1、Standard Method of Test for Instrumental Photometric Measurements of Retroreflective Materials and Retroreflective Devices AASHTO Designation: T 257-96 (2013) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-4d T 25
2、7-1 AASHTO Standard Method of Test for Instrumental Photometric Measurements of Retroreflective Materials and Retroreflective Devices AASHTO Designation: T 257-96 (2013) 1. SCOPE 1.1. This standard covers procedures for instrumental determinations of photometric characteristics of retroreflective ma
3、terials and retroreflective devices. 1.2. The values stated in SI units are to be regarded as the standard. 2. REFERENCED DOCUMENTS 2.1. ASTM Standards: E184, Standard Practice for Effects of High-Energy Neutron Radiation on the Mechanical Properties of Metallic Materials, E706 (IB) (withdrawn 2002)
4、 E308, Standard Practice for Computing the Colors of Objects by Using the CIE System E809, Standard Practice for Measuring Photometric Characteristics of Retroreflectors E810, Standard Test Method for Coefficient of Retroreflection of Retroreflective Sheeting Utilizing the Coplanar Geometry 3. SIGNI
5、FICANCE AND USE 3.1. This method describes procedures used to measure photometric quantities that relate to the visual perception of retroreflected light. The most significant usage is in the relation of the nighttime vehicle headlamps, retroreflector, and drivers eye geometry. For this reason, the
6、CIE Standard Source A is used to represent a tungsten vehicle headlamp and the receptor has the photopic spectral responsivity at the presentation angle, V(), corresponding to the light-adapted human eye. (See Section 4.2.10.) Although the geometry must be specified by the user, it will, in general,
7、 correspond to the relation between the vehicle headlamp, the retroreflectometer, and the drivers eye position. 4. TERMINOLOGY 4.1. Retroreflective Terms: 4.1.1. retroreflectora surface or device that reflects and returns a relatively high proportion of light in a direction close to the direction fr
8、om which it came. This characteristic is maintained over a wide variation of the angle made by the incident light ray and the normal to the retroreflective surface. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of appli
9、cable law.TS-4d T 257-2 AASHTO 4.1.2. retroreflective elementone optical unit that by refraction and/or reflection produces the phenomenon of retroreflection. 4.1.3. retroreflective devicea complete device, ready for use, consisting of one or more retroreflective elements (for example, a device cont
10、aining cats eyes, a cube corner device, or a safety retroreflective device). 4.1.4. retroreflective materiala retroreflective material that consists of a thin continuous layer of small retroreflective elements on or very near the exposed surface (for example, retroreflective sheeting, beaded paint,
11、highway sign surfaces, or pavement striping). 4.1.4.1. retroreflective sheetinga retroreflective material preassembled as a thin film ready for use. 4.2. geometric terms(Figures 1 through 3). Figure 1Geometry of Retroreflective Elements Cats Eye ElementSpherical ElementCube Corner Element 2015 by th
12、e American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-4d T 257-3 AASHTO Figure 2Pictorial View with the Presentation Angle () Illustrated at 90 Degrees (A presentation angle of 0 degrees is normally used.) Figure 3Pl
13、ane View from above with the Presentation Angle Illustrated at 0 Degrees 4.2.1. reference center (O)the defined center of a retroreflector. 4.2.2. reference axis (ON)the defined axis used to determine the entrance angle in photometric measurements and in practical use. This axis passes through the r
14、eference center (O) (Note 13). PhotoreceptorRSAxis ofIncident LightPresentationAngleViewingAngleObservationAngleOrientation AngleDatumMarkObservationAxisReferenceAxisEntrance AngleEntrancePlaneSourceLateralDistance(d)NPhotoreceptorRSNAxis of Incident LightReceptor Aperture AngleSource Aperture Angle
15、Illumination Distance, DReference CenterRetroreflectorViewingAngleObservationAngleObservation Distance,DEntranceAngleReferenceAngleSourceLateral Distance (d) 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable l
16、aw.TS-4d T 257-4 AASHTO 4.2.3. axis of incident light (OS)the line between the reference center and the center of the exit aperture of the light source. 4.2.4. observation axis (OR)the line between the reference center and the center of the entrance aperture of the photoreceptor. 4.2.5. entrance ang
17、le ()the angle between the reference axis and the axis of incident light. Counterclockwise rotation of the reference axis relative to the axis of incident light is considered positive as shown in Figure 3. Note 1Entrance angles are normally in the range of 0 to 90 degrees. However, negative entrance
18、 angles can be used to indicate a change of 180 degrees in the presentation angle, provided the 0-degree orientation of the datum mark is defined relative to the observation plane. 4.2.6. viewing angle ()the angle between the observation axis and the reference axis. Note 2Since this angle is determi
19、ned by other defined angles, the viewing angle is introduced simply for convenience in defining the specific luminance and the luminance factor. 4.2.7. observation angle (a)the angle between the axis of incident light and the observation axis (“divergence angle” is an obsolete term for this angle).
