Fluke Biomedical 07-444 - Manual (Page 12)

Nuclear Associates 07-444
Operators Manual
3-2
The Contrast Index is defined as the density difference between the low contrast reference step and high
contrast reference step, a second point near the top of the straight-line portion of the curve. For most film
screen combinations, the High contrast reference point is step number 13 or 14, and the Low contrast
reference point is step 8.
The Speed Index density step is that step which produces a density closest to 1.0 above base + fog. For
most film screen combinations, the Speed Index is step number 10 or 11 and represents a density of
approximately 1.20. The same step is used for all measurements using the same film type.
The speed reference step and contrast reference steps for each file in the data log is set from the Setup
Program (refer to Section 4 - Setup).
3.3 Displaying Data
When operating the Little Genius in the stand-alone mode (i.e., Control Charts on), an automatic
comparison is made between the control chart items and the control chart limits defined in the chart file.
A value that exceeds its limit is flagged with either an "H" if the value is high or an "L" if the value is low.
When this condition is detected Chart Rng Error is displayed. See the Control Chart Setup on page 4-9
for further details.
On completion of a measurement cycle, the following values are available for display using the ∆ and ∆
front panel keys:
1. The Density Value at each of the 21 steps, displayed as:
Step XX X.XX ∆ .
2. The base + fog value, displayed as:
Base + Fog X.XX ∆ .
Table 3-1. Explanation of Terms
Term Explanation
Densitometer Instrument which measures optical density of exposed X-Ray film.
Optical Density Diffuse transmission density; measure of light blocked by exposed film.
Base + Fog (BF) Inherent density of a processed film plus additional density due to age or
environmental conditions.
Speed Index Density step with optical density closest to 1.00 + BF; generally Step 10 or 11.
Contrast Index Density difference between contrast reference point (generally Step 13 or 14) and
speed index.
Sensitometer Instrument used with a step wedge to precisely expose X-Ray film.
Step Wedge Step tablet; wedge which provides varying optical densities, each marked with a
step number; the density of each step increases with increasing step numbers.
The Nuclear Associates step tablet provides 21 steps with a step marker at the
mid-point of each step; Step No. relates to the log relative exposure as:
Log Relative Exposure =
Step No. X 0.15
RS Relative Speed.
LRE Log Relative Exposure; related to Step No. for Nuclear Associates step wedge
as:
Log Relative Exposure =
Step No. X 0.15
AG Average Gradient.
D max. Maximum Optical Density; largest optical density reading.