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Characterisation of Industrial Film Viewers According to the UNI EN 25580 Standard Giovanni Calcagno, Michele Murgia, Giuseppe Orlando Italian Institute of Welding - Genova Giuliano Succetti Cofar S.r.l. - Arcore (MI) Contact

INTRODUCTION

After 2 years from the definition of UNI EN 25580 "Film Viewer for Industrial Radiography - Minimum requirements" is already possible to have a first balance concerning the application of the specification to a significant numbers of film viewers.
The above mentioned specification, even if do not introduce substantial changes if compared to the ISO 5580 "Industrial radiographic illuminators - Minimum requirements", has shown some doubts for burdens related to its applications because of necessity, for functional test on film viewer, of measurements of optics luminance with expensive instruments and difficult to find and to calibrate.
This matter presents renewed interest in the light of UNI EN 444 norm "General principles for radiographic examination of metallic materials with X and Gamma rays" and the following UNI EN 1435 "Radiographic Control of weld joints". These norms do not settled any maximum value for radiographic density of films, but for its reading it must be used a film viewer according to UNI EN 25580; this condition is precisely indicated on UNI EN 1435, point 6.10.

2. CONTROL OF OPTICS CHARACTERISTICS: THEORETICAL PART

Due to the complexity of this matter, it can be useful a little introduction to main quantities directly or indirectly used in optics measurements, because the recent norm EN 25580, as other previous norms, impose the execution of luminance measurements, different from others that want the characterisation of film viewers with illumination measurements.

Luminance Intensity: provided a luminous source S, its luminance intensity in the direction of ray's propagation direction is the relation [1] I between the issued flux DF and the solid angle dw.
The luminance Intensity is measured in lumen per steradiant , unity is usually told Candle (Cd).

I = dF/dw   [1]


The steradiant is the subtended angle , in the centre of a sphere, from an area of arbitrary form with extension equal to the square of ray of the same sphere.

It must be considered that practically the intensity of a luminous source change according the different positions; for many substances and practical situations the element luminous intensity can be stated in function of direction by the following relation:

dIq = dI_n * cosq   [2]

This relation is the Lambert's Law. A surface element that operates according to this law is called Perfectly Diffused. To verify the appliance of this law, some film viewers for industrial radiography have been verified to value the decrease of luminance intensity emitted in function of direction q; tests made have given uniform results, as per example below in which luminous intensity = 1 for q=90° (e.g. on normal in the centre of film viewer).

Angle q	Measured values	Theoretical values
0	0.052631	0
10	0.263157	0.173648
20	0.403508	0.342020
30	0.578947	0.5
40	0.719298	0.642787
50	0.807017	0.766044
60	0.877192	0.866025
70	0.947368	0.939692
80	0.982456	0.984807
90	1	1
100	0.982456	0.984807
110	0.947368	0.939692
120	0.929824	0.866025
130	0.789473	0.766044
140	0.675438	0.642787
150	0.578947	0.5
160	0.385964	0.342020
170	0.263157	0.173648
180	0.070175	0

Illumination : it is a physical quantity relative to a surface with luminous flux; considering an infinitesimal part dA of this surface, if dF is the luminous flux incident on his area, the illumination E defined as the relation [3] between the luminous flux and the considered area.

E = dF/dA   [3]

If the illumination do not change from point to point, on the considered surface, we can define the total illumination relative to the same surface, if F in the total incident flux, as E=F/A.
The unit of measure of illumination is lumen/m², usually indicated as lux.
In country of English language is often used a different unit of measure, the foot-candle, that is the illumination given from a flux of a lumen on the area correspondent to a square foot.
The numerical relation [4] between lux and foot-candle is:

1 lux » 10.76 foot-candle    [4]

If the luminous source is punctiform, the illumination [5] given by this source on a surface of dA extension, which normal is inclined of q degree as regards the joint with the source, at r distance, is:

E = dF / dA = I cosq / r²    [5]
As :
dF = I dw = I dA cosq / r²

Luminance or Brilliance: considering an element of surface DA that emit a quantity of luminous flux d(DF) in q direction, through the solid angle dw, is called luminance B (or brilliance) of that element the relation between the intensity Dl_q of the same in q direction and the projection of the surface on area perpendicular to this direction. If the luminance of the element in q direction is indicated with Bq, so [6]:

Bq = Dl_q / DA * cosq    [6]

For perfectly diffused surface, [2], the luminance do not change [7] at changes of direction

B_q = Dl_q / DA    [7]

If you consider a luminous extended source, even if it is perfectly diffused, it is not possible value the illuminance given in any point P with relation [5], but it is necessary go on analytically, through an integration.

