International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
  
Digital images produced with one of both methods have two 
important common features: 
* . Discrete spatial information. The spatial information 
is sampled and stored in a fixed pattern array, 
determined by pixel size and pixel number. 
Compared with film, it is a strongly periodical 
equidistant array. The array is produced by the sensor 
of the digital camera or the sensor and mechanical 
system of the scanner. 
* All grey values are digitised. On the sensors 
themselves the images are available as analogue 
values, cven on CCD or CMOS sensors. A/D 
conversion always is made after image capture. The 
only difference between systems is the point, when 
this conversion it is done and where. 
Images can be digitised, after they have been stored on film, or 
after they have been stored in a CCD or a CMOS array. 
[mage quality can be defined by the determination of the 
system component’s quality. The quality definition has to be 
independent from image information. 
The question cannot be “What is a good image" but *What is a 
system, that produces images with minimum difference to the 
physical objects" 
2. IMAGE QUALITY, INFLUENCES AND 
DETERMINATION 
Within the following text the process, starting with light 
reflection on the ground to the digital information storage will 
be analysed. Most of these steps can be controlled and 
optimised by the user, in order to get optimum imaging quality. 
Each step will be explained with all influences and minimum 
requirements for used system will be announced. For most of 
these steps above mentioned DIN standards define objective 
criteria and limits. 
We do not want to publish these definitions and limits here, but 
want to enhance the understanding for their importance. 
In all cases we have to look for geometrical effects, which 
change dimensions of an object and radiometric effects, which 
influence the grey values. 
2.1 Procedures of film and scanner based imaging 
When a film based camera and a scanner are used, the steps of 
information transfer from an object to its image are: 
* [Illumination with sun light 
* Light reflection and diffraction at the object 
* Information through the 
atmosphere 
e Optical transformation by means of a lens system 
* Optical filtering 
* Storage on film, including 
O exposure control 
o mechanical treatment of the film 
transfer using light, 
e Chemical development process 
e Scanning 
Illumination within the scanner 
© Optical transformation with scanner lens 
system 
o Spatial sampling with photo diodes or CCD 
line elements 
o Mechanical transportation of the film 
o A/D conversion for digital storage 
e Data compression 
(e 
Q 
2.2 Procedures of digital cameras 
Digital cameras and scanners use more or less the same 
processes for grabbing images into a digital storage device. In 
case of digital cameras the storage media are transported with 
the camera and connected for online storage during flight 
mission. 
So all steps for image storage on film are not needed. Now 
there is no influence from mechanical film transportation, 
chemical development process and optical influences from the 
scanner illumination an lens onto. quality of the end product . 
On the other hand available CCD sensors are smaller than film, 
so lines sensors or smaller array sensors have to be combined 
to form a large sized image. These cameras use one ore more 
mechanical shutters for exposure control. This combination of 
smaller images gives an additional step in image processing and 
may influence the quality of the image. 
The process steps for “digital cameras” are: 
e Illumination with sun light 
* Light reflection and diffraction at the object 
e Information transfer using light, 
atmosphere 
e Optical transformation by means of a lens system 
e Optical filtering 
e Spatial sampling with photo diodes or CCD elements 
e A/D conversion for digital storage 
* Combination of camera heads, built with two 
dimensional arrays or lines. 
e Data compression 
through the 
2.3 Geometrical and radiometric quality criteria 
2.3.1 Natural influences 
The first three steps, "Illumination with sun light", "Light 
reflection and diffraction at the object", "Information transfer 
using light, through the atmosphere" are the physical 
circumstances which are given by nature. 
They depend on the weather and on the time during the day, 
and also on the season. 
Here the size of the shadows, the existence of clouds and the 
turbulence in the air influence the geometry and grey value 
distribution within the images. 
All these parameters are defined in DIN 18740, part 1. Herein 
the preconditions for an aerial flight are defined. 
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