative ratio of the thickness decrease divided by the length increase resulting from a tensile stress applied to a material.
Porosity
The proportion of the non-solid volume to the total volume of material.
Shear Modulus
The proportionality constant between elastic shear stress and elastic shear strain of a solid material subjected to shear loading.
Shore Durometer
Shore Durometer is a measure of hardness commonly used with rubbers, elastomers and polymers. Like all hardness testers, the Shore Durometer test measures the depth of an indentation under a given test force. There are no less than 12 Shore Durometer scales, with the two most common being the Durometer Shore A and D scales. The Shore A scale used for softer materials and the Shore D scale is for harder ones. The primary differences between the two scales are the force range and indenter shape used during the tests.
All scales range in numbers from 0 to 100, with higher numbers indicating harder materials. For reference, provided below are the Durometer values for a few common materials:
Tensile Strength
The maximum tensile stress a material can withstand before rupture.
Torsional Strength
The maximum torsional stress that a material can withstand before rupture.
Yield Strength
Maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of elastic limit. The amount of permanent deformation used depends on the material (for metals it is 0.2% strain).
Young’s Modulus
The proportionality constant between elastic stress and elastic strain for a solid material subjected to uniaxial loading. This property describes the inherent stiffness of a material.
Reference MIL-O-13830 Optical Components for Fire Control Instruments; General Specification Governing The Manufacturing, Assembly, and Inspection of Glass and BSR/OEOSC OP1.002 Optics and Electro-Optical Instruments – Optical Elements and Assemblies – Appearance Imperfections.
Scratch-Dig refers to the quality of optical surfaces. Owing to their relative complexity, many misconceptions exist about the meaning of scratch-dig specifications, and how they are applied. Conceptually, scratch-dig specifications attempt to set a limit on the amount of area surface defects occupy relative to the overall clear aperture of the optical element. Because assessments are made relative to the size of the part, a scratch that is unacceptable for a small part may be acceptable for a large part.
Much of the confusion about scratch/dig requirements finds its origins in the fact that there are actually two distinct standards – the “visibility method” and the “dimensional method”. These two standards differ only in the way scratch widths are categorized. The dimensional method, which is less prevalent, characterizes scratches by width measurements; whereas, the visibility method uses comparisons to commercially available visual references.
Scratch: Any marking or tearing of the part surface.
Dig: A small rough spot on the part surface similar to a pit in appearance. A bubble is considered a dig. Surface stains are also considered digs.
The second number of the Scratch-Dig specification refers to digs, and establishes a limit to the actual size (diameter) of the digs in hundredths of a millimeter. The dig part of the specifications includes the following three requirements:
The second letter of the Scratch-Dig specification refers to digs, and establishes a limit to the actual size (diameter) of the digs in hundredths of a millimeter. The dig part of the specifications includes the following three requirements:
(*) – Note that the length descriptor of the clear aperture is not always simple. MIL-O-13830 defines the length descriptor as the diameter of a circle with the same area as the clear aperture of the part being evaluated. Often times, however, optical industry prefers to use the smallest dimension of the clear aperture.
| Dig ID | Maximum Dig or Bubble Diameter | Dig or Bubble Separation Distance | ||
| Letter | mm | inch | mm | inch |
| A | 0.05 | 0.0020 | 1.0 | 0.040 |
| B | 0.010 | 0.0039 | 1.0 | 0.040 |
| C | 0.20 | 0.0079 | 20 | 0.787 |
| D | 0.40 | 0.0158 | 20 | 0.787 |
| E | 0.60 | 0.0236 | 20 | 0.787 |
| F | 0.80 | 0.0315 | 20 | 0.787 |
Surface roughness is a measure of the texture of a manufactured surface. Although there are many definitions of surface roughness, all of them are based on a statistical representation of the high frequency surface deviations (peaks and values) from the local mean surface height. Filtering is used to separate the high frequency texture data from lower frequency machining features.
Surface roughness can be measured using contact methods involving dragging a stylus across the part, or using non-contact optical methods. CiDRA® Precision Serivces, LLC uses both measurement methods.
Ra, the most commonly used surface roughness definition, is expressed mathematically by
where n is the total number of data points used in the calculation and Y is the vertical surface position measure from the average surface height. To be applied properly, this line measurement should be made perpendicular to machining lay marks. Visit Wikipedia.org/wiki/Surface_roughness for a more in-depth explanation of surface roughness.
For a list of definitions of terms used in the material charts on each page, please visit our Material Property Definitions section above.