If the emitting part is a flat disk of ray a, place to a b distance respects to the P point, the luminous intensity of an annular element of ray r and width dr, in P direction, it will be:

dl_q = B * dA * cosq

If the B luminance do not change with q direction.

Also:

dA = 2 p r dr
l = b secq
r = b tanq and so dr = b sec²q dq

so: dE = 2 p B senq cosq dq

integrated the expression between q =0 and q =a:



From this relation we can immediately understand that it can be considered punctiform with a margin of mistake quite limited a source which ray is sufficiently inferior at the distance of measure B; if for example a = b / 10, it is an approximation of 1% in neglecting a² in relation to b² and note that p a² is the area A of the surface, the [8] is reduced to the universal law of the inverse of the square of the distance, because, as per definition, B A = 1.

3.FILM VIEWERS TEST ACCORDING UNI EN 25580

The "Italian Institute of Welding", in collaboration with Cofar S.r.l., studied from some years the characterisation of film viewer for industrial radiography referring to the main norms in operation.
In Italy and Europe in general, the main norm is UNI EN 25580 published from UNI in January 1993; this norm do not create relevant changes from analogue documents already in operation, on the contrary is quite the same as ISO 5580 "Non Destructive Testing - Industrial radiographic illuminators - Minimum requirements".
The more important technical aspects have to be found in test concerning optic service and thermal insulation of film viewer.

3.1 Density of film
As far as it concerns reading film, according to the norm the film viewer must supply at least 30 Cd/m² for reading of film with density inferior or equal to 2.5 and 10 Cd/m² of greater intensity: as a consequence, to verify this quantity, is possible to refer to a simple correlation between the variable Density and Luminance, because, in practical controls, this last one will be used as independent variable; this correlation can be indicated using the following diaphragm.

In the diagram has been indicated the curve relative to luminance value of 100 Cd/m² suggested by the norm, when it is possible to have it possible.

3.2 Light divergence and dispersion
The second parameter described in the norm is the diffusion factor s', given by a relation between luminance measurements made by a certain distance from the film viewer and with a specific angle respective to the normal in the centre of the screen, as from the following scheme.

The s' factor is calculated as (L₄₅ + L₂₀) / 2L_s, because the measurements made on a semicircumpherence have a diameter as the maximum dimension of the screen and in any case not inferior to 500 mm.
The measurements must be taken in both directions of rotation.
The minimum diffusion factor should be at least equal to 0.7.
The norm prescribed the control of this parameter to assure to eyes of operator the adequate level of light.

3.3 Luminance uniformity of the screen
It is a test made dividing the screen in square each with side of 35 mm, to allow a measurement of each square; it must be calculated the average of the four higher readings and that of the four lower readings, in order to have the value of the uniformity factor, that is
g = L_min / L_max
Being: Lmin the average of the four lower readings and L_max the average of the four higher readings.
The value must not to be inferior to 0.5.

3.4 Heating of housing
The temperature of housing must not to be superior to 60° C on usual contact surfaces after an hour of intermittent operation (50% of time for putting in the circuit with a maximum of 15 s, with ambient temperature of 20°C). The radiographs with density2 do not roll up after 1 minute of continuous examination and an intermittent working of viewer for an hour.

3.5 Results of test
Thanks to the collaboration activity between "Italian Institute of Welding" and Cofar S.r.l. it has been possible to have lots of dates concerning film viewer on Italian and European market, dates that have been controlled according to the actual European norm.

Referring to optics characteristics of film viewers, below is a resumed prospect of measurements made on about 20 instruments chosen from the most used on market, all with rectangular screen (size 100 x 240 mm or 100 x 480 mm). As you can read after, it has been decided not to analyse film viewer with circular screen (spot) or of higher dimensions (for example 300 x 400 mm) because these instruments have often characteristics very different from those above mentioned.
The chosen parameters for characterisation of instruments are the maximum reading density, the factor of screen uniformity g and the factor of diffusion s'.

