ANSI AIIM 7565-1993 Micrographics Readers for transparent microforms Measurement of characteristics.pdf

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1、Micrographics Readers fortransparentmicroforms Measurementof characteristics1100 Wayne Avenue, Suite 1100Silver Spring, Maryland 20910 USATel: 301-587-8202www.aiim.orgA Publication ofANSI/AIIM/ISO 7565-1993ANSI/AIIM/ISO 7565-1993G115G116G97G110G100G97G114G100G115G116G97G110G100G97G114G100G65G112G112

2、G114G111G118G101G100G32G97G115G74G117G108G121G32G50G53G44G32G50G48G48G48G65G109G101G114G105G99G97G110G32G32G78G97G116G105G111G110G97G108G83G116G97G110G100G97G114G100G115G32G73G110G115G116G105G116G117G116G101G32G40G65G78G83G73G41G83G116G97G110G100G97G114G100G115INTERNATIONAL STANDARD IS0 7565 First e

3、dition 1993-09-01 Micrographics - Readers for transparent microforms - Measurement of characteristics Micrographic - Appareils de lecture de microformes - Mesurage des caract or if the screen is curved, place the meter at point E on a line that is perpen- dicular to a plane tangent to the centre of

4、the screen. The distance between X and E shall be 380 mm or equal to the screen diagonal, whichever is greater. Measure screen luminance at other positions speci- fied in 7.1.3 if required. 7.2.2 Front-projection readers The design of front-projection readers does not al- ways allow the luminance me

5、ter to be placed on a line perpendicular to the screen. The meter shall, there- fore, be placed at a point that is normally used by the observer and the reading taken from this location. This location should be documented. 8 Screen contrast Screen contrast is the ratio of the luminance of the light

6、areas to the dark areas of an image projected onto the screen. 8.1 Apparatus 8.1 .l Meters Use the same type of meters for measuring ambient illumination and screen luminance as described in 7.1 .l. IS0 7565:1993(E) Dimensions in millimetres -_-_- , 1. 1. 1. 1. 1. I 1. t 4 1. I . 1. A B 1. C +) or s

7、creen diagonal. whichever iS greater Figure 4 - Arrangement of measuring equipment and lOCatiOn of measurement Positions 8.1.2 Test mask For determining screen contrast, use a square or rec- tangular sheet of opaque material larger than the pro- jection or film gate area of the microform holder and

8、not greater than 0,18 mm thick. The four edges of the test mask shall be clean and smooth. Suitable test masks can be produced from black card stock, black plastic sheet (such as a film with a minimum density of 2,O) or blackened metal foil. 8.1.3 Location of contrast measurement points Measure the

9、contrast at positions F and G as shown in figure4. The centres of these positions are located to the left and right of the centre of the screen on the horizontal centreline, and midway between the centre and side boundaries of the screen. 8.2 Screen contrast measurements 8.2.1 Operation of reader Op

10、erate the reader at the voltage specified in 7.1.2 and with the reader at best focus. Locate a tungsten light source simulating ambient illumination on a line which passes through the centre of the screen, and is normal to the horizontal centreline of the screen and is elevated at an angle approxima

11、tely 45” from the normal to the plane of the screen (see figure4). Place the light source 2 m or more from the centre of the screen to provide an incident illumination of 540 lx + 20 lx with a colour temperature between 2 800 K and 3 200 K when measured normal to the incident light rays. Take measur

12、ements at the centre of the screen, or at the closest possible point which is not shadowed by hoods, cabinet projections or other portions of the reader itself. All other ambient illumination shall produce less than 10 lx illumination incident upon the screen. If necessary, the direction of the ligh

13、t source, as viewed from the screen may be varied sufficiently in 7 IS0 7565:1993(E) any direction to avoid specular reflections of the light source into the luminance meter or to avoid casting shadows on the contrast measurement position of the screen. It is possible for ambient illumination to be

14、reflected from mirrors behind the screen in rear projection readers which would falsify measurements, therefore care is required when positioning the light. Ensure that the simulated ambient illumination covers a sufficiently wide angle to completely illuminate the area occupied by the front of the

15、reader. 8.2.2 Meter position 9.1 Apparatus 9.1.1 Thermocouple and test film Measure the temperature using a meter and thermo- couple having an accuracy of + 1 “C over the range of 40 “C to 90 “C. Maximum thickness of the ther- mocouple junction shall be 0,13 mm. Keep the junction in good contact wit