20、4.2.8. datum markthe mark placed on the sample by the manufacturer which defines the initial (0-degree) orientation position, and from which the orientation angle is measured. 4.2.9. orientation angle ()the angle, when viewed from Point N, through which the sample may be rotated about the reference
21、axis, from the initial 0-degree orientation of the datum mark. The initial 0-degree orientation angle may be defined relative to either the observation plane or the entrance plane. 4.2.9.1. When defined relative to the observation plane, the 0-degree orientation is when the datum mark is in the obse
22、rvation plane and on the same side of the axis of incident light as the photoreceptor. 4.2.9.2. When defined relative to the entrance plane, the 0-degree orientation is when the datum mark is in the entrance plane and on the same side of the axis of incident light as the reference axis. 4.2.10. pres
23、entation angle ()the dihedral angle between the entrance plane formed by the axis of incident light and the reference axis, and the observation plane formed by the axis of incident light and the observation axis. A 0-degree presentation angle is formed when the photoreceptor is placed in the plane f
24、ormed by the axis of incident light and the reference axis, with the receptor on the same side of the source as the reference axis. A presentation angle of 0 degree as shown in Figure 3 is used, unless otherwise specified. Figure 2 shows the presentation angle at plus 90 degrees. 4.2.11. illuminatio
25、n distance (D Equal to OS)the distance between the center of the exit aperture of the light source and the reference center. 4.2.12. observation distance (D Equal to OR)the distance between the reference center and the center of the entrance aperture of the photoreceptor. 4.2.13. lateral distance (d
26、)the distance from the center of the entrance aperture of the photoreceptor to the axis of incident light, measured perpendicularly to the observation axis. It may be computed by multiplying the observation distance D by the tangent of the observation angle. tandD= (1) 2015 by the American Associati
27、on of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-4d T 257-5 AASHTO 4.2.14. source aperture angle ()the angle at the sample subtended by a given dimension of the source aperture. 4.2.15. receptor aperture angle ()the angle at the sa
28、mple subtended by a given dimension of the receptor aperture. 4.3. Photometric Terms: 4.3.1. Commission Internationale de lEclairage (CIE) photopic standard observera receptor of radiation with a spectral sensitivity curve (Table 1) that conforms to the V() distributions specified in Table 1. Table
29、1Spectral Response of the CIE Standard Photopic Observer (Section 13.1) Wavelength (mm) Relative V(), % Wavelength (mm) Relative V(), % 380 0.00 570 95.20 390 0.01 580 87.00 400 0.04 590 75.70 410 0.12 600 63.10 420 0.40 610 50.30 430 1.16 620 38.10 440 2.30 630 26.50 450 3.80 640 17.50 460 6.00 650
30、 10.70 470 9.10 660 6.10 480 13.90 670 3.20 490 20.80 680 1.70 500 32.30 690 0.82 510 50.30 700 0.41 520 71.00 710 0.21 530 86.20 720 0.10 540 95.40 730 0.05 550 99.50 740 0.02 555 100.00 750 0.01 560 99.50 4.3.2. illuminance (E)the ratio of the luminous flux to the area of the surface, when the lat
31、ter is uniformly illuminated. EA= (2) where: = luminous flux (lumen), and A = area of the surface. 4.3.2.1. normal illuminance (En)normal illuminance is an expression used in the photometry of retroreflectors to designate the normal illuminance from the source on a retroreflective surface, and is me
32、asured in the plane that passes through the reference center and is perpendicular to the axis of incident light. In SI units, normal illuminance is measured in lux; in U.S. units, it is measured in foot-candles. 4.3.3. inverse-square lawthe normal illuminance that a point source produces at a point
33、on a surface varies directly with the luminous intensity of the point source and inversely as the square of the distance between the source and that point, expressed as follows: 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a viola
34、tion of applicable law.TS-4d T 257-6 AASHTO 2nIED= (3) where: I = luminous intensity. 4.3.4. photoreceptoran instrument for measuring luminous flux. 4.3.4.1. photometera photoreceptor used for determining illuminance at a surface, and usually calibrated in units of foot-candles or lux. 4.3.4.2. tele
