CIE 111-1994 Variable Message Signs (E)《可变信息信号(E)》.pdf

1、 - CIE 111 94 E 900b145 0005083 32T ISBN 3 900 734 54 2 COMMISSION INTERNATIONALE DE LCLAIRAGE INTERNATIONAL COM M ISSION ON ILLUMINATION INTERNATIONALE BELEUCH JUNGSKOMM ISSION VARIABLE MESSAGE SIGNS CIE ill - 1994 UDC: 628.975 Descriptor: Signal lighting 656.25 Safety signals COPYRIGHT Internation

2、al Commission on IlluminationLicensed by Information Handling ServicesCIE 111 94 H SDb145 0005084 266 This Technical Report has been prepared by CIE Technical Committee4-17 of Division 4 Lighting and Signalling for Transport and has been approved by the Board of Adminstration of the Commission Inter

3、nationale de IEclairage for study and application. The document reports on current knowledge and experience within the specific field of light and lighting described, and is intended to be used by the CIE membership and other interested parties. It should be noted, however, that the status of this d

4、ocument is advisory and not mandatory. The latest CIE proceedings or CIE NEWS should be consulted regarding possible subsequent amendments. Ce rapport technique a t prpar par le Comite Technique CIE 4-17 de la Division 4 Eclairage et signalisation pour les transports et a t approuv par le Bureau dAd

5、ministration de la Commission Internationale de IEclairage, pour tude et application. Le document traite des connaissances courantes et de exprience dans le domaine spcifique indiqu de la lumire et de Ictairage, et il est tabli pour lusage des membres de la CIE et autres groupements intresss. II fau

6、t cependant noter que ce document est indicatif et non obligatoire. Pour connaitre dventuels amendements, consulter les plus rcents comptes rendus de la CIE ou le CIE NEWS. Dieser Technische Bericht ist vom CIE-Technischen Komitee 4-1 7 der Division 4 Beleuchtung und Signale fur den Verkehr ausgearb

7、eitet und vom Vorstand der Commission Intemationale de IEclairage gebilligt worden, Das Dokument berichtet ber den derzeitigen Stand des Wissens und ErFahrung in dem behandelten Gebiet von Licht und Beleuchtung; es ist zur Verwendung durch CIE-Mitglieder und durch andere Interessierte bestimmt. Es s

8、ollte jedoch beachtet werden, da das Dokument eine Empfehlung und keine Vorschrift ist. Die neuesten CIE-Tagungsberichte oder das CIE NEWS sollten im Hinblick auf mgliche sptere hderungen zu Rate gezogen werden. Any mention of organisations or products does not imply endorsement by the CIE. Whilst e

9、very care has been taken in the compilation of any lists, up to the time of going to press, these may not be comprehensive. Toute mention dorganisme ou de produit nimplique pas une prfrence de la CIE. Malgr le soin apport la compilation de tous les documents jusqu la mise sous presse, ce travail ne

10、saurait ALa AL, is the actual luminance difference between target and background; is the threshold luminance difference between target and background. (See also CIE Publication 19.21 and 19.22 (1981).) Adrian (1 989) provides a means of calculating threshold luminance based on physiological characte

11、ristics of the eye and experimental data. Adrians model takes into account the same factors as Blackwells investigation of size, time, threshold, contrast and luminance. However the model goes on to include additional factors such as disability glare and negative and positive contrast and subject va

12、riables such as age. The model provides a convenient method of determining threshold luminance values and can thus be employed to determine values of Visibility Level. Adrians objective was to determine levels of street lighting and suggests that Visibility Levels should be between 10 and 20. Concep

13、t of visibility relates to particular tasks and observers. Sometimes it is necessary to measure the subjects visual ability independent of the task being conducted. A method of measuring visual ability has been suggested which evaluates the eye using a measure of performance which has been applied i

14、n measuring the quality of lenses and other optical systems. 2.2.4 Modulation transfer function and contrast sensitivity Conventional tests of visual acuity, such as the Snellen chart, do not accurately assess how well a person can see. As a measure of the performance of an optical system the abilit

15、y to resolve fine high contrast detail does not predict ability to resolve large images of low contrast. Regan, Raymond, Ginsburg and Murray (1981) in discussing vision anomalies in multiple sclerosis sufferers suggest that visual acuity is insufficient as a measure of form vision. Cornsweet (1970)