16、h the emulsion side of the test film. Place the test film in the reader gate with the emulsion surface away from the light source and the thermocouple junction so that the junction appears in the centre of the normal display Locate the screen luminance measuring meter at the eyepoint E as shown in f

17、igure4. 8.2.3 Procedure area. Place the test mask (see 8.1.2) in the microform holder so that the focused image of one of the edges of the mask lies on a vertical line midway between the points X and F (see figure41, with the left portion of the screen masked. Take luminance measure- ments at test p

18、ositions F and G. The measurements are identified as FD and G, (subscript D signifies dark and subscript L signifies light). While keeping the junction in this position, perma- nently anchor the thermocouple by either cementing or taping its insulated leads to the film at several points outside of t

19、he aperture area and the glass flats. Then apply a very small quantity of cement to the junction and its bare leads (either a clear epoxy ce- ment or some other suitable cement may be used), press into place with a suitable device and hold until set. Take care to ensure that the junction is well bon

20、ded and lies flat with respect to the surface of the film. Adjust the position of the test mask in the microform 9.1.2 Thermometer for measuring ambient holder, or the position of the holder itself, so that the temperature focused image of one of the edges of the mask lies on a vertical line midway

21、between points X and G with the right portion of the screen masked. Take lumi- nance measurements at test positions F and G. The measurements are identified as FL and Go. Measure the ambient temperature with a ther- mometer having an accuracy of ) 1 “C over the range of 15 “C to 30 “C. Screen contra

22、st = :I z : 1 D D 9.2 Procedure Measure the temperature of the film in the film gate after the reader has been operating for 60 min at an ambient temperature of 23 “C k 2 “C and at the volt- 9 Film gate temperature measurements age described in 7.1.2. Measure the film gate temperature at the surface

23、 of a silver-gelatin microfilm test strip which has a mini- mum density of 1,5 and mounted in the reader that is being operated as specified in 7.1.2. 8 IS0 7565:1993(E) Annex A (informative) Importance of the luminance measurement The method specified in clause 7 for the measure- ment of screen lum

24、inance has the advantage of measuring the screen characteristics as they are per- ceived by the observer. A.1 Screen characteristics The screen receives a luminous image, i.e. at each point, a set of luminous rays of variable direction and luminances. At a given point, the illuminance of the screen

25、can be obtained by adding the energies of all the rays falling on this point. The screen scatters this energy according to its structure. For rear-projection screens, the light is dif- fused by an optical coating applied on the surface of the screen on which the image is projected. The thicker the c

26、oating, the greater the diffusion. If the light is poorly scattered, a maximum of energy is concentrated along the axis joining the source (projection lens) to any point of the screen. This yields a bright image in this direction but with a significant fall-off in intensity at wide viewing angles, s

27、uch as in the corners of the screen. If the light is very poorly scattered, the light source is usually visible in the centre of the screen. This phenomenon is known as hot spotting. If the light is nearly perfectly diffused, the hot spot or fall-off of intensity no longer appears obvious but the im

28、age luminance is low because the light is no longer concentrated towards the observer and also because such diffusers generally absorb a greater part of the energy. A.2 Screen observation FigureA. gives an example of how an observer sees a screen. The observer is on the axial beam (normal to the scr

29、een at its centre) at a given distance from the screen. From the corners the observer receives light rays not normal to the screen either in the inci- dent beam or in the diffuse light. The computation of emitted luminances takes into account - the distribution of illuminance received by the screen

30、depending on many factors, such as source objective, condenser and their relative adjust- ments; and - the scattering characteristics of the screen. As a result of imperfections in the optical system and the diffuser - the illuminance is less in a corner than on the axis; and - the emitted luminance

31、 depends on the emission angle. For example, referring to figureA.l, the luminance of the screen centre is 1 027 cd/m* while the luminance of the screen corner is only 88 cd/m*. This variation is caused by the scattering index. This will not be shown by a simple measurement of the illuminance using

32、a luxmeter. This measurement will indicate 1 076 lx in the screen centre and 848 lx at the screen corner. A.3 Evaluation of screen diffusion characteristics The diffusion will be described by comparing the ac- tual luminance in the observers direction with the uniform luminance obtained with a theor