Previous Viewers					New concept Viewers
Visore	Dmax	?' dx	?' sx	g	Visore	Dmax	?' dx	?'sx	g
A	3.3	1.2	0.9	0.47	a	3.92	0.95	0.96	0.57
B	3.3	1.02	1.06	0.4	b	3.61	1.01	1.00	0.51
C	3.7	1.1	1.2	0.6	c	4.63	0.74	0.76	0.81
D	3.54	1	0.93	0.37	d	3.71	0.96	0.96	0.56
E	3.44	0.95	0.94	0.39	e	3.71	0.95	0.97	0.69
F	3.35	0.92	0.89	0.76	f	3.83	0.96	0.96	0.54
G	3.5	1.1	1.2	0.59	g	3.66	0.96	1.00	0.55
H	3.4	1	1.1	0.41	h	3.74	0.97	0.96	0.52
I	3.5	0.92	0.97	0.4	i	3.93	0.96	0.98	0.60
L	3.5	0.94	1.03	0.27	l	3.60	0.96	0.98	0.58
M	3.4	0.91	0.99	0.46	m	4.73	0.78	0.80	0.84
N	3.6	0.88	0.91	0.56	n	3.92	0.97	0.97	0.59
P	3.7	0.86	0.92	0.58	p	3.52	0.99	0.99	0.65
Q	3	0.9	0.92	0.36
R	3.7	0.9	0.87	0.6
S	3.8	0.89	0.9	0.59
T	3.5	0.9	0.8	0.53
U	3.5	0.98	0.95	0.66
Media	3.48	0.96	0.97	0.49	Media	3.88	0.94	0.95	0.62

Note: Bold character for spot Viewers

3.6 Technical consideration on UNI EN 25580
From the prospect below and from the experience acquired in these years, it is clear that:

1. Values of maximum reading density are often inferior to those indicated on catalogues or commercial publications in which you can often find density higher than 4.0;
2. In general the less satisfying characteristic is the uniformity of luminance on screen surface, because more than 50% of type analyzed do not achieve the minimum value indicated in the norm (g=0.5); from this point of view there is an improvement for film viewer recently built, for which it has been provided a better distribution of light on all the surface of film viewer, while on others exemplars it was more important to have powerful luminose sources but often too much concentrated in small area, in the centre of the screen;
3. It has been verified that viewers with screen of big dimensions (for example 300 x 400 mm) find difficulties in easily satisfy what previewed, especially for characteristics of uniformity, up to suggest their removal from the above scheme; also consider the remarkable change of g factor in junction of different dimensions of measure lattice described in the following diagram. For extended surfaces it is very difficult to respect the requirement of uniformity, perhaps because it is technically more correct change the frequency of measurements in function of screen dimensions;
4. Film viewer with circular screen, even if considered by the norm as the rectangular one, have often reduced diameters (for example 70 mm), so the measurements made to control the uniformity is poorly relevant; to visualise the result of a characterisation made with a reduced pitch-scanning (equal to 5 mm) attached is the three-dimensional elaboration and for curve of level of a circular film viewer;
5. The norm, in some points, can be read in a different way: for example it refer to "special photometers or instruments suitable for the purpose" to verify diffusion and uniformity, even if the units of measure considered (Cd/m²) clearly refer to luminance;
6. Referring to the use of luminance meter, these measuring instruments are usually supplied with different angular areas, typically 3°, 1°, 20' and 6' correspondent circular reading areas with diameter of 27, 9, 3 and 1 mm. decreasing the value of angular area (making more precise measurements) it is possible to notice fluctuations of readings; this aspect of controls is absent in the norm;
7. In the norm there are no indication of quantity concerning spectral characteristics of luminous bundle from the film viewer. Only the German law DIN 54116 refers to this matter, tresholding the window of emission to the interval between 520 and 600 nm. Even if this characteristics seems to be of secondary importance, it must be considered that the measuring instruments, in this case the luminance meter, have a specific spectral sensitivity (as shown in diagram below) that advise to take measurements above all where the sensibility is higher.
8. Referring to thermal insulation of housing, it has been noticed that this aspect - practically and operatively of extreme importance - is often not considered by manufacturers, being verified maximum temperatures on surfaces also higher than 80°C;
9. The norm does not refer to the frequency to use for controls of functional characteristics of film viewers. Film viewers can change during time the functional characteristics, both for components and materials used in their building and for absence of maintenance and tidiness.

4. COMPARISON WITH OTHER NORMATIVES

In this paragraph you can find some comparisons with other actual used normative, in order to compare them with UNI EN norm.

