Factors Controlling the Image Quality of Aerial Photographs, the Lens System, the Film Type, the Development and Printing Process (Especially for GATE-Geospatial 2022)

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The aerial photograph is the product of a precision surveying instrument - the aerial camera. As the exposure has to be made whilst the aeroplane is in motion, the aerial camera requires a fast lens, a quick, efficient and dependable shutter and a high-speed emulsion for the film.

  • Hence, the image quality depends on (1) the lens of the camera system, (2) the film type used and (3) the development and printing processes.
Image Quality of Aerial Photographs

The Lens System

  • The function of the lens in to gather a selected bundle (A bundle of light rays is a collection of rays spreading out in three dimensions (i.e.. in different planes) like the ribs of an umbrella, and should be differentiated from a pencil of rays which is a number of rays radiating from one point in two dimensions, i.e.. in one plane like the spokes of a wheel) of light rays for each or an infinite number of points on the terrain and to bring each bundle to focus as a point on the focal plane.
  • In the aerial camera, the focus is always set for an object distance of infinity (i.e. a fixed-focus camera) . It should be noted; however, that the actual lens system is rather complex and that gives only a simplified representation.
The Lens System

The lens system, in fact, contains two nodal points on the optical axis: the front nodal point (N1) on the object-space side and the rear nodal point (N2) on the image-space side.

Contains Two Nodal Points on the Optical Axis

Some actual examples of commonly used lens systems are shown in Figure below.

Sections of Some Commonly Used Zens Systems

Sections of some commonly used Zens systems: (a) Bausch and Lomb Metrogon, f/6.3; (b) Zeiss Pleogon, f/5.6; (c) WiZd Aviogon, f/5.6; (d) WiZd Super-Aviogon, f/5.6; and (e) Wild Aviator, f/4.

  • The amount of light which is admitted through the lens is controlled by the diaphragm and shutter. The diaphragm is a physical opening the size of which is adjustable by rotating a series of leaves. When the diaphragm is wide open, the lens resolution is greatest. The shutter, which is located between the lens elements and close to the plane of the diaphragm, controls the interval of time during which light is allowed to pass through the lens (i.e. exposure time) . The intra-lens shutter is usually preferred for the aerial camera because it admits light to all parts of the negative instantly upon opening and cuts off light from all parts of the negative instantly at the end of the pre-set exposure time interval. Thus, it gives rise to a central projection which helps to preserve in the negative the precise relationship of all object points photographed.
  • The lens design involves lengthy computations, after which the lens has to be ground to the exact specifications and then the various elements assembled together. Human errors inevitably occur in the manufacture of the lens, for example, in grinding and polishing the surfaces of the optical glass and in maintaining a high consistency in its refractive index. For a wide-angle camera with 152mm focal length, the effects caused by differences in the refractive index could result in a distortion of 3 - 4pm in the image plane.
  • Also, in assembling the lens elements together, the accuracy of the mechanical alignment is influenced by the accuracy of machining the necessary metal parts and by the fact that the mechanical assembly must leave some space to allow for differential expansion of glass and metal. All these errors cause residual distortions which result in the lens not being ′ able to bring an image to focus on its theoretically correct position in the focal plane.
  • Most of these distortions occur radially from the centre of the focal plane (radial distortions) , but there are also distortions which occur in a direction perpendicular to the radial line (the tangential radiation) , apparently caused by faulty centring of the lens assembly.
  • Apart from the lens assembly which is a source of distortions in the resultant image, the film flattening device and the film transport inside the magazine can give rise to film deformations. It is essential that the film is kept flat on the focal plane during exposure. This may be achieved either by a pneumatic method using over-pressure by pumping air out from the back of the film to create a vacuum or by mechanical flattening by means of pressure being applied on the back of the film over a glass plate on the focal plane.

Film Flattening

  • But the film flattening device is never perfect. In the pneumatic flattening method, for example, the contact between the back surface of the film and the reference plate may be prevented or disturbed by improper functioning of vacuum pumps or by particles trapped between the two surfaces.
  • When this is the case, image shift occurs, i.e.. a point to be imaged in A is actually imaged at A ′ and, if parallel projection is used in the mensuration process, appears to be located at A ″ , a map of the displacement vectors over the photograph reveals that this occurs radially, and large localised errors will show up clearly at those parts where improper flattening occurs.
  • As for the film transport in the magazine, an appreciable force is continually applied to pull the film across the focal plane, thus leading to an elastic extension of the film prior to the exposure and to contraction afterwards.
Deformation Pattern of an Aerial Photograph
Deformation Pattern of an Aerial Photograph

Deformation pattern of an aerial photograph including two large local errors resulting from the improper flattening of the film in the aerial camera (after Ziemann, 1972

Because of the presence of these sources of distortion, the aerial camera has to be calibrated before being used to obtain aerial photographs for photogrammetric purposes. This calibration makes use of elaborate instruments under controlled laboratory conditions to determine the principal point or the collimation centre on the focal plane, the focal length and the projection centre of the lens system. Thus one establishes the mathematical relationship between points on the image surface of a real camera and those of an ideal camera, so that appropriate correction can be made to their position.

