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About Brightness or Luminance   

Film viewers / illuminators

Luminance is brightness of light source -. If you are looking at a particular density of X-Ray film you need a given level of light to meet the code requirements. Here are the units that luminance is measure.

  SI - Metric Imperial (US)
Luminance Candela Per Square Meter (cd/m2) Foot Lambert (fl)

Other visual inspection work including Ultraviolet

Illumination is ambient light level like you would measure in a room or work place. If you are doing visual inspection (or under UV light) you would need a certain level of light. Here are the units that illumination is measure.

  SI - Metric Imperial (US)
Illumination lux (lx) foot-candle (fc)

In both cases - inadequate light levels can mean defects are missed.

Codes / standards for film viewers / illuminators -

There are several codes / standards which apply to film viewers / illuminators but these are the most commonly quoted EN25580 1992, ASTM E 1390-90 (2000) and ISO 5580 1995. These are for the manufacture and show the limits of film densities that the viewer can be used for. For example if the brightness of the film viewer is 25,000cd/m2 then it can only be used for film density 2 or less.. A film viewer does not have to achieve 4 density to meet code requirements - it may be designed for only lower density films.

There are two US codes i.e.

ASTM E1390 - 90(2000) Standard Guide for Illuminators Used for Viewing Industrial Radiographs it starts...

This guide provides the recommended minimum requirements for illuminators used for viewing industrial film radiographs using transmitted light. 1.2 The illuminator has to ensure the same safety for personnel, or users of any electric apparatus….

ASTM E1390 - 06 Standard Specification for Illuminators Used for Viewing Industrial Radiographs it starts...

Abstract This specification provides the recommended minimum requirements for industrial radiographic illuminators used for viewing industrial radiographic films using transmitted light sources. The function of the illuminator is to provide sufficient illumination and viewing capabilities for the purpose of identification and interpretation of radiographic images...

Both codes mention luminance and candela - even though they are US codes...

Interestingly

Film viewer manufactures talk about luminance or brightness of their film viewers but it is the transmitted light ( that transmitted from the viewer through the film to operator) that is most important. Both codes give these requirements. Quoted....

The luminance of the transmitted light shall not be less than 30 candelas per square meter for film densities equal to or less than 2.5 optical density and not less than 10 candelas per square meter for film densities greater than 2.5 optical density.



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