35、photometera photoreceptor used for determining luminance at a distance or illuminance from a restricted field of view. The device is equipped with an objective lens that may be focused on a target. 4.3.5. goniometeran instrument for measuring or setting angles. 5. QUANTITIES AND UNITS FOR RETROREFLE
36、CTIVE PHOTOMETRIC MEASUREMENTS 5.1. Photometric quantities used to specify the performance of retroreflective materials and devices are specific intensity (SI), specific luminance (SL), specific intensity per unit area (SIA), specific intensity per unit length (SIL), and luminance factor (LF). 5.1.1
37、. Specific Intensity (SI)The ratio of the luminous intensity of the retroreflector to the normal illuminance. ( )2nEDSIE= (4) where: E = illuminance at the observation position, and D = distance between the center of the photoreceptor entrance aperture and the reference center. Specific intensity is
38、 expressed in candelas per foot-candle (cd fc1). Note 3The CIE vocabulary defines the above relationship as the coefficient of luminous intensity (CIL), which is expressed in metric units of candelas per lux. Note 4The quantity SI is recommended for determining the performance of such retroreflector
39、s as button reflectors, delineators, or automotive reflectors, since it depends on a unit device and the area need not be measured. 5.1.2. Specific Luminance (SL)The ratio of the luminous intensity of the projected surface to the normal illuminance at the surface on a plane normal to the incident li
40、ght. The ratio is expressed as follows: ( ) ( ) ( )2cos coscosnnED A I ASISLE EA = = =(5) where: I = E(D)2= retroflective luminous intensity of the sample, A = surface area of the sample, and = viewing angle. 2015 by the American Association of State Highway and Transportation Officials.All rights r
41、eserved. Duplication is a violation of applicable law.TS-4d T 257-7 AASHTO Specific luminance is expressed in candelas per square meter per lux (candelas per square foot per foot-candle). Candela replaces the term candlepower. It is used in both SI and U.S. units. Note 5The CIE vocabulary defines th
42、is relationship as the coefficient of luminance, which is expressed in metric units of candelas per square meter per lux. Note 6The quantity SL treats the retroreflector as a surface source whose projected area is visible as an area at the observation position. The quantity SL relates to the way the
43、 effective retroreflective surface is focused on the retina of the human eye and to the visual effect thereby produced. It is recommended for describing the performance of highway signs and striping, or large vehicular markings, which are commonly viewed as discernible surface areas. 5.1.3. Specific
44、 Intensity per Unit Area (SIA)The ratio of the luminous intensity of the surface to the normal illuminance and to the area of the retroreflective surface. ( )2nED ESIAA= (6) Specific intensity per unit area is expressed in candelas per lux per square meter (candelas per foot-candle per square foot).
45、 Note 7The CIE vocabulary defines this relationship as the coefficient of luminous intensity (CIL) per unit area, which is expressed in SI units of candelas per lux per square meter. Note 8The quantity SIA treats the retroreflector as an apparent point source whose retroreflected luminous intensity
46、is dependent on the area of the retroreflective surface involved. It is a useful engineering quantity for determining the photometric performance of such retroreflective surfaces as highway delineators or warning devices. SIA may also be used to determine the minimum area of retroreflective sheeting
47、 necessary for a desired level of photometric performance. 5.1.4. Specific Intensity per Unit Length (SIL)The ratio of the luminous intensity of the sample to the normal illuminance and to the length of the retroreflective sample. ( )2nED ESILL= (7) where: L = length of the sample. Specific intensit
48、y per unit length is candelas per lux per meter (candelas per foot-candle per foot). Note 9The CIE vocabulary defines the above relationship as the coefficient of luminous intensity (CIL) per unit length, which is expressed in metric units of candelas per lux per meter. 5.1.5. Luminance Factor (LF)T
49、he ratio of the luminance of the surface to that of a perfect diffusing surface such that: ( ) ( )2coscos coscosnE D SIALFAE= =(8) The luminance factor has no dimensions. Note 10In this formula, the dimensions associated with the reflectance of the perfect diffusing surface are the same as those of SIA, and are thus canceled. 6. REQUIREMENTS TO BE STATED IN SPECIFICATIONS 6.1. When stating photometric retroreflective requirements, the following shall be specified