16、describes a method which provides a more general means of measuring the quality of the visual system. The quality of an optical system might be described in general terms as its ability to resolve images. Such a very general definition is of little use, however it may be refined to provide a useable

17、 definition that allows measurements of the human optical system to be made. An image, for example of stripes, can be shown to be the sum of a number of sine waves by means of Fourier analysis. To produce a striped pattern an infinite number of sine waves must be added of higher and higher frequency

18、. In fact any image might be considered to be the sum of a number of sine waves. The ability of a optical system to resolve sine wave gratings will therefore be a measure of its ability to resolve any image. If the output intensity (amplitude) of a sine wave grating were exactly the same as the inpu

19、t intensity then the transfer is perfect or unmodulated. There has been no modulation in the transfer of the sine wave grating. Typically modulation does occur and with a lens the degree of modulation decreases with increasing frequency of the input sine wave grating until at very high frequencies t

20、he modulation is zero. The output image at this point is a uniform distribution as the original input sine wave grating is “blurred out“. The plot of modulation against frequency can describe the quality of any optical system and is termed the Modulation Transfer Function. A practical means of measu

21、ring the modulation transfer function of the human visual system is by employing thresholds. The subject is presented with sine wave gratings the intensity of which is reduced until it is only just visible. The contrast at which this occurs is related to the modulation. However modulation occurs at

22、low as well as high frequencies unlike a simple lens which modulates only at higher frequencies. Clearly the human visual system does not behave linearly and the resulting curve cannot strictly be termed a modulation transfer function. 13 COPYRIGHT International Commission on IlluminationLicensed by

23、 Information Handling ServicesCIE LLL 94 B 9006145 OOOSLO2 O01 m CIE 111 - 1994 This response curve is referred to as the contrast sensitivity function and in conjunction with normative data can be used to assess the quality of a persons visual system without recourse to generating a complete modula

24、tion transfer function. Contrast sensitivity is a measure of someones ability to distinguish objects under different levels of contrast. A more rigorous definition is provided by the CIE International Lighting Vocabulary (1 987). Contrast Sensitivity is the reciprocal of the least perceptible (physi

25、cal) contrast, usually expressed as UAL, where L is the average luminance and AL is the luminance difference threshold. The actual value of contrast sensitivity therefore depends on the luminance and the viewing condition, including the state of adaptation. It has been considered (Ginsburg, Evans an

26、d Canon, 1984) that a measure of a persons contrast sensitivity at a number of visual frequencies provides a more effective measure of the ability to see. Systems are available which allow the contrast sensitivity curve to be determined easily and rapidly. One such system is a photographically produ

27、ced chart which presents to the subject “patches“ containing sine wave gratings of dark and light bands in five rows of increasing levels of frequency. The contrast of the “patches“ is reduced across nine columns the final column of “patches“ being completely blank. The subject is required to determ

28、ine the orientation of the bands in each of the patches, the limiting level of contrast for each frequency giving the subjects contrast sensitivity function. (See also CIE 95-1992.) Ginsburg, Evans, Sekule and Harp (1982) in a study of Il instructor pilots found that the ability to recognise a parke

29、d MlG fighter was predicted by the pilots contrast sensitivity and not by acuity. In the experiment an F-16 simulator was used to provide a simulation of an air to ground recognition task. The simulator however used a video simulation of the visual scene which cannot necessarily be considered to be

30、directly comparable to a real image. Evans and Ginsburg (1985) later went on to use contrast sensitivity to accurately predict the performance of 20 subjects on a motion picture simulation of a highway sign discrimination task. The motion picture projection again is not an exact representation of a

31、drivers view. However Evans and Ginsburgs (1985) work repeats that of Sivak, Olsen and Pastalan (1981) who used signs viewed from a moving automobile, and generated very similar results. It is reasonable to assume that the movie simulation was effective and the persons contrast sensitivity does accu

32、rately predict actual visual task performance. Some pathologies causing subjective complaints and actual reductions in visual performance can leave visual acuity unaffected. Regan, Silver and Murray (1977) found that of 48 multiple sclerosis sufferers, all having 20/20 Snellen acuity, 33 displayed a

