Visual Contrast of Text Subgroup/Resources

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Moved from the Visual Contrast Subgroup page. This subpage includes the glossary, bibliography, and links to offsite team member materials. Also, "the lighter side of light" a light hearted look at lighting.


"Normal" Vision

Normal Vision is a specific definition, and a clinical definition:

  • Snellen acuity of 20/20 or lower (20/16 is "perfect" acuity, 20/200 is SSA disabled)
  • Peli-Robson contrast sensitivity of 1.95 or higher (2.25 is "best")
  • Farnsworth Munsell Hue Color TES of 60 or less (TES 0 is perfect)
  • 100% Visual field with MD no lower than -2db (0 is best, -20db is SSA disabled)
  • The age-related baseline normal is ages 20 thru 40.
  • Below 20, contrast sensitivity is still developing, so _young normal_ includes a lower contrast sensitivity.
  • Above 40, presbyopia is a normal development, so _mature normal_ includes a lack of near-distance acuity.1

Depending on the study, "normal" often includes "with refractive correction" if the correction can acheive the above scores. An example: "normal with correction needed for presbyopia". Presbyopia being "normal" for over age 40 for instance.

"Normal" is not "most perfect"

Because normal vision is specifically defined, it also provides a useful baseline relating to user needs. Normal vision has natural limitations that define minimum requirements for readability. From this foundation of normal vision user needs, we can then indicate the degree of additional accommodation needed for varying degrees of impairment(s).

Footnote (1) Presbyopia is not due to disease or degeneration, but due to the enlarging of the eye's lens to a point that prevents near focusing due to the reduced available distance for the ciliary muscles to work and pull the flexible lens into focus. Because it is cartilage, the lens grows throughout our lifetime (as do the ears and nose) increasing size and reducing the flexibility, resulting in presbyopia by the early 40s.


The CIE's canonical glossary can be seen here: and it contains the definitions and equations for CIE color spaces such as CIELUV.


  • Light — visible light is energy in a narrow range of frequencies or wavelengths that can be detected or sensed by “photo sensitive cells” in the back of the eye.

Color Perception

  • Color — color is not "real", but a perception or sensation created by visual processing in the brain (in the brain’s visual cortex) from photosensitive cells in the eye as they respond to different frequencies of light.
  • Hue — refers to a particular color sensation, i.e. red, green, yellow, blue, etc. Hue does not exist in reality, it is solely the perception of the visual system responding to light of different frequencies or combinations of frequencies.
  • The different frequencies of light can be compared to different frequencies of sound waves, where the left of a piano keyboard creates low frequency sound for instance.
  • For visible light, red is a low frequency and blue is a high frequency, and green is about in the middle.
  • Saturation — the color intensity or purity, reduced by:
  • tint (add white),
  • shade (add black),
  • tone (add grey),

Brightness Maths

  • Brightness — a relative perception, see also perceptual lightness. Brightness is a subjective sensation of an object being observed and one of the color appearance parameters of many color appearance models, typically denoted as Q. Brightness refers to how much light appears to shine from something. This is a different perception than lightness, which is how light something appears compared to a similarly lit white object. The antonym of brightness is dimness.
  • Luminance (Y or L) — a physical measure of visible light intensity. Luminance is mathematically linear as light is in the real world.
  • Perceived Lightness (L*) — the perception of physical light intensity, i.e. how light something appears compared to a similarly lit white object. Perceptual lightness is mathematically nonlinear in regards to light in the real world, however, some perceptual models attempt to provide a mathematically linear version of perception which then presents light as non-linear. The symbol L* refers to CIE L*a*b*, and should not be confused with luminance L. The antonym of lightness is darkness.
  • Luma (Y´ prime) — is a gamma encoded, weighted signal used in some video encodings. It is not to be confused with linear luminance.
  • Gamma — or transfer curve (TRC) is a curve that is commonly applied to image data for storage or broadcast to reduce perceived noise and improve data utilization.