The Film Type

  • The film type used also plays an important part in determining the quality of the resultant image. The photographic emulsion is silver halide salt suspended in gelatin and is coated on some suitable supporting base material. When light, a form of radiant energy of specific wavelengths in the electromagnetic spectrum, falls on this, a chemical reaction occurs this converts the emulsion into a latent image.
  • This latent image is composed of a small aggregation of silver atoms formed on the surface or the interior of the grains. They are the ‘sensitivity specks’ acting as development centres. In developing, some chemical reducing agents are used so that the small black sensitivity specks of silver are precipitated. Since unexposed areas are not reduced to silver at all, the reaction is totally proportional to the amount of light falling on the emulsion.
  • The result of the development is a negative image, from which a positive point can be obtained. The variations in the intensities of the light on the sensitised materials give rise to different degrees of darkness, the tones of which can be measured and expressed by a number called the density. The density is defined in terms of opacity which measures the proportion of light passing through the film. Thus, density can be expressed mathematically as
The Film Type
  • Where D is the density, 0 is the opacity, I0 is the incident light and it is the transmitted light. The higher the density, the darker the film.
  • Closely related to density is the concept of contrast which measures the actual difference in density between the highlights and low-lights or shadows on a negative or photographic print. This can be studied together with the speed of the photographic material by means of the characteristic curve, which portrays graphically the relationship between density and the time of exposure, (measured on a logarithmic scale) . The result is an elongated S-shaped curve which shows three distinct Portions as (a) the toe, (b) the slope or Gamma (Y) and (c) the shoulder.
  • The slope portion which is more or less a straight line can be extended to meet the abscissa to form an angle . This is generally known as Garina (y) which is, in fact, the tangent of angle θ. The steeper the slope (i.e.. the higher the Gamma) , the greater the contrast. Thus a fast film (with hard gradation, short exposure time and large density range) has a bigger Gamma than a slow film (with soft gradation, long exposure time and small density range) .
The Slope Portion Which is More or Less a Straight Line
  • These observations lead to other considerations of the properties of the sensitised materials used in aerial photography such as the speed, spectral sensitivity and the resolving power. The speed of the emulsion may be thought of as a measure of its sensitivity, and as yet there are no satisfactory means of measuring the speed of the films used for aerial photography, since the ASA (American Standards Association) or DIN (German Institute) methods of measuring film speed do not relate to the special conditions under which the film is used. The Eastman Kodak Company has developed an Aerial Exposure Index in which the speed point is based on the point on the characteristic curve where the slope is 0.6 of the value of Gamma.
  • The emulsion used is not sensitive to all parts of the visible light spectrum, and its response to light of different wavelengths depends on its spectral sensitivity. All silver halide photographic materials have inherent sensitivity only to the short-wavelength blue light. For lights of longer wavelengths (such as green, yellow and red) , sensitising dyes must be added before they can be sensed.

Monochrome Films

  • Thus, in the class of monochrome films, there are three types of sensitising: (a) orthochromatic, (b) panchromatic and (c) infra-red. The orthochromatic will respond to blue and green lights (wavelengths from about 400 to 600nm) ; the panchromatic will respond to blue, green and red lights (from below 400 to 700nm) ; the infra-red will respond to all lights in the visible spectrum as well as the invisible near infra-red (from below 400 to about 900nm) .

Panchromatic Films

  • Each of these types of film is useful in aerial photography, but for more general purposes the panchromatic film is preferred. There is still another class of film - the colour film -which can depict the natural colours of the landscape. It is a more complex type of film than the monochrome, being made up of three layers of emulsion sensitised separately to blue, green and red lights. Its use in aerial photography is relatively recent and still requires further investigation. Obviously, the types of film used can significantly affect the quality of the resulting image.
To Evaluate the Quality of the Image

Resolving Power of Lens

  • To evaluate the quality of the image it is usually necessary to consider the resolving power of the lens as well as the film. The resolving power of a film is a subjective measure of image ‘sharpness’ , expressed as the number of line pairs per millimetre which can be distinguished in the image. This requires the use of a test pattern which usually consists of three thick parallel lines on a contrasting background. The width of the lines is equal to the space between them.
  • A basic pattern of vertical lines is placed next to a pattern of horizontal lines, and the combination is repeated in a range of sizes over the target area, as shown by the US Geological Survey example which has been employed for camera calibration. For aerial photography, the low-contrast resolution is more significant and a ratio of line-to-space luminance of 1.6 to 1 is used.
  • But this subjective measure is not entirely satisfactory as it is dependent on the contrast between the lines and the background and does not therefore reliably indicate visual image sharpness. Thus, an alternative method for evaluating image quality - the Modulation Transfer Function (MTF) - is now preferred. The MTF or sine-wave response is a curve indicating the degree to which image contrast is reduced as spatial frequency is increased.
A Resolving-Power Test Pattern for Camera Calibration

