Glossary (Definitions) of Terms Related to Retro-Reflectivity and Photo-Metrics
In order to assist our customers in understanding all aspects of visibility, photo-metrics, and retro-reflectivity, we have put together a glossary of relevant terms. Words that you will see in specification sheets, regulations, and accident analysis reports. This list will hopefully help you with clarity as you read through these documents.
Adhesive (for retro reflective sheeting) – A sticky substance that is used to join two things together. This is normally permanent, but can also be temporary (removable). For retro reflective tape there are 5 classes of adhesive backings. Each is for a specific purpose. Class 1 is the most common and is the backing found on most reflective tapes.
ASTM D4956 Type 1 through 11 – This specification includes flexible, non-exposed glass-bead lens and microprismatic, retroreflective sheeting for traffic control signs, delineators, road barrels, and devices. ASTM 4956 does not address inks, overlays, or other printing methods that may be applied to retroreflective sheeting material to create traffic signs or other devices.
ASTM E810 – A standardized method for measuring retro reflectivity. Measurements made by this test method are related to visual observations of retroreflective sheeting as seen by the human eye when illuminated by tungsten-filament light sources such as a motor vehicle headlamp.
Brightness – Attribute of a visual perception according to which an area appears to emit more or less light. The quality or state of giving out or reflecting light. The opposite of darkness. One object being substantially brighter than another, creates contrast and conspicuity.
Candelas / Lux / Square Meter – A unit of luminous intensity expressed as one candela in one second of light emittance per square meter of area. This is the unit of measurement used to measure the intensity of brightness of retroreflective sheeting and other lighting. This measurement is also used to set minimum standards for reflectivity.
Chromaticity – Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness. It is the quantification of color.
Chromaticity Coordinates – A numeric description of color. Or color quantified with numeric coordinates. The chromaticity coordinates can be expressed on a 2D chromaticity diagram. These coordinates assure that a retro reflective tape is the proper color.
Chromaticity Diagram – This diagram shows all the hues perceivable by the standard observer for various (x,y) pairs and also indicates the spectral wavelengths of the dominant single frequency colors. It is an image that displays all colors and their corresponding xy coordinate.
Coefficient of Retro-Reflection – The ratio of the coefficient of luminous intensity of a plane retroreflecting surface to its area expressed in candelas per square meter.
Color – The aspect of any object that may be described in terms of hue, lightness, and saturation. Expressed by chromaticity coordinates according to the CIE (Commission Internationale de l’Eclairage 1931 ) standard colorimetric system.
Competitive Lighting – Lighting that overshadows or drowns out other lights or retro reflective surfaces to where they become less conspicuous or invisible. An example would be a trucks headlights shining at 15,000 candelas, near retro reflective tape shining at 500 candelas.
Cone of Retro Reflectivity – light returning from a sheet of reflective tape expands as it moves towards the light source. As it expands, it forms a cone of reflectivity shaped like a megaphone. As the light expands, it becomes dimmer.
Conspicuity – the characteristics of an object that determine the likelihood that it will come to the attention of an observer. This is achieved through color, contrast, patterns, and brightness.
Department of Transportation (US-DOT) – The federal agency in charge of all transportation related affairs in the US. This agency is over the FMCSA (Federal Motor Carrier Safety Association), NHTSA (National Highway Traffic Safety Association), FHWA (Federal Highway Association), and many other transportation agencies. (FAA, FTA, FRA, MARAD, PHMSA, etc.)
Diffused or Scattered Reflectivity – light is scattered or diffused in multiple directions. Almost all objects are seen in this way. If you look at your car in daylight, you are seeing it because of diffused or specular reflectivity.
Dimensional Stability – in terms of retro reflective tape or any other adhesive film, this is the ability of a film to hold its original size. The more dimensionally stable a film is, the longer it lasts on a surface, given good adhesion.
Entrance angle – angle between the illumination axis and the retroreflector axis. This is the angle that light enters a retro reflective film. At a perpendicular 0 degree angle, reflectivity is at its highest. Specification sheets measure maximum reflectivity at 4 degrees entrance angle. At a 15 degree entrance angle, retro reflectivity is reduced by approximately 20%. At 30 degrees, reflectivity drops by approximately 50%. At 45 degrees, reflectivity drops by approximately 85%. And at a 60 degree entrance angle, retro reflectivity is reduced by 86 – 99% depending on the tape type and brand.
Federal Motor Carrier Safety Administration (FMCAS) – established within the Department of Transportation on January 1, 2000, pursuant to the Motor Carrier Safety Improvement Act of 1999 [Public Law No. 106-159, 113 Stat. 1748 (December 9, 1999)]. Formerly a part of the Federal Highway Administration, the Federal Motor Carrier Safety Administration’s primary mission is to prevent commercial motor vehicle-related fatalities and injuries. (this agency is under the US DOT)
Flexibility – the ability of an object or film to stretch, conform, or bend. The quality of bending easily without breaking.