  • Contrast — is a perception of the difference between two objects/elements. There are many forms of contrast, and the different types of contrast interact with and are affected by each other as well as being affected by other aspects of vision.
  • Lightness contrast: the difference in lightness and darkness between two items. This is a particularly important form of contrast for information such as text.
  • Spatial contrast: in other words contrasts of size. Size contrasts directly affect the perception of lightness contrasts.
  • Hue contrast: the perception of different light frequencies. Hue contrasts are three times weaker than lightness contrasts, and some people have problems perceiving some hues, so hue should never be a primary design contrast.
  • Positional contrasts: the distance and/or orientation between objects is important in object recognition and identification.
  • Temporal contrasts: contrasts of time, speed, and change.

Sensitivity Acuity and Impairments

  • Contrast Sensitivity — usually refers to an individual's ability to perceive lightness contrast. At birth, contrast sensitivity is very low, and it takes about 20 years for an individual to develop peak contrast sensitivity.
  • Contrast sensitivity impairments can be age related, the result of medications, neurological issues, retinal diseases, ocular degeneration such as cataracts, and other causes.
  • Visual Acuity — acuity refers to the ability of the eye’s optics to focus light onto the photoreceptors on the back of the eye.
  • Poor acuity is usually understood as blurry vision or an inability to focus.
  • Acuity can be affected similarly to contrast sensitivity. And poor acuity can also reduce contrast sensitivity. Though contrast sensitivity impairments do not necessarily affect acuity.
  • Spatial Frequency — in a practical sense, this refers to the weight and size of a font, or the stroke width. A thinner font or narrower stroke width is a higher spatial frequency than a bolder or thicker stroke. Higher spatial frequencies require more luminance contrast to be visible than lower frequencies, such as a very bold large headline.

Additional Useful Resources

Links to Offsite Resources Created by Team Members

  • Evaluating Fonts:(Andy) - Evaluating Fonts: Font Family Selection for Accessibility & Display Readability. This is an informal preprint with many font samples evaluated under experimental conditions at the Myndex lab.

Selected Experiments

  • Perception Experiments Page(Andy) - List of some of the experiments and related discussion that led to the development of SAPC. Some direct links to key findings:
  • Direct link to the CE14 weight experiment results and discussion.
  • Direct link to the CE17 results and SAPC overview.

Web Apps and Code Repositories

  • APCA WebApp(Andy) - APCA simplified contrast tool, for live guidance to designers and developers.
  • SAPC WebApp(Andy) - SAPC (APCA) contrast tool, lookup table for font size and weight, and examples.
  • CVD Simulator WebApp(Andy)- Color Vision Deficiency simulator based on the Brettel research.
  • Andy's Github repo - The APCA Source Code and Related.

Articles, Discussions


These info graphics were created by A.Somers to help explain some of the concepts related to readability of web content.

Critical Font Size Chart

Critical Font Size for Readability. Notice that average vision (20/20) indicates a minimum critical size of about 14px minimum, 19px as preferred. And for 20/16 "normal good vision) the minimum is over 11px.

Screen Pixel Density vs View Distance

This chart shows screen pixel density vs distance to maintain the CSS reference px relationship to arc minutes.

Screen pixel density vs distance to maintain the CSS reference px relationship to arc minutes..

As can be seen, devices that are designed to be used closer such as a phone, is also designed with a higher pixel density, such that the relative visual angle of one CSS px remains the same. The reference is a desktop monitor at 28" with 96ppi or a phone at 12" with 224ppi all dresult in the same visual angle for a CSS px.

Contrast and Spatial Frequency

Contrast Sensitivity Curve vs Spatial Frequency, with sample fonts.

Conformance for Visual Contrast

Contrast Values Lookup

Additional Examples

The following examples demonstrate how much luminance each of the three sRGB primaries provides to the total. Blue hardly eny at all. Againt black, even maximum blue is unreadable.