A resolving-power test pattern for camera calibration employed by the United States Geological Survey. Numerals indicate the number of Zine pairs per millimetre

MTF Concept

  • The MTF concept makes use of the fact that the brightness variation across a line chart results in a series of square waves. From the Fourier Theorem, these square waves can be synthesised from a series of sine waves and they correspond to sine waves of a fundamental frequency, together with their associated harmonics.
  • It should be noted that the photographic image of a single narrow band of light near the limit of resolution will show a density such as that shown in the unbroken line in which the ideal image is shown dotted.
Brightness Variation Graph: A Square Waveform
  • Brightness variation graph: a square waveform
  • Breakdown of Zine test-object into sine-curve components
  • Density distribution of the photographic image of a single narrow band of light near the limit of resolution: real and ideal images compared (after Mlins, 1965) .
  • Thus, in determining the modulation transfer, a sinusoidal target is used which consists of a series of density variations between a constant maximum and a constant minimum, with the high-density peaks coming closer together as we move across the density variations. The density profile in each element of the test target is sinusoidal - hence the term sine-wave response.
  • If the film or lens under testing is perfect, all the peaks will be resolved. But this is not the case and at some stage, the film or lens will fail to resolve the peaks of density. The density variations in the image of the test become as shown in below Figure.
As the Peaks Get Closer
  • As the peaks get closer, the ability of the film or lens to resolve the peaks of density becomes less and less. A photometer called the microdensitometer is required to measure the density difference between peaks and troughs at each line frequency.
  • The values of maximum density (D max) and minimum density (D min) at each frequency, which can be determined directly from the microdensitometer trace, are converted to linear relative exposures by means of the characteristic curve (i.e.. density against log E) to give maximum and minimum relative exposure values (I max and I min) for the following formula of modulation transfer (MT) factor:
Modulation Transfer (MT)
  • The normalised MT factors are plotted against spatial frequency to produce the sine-wave response curve. Thus, the curve indicates the response at all frequencies, whereas the resolving power only indicates the highest frequency visible.
The MTF Components of an Aerial Photograph

The MTF components of an aerial photograph (after De Belder et al, 1972)

  • The major merit of the MTF lies in the possibility of its being employed to evaluate the whole photographic system because individual MTF curves can be produced for lens, films, image motion, and other relevant variables in the camera system.
  • A total system MTF is derived by multiplying the responses of the appropriate lens, film, and other curves frequency by frequency in a process known as cascading. The result is a single MTF for that combination of lens and film, etc. An actual example is shown in above Figure where five components of an aerial photograph are cascaded.

The Development and Printing Process

  • The development process detects and amplifies the latent image of the exposed film to produce a visible silver image, which is in negative form. The procedures involve wetting and drying, both of which can have harmful effects on the overall stability of the resulting negative. At a constant relative humidity, film dimensions increase within temperature and decrease with a drop in temperature. Since the film is an elastic material coated with a gelatin layer, this overlying layer of emulsion exerts a compressive force on to the support at lower relative humidity (thus causing the emulsion to dry up quickly) , which results in irregular (or non-linear) changes. In developing, the structure of the emulsion is modified by wetting, removal of unexposed silver halide and drying, thus resulting in a change of thickness and modulus of the emulsion layer, which reduces the compressing forces exerted by the emulsion. Also, the drying needs to be carefully controlled because overdriving or forced drying results in lower relative humidity and tends to expand the film, whilst slowly dried film will usually show shrinkage. Today films are usually considered stable with the supporting base being made of polyester material, such as cellulose acetate butyrate in Kodak՚s Estar-base films whose non-uniform dimensional change should not exceed 31.1m in a 24Omm frame on average and is less than 84m at any point in maximum. This is generally known as the Topo Base Film. 11 But even the Estar-base film is intentionally stretched in manufacture in both length and width to avoid a systematic orientation of its molecules. Further deformation occurs as aerial films are processed in the roller-transport processing machine, causing a lengthwise tension whilst wet and during drying.
  • From the negative, it is possible to obtain a positive print on glass (known as a diapositive) or on paper. It can be produced in printers which project the negative through a lens or in contact printers. Projection printing gives rise to distortion due to displacements. Generally, contact printers with centrally projected light are used.
  • These often allow automatic dodging of the photographs, a procedure which reduces highlights and shadows in the negative in order to improve the readability and interpretability of the printed diapositive by bringing out finer details. 12 Contact printing with automatic dodging can easily give rise to errors due to, for example, the film not being made to lie completely flat on the printer stage.
  • On average, the magnitude of the error is from approximately ±2 to 4µm, Experiments carried out by Corten at ITC in the Netherlands have shown that after positive contact copying (on paper or on glass) , degradation in image detail occurs. “It has been suggested by Ziemtinn that printing with parallel light (instead of centrally projected light) can reduce the printing error by almost 50 per cent.”

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