From Monochrome Panchromatic to Colour Films: Electromagnetic and Visible Spectrum (Especially for GATE-Geospatial 2022)

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The various applications of aerial photography in geography have centred mainly on, standard monochrome film type - the black-and-white panchromatic film which is equally sensitised to the whole visible spectrum with wavelengths ranging from below 400nm to about 700nm. This makes it suitable for general-purpose uses. There are other monochrome film; such as the orthochromatic film which is sensitised for wavelengths from below 400nm to about 600nm and the Infra-red film which is sensitised for wavelengths from below 400nm to about 900nm. All these may be regarded as special-purpose films because they are especially sensitised to some particular wavelengths so that they can record some objects more distinctly than others. This usually means that the photographic tone contrast or difference in brightness between the image and its background is enhanced, thus allowing the image to be more easily interpreted. Apart from these monochrome non-panchromatic films, there are also colour films which are capable of depicting our terrestrial environment either in true I colour, which is more familiar to the human eye, or in false colour which attenuates the effect of colour on certain objects at the expense of others, to facilitate detection. All these different types of special film have significant impacts on the geographical applications of aerial photography today.

Electromagnetic Spectrum

The electromagnetic spectrum is an effect produced when electromagnetic radiations are resolved into their constituent wavelengths or frequencies. Within the spectrum, the radiant energy, whose principal source is the sun, moves with the constant velocity of light (i.e.. 3x108m/sec) in a harmonic wave pattern so that a reciprocal relationship exists between wave frequencies and wavelengths. The term ‘harmonic’ is applied to waves which are equally and repetitively spaced in time, and the energy is measured in units known as photons. Each part of the electromagnetic spectrum has unique energy characteristics and wavelengths.

The Visible Spectrum

In photography, the major form of radiant energy which is detectable is white light - the visible radiation which can be dispersed by a glass or a crystal prism into a band of colours ranging from violet through blue, blue-green, green, yellow, orange-red and deep red within wavelengths from about 400nm at the blue end to about 700nm at the extreme-red end, thus forming the visible spectrum. It should be noted that although this visible spectrum occupies only a very small portion of the whole electromagnetic spectrum, its energy is probably the most important as a source of information of our terrestrial environment when compared with other new remote sensing techniques recently developed to exploit the invisible portions of the spectrum. This is because the human eye is more accustomed to seeing the significance of the resultant photographic images through their tone contrast sharpness and stereoscopic impression. Of these three characteristics, the special importance of tone contrast as a clue in photo-interpretation has already been amply demonstrated. Therefore, again in interpretability will result from the procurement of photography in the proper part of the visible spectrum through the effects of greater tone contrast. This realisation has led in recent years to impressive growth in the range of photographic materials for interpretation.

Visible Portion

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