Maximum red interestingly is in the perceptual middle of dark/light, so for normal vision it is about equally readable on either black or white. However, it is to be noted that some forms of CVD see sRGB red as 35% darker, so red on black may be a problem for them.

Green makes up the vast majority of luminance, so as can be seen, full green is much brighter than either full red or full blue. As a result, full green against white is unreadable, and it must be lowered substantially, by about 75% luminance.

Font and Color Examples


Smaller Examples This is just a smaller version of the example.


Partial Bibliography and Reference Cites

The following references are part of the work product of A. Somers 2019/2020 research into visual contrast and accessibility.

Key References

J. C. Stevens and S. S. Stevens
Brightness function: Effects of adaptation
J. Opt. Soc. Am., vol. 53, pp. 375-385, 1963

C. J. Bartleson and E. J. Breneman
Brightness Perception in Complex Fields
J. Opt. Soc. Am., vol. 57, pp. 953-957, 1967

C. A. Poynton
A Technical Introduction to Digital Video
John Wiley & Sons, New York, 1996

C. A. Poynton
Gamma and its disguises: The nonlinear mappings of intensity in perception, CRTs, film and video
J. SMPTE, pp. 1099-1108, 1993

Nooree Na; Hyeon-Jeong Suk
Adaptive luminance contrast for enhancing reading performance and visual comfort on smartphone displays
3 November 2014

M.D. Fairchild
Color Appearance Models
John Wiley and Sons, 3 edition, 2013.

L.A. Olzak and J.P. Thomas
Seeing spatial patterns
In Handbook of perception and human performance. Wiley, 1986.

Maureen Stone
In color perception, size matters
IEEE Computer Graphics and Applications, 32(2):8–13, March/April 2012.

Maureen Stone, Danielle Albers Szafir, and Vidya Setlur
An engineering model for color difference as a function of size
In 22nd Color and Imaging Conference. Society for Imaging Science and Technology, 2014.

Nooree Na and Hyeon-Jeong Suk
Adaptive luminance contrast for enhancing reading performance and visual comfort on smartphone displays
Optical Engineering 53(11), 113102 (November 2014)

David Kane, Marcelo Bertalm
The influence of lightness, and the crispening effect on the perceived contrast of textured images
Dept. of Information and Communication Technologies, Universitat Pompeu Fabra, Barcelona, Spain

Gaurav Sharma,1* Wencheng Wu,2 Edul N. Dalal2
The CIEDE2000 Color-Difference Formula: Implementation Notes, Supplementary Test Data, and Mathematical Observations
COLOR research and application Volume 30, Number 1, February 2005

by M. Ronnier Luo and Changjun Li
Cie Colour Appearance Models And Associated Colour Spaces
Chapter 11 of

Knoblauch K1, Arditi A, Szlyk J.
Effects of chromatic and luminance contrast on reading.
J. Opt. Soc. Am. A/Vol. 8,No. 2/February 1991

Legge GE1, Parish DH, Luebker A, Wurm LH.
Psychophysics of reading. XI. Comparing color contrast and luminance contrast.
Minnesota Laboratory for Low-Vision Research, University of Minnesota, Minneapolis 55455.

Thomas E. White1, Bibiana Rojas2, Johanna Mappes2, Petri Rautiala3 and Darrell J. Kemp1
Colour and luminance contrasts predict the human detection of natural stimuli in complex visual environments
Biol. Lett. 13: 20170375.
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Neurological / Visual Cortex:

Peter NeriID
The Empirical Characteristics Of Human Pattern Vision Defy Theoretically-Driven Expectations
PLOS Computational Biology December 4, 2018
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Flanagan P1, Zele AJ.
Chromatic And Luminance Losses With Multiple Sclerosis And Optic Neuritis Measured Using Dynamic Random Luminance Contrast Noise.
Department of Psychology, Deakin University, Victoria 3217, Australia.

