Panchromatic film is a type of black and white photographic film whose emulsion is sensitised to respond to all colours across the full visible spectrum - from violet and blue at the short wavelength end through green and yellow in the middle, to orange and red at the long wavelength end - producing tonal renditions of coloured subjects that broadly correspond to the relative luminance values that the human eye perceives when viewing the same subject. The word panchromatic derives from the Greek pan meaning all and chromatic meaning colour, accurately describing the film's comprehensive spectral sensitivity.
The development of panchromatic film represented a significant advance over earlier orthochromatic emulsions, which were sensitive only to blue, violet, and to a limited extent green light, but were almost entirely insensitive to red and orange wavelengths. The practical consequence of orthochromatic sensitivity was that red and orange coloured subjects - including human skin, red fabrics, autumn foliage, and red flowers - were rendered as very dark or almost black tones in the finished print, while blue skies appeared almost white due to the film's strong blue sensitivity. This tonal distortion placed significant limitations on the ability of orthochromatic film to produce natural looking black and white renditions of coloured subjects, particularly in portraiture where the unnatural darkening of skin tones and lips was a persistent aesthetic problem.
Panchromatic sensitivity is not achieved by the silver halide emulsion alone, as silver halides are naturally sensitive only to blue and ultraviolet light and require chemical treatment to extend their sensitivity across the rest of the visible spectrum. The extension of emulsion sensitivity to green, yellow, orange, and red wavelengths is achieved through the incorporation of spectral sensitising dyes - organic compounds that absorb light at specific wavelengths and transfer the absorbed energy to the silver halide crystals, triggering the same photochemical response as direct blue light exposure. The precise spectral sensitisation achieved depends on the specific combination and concentration of sensitising dyes incorporated into the emulsion during manufacture, and different panchromatic films may exhibit subtly different relative sensitivities to different parts of the spectrum depending on their particular formulation.
The comprehensive spectral sensitivity of panchromatic film enables much more natural and tonally accurate black and white renditions of coloured subjects than orthochromatic film could produce. Human skin tones are rendered with natural luminosity and delicacy, blue skies are recorded as mid-toned rather than washed out, and the full range of colours in a scene is translated into a graduated tonal scale that reflects their actual relative brightnesses more faithfully. This natural tonal rendering also provides the photographer with meaningful creative control through the use of coloured filters, which selectively transmit or absorb specific wavelengths and alter the relative tonal values of differently coloured subjects in the finished print - a technique that is most effective and predictable when used with panchromatic film whose response spans the full visible spectrum.
Because panchromatic film is sensitive to red light as well as blue and green, it cannot be handled or processed under the red or orange safelights that were used with orthochromatic materials in the darkroom. Panchromatic film must be loaded, handled, and processed in complete darkness, or in some cases under a very dim green safelight to which panchromatic emulsions have limited sensitivity, making darkroom handling somewhat more demanding than the relative convenience of orthochromatic materials allowed. This requirement for complete darkness during handling was for many years one of the practical considerations that influenced the choice between panchromatic and orthochromatic materials for different applications, though the superior tonal quality of panchromatic film made it the dominant choice for virtually all serious black and white photography once the technical challenges of panchromatic sensitisation had been fully resolved.