33、bnormal contrast sensitivity to low and medium frequency figures. Regan, Raymond, Ginsburg and Murray (1981) in an experiment with 7 multiple sclerosis sufferers found, though all had 20130 Snellen acuity or better, all showed abnormally low contrast sensitivity in medium frequencies. More practical

34、ly this abnormality manifested itself as a diminished ability to recognise relatively larger low contrast characters of the alphabet. In general Ginsburg (1981) found that, as target size decreased, the necessary contrast for identification must be increased. Additionally, that the necessary contras

35、t was greater for older observers. The implication for matrix variable message signs might be that contrast should be maximised to overcome the effects of the ageing observer and reductions in contrast caused by veiling reflections, fading elements, fog, front screens and other contrast reducing fac

36、tors. However in a study of light emitting variable message signs Kerr, Snelgar, Jordan, Emmerson and Linfield (1988), using simulated upper and lower case characters, indicated that there may be an optimum contrast which depends upon the age of the observer and the configuration of the sign. 2.2.5

37、Perceived brightness The CIE International Lighting Vocabulary (1987) defines brightness or the now obsolete term of luminosity as that attribute of visual sensation according to which an area appears to emit more or less light. Intuitively the apparent brightness of a point in the visual field woul

38、d seem to be determined by the excitation of the corresponding receptors in the eye. Though this assumption appears to be reasonable it is fallacious. The perceived or apparent brightness of a target is modulated by inhibitor 14 COPYRIGHT International Commission on IlluminationLicensed by Informati

39、on Handling ServicesCIE 111 94 I 9006345 0005103 T48 CIE 111 - 1994 signals between the receptors in the retina of the eye. Receptors that receive a higher illumination inhibit the firing of the receptors surrounding them and of each other. Hence as was mentioned an area appears to emit an amount of

40、 light which in absolute physical terms it may not be doing. The effect is to emphasise the boundaries between light and dark areas of the visual field. Brightness is therefore partly determined by the pattern of excitation on the retina (Cornsweet, 1970). Attempts to relate perceived brightness to

41、a physical measure of luminance are further confounded by accommodation of the eye. When the visual field contains objects of higher luminance the annular muscle in the iris automatically contracts and reduces the size of the pupil (Gregory, 1966), reducing the amount of light that is entering the e

42、ye. Lateral inhibition and accommodation of the retina in normal circumstances serve to enhance the performance of the eye. However in some particular situations the two mechanisms can reduce an observers ability to read a sign. For example if the sun is low and behind a sign the inhibition between

43、receptors and the closing of the pupil effect to make the sign uniformly dark. In order to ameliorate this effect the sign might be equipped with a backing board which would reduce the luminance of the visual field. Another means of reducing this effect might be to increase the luminance, and hence

44、the brightness of the sign or message. Lotens and Van Leeuwen (1976) comment that lighter areas of the visual field tend to spill light on to darker areas. This effect is often referred to as halation or irradiation. It is also noted in the CIE Technical Report on Roadsigns (1988) where it is mentio

45、ned that “white letters appearing to spread so that the stroke width appears greater“. Despite wide acceptance of the need to take into account the infiuence of halation no mechanism is suggested whereby it occurs. The effect results in dark letters on a light background appear narrower than similar

46、ly sized light characters on a dark background. In order to compensate for this effect conventional character sets or fonts for use on road signs are commonly provided in two forms. One slightly narrower than the other. In the United Kingdom these are Transport Heavy and Transport Medium (Ministry o

47、f Transport, 1963). The effect is utilised in a matrix variable message sign to provide apparently continuous characters from discrete elements. Mazoyer and Colomb (1987) found that the possible separation between elements is related to their luminance, presumably as a result of the greater halation

48、 associated with the higher luminance level. 2.2.6 Glare sensitivity Glare is produced when some part of the visual field has a sufficiently higher luminance than the background to cause annoyance or to reduce the visual performance of a normally adapted eye and can either be direct or by reflection

49、 (McCormick and Sanders, 1982). Potential sources of glare may be the emitters themselves on a light emitting sign or the sun when it is low behind the sign. The sun might provide a source of reflected glare in cases where the sign has a flat specular screen covering the sign face. Glare may cause discomfort, disability or both (Galer, 1987). In a situation of discomfort glare the observer is made uncomfortable but there is no measurable reduction in the observers visual ability. In the case of disability glare the observers performanc