Infrared vs. Panchromatic - Mt. Reynolds

Panchromatic film is a normal black-and-white film which you can buy in the store and is what your grandparents probably used. It is called panchromatic because it is sensitive to light of all visible colors (electromagnetic wavelengths approximately 0.4mm to 0.7mm). Objects of different colors are distinguishable in panchromatic film, with grayscale tones closely approximating the brightness of the photograph's subject. Panchromatic film is typically not as sensitive to subtle variation in green light as some other colors, making it difficult to differentiate between similar vegetation types. To reduce atmospheric haze, a yellow filter is normally used. For these images the filters believed to have been used were a K2 (Wratten No. 8) and #12 deep amber (minus blue member of the cyan-magenta-yellow set of subtractive primaries).

Infrared film absorbs different wavelengths of energy than panchromatic film and was used to help differentiate among vegetation types. A photographic image taken with infrared film displays naturally-occurring, reflected electromagnetic wavelengths in the near-infrared range, between approximately 0.7mm to 0.9mm (a filter is used to block wavelengths in the visible spectrum, typically those below 0.7mm). For these images the filters believed to have been used were A-red (Wratten No. 25) and F-red (Wratten No. 29). The reflectance characteristics of vegetation are considerably different in the infrared range than the visible range, with maximum reflection for vegetation occurring in the near-infrared range. The cell-wall/air-space interfaces are responsible for reflecting most infrared radiation, and these interface structures vary with age, plant health, and between vegetation types. On infrared film, healthy deciduous vegetation is typically very light (nearly white), while healthy coniferous vegetation reflects less infrared radiation and appears in gray tones. Dead or senescent vegetation appears black. On infrared film, water absorbs infrared radiation and appears very dark (it reflects very little) and is smooth in contrast with the surrounding land. Also, moist soils appear in dark tones, while dry soils appear in lighter tones. Infrared film is also much better at providing a clear picture through haze or other forms of air pollution as infrared radiation is not scattered by particles in the atmosphere as readily as wavelengths in the visible spectrum. For more detail regarding infrared film and photographing vegetation, several excellent aerial photography and remote sensing texts exist and can be found at most libraries.

Panchromatic & infrared black-and-white images from 1935 at Mt. Reynolds Lookout in Glacier National Park, MT, USA.

180-300° arc sector image, panchromatic

180-300° arc sector image, infrared

300-60° arc sector image, panchromatic

300-60° arc sector image, infrared

60-180° arc sector image, panchromatic

60-180° arc sector image, infrared

Photographs courtesy of Glacier National Park archives.