Yu Han; Yunze Cai
Contrast Sensitivity Function Calibration Based On Image Quality Prediction
11 November 2014 Optical Engineering, 53(11), 113107 (2014)
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Stuart Jackson
Encoding-Decoding Models Of Luminance Contrast Processing
a dissertation @ Center for Neural Science New York University

Daniel P.SpiegelabAlexandreReynaudbTatianaRuizbMaudeLaguë-BeauvaisacRobertHessbRezaFarivarab
First- and second-order contrast sensitivity functions reveal disrupted visual processing following mild traumatic brain injury
Vision Research 122 (2016) 43–50

Dajun Xing, Chun-I Yeh, James Gordon, Robert M. Shapley
Cortical brightness adaptation in macaque V1
Proceedings of the National Academy of Sciences Jan 2014, 111 (3) 1210-1215; DOI: 10.1073/pnas.1314690111

Ji Dai1,2 and Yi Wang1
Representation of Surface Luminance and Contrast in Primary Visual Cortex
Cerebral Cortex April 2012;22:776-787 doi:10.1093/cercor/bhr133

Monitor Technology Related

Federal Aviation Administration (FAA). (2003)
Human factors design standard (HFDS)
HF-STD-001. Available for download at
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Larry Arend, with contributions from Alex Logan and Galina Havin
Using Color In Information Display Graphics
NASA Website, NASA Ames Research Center, Human Systems Integration Division
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Ulf Ahlstrom, FAA and Larry Arend NASA Ames Research Center
Color Usability On Air Traffic Control Displays
Proceedings Of The Human Factors And Ergonomics Society 49Th Annual Meeting—2005 93

Rafa􏱈l K. Mantiuk
Perceptual Display Calibration
Postprint version _ Displays: Fundamentals & Applications, Second Edition by Rolf R. Hainich and Oliver Bimber, A K Peters/CRC Press 2016.

Department Of Defense Handbook
Electronically / Optically Generated Airborne Displays

Sumanta N. Pattanaik James A. Ferwerda Mark D. Fairchild∗ Donald P. Greenberg
A Multiscale Model of Adaptation and Spatial Vision for Realistic Image Display
Program of Computer Graphics†, Cornell University


D. A. Burkhardt, J. Gottesman, D. Kersten, G. E. Legge,
“Symmetry and constancy in the perception of negative and positive luminance contrast”
JOSA A, 1(3), 309-316 (1984).

Gordon E. Legge, Gary S. Rubin, Denis G. Pelli, And Mary M. Schleske
Psychophysics Of Reading Ii. Low Vision

Anusha Y Sukhaa and Alan Rubinb
Psychophysical aspects of contrast sensitivity*
S Afr Optom 2013 72(2) 76-85

Gordon E Legge
Color Improves Object Recognition in Normal and Low Vision
Article in Journal of Experimental Psychology Human Perception & Performance
<a href="September 1993 DOI: 10.1037/0096-1523.19.4.899 · Source: PubMed">September 1993 DOI: 10.1037/0096-1523.19.4.899 · Source: PubMed</a>

Anthony J. Calabria and Mark D. Fairchild
Perceived image contrast and observer preference I. The effects of lightness, chroma, and sharpness manipulations on contrast perception
Munsell Color Science Laboratory—Rochester Institute of Technology

Chung ST, Tjan BS
Spatial-Frequency And Contrast Properties Of Reading In Central And Peripheral Vision
J Vis. 2009;9(9):1–19. Published 2009 Aug 28. doi:10.1167/9.9.16

Kim, M. H., Ritschel, T., and Kautz, J. 2011
Edge-Aware Color Appearance
ACM Trans. Graph. 30, 2, Article 13 (April 2011), 9 pages. DOI = 10.1145/1944846.1944853
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Haun, A. M., & Peli, E. (2013)
Perceived contrast in complex images
Journal of Vision, 13(13):3, 1–21, doi:10.1167/13.13.3.
<a href="http://www.">http://www.</a>

