Principles Of Colour Sensitometry Notes

Notes from “Principles of Colour Sensitometry”

1. Introduction

The measurement of film must result in the quantitive knowledge of:

• the characteristics of materials in use
• the effects of laboratory handling
• the nature of the photographic images formed
• knowledge sufficiently accurate and comprehensive ensure efficient production of best results

Photographic sensitometry is the science that specifies most of these measurements and the ways of making them.

The principal concern of photographic sensitometry is the determination of a quantitive relationship between photographic exposures and the images they produce. However, it also deals with the entire process from subject to observer.

• It examines the kind and amount of radiant energy the film should and does receive.
• It tests and controls the sequence of chemical treatments that form in exposed film.
• It measures the processed images determining the character and contents in terms of what is most useful for the application.
• Provides means for observers to relate their quality judgements to the physical characteristics of photographic material.

Colour Sensitometry is used in the motion picture film industry in two broad fields:

1. Evaluation of techniques and materials
2. The control of the above.

Field one provides straightforward objective descriptions of colour film images and indirectly describes the characteristics of film and processing operations or printing systems. It also provides a common language to describe the quality and performance of those systems. It can be used to compare photographic film with:

• An ideal
• A practical aim point
• A tolerance limit
• Alternative products (other film stocks etc.)

There are many procedures but the common one is as follows:

1. Exposure imposed on a sample film
2. Sample is processed
3. Resulting image is processed by some densitometry technique
4. The data is plotted and interpreted

2. Fundamentals of Sensitometry

Photographic Sensitometry:

The science of measuring the sensitivity of photographic materials.

Original paper written by Ferdinand Hurter and Vero Driffield titled Photochemical Investigations and a New Method of Determination of the Sensitivities of Photographic Plates asked the following questions:

1. What is the relationship between exposure and opacity?
2. What is “correct” exposure?
3. How is emulsion speed expressed?
4. What is the effect of development?
5. How is the degree of development measured?
6. What is “correct” development?

Concept of Density

Several methods can be used to measure silver deposits of processed sensitometric strips it is logical to use one based on light transmission. The value should go up as deposits become darker/ light transmission reduces. It should also show similar differences as one perceives visually. Measuring density (the log of reciprocal light transmittance) does all this.

Imagine sheet of material that allowed one tenth of light incident upon it to be transmitted. If we stacked two layers it would only allow one hundredth of the incident light to be transmitted and so on. Likewise, it would take ten and one hundred units of light respectively to transmit exactly one unit of light. This can be shown in the table below:

Number of layers	Output from Unity Input	Input for Unity Output	Log of column three
1	1/10	10	1
2	1/100	100	2
3	1/1000	1000	3

Hurter and Driffield stated column one consisted of numbers proportional to the quantity of developed silver (per unit are) in the silver deposit. Column two is the Transmission (T) of the silver deposit (can be expressed as a decimal or percentage too). The third column is the reciprocal of transmission and is called Opacity (O). The fourth column is defined by Hurter and Driffield as Density (D) and is the log of opacity. We use the reciprocal of transmission so that density becomes larger as deposits become darker.

\[\text{Factional Output} = \text{Transmission} = T\]

\[\text{Reciprocal of Fractional Output} = \frac{1}{T} = \text{Opacity} = O\]

\[\text{Log of Opacity} = \text{Density} = D\]

\[D = log(\frac{1}{T})\]

3. Sensitometric Exposures

Guiding principle

Sensitometric exposures should duplicate as closely as possible the conditions of actual use

The machine provides exposures in a systematic and reproducible way. It creates a step chart from clear base to black patches and must be big enough to easily and accurately measure silver/dye deposits. This machine is called a sensitometer.

Sensitometers

Three main parts to a sensitometer:

1. Light source of constant intensity and specific SPD
2. A shutter for exact timing
3. An exposure modulator to put different exposures in different places.

Generally helpful for exposures to be a constant factor times the previous exposure. Usually $ \sqrt{2}$ or $\log(\sqrt{2}) \approx 0.15$.

Exposure is illuminance times time:

\[E = It\]

Types of Sensitometry

Two main classes:

1. Time scale. Constant intensity with varying exposure time
2. Intensity scale. Exposure time is constant, intensity is varied.

Time scale exposures can suffer from reciprocity law failure and given the three emulsions of colour film are not identical they may have different reciprocity law characteristics. The colour shifts this can cause mean time scale machines are no good for colour sensitometry.

Step Wedges

Ideal light source for intensity scale machines would not change its SPD as it was dimmed. Moving the light would be ideal but is not practical, even optically. Therefore, step wedges are most practical. Step wedges should have a density of $0.15$ and cover a range of $3.0$. Must also be neutral density and within $0.15\%$ of each other. To avoid reciprocity law issues use an exposure time as close to the expected use case as possible.