Spectral sensitivity is the relative efficiency of detection, of
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
or other signal, as a function of the
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
or
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
of the signal.
In
visual
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (th ...
neuroscience
Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, developme ...
, spectral sensitivity is used to describe the different characteristics of the
photopigment
Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and pho ...
s in the
rod cells and
cone cells in the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
of the
eye. It is known that the rod cells are more suited to
scotopic vision
In the study of human visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from Greek ''skotos'', meaning "darkness", and ''-opia'', meaning "a condition of sight". In the human eye, ...
and cone cells to
photopic vision
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visu ...
, and that they differ in their sensitivity to different wavelengths of light. It has been established that the maximum spectral sensitivity of the human eye under daylight conditions is at a wavelength of 555
nm, while at night the peak shifts to 507 nm.
In
photography
Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employe ...
, film and sensors are often described in terms of their spectral sensitivity, to supplement their
characteristic curves that describe their
responsivity Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, it measures the electrical output per optical input.
A photodetector's responsivity is usually expressed in units of amperes or volts per ...
. A database of camera spectral sensitivity is created and its space analyzed. For
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
films, the spectral sensitivity is chosen to be appropriate to the phosphors that respond to X-rays, rather than being related to human vision.
In
sensor systems, where the output is easily quantified, the
responsivity Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, it measures the electrical output per optical input.
A photodetector's responsivity is usually expressed in units of amperes or volts per ...
can be extended to be wavelength dependent, incorporating the spectral sensitivity. When the sensor system is linear, its spectral sensitivity and spectral responsivity can both be decomposed with similar basis functions. When a system's responsivity is a fixed monotonic nonlinear function, that nonlinearity can be estimated and corrected for, to determine the spectral sensitivity from spectral input–output data via standard linear methods.
The responses of the rod and cone cells of the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
, however, have a very context-dependent (coupled) nonlinear response, which complicates the analysis of their spectral sensitivities from experimental data. In spite of these complexities, however, the conversion of light energy spectra to the effective stimulus, the excitation of the
photopigment
Photopigments are unstable pigments that undergo a chemical change when they absorb light. The term is generally applied to the non-protein chromophore moiety of photosensitive chromoproteins, such as the pigments involved in photosynthesis and pho ...
, is quite linear, and linear characterizations such as spectral sensitivity are therefore quite useful in describing many properties of
color vision.
Spectral sensitivity is sometimes expressed as a
quantum efficiency, that is, as probability of getting a quantum reaction, such as a captured
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
, to a quantum of light, as a function of wavelength. In other contexts, the spectral sensitivity is expressed as the relative response per light energy, rather than per quantum, normalized to a peak value of 1, and a quantum efficiency is used to calibrate the sensitivity at that peak wavelength. In some linear applications, the spectral sensitivity may be expressed as a spectral
responsivity Responsivity measures the input–output gain of a detector system. In the specific case of a photodetector, it measures the electrical output per optical input.
A photodetector's responsivity is usually expressed in units of amperes or volts per ...
, with units such as
amperes per
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
.
See also
*
Frequency response
In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and phase of the output as a function of input frequency. The frequency response is widely used in the design and analysis of s ...
*
Orthochromasia
In chemistry, orthochromasia is the property of a dye or stain to not change color on binding to a target, as opposed to metachromatic stains, which change color. The word is derived from the Greek '' orthos'' (correct, upright), and chromatic ( ...
References
{{Reflist
Color vision
Radiometry