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MICROFILM DENSITOMETRY

Densitometry is the key quality control tool in Micrographics. Properly used, it provides the feedback for proper exposure and consistent development. Without densitometry few of us could maintain product quality both film vendor and user alike. The purpose of this paper is to de-mystify densitometry and raise the readers awareness to the importance of densitometric control.

For most photographic materials there are direct relationships between exposure and density (darkness). For films which reverse sign (negative working) greater exposure produces more density, less exposure equals less density. Furthermore, the development process will effect the maximum density (D-max) and the minimum density (D-min) plus the contrast of the film. The measurement of film density will provide feedback on proper exposure and the consistency of development.


WHAT IS DENSITY?

When a casual observer picks up a piece of film they would note that the dark areas hold back the light while the clear areas transmit the light. The dark areas appear dark because they are opaque to the viewing source. Often we will use the word "opaque" or more frequently, "opacity" to describe dark areas on a film. Imagine that our casual observer wishes to devise a measurement system that will provide more objective data concerning his film sample. Presently the sample can only be described in terms of light, lighter, dark or darker. One obvious method of measuring the film’s opacity would be to illuminate the film with a source of a known intensity (100 foot candles) and then measure how much light passed through the film. The dark areas, high in opacity, would block most of the light while the light areas would transmit most of the light. This system could be described in terms of percent; 100 foot candles strike the film (100%) while only a certain percentage will pass through depending on the films darkness--only 10% of the light is transmitted for example. In this example, if 10% is transmitted it means the film has blocked 90% or has a 90% opacity. With this system, if I knew either the percent transmission or the percent opacity I would automatically know the other value because the sum of the two numbers will always have to equal 100% thus a 70% opacity would equal a 30% transmission.

Conceptually, all it would take is a rather simple device, merely comparing how much light strikes a film sample with how much light is transmitted. In practice, densitometers are a bit more complicated, as we shall see.

A densitometer consists of two key ingredients--a light source and a receptor. If the light source illuminates the film with a known amount of energy, all the instrument must do is compare what went in with what came out. Obviously, this is a gross simplification.

We discussed the concepts of transmission and opacity. For example, a film that has an opacity of 50% would have a transmission of 50%. This could be written:

incident light
opacity = transmitted light

Opacity/transmission is expressed in terms of percent. Density is the logarithmic measure of opacity. You may wish to brush up on your high school logarithm class at this point. Using logs and our 50% example it would be written as follows:

D = Log 100

Thus, density equals Log 2, the logarithm for 2 equals’ 0.30. Practically speaking, this means that a film with an opacity of 50% has a density of .30.


Most who use densitometers have little idea of what the readings relate to. They know that higher numbers indicate more density but few would appreciate that a density reading of .30 equals 50% transmission. Furthermore, every additional .30 is a further reduction of transmission by one half.

% Transmissions

Density

100

0.0

50

0.30

25

0.60

12.5

0.90

6.25

1.20

3.12

1.50

1.56

1.80

.78

2.10

 

Once you understand the premise behind density, a number of common practices make much more sense, for example: in source document microfilming we usually aim for a document background density of somewhere between .90 and 1.10, or around 90% opacity. (The best reference for which density to use for which application can be found in ANSI/AIIM MS-23). Little would be gained by much higher aim-points. COM films typically do have higher density aim-points to increase visual contrast. A case of diminishing returns is reached by exceeding a density of 2.0 (99% opacity) because the eye, with normal viewing sources, cannot distinguish differences higher than 2.00. A few important reminders on density: every .30 is, in fact, a doubling of usual density. This can have real significance on minimum densities and the duplication process. For example, a minimum density of .15, versus a more desirable .05, would require 30% more exposure during the duplication process. In other words, the seemingly insignificant .10 point density difference could cause your duplicator to run 30% slower. Some silver duplicating films have very high minimum printing densities that can send reprint energy requirements soaring.

 

FILTERS:


We have discussed at some length the mechanics of light collection, but there is another aspect of densitometry that is equally important--the quality of light. Most densitometry is designed to relate to the human response. The human eye, though sensitive to all the colors in the visual system, is most sensitive to yellow/green. All densitometers contain a filter, the wratten 106, which tunes the densitometer to see as the eye does. This filter is frequently referred to a "the visual filter:' This filter is used for all visual readings on silver, diazo and vesicular films

USE GUIDELINES:

1. Follow the recommendations of the manufacturer for zeroing the instrument.

2. When making your density readings, it is good practice to make multiple readings and average the results.

3. Calibration should be checked once per day or when ever the densitometer is turned on.

4. Calibration strips and filters should be replaced at least once a year, due to fading and scratching.