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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004 
  
The light source should be chosen to produce light of the spec- 
tral characteristics (expressed by means of its colour tempera- 
ture) of the air-photo daylight (—5500 — 6500 K). Since the 
normally used tungsten bulb (— 2800 K) does not meet this 
requirement, an adapter filter is necessary. This filter may also 
be used to modify the light further for the transmission charac- 
teristics of the photogrammetric lens in order that the illumina- 
tion for the exposure of the step wedge resembles that reaching 
the image plane of a photogrammetric camera. The shutter of a 
sensitometer is usually a sector disk operated by a synchronous 
motor at a fairly high speed in a repeatable fashion. 
The step wedge is a series of calibrated filters with uniform 
density increments. It converts a single level of light produced 
by lamp, filter and shutter into many different levels or illumi- 
nances; this simulates the condition in photography were many 
different illuminances reach the image plane. Step wedges for 
sensitometry need to be spectrally neutral, i.e. their transmit- 
tance must not vary as a function of wavelength; such step 
wedges can not be produced photographically but are made by 
incorporating carbon particles in gelatine and copying them 
onto film base. Step wedges are produced with different density 
ranges and increments. Common in photogrammetry is the use 
of tablets covering a range of 3.0 between maximum density 
and the density of clear film (having an uncoated density in the 
order of 0.05 to 0.10); this range is than divided into increments 
of 0.15. Since densities are logarithmic values (see section 4), 
the illuminances reaching the image plane have a logarithmic 
range of 3.0 representing an illuminance ration of 1000:1, and a 
density decrease of 0.3 results in a doubling of the illuminance 
in the image plane. — The density range of 3.0 is far greater than 
that typically desirable for aerial images. 
Densitometry is the measurement of transmission or reflection 
characteristics of objects and photographic images. While sen- 
sitometry assures that an exposure will be correct, the task of 
densitometry consists of checking whether the exposure was 
correct: the exposures taken with a sensitometer will be meas- 
ured with a densitometer and evaluated. Hence, densitometry is 
the last step in a sensitometric process. 
4 
D 
  
e 
Log H = Log (E't) 
  
Figure 2. Characteristic curve for a monochrome negative 
emulsion 
The characteristic curve for photographic materials, Figure 2, 
also called sensitometric, HD- or D logH-curve, shows the re- 
lationship between the exposure of the material and the re- 
sulting density of a photographic image. It may be assumed to 
have the shape of a stretched S with the following parts: 
A: minimum density or base-plus-fog, 
B-C: toe of the curve, 
C-D: straight-line section of the curve, 
D-E: shoulder of the curve, 
E: maximum density, and 
E-F: area of solarization. 
The slope of the characteristic curve at any given point is equal 
to the slope of the tangent at this point. The slope a of a curve 
at any given point is called the gradient and it varies along the 
curve. The maximum gradient of the characteristic curve is 
called "gamma" and corresponds with the slope of the straight- 
line portion, hence: y 7 tana. 
When an unexposed photographic emulsion is processed, there 
will always be a certain amount of silver formed, i.e. the film 
gets slightly darker. In addition, even the clear film used as 
support is not completely clear. The sum of both effects should 
not increase beyond D = 0.2 if the photographic material has 
been handled correctly. 
The entire toe of the curve is often considered to be the area of 
underexposure, however, there are indications that this is not 
valid for all the range between B and C. For example, the point 
of minimum useful density may be defined by placing a tangent 
with a gradient of 0.2 at the characteristic curve in the toe 
section or by defining its gradient as a fraction of y, e.g. 30%. 
The straight-line section (C-D) is of particular significance 
since y is used e.g. in datasheets to show the effects of changes 
in development conditions in the form of a y/time curve. It is 
sometimes also referred to as the contrast area because of the 
constant ratio between logH-difference and density difference, 
and y as contrast factor. 
The shoulder of the curve is often considered to be the area of 
overexposure, however, there are indications that this is not 
valid for all of the range between D and E; the point of maxi- 
mum useful density could be found using a tangent parallel to 
the for defining that of the minimum useful density. 
Increasing exposure increases density until a maximum density 
is reached. Extreme overexposure can result in solarization, a 
reversal of gradation in heavily exposed areas. The degree of 
solarization depends on the particular emulsion and on the de- 
veloper used. 
Color films have three separate emulsion parts (B-G-R- or 
G-R-IR-sensitive). Each of these needs to be described with a 
separate characteristic curve; however, the curves need to be 
practically identical for films where the original images are 
used for evaluation (usually reversal films) and at least parallel 
for films where copies are made under the utilization of filters. 
Non-parallel curves will result in unwanted color changes with 
density changes; such changes occur with color-infrared emul- 
sions. 
3. DIGITAL SENSORS FOR PHOTOGRAMMETRY 
Digital sensors used in photogrammetric applications today are 
CCDs (Charge-Coupled Devices) made in a dedicated semi- 
conductor process. Semiconductors can conduct electrical cur- 
rent under certain circumstances. 
Solid-state imaging is based on the physical principle of con- 
verting light (photons) into a measurable quantity (electrical 
voltage, electrical current, density for photographic emulsions). 
The link between the photons at the input of an imager and the 
output of the device (voltages, metallic silver or colour dyes) 
are the electrons. The collection and transport of those electrons 
is of great importance in this chain. It is almost impossible to 
transport a single electron to an output stage and to convert it 
into a measurable quantity because its energy content is much