Orthochromatic Photography: Properties and Applications (Especially for GATE-Geospatial 2022)

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Orthochromatic photography refers to a photographic emulsion that is sensitive to only blue and green light, and thus can be processed with a red safelight

The orthochromatic film is a monochrome film most sensitised to light with a wavelength of 560nm in the middle of the green band of the visible spectrum. Since within its limits of sensitivity its coverage includes the shorter wavelengths at about 400nm, it is also sensitive to blue light.

In the past, it was used as a standard film for aerial mapping in the United States to simulate the orthochromatic effect from the panchromatic emulsions by means of a minus-red filter which can cut down the long wavelength (red light) as well as the short wavelength (blue light) within the visible portion.

Illustration 2 for Orthochromatic Photography: Pro …

Useful Properties of Orthochromatic

Film of low contrast is used to produce pictures of continuous tone as are the photographs. Orthochromatic film is of high contrast and renders only a line tone of either black or white tone. Thus, lines shown in map manuscript are mapped in the film as clean lines without alteration of tone. It allows creation of positive or negative copies from the manuscript with precise enlargement or reduction without introducing deformities and alterations.

Applications of Orthochromatic Photography

This special sensitivity to the light in the spectrum between wavelengths of 400 and 600nm has been found particularly useful in two types of application:

  • Coast studies: For the former, Sonu has already pointed out that at such a range the film allows maximum depth penetration of water bodies, 3 whilst for the latter, green vegetation registers a good tonal contrast on orthochromatic film with its strong sensitivity to green light, thus facilitating the identification of details about the vegetation.
  • Vegetation surveys: This latter use is particularly favoured by the Russians who discovered that orthochromatic photographs were taken in summer and early spring gave greater tone contrast than panchromatic for such commercially important tree species like pine and Alnus whilst no difference was noted in autumn between the two films.

Orthochromatic film is now only used in test flights. But more recently, the growing interest in multispectral or multiband photography has attracted more attention to this type of film.

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