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Film Viewer Intensity in Accordance with AS2177

AS2177- 2006 requires that the output of a film viewer be such that brightness of an illuminated radiograph shall not be less then 30 cd/m² and shall preferably be greater than 100 cd/m².

Since the density of a film directly effects the transmission of light it is important to understand the relationship between Film Density and Viewer Brightness. Refer to Table 1.

To calculate density the formula is:

Density = Log¹⁰ (Intensity of Incident Light / Intensity of Transmitted Light)

Example 1:     What would be the maximum film density viewable, with a viewer with an output of 48000 cd/m².

         Density = Log¹⁰ (48000/30)               

         Maximum Density Viewable = 3.2

Example 2:     What would be the recommended maximum film density viewable, with a viewer with an output of 48000 cd/m².

          Density = Log¹⁰ (48000/100)            

          Recommended Maximum Density Viewable = 2.68

Film Density	MinimumOutput cd/m2	RecommendedOutput cd/m2
2.0	3000	10000
2.5	9487	31623
3.0	30000	100000
3.5	94868	316228
4.0	300000	1000000

Table 1 – Density Vs Viewer Output

Advertised Density Capability

As most viewers are manufactured overseas, they are commonly supplied with specification sheets with nominated density capabilities that refer to other international standards. The requirements vary from standard to standard, it is therefore important to determine the correct output based on cd/m².

For example a viewer rated to D3.6 to EN25580, has a maximum density of D3.1 to AS2177.

Measurement of Output

Confirmation of output is required annually and can be performed in a variety of ways including:

a)         Direct reading from calibrated luminous intensity meter.

b)         Conversion from illuminance (lux) to luminous intensity (cd/m²)