Spectral sensitivity
{{short description|Relative efficiency of detection of a signal as a function of its frequency or wavelength}}
File:Cones SMJ2 E.svg spectra) of human cone cells, S, M, and L types]]
File:Spectral sensibilities.png
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal.
In visual neuroscience, spectral sensitivity is used to describe the different characteristics of the photopigments in the rod cells and cone cells in the retina of the eye. It is known that the rod cells are more suited to scotopic vision and cone cells to photopic vision, and that they differ in their sensitivity to different wavelengths of light.{{cite book | title = Fundamentals of Sensation and Perception | author = Michael Levine | publisher = Oxford University Press | date = 2000 | edition = 3rd}}{{cite book | title = Visual Perception: A Clinical Orientation | author = Steven H. Schwartz | publisher = McGraw-Hill Professional | date = 2004 | isbn = 0-07-141187-9 | url = https://books.google.com/books?id=Oyk0faySQfUC&dq=spectral-sensitivity+intitle:perception&pg=PA32 }} 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.{{Cite book
|title=Handbook of optical systems
|last1=Gross
|first1=Herbert
|last2=Blechinger
|first2=Fritz
|last3=Achtner
|first3=Bertram
|editor-last=Gross
|editor-first=Herbert H.
|volume=4
|date=2008
|page=40
|publisher=WILEY-VCH
|place=Weinheim, Germany
|url=https://books.google.com/books?id=wrtFcUEIWTgC&dq=%22spectral+sensitivity%22+eye&pg=PA40
|isbn=978-3-527-40380-6
}}
In photography, film and sensors are often described in terms of their spectral sensitivity, to supplement their characteristic curves that describe their responsivity.{{cite book | title = Advanced Photography | author = Michael Langford | publisher = Focal Press | date = 1998 | isbn = 0-240-51486-6 | url = https://books.google.com/books?id=Z92UjOE93SIC&dq=%22spectral+sensitivity%22+intitle:photography&pg=PA73 }} A database of camera spectral sensitivity is created and its space analyzed.{{cite book | title = What is the space of spectral sensitivity functions for digital color cameras? | author = Jun Jiang | author2 = Dengyu Liu | author3 = Jinwei Gu | author4 = Sabine Süsstrunk | name-list-style = amp | publisher = IEEE | date = 2013 | isbn = 978-1-4673-5053-2 | url = http://www.cis.rit.edu/~jxj1770/camSpec/}} For X-ray films, the spectral sensitivity is chosen to be appropriate to the phosphors that respond to X-rays, rather than being related to human vision.{{cite book | title = Chesneys' Radiographic Imaging | author = John Ball | author2 = Tony Price | name-list-style = amp | publisher = Blackwell Publishing | date = 1995 | isbn = 0-632-03901-9 | url = https://books.google.com/books?id=jpwNLOhYRzgC&dq=%22spectral+sensitivity%22&pg=PA67 }}
In sensor systems, where the output is easily quantified, the responsivity 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.{{cite book | title = Physics-Based Vision | author = Glenn E. Healey | author2 = Steven A. Shafer | author3 = Lawrence B. Wolff | name-list-style = amp | publisher = A. K. Peters Ltd. | date = 1992 | isbn = 0-86720-295-5 | url = https://books.google.com/books?id=jdwLY6_r0ngC&dq=sensor-responsivity+%22spectral+sensitivity%22&pg=PA12 }} 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.{{cite book | title = The Science of Color | author = Steven K. Shevell | publisher = Elsevier | date = 2003 | isbn = 0-444-51251-9 | url = https://books.google.com/books?id=G1TC1uXb7awC&dq=sensor-responsivity+%22spectral+sensitivity%22&pg=PA337 }}
The responses of the rod and cone cells of the retina, however, have a very context-dependent (coupled) nonlinear response, which complicates the analysis of their spectral sensitivities from experimental data.{{cite book | title = Adaptive mechanisms in the ecology of vision | author = S. N. Archer | publisher = Springer | date = 1999 | isbn = 0-7923-5319-6 | url = https://books.google.com/books?id=4zUpQatwhwUC&dq=retina++%22spectral+sensitivity%22+cone+rod+interaction&pg=PA340 }} In spite of these complexities, however, the conversion of light energy spectra to the effective stimulus, the excitation of the photopigment, is quite linear, and linear characterizations such as spectral sensitivity are therefore quite useful in describing many properties of color vision.{{cite book | title = Light Vision Color | author = Arne Valberg | publisher = John Wiley and Sons | date = 1995 | isbn = 0-470-84902-9 | url = https://books.google.com/books?id=OoESifAi9ZsC&dq=cones+%22spectral+sensitivity%22++linear+nonlinear&pg=PA152 }}
Spectral sensitivity is sometimes expressed as a quantum efficiency, that is, as probability of getting a quantum reaction, such as a captured electron, to a quantum of light, as a function of wavelength.{{cite book | title = Digital Image Analysis of Microbes: Imaging, Morphometry, Fluorometry and Motility Techniques and Applications | author = M. H. F. Wilkinson | author2 = F. Schut | name-list-style = amp | publisher = John Wiley and Sons | date = 1998 | isbn = 0-471-97440-4 | url = https://books.google.com/books?id=8JJwuU13YPAC&dq=%22spectral+sensitivity%22++%22quantum+efficiency%22&pg=PA60 }} 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.{{cite book | title = Contrast Sensitivity of the Human Eye and Its Effects on Image Quality | author = Peter G. J. Barten | publisher = SPIE Press | date = 1999 | isbn = 0-8194-3496-5 | url = https://books.google.com/books?id=kPyyBAomC4cC&dq=%22spectral+sensitivity%22++%22quantum+efficiency%22&pg=PA32 }} In some linear applications, the spectral sensitivity may be expressed as a spectral responsivity, with units such as amperes per watt.{{cite book | title = Optics and lasers: including fibers and optical waveguides | author = Matt Young | publisher = Springer | date = 1993 | isbn = 3-540-65741-X | url = https://books.google.com/books?id=W3030b9GlZQC&dq=%22spectral+sensitivity%22++per-watt&pg=PA92 }}{{cite book | title = Survey of Instrumentation and Measurement | author = Stephen A. Dyer | publisher = Wiley-IEEE | date = 2001 | isbn = 0-471-39484-X | url = https://books.google.com/books?id=PMZhn82rXvIC&dq=%22spectral+sensitivity%22++per-watt&pg=PA922 }}{{cite book | title = Analysis and Application of Analog Electronic Circuits to Biomedical Instrumentation | author = Robert B. Northrop | publisher = CRC Press | date = 2004 | isbn = 0-8493-2143-3 | url = https://books.google.com/books?id=CSOva1nQsUEC&dq=%22spectral+sensitivity%22++per-watt&pg=PA102 }}