Fluke Biomedical 07-444 - Manual (Page 11)

Functional Description
Optical Density Measurement
3
3-1
Section 3
Functional Description
This Section provides an overview of the functions performed by the Little Genius. Table 3-1 provides an
explanation of some of the terms used throughout this Instruction Manual.
3.1 Optical Density Measurement
The primary function of the Little Genius is to measure the optical density of test film that has been
exposed on a Nuclear Associates or X-Rite sensitometer. This consists of “21 step” measurements and a
base + fog measurement. The Densitometer is also capable of single-point measurements. In the single
measurement mode, the film is manually positioned with continuous display update.
3.2 Calculation of Significant Values
The Little Genius uses the readings taken during a measurement cycle to calculate/determine:
• Average Gradient
• Relative Speed
• Contrast Index
• Speed Index
• Maximum optical density
Average Gradient (AG) is defined as:
Relative Speed (RS) is defined as:
Calculating Average Gradient (AG) and Relative Speed (RS) is performed as follows:
1. Calculate (BF + AG Hi OD), (BF + AG Lo OD), and (BF + 1.0) where:
BF = Base Fog value.
AG Hi OD =
AG Lo OD =
2. Using interpolation if necessary, find the log relative exposure (LRE) corresponding to the each of
the values calculated in Step 1.
3. Determine AG and RL using the values calculated in Steps 1 and 2 in the appropriate equation.
AG =
BF + AG
HiOD
( )
− BF + AG
LoOD
( )
LRE
BF+ 2.0
( )
− LRE
BF +0. 25
( )
RL =
1
LRE
BF +10
( )