Hyuk-Ju Kwon, Sung-Hak Lee†, Seok-Min Chae†, and Kyu-Ik Sohng †
Tone Mapping Algorithm for Luminance Separated HDR Rendering Based on Visual Brightness Functions
National Research Foundation of Korea
<a href="(NRF) funded by the Ministry of Education, Science and Technology (2011-0025905).">(NRF) funded by the Ministry of Education, Science and Technology (2011-0025905).</a>

Cédric Bertolus, Daniel Bailleul, Marc Mersiol.
Viewing distance requires large characters to ensure legibility on TV-set.
AFIHM. 29ème conférence francophone sur l’Interaction Homme- Machine, Aug 2017, Poitiers, France. ACM, IHM-2017, 10 p., 2017,
<a href=""> 10.1145/3132129.3132133. hal-01578483</a>

By R. D. Freeman And L. N. Thibos
Contrast Sensitivity In Humans With Abnormal Visual Experience
J. Physiol. (1975), 247, pp. 687-710 687 With 10 text-ftgure8

Gary S. Rubin1,2, Mary Feely1, Sylvie Perera3, Katherin Ekstrom3 and Elizabeth Williamson4
The effect of font and line width on reading speed in people with mild to moderate vision loss
Ophthal. Physiol. Opt. 2006 26: 545–554

Nafiseh Hojjati & Balakrishnan Muniandy Universiti Sains Malaysia, Malaysia
The Effects of Font Type and Spacing of Text for Online Readability and Performance
Contemporary Educational Technology, 2014, 5(2), 161-174

Jonathan S. Pointer
Recognition versus Resolution: a Comparison of Visual Acuity Results Using Two Alternative Test Chart Optotype
J Optom 2008;1:65-70

João LourençoJoão LourençoStephanie MroczkowskaStephanie MroczkowskaPaul H ArtesPaul H ArtesLuis Garcia-SuarezLuis Garcia-Suarez
Luminance contrast sensitivity for achromatic and chromatic parafoveal stimuli under mesopic conditions (Mesopic visual function in healthy and Aged-related Macular Degeneration(AMD) subjects: relating structure to function)
British Congress of Optometry and Visual Science 2019 (BCOVS 2019)
<a href="DOI: 10.13140/RG.2.2.15959.83367">DOI: 10.13140/RG.2.2.15959.83367</a>

William V. Good,1 Chuan Hou,1 and Anthony M. Norcia1
Spatial Contrast Sensitivity Vision Loss in Children with Cortical Visual Impairment
Investigative Ophthalmology & Visual Science, November 2012, Vol. 53, No. 12

Kaisa Tiippana a,*, Risto Na ̈sa ̈nen b
Spatial-frequency bandwidth of perceived contrast
Vision Research 39 (1999) 3399–3403
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Psychophysics Of Reading: Xi. Comparing Color Contrast And Luminance Contrast1
Gordon E. Legge, David H. Parish, Andrew Luebker And Lee H. Wurm

Clinical And Experimental

Cheong, Lovie-Kitchin and Bowers
Reading in low vision

Allen MY Cheong* BSc (Optom) Jan E Lovie-Kitchin* PhD MSc Alex R Bowers† PhD
Determining magnification for reading with low vision

AMAChakrabarti K, Kaczmarek RV, Thomas JA, Romanyukha A.
Effect of room illuminance on monitor black level luminance and monitor calibration.
J Digit Imaging. 2003;16(4):350–355. doi:10.1007/s10278-003-1720-5

Arditi A.
Rethinking ADA signage standards for low-vision accessibility
J Vis. 2017;17(5):8. doi:10.1167/17.5.8