4.1 MIL STD 453-C Norm
From a technical point of view and referring to optics characteristics of film viewer, all the norms considered accord the carrying out of luminance measurement, except MIL STD 453 - C, that only indicate the use of a illumination of minimum 2 foot-candle (light emit by the film, independently from its optics density). This difference has in practice important consequences, because the instrument that can be used for the control (Luxmeter) is much cheaper in comparison with a luminance meter.

It must be considered that Luxmeter available on market have always ground scale sufficiently dimensioned to make necessary measurements only from a certain distance from the screen; in this case, it is necessary to introduce an adjustment in measurements because of its changes with distance square from source (see point 3, rel. [8]).
Making measurements in contact (technically questionable because in this case there is any punctiform source) it is often used a film with known density applied on sensor photo-sensitive to extenuate the intensity of the emitted bundle: in this case it is necessary a correct test of optics density of film (through densitometer verified and/or calibrated, for example referring to the norm ASTM E 1079, that obliged to control with primary step-tablet).

This norm do not accord any prescription concerning the divergence and diffusion of light and its uniformity, but only suggest, an uniform level of illumination; there is also any reference for maximum temperature of housing, being requested only the presence of an adequate cooling system.

This norm, used also in aeronautical field, is only apparently easy to be applied for its conditions in using Luxmeter.

4.2 ASTM 1390-90 and ISO 5580 norms
These two norms have technically the same content. As noticed before, UNI EN 25580 norm is connected with ISO 5580.

According to ASTM E 1390-90 norm, the test of uniformity of illumination through a lattice of 2 in. (about 50 mm) must be made, instead of 35 mm; this test must also be remade for the three different levels of illumination (low, medium and high), making the control more severe and onerous.

4.3 ISO 2504 norm
This norm has some considerable differences if compared with the others; first of all it requires that the film viewer, for any value of density, a luminance transmitted from the film of at least 30 Cd/m², making more sever the controls for value of density higher than 2.5.
On the other side, there are no indication for characteristics of divergence and diffusion of emitted light (it is sufficient that the film viewer contains an thin opal glass), and any indication for uniformity of luminance on the screen (we suggest to mask the film, if there are area with remarkable change of density).
The norm do not accord any requirements for thermal insulation of instruments housing and for deflections on film after a continuous reproach on film viewer.

4.4 DIN 54116 Norm Part 1 and 2
These two norms contain elements with any correspondence with the other norms taken into consideration.
As already noticed, in the second part is clearly indicated the spectral interval of emission in which can operate the viewer (wavelength l between 520 and 600 nm), while the other norms give quality indication about colour light.
The control of uniformity of illumination on the screen has to be made using a lattice with square link of about 10 cm² surface: as a consequence, with the same film viewer it is more difficult to respect the minimum value g = 0.5 because the measurements become more exact.

DIN 54116 norms Accord also a test for acoustic emission of film viewer and require to make acoustic measurements from an altitude of 1.5 m and a distance of 0.5 m from the viewer, in three different points, in front of three visible sides of instrument, from the work place of the operator. It is required not to exceed 40 dB (A).

Table: (0)Values transmitted by film, independent from its optics density (1)Divergence factor s' = L45+L20/2*L5 (2)Uniformity factor g = Lmin / Lmax (3)Conditions controlled after an hour of intermittent working, if not different specified (4)Conditions concerning film density D=2, after a minute of continuous observation on viewer

5. CONCLUSIONS

Thanks to the experience and in consideration of low knowledge of users of this matter, is possible to express some evaluations to complete what already written in point 3.6:
• The norm refers to a physical quantity that requires the use of a specific measurement instrument (the luminance meter); in order to guarantee to measurements the sufficient degree of repeatability is also proper to calibrate this instrument periodically. It is not always attendible and technically justifiable the control of film viewer through different photo-meters, as the norm accord this possibility; it should be also necessary specifications of minimum requirements of measuring instruments used, referring to norms or standard of affirmed technical validity (for example DIN 5032 norm, part 7).
• Film viewer available on market, tested, only in few cases have the requirements for reading film of high intensity (3.5 ¸ 4); it is necessary to distrust values often declared but technically not attendible (a duplication of the luminance value, permit only an increase of reading densities only of 0.3); in any case in these last years, the controls made in "Italian Institute of Welding" on about 300 film viewer from different manufacturer has permit to underline an amelioration of qualities of the same film viewers, especially referring to diffusion and illumination;
• It is necessary that the viewer (point 4 of norm) has a marking with an apposite target complete of more relevant technical data (voltage and power supply frequency, power input, maximum luminance);
• The users should fix periodical controls, referring to at least the maximum value of luminance given, because during time the film viewer can loose its optics performances; just for this reason Cofar S.r.l. has carried out a researching activity, in collaboration with Italian Institute of Welding, for evaluation of decay of optics performances of viewers, in order to test the instruments in a correct way.

