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Today film viewing light boxes with LEDs are common, which provides increasing luminances...
Is there any physical limit on the luminance, measured in cd/m² [Candela/square meter] and NOT lx [Lux]?
Yes, this is the X-ray film itself!
The maximum film temperature during film inspection should not be much higher than 40 centigrades. Otherwise,
the film emulsion will be destoyed, which is based on gelatine (cooked from bones of cows and so a non-vegitarian product). So the temperature increasement during film viewing should not be higher than 30K (K for Kelvin as temperature difference).
Experiments in the last years had shown, that light boxes with too high light output let the film "grill" itself, because all the light is absorbed nearly completely by the fine distribued silver in the film emulsions and all this energy is converted into heat as any black body radiator it does. As a result, the film gots too hot!
So optical film densities above D>5 are difficult to view, even when today viewing boxes with more than 1 mio cd/m² are commercially available. This film viewing has to be fast not to destroy the film emulsions! You can easily measure this effect with an non-contact IR thermometer, which are widely in use in these Corona times...
But this effect can be calculate too. From the definitions of optical units you find:
1 cd = 1 lm/sr = 1/2pi lm for diffuse lightboxes (lm is the Lumen and sr the steradiant, which is 2pi for a light box, which does light only in the forward direction). From the definition of lumen is follows that 1 W = 683 lm for visual radiation.
For example, the radiation power of visual light of a light box with 1 mio cd/m² luminance is calculated as following:
1 mio cd/m² = 10^6 cd/m² = 10^6/2pi lm/m² = 10^6/(683*2*3.14) W/m² = 233 W/m²
This is the light power output of the light box per square meter measured in Watt.
The film absorbs this light power nearly compeletely (an optical density of D=3 absorbs 99,9% of the light, only 0,1% is passed through). The film converts this power into heat and increases its temperaure according to its specific heat capacity. Beside two very this emulsion layers on both sides of the NDT film (each only 10 microns thick) the film has a base of blueish tint made from polyester, about 150 micron thick. From tables it could be found that the specific heat capacity c of polyester is around c = 1.1 W/(g*K) [Watt/(Gramm*Kelvin)].
The specific heat capacity c is defined as following:
c = P / (m * DeltaT) [P - power absorbed in Joule (1J=1Ws), m - Mass, DeltaT - temperature increasement by absorbing the Power P]
Therefore: DeltaT = P/(m*c)
Considering a typical NDT film with 10x24cm² = 0.024m² and a weight of 6.2g we finally calculate the increase in film temperature for this light box viewing of 1 mio cd/m²:
DeltaT = 233 W/m² * 0.024 m² * 1s / (6.2 g * 1.1 W/(g*K)) = 5.6 W *1s * 0.147 K/W = 0.82 Ks
The film will increase its temperaure by absorbing the viewing light by 0.82 K per second! After 20s film vieweing it has already a temperature of 16 K above room temperature, after 36s it has a temperature of 30K above room temperature, which is too hot for good workmanship of film viewing.
Therfore, for dark films and bright light boxes the film viewing should be rather short!
To solve this problem, several approaches could be possible:
1. use cooled films from the fridge before viewing, so the temperatrue increasement to reach the max. of 40 centigrades is higher. I never tried this.
2. Enhance your viewing sensitivity for dark film viewing. Take strychnine, but carefully, only a little bit, its poisoning, but reports are available from the beginning of scintillation techniques, that experimentators are more sensitive very small light flashes. I also did not try this.
3. Use for film inspection not your naked eye, but a modern CMOS camera (e.g. SONY Pregius with backside illumination). These cameras are much more sensitive against light then the human eye (at a integration time of several seconds, the human visual system has only about 40ms integration) and need one to two orders of lower light intensities for a similar SNR than the human visual system. This aproach I tried with some success, a report is under development.
From the above numbers it is clear, why such effects of increased film temparatures during inspection became significant only with the newer LED film viewers. The old style incandelescent bulb viewer had a light output efficiency lower by at least one order of magnitude than LEDs today, therfore, the film viewing time could be longer by 10 times for the same effect. Also, the incandelescent bulbs of several kW electrical Power generated another cooling problem for the viewer itself, not for the film!
_________________ Dr. Uwe Zscherpel Federal Institute for Materials Research and Testing (BAM) Division 8.3 Radiological Methods 12200 Berlin uwez@bam.de
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