TRANSMITTANCE OR DENSITY - A QUESTION OF BITS?
H. Ziemann, D. Grohmann
Anhalt University of Applied Sciences, POB 2215, 06818 Dessau, Germany
(hziemann, grohmann)@vw.hs-anhalt.de
Commission I: PS WG I/2 *Sensor Calibration and Testing”
KEY WORDS: Digitization, Radiometry, Film, Image, Scanner
ABSTRACT:
The question of radiometric resolution is an important one in connection with the introduction of digital systems. The number of
available bits is generally given with the note that the achieved radiometric resolution is much superior to that of photographic
emulsions. In this comparison it is usually overlooked that film records densities, i.e. logarithmic values, while CCDs record trans-
mittance or reflectance, i.e. linear values. To be able to derive from transmittance or reflectance densities reaching from 0 to 3 with
an 8 bit resolution —13 bits of resolution are needed for transmittances or reflectances. The paper presents results for a black-and-
white digital image recorded using 12 bits which was recalculated from the recorded reflectances to densities assuming different
exposures and film emulsion properties defined by the Standards characteristic curve for black-and-white negative silver-based film.
It is shown that film, when properly exposed and processed, is able to reproduce the information present in a digital image recorded
with 12 bit radiometric resolution.
KURZFASSUNG:
Die Frage der radiometrischen Auflósung ist im Zusammenhang mit der Einführung digitaler Systeme wichtig. Die Angabe der An-
zahl der zur Aufzeichnung zur Verfügung stehenden Bits wird im allgemeinen mit dem Hinweis verbunden, dass die radiometrische
Auflósung digitaler Systeme (unter Verwendung von CCDs) wesentlich besser ist als die von photographischem Film. Dabei wird in
der Regel übersehen, dass Film (logarithmischen) Dichtewerte aufzeichnet, und dass zur Aufzeichnung von (linearen) Transmittanz-
oder Reflektanzwerten mit gleicher Auflósung in bereichen hoher Dichte bis zu ~ 13 Bits benótigt werden. Es werden Ergebnisse für
ein digitales, mit 12 Bit aufgezeichnetes Bild berichtet. Dieses wurde zunächst aus linearen in logarithmische Objektumfangswerte
umgerechnet, die dann unter Annahme unterschiedlicher Belichtung und einer Normschwürzungskurve in Dichtewerte für eine
photographische Aufnahme umgerechnet wurden. Es zeigt sich, dass bei richtiger Belichtung alle im digitalen Bild vorhandenen
Information auch im photographischen Bild vorhanden ist.
1. INTRODUCTION Figure 1. The sensitivity of a photographic material depends on
the emulsion type and the development conditions.
This paper summarizes a number of sensitometric concepts and
the review the relationship between density and transmittance.
It discusses density considerations valid for analogue photo-
grammetric images and leads from there to characteristics of
digitized and digital images. An image taken with a digital
aerial camera and recorded using 12 bits has been transformed
from linear values obtained as output from a CCD (representing
transmittances in the case of scanning an image and reflec-
tances in the case of taking an image with a digital camera) to
logarithmic (density) values and investigated in regard of its
detail in shadow areas.
Light Source
Shutter Step Tablet
Figure 1. Schematic presentation of a sensitometer
2. SENSITOMETRY, DENSITOMETRY AND
CHARACTERISTIC CURVE A sensitometer consists of the three major parts shown in Fig-
ure 1: (1) a suitable light source calibrated in produce a precise
illumination at a standard colour temperature, (2) a calibrated
shutter and (3) a transparent step wedge covering a density
range from 0 to 3 in equal density increments. The sensitometer
should be used with settings simulating actual aerial photo-
graphic conditions.
Sensitometry is the science of measuring the sensitivity of
photographic materials; it is used to control the operations of
exposing and processing photographic materials. The sensitivity
or photographic speed refers to the response of a photographic
material to incident light. In practice, it is the amount of light
exposure required to get a certain density in the processed
photographic material as determined with a sensitometer,
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