Thanks to this activity it has been possible do notice a certain decay of lamp performance during the use of viewer in one year: this decay has been noticed in about 30% of viewer, so the periodicity of test should not exceed 12 months;
• In addition to optics characteristics it is important to consider also the other parameters indicated in the norm, as the maximum temperature of housing and the absence of deformation on film whit prefixed working condition (see point 2.9)

6. INSTRUMENTS USED FOR TESTING

To make test described in this section the following instruments have been used:
1. Luminance meter LMT mod. L 1009 S/N 89504 classified A (for angular field 3° and 1° according to DIN 5132/7 with the following specifications:
1. angular field 3°, 1°, 20', 6' (and additional optics);
2. Reading area F 27mm, 9.0 mm, 3.0 mm, 1.0 mm;
3. Distance of measure: from 0.5 m to infinity;
4. Photometry sensor calibrated to SI
5. Relative errors to approximation of spectral sensibility f1 < 2.0%
6. Total error f_ges < 6.1%
2. Automatic Film Processing unit AGFA NDT M
3. Step Wedge NIST
4. X-Rite 301 densitometer
5. Luxmeter Spectroline mod. DSE 100 X S/N 307994 (control unit) and S/N 308004 (sensor)
6. Viewer Cofar S.r.l. size 10x24 cm and Spot

7. BIBLIOGRAPHY AND NORMATIVES

1. ASTM E 1390-90 "Standard guide for Illuminators Used for Viewing Industrial Radiographs"
2. DIN 5032 Teil 1 - Photometrische Verfahren
3. DIN 5032 Teil 6 - Lichtmessung - Photometer - Begriffe, Eigenschaften und deren Kennzeichnung
4. DIN 5032 Teil 7 - Lichtmessung - Klasseneinteilung von Beleuchtungsstärke und Leuchtdichtenmeb gerte
5. DIN 5032 Teil 8 - Lichtmessung - Datenblatt für Beleuchtungsstärkemeb geräte
6. DIN 54116/1 "Betrachtung von Durchstrahlunsaufnahmen - Betrachtungsbedingungen"
7. DIN 54116/2 "Betrachtung von Durchstrahlungsaufnahmen - Betrachtungsgeräte"
8. ISO 2504 "Radiographie des soudures et conditions d'observation des film - Emploi des types recommandés d'indicateurs de qualité d'image (IQI)"
9. ISO 5580 "Non Destructive testing - Industrial radiographic illuminators - Minimum requirements"
10. MIL-STD-453C "Inspection, radiographic"
11. Normative Project UNI "Controllo radiografico mediante raggi X di manufatti plastici e/o composti - Parte 1°: criteri generali"
12. UNI EN 444 "Principi generali per l'esame radiografico di materiali metallici mediante raggi X e gamma"
13. UNI EN 1435 "Controllo radiografico di giunti saldati"
14. UNI EN 473 "Certificazione e qualificazione del personale addetto al controllo non distruttivo " <
15. UNI EN 25580 "Visori per radiografie industriali - Requisiti minimi"
16. Francis Weston Sears (Faculty of Physics Institute of Technology in Massachusetts) - Ottica - Casa Editrice Ambrosiana, Milano
17. "Caratterizzazione e normativa per l'affidabilità dei Controlli Non distruttivi" - G. Costa, R. Marmigi - National Conference AIPnD (Ferrara, October 1993)
18. "Caratterizzazione dei negativoscopi per uso industriale mediante tecniche tradizionali ed innovative: primi risultati" G. Calcagno, M. Murgia, G. Orlando; S. Giammartini, R. Marmigi - National Convention AIM (Milan, October 1994)
19. "Caratterizzazione di visori per pellicole radiografiche ad uso industriale mediante sistema computerizzato di acquisizione ed elaborazione dati" M. Murgia - G. Orlando - S. Giammartini - R. Marmigi - C. Cantore - 8°National Convention AIPnD (Torino, October 1995).

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