Spectral sensitivity is a fundamental characteristic of a photographic emulsion or digital image sensor describing its relative response to light of different wavelengths across the electromagnetic spectrum, quantifying how strongly the material or device responds to each colour of the visible spectrum and to adjacent invisible regions including ultraviolet and infrared radiation. It determines which wavelengths of light the photographic material is capable of recording, with what relative efficiency at each wavelength, and therefore has profound implications for how the tones and colours of a subject are translated into the photographic image.
The spectral sensitivity of a photographic emulsion is not uniform across the spectrum but varies considerably with wavelength, reflecting both the intrinsic photochemical properties of the silver halide crystals that form the light sensitive component of the emulsion and the additional spectral sensitisation introduced by the chemical sensitising dyes incorporated during emulsion manufacture. Without any spectral sensitisation beyond the natural response of silver halide, a photographic emulsion responds strongly to ultraviolet and blue-violet light but is largely insensitive to green, yellow, orange, and red wavelengths - a spectral sensitivity profile described as blue sensitive or unsensitised. By incorporating orthochromatic sensitising dyes into the emulsion, manufacturers extend its sensitivity to include the green region of the spectrum while leaving it insensitive to red and orange. By incorporating panchromatic sensitising dyes, the sensitivity is extended further to cover the full visible spectrum from violet through to red, producing a panchromatic emulsion that responds to all colours of visible light.
The specific pattern of spectral sensitivity across the visible spectrum is not equally balanced for all wavelengths even in a panchromatic emulsion - the relative sensitivity at each wavelength depends on the specific sensitising dye formulation used, and different panchromatic films exhibit subtly different spectral sensitivity profiles that influence their tonal rendition of coloured subjects in black and white photography. A film with relatively greater sensitivity to red wavelengths will render red and orange subjects as lighter tones than a film with lower red sensitivity, while rendering blue subjects as relatively darker tones, and vice versa. These differences in spectral sensitivity between films of different manufacture are one of the factors that give different black and white films their distinctive individual character and tonal rendering qualities.
Beyond the visible spectrum, the spectral sensitivity of photographic materials extends into adjacent invisible regions that are of significant practical and scientific importance. Most conventional silver halide emulsions retain some sensitivity to near ultraviolet radiation at wavelengths below approximately 400 nanometres, which is why UV absorbing filters are used on camera lenses to prevent ultraviolet haze from affecting outdoor photographs - the emulsion records the scattered ultraviolet radiation from the sky as additional exposure that reduces contrast and clarity in distant landscapes. Specialised infrared sensitive emulsions extend the spectral sensitivity of conventional panchromatic or orthochromatic materials into the near infrared region beyond approximately 700 nanometres, enabling infrared photography that records the world according to the infrared reflectance of different surfaces rather than their visible colour, producing the characteristic pale sky, dark foliage, and dreamlike rendering associated with infrared photography.
In colour photography, the spectral sensitivity of the three separate emulsion layers in a colour film or the three colour channels of a digital image sensor must be carefully balanced and matched to produce accurate, neutral colour rendition across the full visible spectrum. Each layer or channel must respond to its designated primary colour region - red, green, or blue - with sufficient sensitivity and adequate separation from the adjacent colour regions to enable accurate colour discrimination and reproduction throughout the tonal range of the image. The spectral sensitivity profiles of the three layers or channels are therefore a critical factor in the colour accuracy and fidelity of the photographic system, and the careful engineering of these profiles through sensitising dye formulation and filter design is one of the most important aspects of colour film and sensor design.
In digital imaging, the spectral sensitivity of the image sensor is determined by the combination of the intrinsic quantum efficiency of the silicon photodiode material at different wavelengths - which is naturally most sensitive in the red and near infrared and progressively less sensitive towards the blue end of the visible spectrum - and the spectral transmission characteristics of the colour filter array placed over the sensor to provide colour discrimination, most commonly a Bayer pattern mosaic of red, green, and blue filtered photosites. The sensitivity of digital sensors to near infrared radiation beyond the visible spectrum is typically suppressed by a hot mirror or infrared cut filter placed in front of the sensor, as infrared sensitivity would otherwise cause colour balance errors in images captured under mixed visible and infrared illumination conditions.