Two black-and-white films exposed to the same scene can return different tones for the same colour, because a monochrome emulsion does not record colour at all but only the brightness it perceives at each wavelength. That perception is governed by the film’s spectral sensitivity, and the two historical classes of sensitising define the extremes of the problem: orthochromatic emulsions that are blind to red, and panchromatic emulsions that respond across the full visible spectrum. The difference determines how skin, foliage, lips, and skies translate into grey.

The Native Bias of Silver Halide

A silver halide emulsion, left unsensitised, is not neutral to the spectrum. The crystals absorb energy only at the short-wavelength end, so the raw material responds strongly to ultraviolet and blue light, weakly to green, and effectively not at all to orange and red. This native blue sensitivity is why the earliest plates rendered blue skies as featureless white and any red object as near-black.

The remedy was discovered by the German photochemist Hermann Wilhelm Vogel in 1873, who found that adding certain dyes to the emulsion extended its sensitivity into longer wavelengths. A dye adsorbed to the crystal surface absorbs light at a wavelength the halide itself cannot use and transfers that energy to the grain, a process called spectral or optical sensitisation. The reach of the response depends entirely on which dyes are present.

Orthochromatic: Sensitivity Without Red

An orthochromatic emulsion is sensitised into the green but deliberately, or by limitation, stops short of red. Its response covers roughly the blue and green region and falls away through the yellow, leaving orange and red recording little density. Ilford’s technical data for ORTHO Plus describes the film as sensitive only to blue and green light, which is why it carries a split rating of ISO 80 in daylight but only ISO 40 under tungsten illumination: tungsten light is rich in the red wavelengths the emulsion cannot use, so the same exposure yields less density.

The tonal consequence is severe and predictable. Anything red darkens. Caucasian skin, which reflects strongly in the red, records several stops darker than its visual brightness, so freckles, ruddiness, and lips deepen toward black while blue eyes and blue fabrics lighten toward white. Foliage, reflecting green, holds a middle grey, but red flowers among it sink into shadow. This is the rendering that obliged early motion-picture studios to apply heavy greasepaint to even out skin tone on screen.

One practical advantage follows from the same blindness: because the emulsion does not see red, it can be handled and developed by inspection under a dark red safelight. Ilford specifies its 906 dark red safelight for ORTHO Plus, an option no panchromatic film permits.

Panchromatic: The Full Visible Spectrum

A panchromatic emulsion carries sensitising dyes that extend its response across the entire visible band, from blue through green and yellow into red. The aim is tonal rendering that approximates how the eye weighs the brightness of colours, so that a red and a green of equal visual lightness translate to similar greys rather than to black and grey. Wratten and Wainwright prepared the first commercial panchromatic plates, available from 1906, and Eastman Kodak adopted panchromatic emulsion as a regular motion-picture stock in 1922, after which it became the standard for general photography.

Because a panchromatic film responds to red, it cannot be handled under any visible safelight and must be loaded, processed, and inspected in total darkness. Its full-spectrum response also makes it the foundation for contrast control through filtration. A coloured filter lightens its own colour and darkens its complement only because the underlying emulsion records the whole spectrum to begin with: a red filter on panchromatic film darkens a blue sky and lightens skin precisely because the film can still register the red the filter passes. On an orthochromatic emulsion, blind to red, a red filter would simply block most of the light the film could record and yield little image. Full spectral response is therefore not only a matter of natural rendering but the precondition for the entire system of contrast filtering in black-and-white photography.