Two black-and-white films exposed to the same scene can return different greys for the same colour, because a monochrome emulsion does not record colour at all, only the brightness it perceives at each wavelength. That perception is set by the film’s spectral sensitivity, and the response is not a single flat band but a curve with a definite long-wavelength cutoff. Where that cutoff falls decides how skin, foliage, lips, and skies translate into grey.

The Wavelength Spine

A silver halide emulsion, left unsensitised, is not neutral to the spectrum. The crystals absorb energy only at the short-wavelength end: sensitivity is strong in the ultraviolet and blue and falls away above roughly 500 nm, so unsensitised material is effectively blind to green, orange, and red. This is why the earliest plates rendered blue skies as featureless white and any red object as near-black.

Sensitising dyes push the cutoff outward, and each class of film is defined by how far. An orthochromatic emulsion extends through green and yellow but dies out near 590-600 nm, leaving it insensitive to orange and red; Ilford’s published spectral curve for ORTHO Plus shows exactly this, a response that climbs through blue and green and collapses before the orange. An ordinary panchromatic film carries the response across the whole visible band to about 650-700 nm. Extended-red emulsions such as Ilford SFX 200 reach further still, to roughly 720-740 nm, and the discontinued Kodak High Speed Infrared (HIE) extended out to about 900 nm, well into the infrared. “Into the green” and “the full spectrum” are therefore measurable claims, separated by a hundred nanometres or more.

The mechanism Vogel uncovered is photophysical, not magic. A dye molecule adsorbed to the silver-halide grain absorbs a long-wavelength photon and injects an electron into the crystal’s conduction band, producing the latent-image silver speck the halide alone could never have formed at that wavelength. The dye captures light the crystal cannot; the crystal records the consequence. Mees and James give this electron-injection account in The Theory of the Photographic Process, and it is why the reach of a film depends entirely on which dyes are present.

Vogel, the Dyes, and the Road to Pan

Hermann Wilhelm Vogel (1834-1898) discovered optical sensitisation in autumn 1873, in an experiment dated 25 August, after noticing that English collodion bromide dry plates were unexpectedly green-sensitive because of a yellow dye in the coating. Early orthochromatic plates were built on eosin and erythrosine sensitisers, which carried the response into the green. In 1884 Vogel himself produced near-panchromatic “Azaline” plates using a cyanine-family sensitiser (his “Azalin”, a cyanin and chinolinrot mixture) that reached into the reddish-orange, the first real step toward full-spectrum film.

The commercial chronology follows from there. Wratten and Wainwright of Croydon made the first commercial panchromatic plates available in 1906; Kenneth Mees worked at the firm from 1906 to 1912 developing them before Eastman Kodak acquired it in 1912. Kodak offered panchromatic motion-picture negative on special order from 1913 and released Kodak Panchromatic Cine Film as a regular stock in 1922. The Headless Horseman (1922) was the first feature shot entirely on panchromatic stock, which displaced ortho in cinema over the 1920s.

Orthochromatic in Practice: ORTHO Plus

Ilford ORTHO Plus is the modern reference point, and its datasheet is precise about what the film is. It “was originally designed as a high-resolution copy film”, not a portrait emulsion, though it can be processed to pictorial contrast as a camera film; normal in-camera contrast runs to a Gbar of 0.62-0.70. It is sensitive only to blue and green, so reds and oranges appear much darker than normal. That blindness explains its split rating: DX coded at ISO 80 for daylight but rated ISO 40 under tungsten, because tungsten light is rich in the red wavelengths the emulsion cannot use, yielding less density for the same exposure.

For development the datasheet lists ID-11, Microphen, PQ Universal at 1+9, and Phenisol at 1+4. The same red-blindness that constrains the film also relaxes the darkroom: ORTHO Plus may be handled under an Ilford 906 dark red safelight fitted with a 15 watt bulb, kept a minimum of 1.2 m (4 ft) from the working area to avoid fogging and reduced contrast, or in total darkness. Panchromatic film permits no such latitude. Because it responds to red, it must be loaded, processed, and inspected in complete darkness, with no green-safelight nuance available.

A Worked Skin-Tone Example

Place a front-lit Caucasian face on panchromatic film and meter it to land on Zone VI, where Ansel Adams put average lit skin in The Negative. That face reflects heavily above 600 nm, in exactly the band where orthochromatic film has gone blind. Expose the same scene on ORTHO Plus and the red and near-red reflectance the meter counted simply does not register as density: the skin falls two to three zones, toward Zone III-IV, while blue eyes and blue fabric lighten toward white. Lips, ruddiness, and freckles deepen toward black.

This is precisely the problem early cinema fought before 1922. Orthochromatic stock exaggerated lip and skin tone, and the limitations “could be corrected by makeup, lens filters, and lighting, but never completely satisfactorily” until panchromatic film replaced ortho in the 1920s. The greasepaint was a wavelength patch, not a stylistic choice.

Panchromatic Is Not One Thing

Kodak itself split panchromatic emulsions into Type A (orthopanchromatic: extra blue, reduced red), Type B (roughly uniform daylight response), and Type C (extra red sensitivity), and the modern catalogue maps onto that. Ordinary pan, the Type B workhorses, covers Ilford HP5 Plus, FP4 Plus, and Kodak Tri-X and T-Max. Orthopanchromatic-leaning films such as Fuji Acros II and Adox CHS 100 II and CMS 20 carry a blue lean and a slightly clipped red, which renders skin a touch smoother. Extended-red and superpanchromatic stocks, Ilford SFX 200, Rollei Retro 80s and Superpan 200, and the late Kodak HIE, push hardest into the red, and the smoothing effect on faces becomes pronounced: heavy red sensitivity lifts reddened blemishes and lines toward the surrounding skin tone, so they record paler and read as less distinct. So “panchromatic” is a family, not a flat category, ranging from near-ortho to near-infrared.

Filtration Lives on Full Spectral Response

A coloured filter lightens its own colour and darkens its complement, but only because the underlying emulsion records the whole spectrum to begin with. The standard daylight filter factors are all derived for panchromatic film: a Wratten 8 (K2, medium yellow) costs 2x (1 stop), a Wratten 15 (deep yellow) 2.5x (about 1 1/3 stops), a Wratten 11 (yellow-green) 4x (2 stops), a Wratten 25 (red) 8x (3 stops), and a Wratten 47 (blue) 6x (about 2 2/3 stops). Those factors are referenced to holding a Zone V grey card under a 5500 K daylight and skylight mix.

The worked case: on panchromatic film a Wratten 25 red filter, after its 3-stop compensation, darkens a clear blue sky by roughly two to three zones while holding or even lifting front-lit skin, because the film still registers the red the filter passes. A red filter raises overall contrast index, a blue filter lowers it, and a green filter holds roughly normal contrast. None of this transfers to orthochromatic film. Because ortho cannot record the red a deep-red filter passes, the published factors are useless on it: a Wratten 25 on ORTHO Plus would block most of the light the film could record and yield little image, and any filtration on ortho has to be tested separately. Full spectral response is not just a matter of natural rendering; it is the precondition for the entire system of contrast filtering in black-and-white photography.