33、etically per- fect diffusing screen, that is, where all of the light is scattered. A.3.1 Perfect diffuser In a perfect diffuser, all the energy received is re- emitted equally in all directions so the exitance, M, of the screen is equal to the received illuminance, E. (This can be measured by a luxm

34、eter looking towards the source from the position of the screen of interest.) The light is diffused according to Lamberts law: - the emitted luminance, L, is independent of the direction; and - the exitance, M, and the luminance, L, are linked by the relation M = x x L. 9 IS0 7565:1993(E) Consequent

35、ly, for a perfect diffuser the luminance in any direction is L = E/M A.3.2 Actual diffuser For an actual screen, if L is the luminance measured in a given direction, such as the eyepoint, the Iumi- nance factor is defined as the ratio L/L where L is the luminance of a pressed in cd/m*. perfect diffu

36、ser. L and L are ex- E is the received illuminance, expressed in lx. The ratio L/L can be expressed as L x XL -=- L E or Luminance factor Light emitted by the screen (in cd/m*) = Light received by the screen (in lx) xx The ratio L/L is generally greater than 1 along the axial beam and consequently l

37、ower than 1 at wide viewing angles. A.3.3 Example (for values given in figureA.1) The amount of light striking the centre of the reader screen and perpendicular to the screen is 1 076 lx (as would be measured using a perfectly diffusing receptor). The luminance of the screen, as measured from the po

38、sition of an observer and along the same optical axis, is 1 027 cd/m*. Hence, the luminance factor is 1 027 x IT =3 1076 To achieve this increase in luminance in the direction of the observer (compared to the luminance of a per- fect diffuser), it is necessary to reduce the screens light diffusion f

39、rom that of a perfect diffuser. This re- duction is illustrated in figureA. by the polar plot of luminance distribution that indicates that the lumi- nance is maximum along the line of incidence. The importance of luminance factor is further exemp- lified by the luminance distribution for light stri

40、king the screen near the corners. Firstly, the light intensity striking the screen is less than on-axis due to the op- tical system. Secondly, the screen gain, measured along the line of incidence, is also reduced to 626 x = = 2 3 848 Also, the luminance factor measured normal to the screen is 276xn

41、 =, 848 Of even greater significance is the screen brightness measured in the direction of the observer, 88 cd/m*. Hence, the ratio of brightness between centre and corner, as seen by the observer, is 1 027 -= 11,68 88 which represents a light fall-off of more than 91 % while the fall-off in inciden

42、t light is only 21 %. Further examination of figureA. reveals that the luminance reaching the observer from the corner of the screen is strongly dependent on the viewing pos- ition. The closer the observer is to the screen, the greater the light fall-off. Because of the nature of viewing screens, th

43、e results obtained from luminance measurements are depen- dent on the geometry of the viewing system. This is why taking a simple lightmeter measurement, without a knowledge of screen properties, can be very mis- leading. 10 IS0 7565:1993(E) Lens I- - Corner of screen Luminance distribution /- at10

44、% from corner lO%of diagonal -276cd/m2 normnltoscreen Luminance distribution on centreline Observer Figure A.1 - Effects of the actual scattering properties on viewing luminance 11 IS0 7565:1993(E) Annex B (informative) Field method using a luxmeter B.l Presentation This field method is useful for c

45、omparing reader per- formance from day-to-day and performance of readers of the same design. It is generally not useful in com- paring readers of different designs. The measured values do not indicate the actual luminance or lumi- nance variations experienced by the user (see annex A). It does provi

46、de a simple means for obtaining data useful in monitoring the adjustment of a specific reader and to check the centring of the various optical components. B.2 Method Place the sensitive surface of a luxmeter with the diffuser removed within 20 mm of the reader screen surface. If this is impossible,

47、remove the screen and place the diffuser of the luxmeter at the place where the screen surface would be (this last configuration shall be used for front-projection screens). Readings taken in this manner will be higher than if the screen were in place since the screen characteristics are not include

48、d. Since measurements made by this method may be compared with measurements made by the same method on the same reader at a different time, the exact location of the measured area should be re- corded if meaningful data are to be acquired. Measure the illuminance at the position specified in 7.1.3. 12 IS0 7565:1993(E) UDC 778.148:620.16 Descriptors: micrographics, micrographic equipment, microform readers, tests, performance tests, test equipment Price based on 12 pages