Glass Bead Reflective Sheeting (tape) – retro reflective films that use perfectly round glass sphere for reflectivity. Light enters the bead, is bent, bounced and returned to the light source. There are two kinds of beads, standard and metallized. Glass bead reflective tape is not as bright as prismatic films.
Impact Resistance – the ability of a material to resist permanent deformation caused by high force or shock applied to it over an extremely short period of time.
Light Dispersion – the width of the returning beam of light from a standard or reflective surface. Certain tapes reflect in a tight beam and can be seen at great distances. Others reflect in a wider beam and can be seen closer in but at wider angles.
Luminance Factor – the ratio of the luminance of the body considered to the luminance of a perfect diffuser under identical conditions of illumination and observation.
Micro-Prismatic Reflective Sheeting (tape) – retro reflective tape that achieves reflectivity via man made micro prisms. A prism array is embossed on the back of a film and either metalized like a mirror, or backed with a white film. Prismatic retro reflective tape is brighter than glass bead films.
National Highway Traffic Safety Administration (NHTSA) – responsible for reducing deaths, injuries and economic losses resulting from motor vehicle crashes. NHTSA sets and enforces safety performance standards for motor vehicles and equipment, and through grants to state and local governments enables them to conduct effective local highway safety programs. NHTSA investigates safety defects in motor vehicles, sets and enforces fuel economy standards, helps states and local communities reduce the threat of drunk drivers, promotes the use of safety belts, child safety seats and air bags, investigates odometer fraud, establishes and enforces vehicle anti-theft regulations and provides consumer information on motor vehicle safety topics. (this agency is under the US DOT)
Observation angle – angle between the axes of the incident beam and the observed (reflected ) beam. In retroreflection the angle between the illumination axis and the observation axis.
Photometer – an instrument for measuring light. Often used in photography. These devices do not necessarily measure retro reflectivity, just basic light returning from a surface in daylight.
Photometrics – The study of light. The measurement of the intensity, brightness, chromaticity, or other properties of light.
Retro Reflectivity – the ability of a surface to return light back to the light source. This differs from scattered or diffused reflectivity which is when a surface returns light in all directions, and specular reflectivity which is like a mirror. (AKA – Retro Reflection – reflection in which the reflected rays are preferentially returned in directions close to the opposite of the direction of the incident rays, this property being maintained over wide variations of the direction of the incident rays.)
Refraction – change in the direction of the propagation of radiation determined by change in the velocity of the propagation in passing from one medium to another.
Retro-Reflective Element – A single optical unit which by refraction or reflection, or both, produces the phenomenon of retroreflection.
Retro-Reflective Material – A material that has a thin continuous layer of small retroreflective elements on or very near its exposed surface ( for example, retroreflective sheeting, beaded paint, highway sign surfaces or pavement striping).
Retro-Reflective sheeting – A retro-reflective material preassembled as a thin film ready for use.
Retroreflectometer – In the most simple of terms a retroreflectometer is an instrument that assesses the nighttime visibility of a retro reflective surface. Reflective surfaces such as sign faces, DOT C2 tape, and pavement markings, can be tested. The readings from these devices are in candelas, which is a measure of light. The devices are first calibrated using a known reading from a sample swatch of retro reflective material, and then used to test the subject film.
Retro-Reflector – A surface or device that reflects light back along the incident path, irrespective of the angle of incidence. This simply means that it returns light back to the source. These devices can normally accomplish this at angles of up to 60 degrees, with reflectivity declining as the angle increases.
Rotation Angles – An angle of rotation is the measure of the amount that a figure is rotated about a fixed point called a point of rotation. Reflective tape brightness is measured at different rotation angles.
Shrinkage – the reduction in size of a film compared to its original size. In terms of tape, it is how much the film pulls back over time. This process can affect the longevity of a film. A lack of shrinkage is called dimensional stability.
Sight Distance – Sight Distance is a length of road surface which a particular driver can see with an acceptable level of clarity.
Solvent Resistance – the ability of a surface to resist corrosion or degradation from solvents such as alcohol, mineral spirits, etc.. The more solvent resistant a surface is, the longer it lasts in a harsh environment. Generally speaking.
Specular Gloss – the measure of light reflecting off of the surface of an object or film. A polished surface has more gloss than a matte surface.
Specular Reflectivity – Mirror like reflectivity. Light return depends on the angle of the surface.
Source – An object that produces light or other radiant flux, such as a vehicles headlights.
Stopping Sight Distance – Stopping Sight Distance (SSD) is the viewable distance required for a driver to see so that he or she can make a complete stop in the event of an unforeseen hazard.
Viewing angle – The angle between the retroreflector axis and the observation axis. The angle in which we view an object.
Visibility – The properties and behavior of light waves and objects interacting in the visual environment to produce light signals capable of evoking visual sensation.
