·Table of Contents
Characterisation of Industrial Film Viewers According to the UNI EN 25580 StandardGiovanni Calcagno, Michele Murgia, Giuseppe Orlando
Italian Institute of Welding - Genova
Cofar S.r.l. - Arcore (MI)
Luminance Intensity: provided a luminous source S, its luminance intensity in the direction of ray's propagation direction is the relation  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 
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 = dIn * cosq 
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|
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  between the luminous flux and the considered area.
E = dF/dA 
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  between lux and foot-candle is:
1 lux » 10.76 foot-candle 
If the luminous source is punctiform, the illumination  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 
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 Dlq 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 :
Bq = Dlq / DA * cosq 
For perfectly diffused surface, , the luminance do not change  at changes of direction
Bq = Dlq / DA 
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 , 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:
dlq = B * dA * cosq
If the B luminance do not change with q direction.
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  is reduced to the universal law of the inverse of the square of the distance, because, as per definition, B A = 1.
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) / 2Ls, 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 = Lmin / Lmax
Being: Lmin the average of the four lower readings and Lmax 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|
3.6 Technical consideration on UNI EN 25580
From the prospect below and from the experience acquired in these years, it is clear that:
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. ).
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).
(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
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