Chung ST, Tjan BS
Spatial-frequency and contrast properties of reading in central and peripheral vision
J Vis. 2009;9(9):1–19. Published 2009 Aug 28. doi:10.1167/9.9.16

Silvia Zuffia, Carla Brambillab, Giordano Berettac, Paolo Scalaa
Human Computer Interaction: Legibility and Contrast
14th International Conference on Image Analysis and Processing (ICIAP 2007)

Aries Arditi *, Jianna Cho
Letter case and text legibility in normal and low vision
Vision Research 47 (2007) 2499–2505

Long To, Russell L. Woods, Robert B. Goldstein, Eli Peli ⇑
Psychophysical contrast calibration
Vision Research 90 (2013) 15–24

Maureen Stone, Sharon J. Laskowski
Guidelines for Using Color in Voting Systems
NIST—Information Access Division Information Technology Laboratory National Institute of Standards and Technology NISTIR 7537

Barten, P. G. J. (1999)
Contrast sensitivity of the human eye and its effects on image quality
Eindhoven: Technische Universiteit Eindhoven DOI: 10.6100/IR523072

Alex D. Hwang and Eli Peli
Positive and negative polarity contrast sensitivity measuring app
IS&T Int Symp Electron Imaging. 2016 ; 2016: .

Visual Calibration and Correction for Ambient Illumination
ACM Transactions on Applied Perception, Vol. 3, No. 4, October 2006.

Color Vision Related Resources:

Hiroshi Fukuda, Shintaro Hara, Ken Asakawa, Hitoshi Ishikawa, Makoto Noshiro, Mituaki Kat u ya
Computer Simulation of Color Confusion for Dichromats in Video Device Gamut under Proportionality Law
IPSJ Transactions on Computer Vision and Applications Vol.7 41–49 (May 2015)
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G. Machado, M. Oliveira, and L. Fernandes (2006)
A Physiologically-based Model for Simulation of Color Vision Deficiency
IEEE Transactions on Visualization and Computer Graphics ( Volume: 15 , Issue: 6 , Nov.-Dec. 2009 )
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V Smith, J Pokorny (1972)
Spectral Sensitivity of Color-Blind Observers and the Cone Photopigments
Yision Res. Vol. 12, pp. 2059-2071. PergamonPress 1972. Printed in Great Britain.
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Deane B. Judd (1949)
Response Functions for Types of Vision According to the Muller Theory
Part of Journal of Research of the National Bureau of Standards, Volume 42, January 1949
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Interesting Concepts in Improving Accessibility:

David R. Flatla (2011)

Accessibility for

Individuals with Color Vision Deficiency
Doctoral Symposium UIST’11, October 16–19, 2011, Santa Barbara, CA, US
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Huei-Yung Lin, Li-Qi Chen, & Min-Liang Wang (2019)
Improving Discrimination in Color Vision Deficiency by Image Re-Coloring
Sensors (Basel). 2019 May; 19(10): 2250.
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Jia-Bin Huang, Sih-Ying Wu, and Chu-Song Chen (??)
Enhancing Color Representation for the Color Vision Impaired
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Gretchen M. Culp (2016)
Increasing Accessibility for Map Readers with Acquired and Inherited Color Vision Deficiencies: A Re-Coloring Algorithm for Maps
CUNY Academic Works.
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Huang, Tseng, Wu, Wang (2007)
Information Preserving Color Transformation for Protanopia and Deuteranopia
October 2007IEEE Signal Processing Letters 14(10):711 - 714 DOI: 10.1109/LSP.2007.898333
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M. Madalena, G. Ribeiro, Abel J.P. Gomes (2019)
Contour Enhancement Algorithm for Improving Visual Perception of Deutan and Protan Dichromats
International Journal of Interactive Multimedia and Artificial Intelligence, Vol. 5, No 5
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  • Chris
  • Bruce
  • Andy
  • Todd L.

Former Participants

  • Cybelle
  • Chuck

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