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The photopigment absorption curve [1] shows that the red (L) cones have a slight rise at 400nm (violet wavelength). Also, Wikipedia's image shows it [2]. That'

The photopigment absorption curve [1] shows that the red (L) cones have a slight rise at 400nm (violet wavelength). Also, Wikipedia's image shows it [2]. That's why cameras also have a bump at 400mn, as shown on this manufacturer's page (under the relative sensitivity tab) [3]. Cameras must have it so that they can capture light at 400nm the same way our eyes do. Otherwise those colors would look wrong in pictures. That is also how computer monitors can display a color that we see as violet. They output a bit of red and lots of blue, which excites our red and blue cones and give the same stimulus as violet at 400nm wavelength. So, monitors can't output violet, but they can trick our eyes to think that there is violet. The same happens for yellow. Monitors can't output yellow wavelength (590nm), but they output red and green, which excites our red and green cones and we think we see yellow. So, you could also say that when you see yellow on web pages, it isn't really yellow. It is just a mix of red and green. [1] http://www.handprint.com/HP/WCL/color1.html http://www.handprint.com/HP/WCL/color1.html [2] https://en.wikipedia.org/wiki/Photoreceptor_cell#/media/File:1416_Color_Sensitivity.jpg https://en.wikipedia.org/wiki/Photoreceptor_cell#/media/File... [3] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=4024 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=40...

jacobolus 8y ago
> That's why cameras also have a bump at 400mn, This explanation is incorrect. Your graph is on a log scale. You can make a camera out of any invertible linear combination of human cone cell responses and you will end up recording basically the same information as far as an idealized human observer is concerned (that’s the basic idea of the CIE system of colorimetry). The specific choice of computer display primaries has to do with maximizing the possible gamut (as well as other technical and economic criteria), not precisely targeting specific cone cells. > when you see yellow on web pages, it isn't really yellow You are using a non-standard definition of “yellow” not accepted by color scientists or color measurement experts.
- cosarara 8y ago
  This graph, from [1] shows that the L cone's responsivity doesn't go back up at all toward the left: https://en.wikipedia.org/wiki/File:Cones_SMJ2_E.svg https://en.wikipedia.org/wiki/File:Cones_SMJ2_E.svg It is lower at 425 than at 450, which makes sense. Now what does this one from [2] mean https://en.wikipedia.org/wiki/File:1416_Color_Sensitivity.jpg https://en.wikipedia.org/wiki/File:1416_Color_Sensitivity.jp... (logarithmic scale or not)? The "Red cones" Long line says "absorbance" is higher at 425 than at 450. [1] https://en.wikipedia.org/wiki/Cone_cell https://en.wikipedia.org/wiki/Cone_cell [2] https://en.wikipedia.org/wiki/Photoreceptor_cell#Humans https://en.wikipedia.org/wiki/Photoreceptor_cell#Humans
  - jacobolus 8y ago
    Your second link is on a log scale. The left tail of that long-wavelength cone cell response is not related to the bump in the X of the XYZ color matching functions.
    - cosarara 8y ago
      I know it is on a log scale. This shouldn't change whether it goes up or not, it should just change how much it goes up (or down), visually.
- blincoln 8y ago
  > Your graph is on a log scale. Have you considered the possibility that human vision might have a more-or-less logarithmic response, and that this is why so many graphs related to it use a log scale?[1][2][3] [1] "Brightness as perceived by the [human visual system] is a logarithmic function of the light intensity incident on the eye.", https://people.cs.umass.edu/~elm/Teaching/ppt/691a/CV%20UNIT%20Light/691A_UNIT_Light_1.ppt.pdf https://people.cs.umass.edu/~elm/Teaching/ppt/691a/CV%20UNIT... [2] https://spectrum.ieee.org/podcast/biomedical/bionics/does-the-brain-work-logarithmically https://spectrum.ieee.org/podcast/biomedical/bionics/does-th... [3] https://learn.sparkfun.com/tutorials/light/visible-light https://learn.sparkfun.com/tutorials/light/visible-light
  - jacobolus 8y ago
    Human vision is non-linear, but it is not logarithmic. What it does have is several different mechanisms of adaptation which allow it to respond to very dim or very bright contexts. Some of these adaptations take 30 minutes or more. But what we are talking about here is the wavelength-responses of a particular type of cone cell. Individual cone cells are activated when some photon excites a little protein and causes a chemical reaction. You can read about the details here https://en.wikipedia.org/wiki/Opsin https://en.wikipedia.org/wiki/Opsin The spectral sensitivity charts we are talking about here show more or less a summary of how likely that signaling process is to happen for a particular type of cone cell for light coming in at a particular wavelength. To figure out the eye’s overall cone response to some particular light source, you can more or less take the spectrum of the light source, multiply by the spectral sensitivity of the type of cone cell, and then integrate. Notice that that part of our process is linear in terms of the charts we are talking about. The non-linear part has to do with various types of adaptation which attenuate or discount those cone sensitivities more or less uniformly across wavelengths (for each cone). This doesn’t suddenly make the left part of the long-wavelength cone response more relevant.
    - blincoln 8y ago
      Let's assume you're correct, and that the more-or-less logarithmic aspect to human vision is inconsequential here, and also that the hump in absorption around 410nm by the red cones is also irrelevant. How would you explain the human perception of violet light as a purple colour, then?

jacobolus 8y ago

> That's why cameras also have a bump at 400mn, This explanation is incorrect. Your graph is on a log scale. You can make a camera out of any invertible linear combination of human cone cell responses and you will end up recording basically the same information as far as an idealized human observer is concerned (that’s the basic idea of the CIE system of colorimetry). The specific choice of computer display primaries has to do with maximizing the possible gamut (as well as other technical and economic criteria), not precisely targeting specific cone cells. > when you see yellow on web pages, it isn't really yellow You are using a non-standard definition of “yellow” not accepted by color scientists or color measurement experts.

cosarara 8y ago
This graph, from [1] shows that the L cone's responsivity doesn't go back up at all toward the left: https://en.wikipedia.org/wiki/File:Cones_SMJ2_E.svg https://en.wikipedia.org/wiki/File:Cones_SMJ2_E.svg It is lower at 425 than at 450, which makes sense. Now what does this one from [2] mean https://en.wikipedia.org/wiki/File:1416_Color_Sensitivity.jpg https://en.wikipedia.org/wiki/File:1416_Color_Sensitivity.jp... (logarithmic scale or not)? The "Red cones" Long line says "absorbance" is higher at 425 than at 450. [1] https://en.wikipedia.org/wiki/Cone_cell https://en.wikipedia.org/wiki/Cone_cell [2] https://en.wikipedia.org/wiki/Photoreceptor_cell#Humans https://en.wikipedia.org/wiki/Photoreceptor_cell#Humans
- jacobolus 8y ago
  Your second link is on a log scale. The left tail of that long-wavelength cone cell response is not related to the bump in the X of the XYZ color matching functions.
  - cosarara 8y ago
    I know it is on a log scale. This shouldn't change whether it goes up or not, it should just change how much it goes up (or down), visually.
blincoln 8y ago
> Your graph is on a log scale. Have you considered the possibility that human vision might have a more-or-less logarithmic response, and that this is why so many graphs related to it use a log scale?[1][2][3] [1] "Brightness as perceived by the [human visual system] is a logarithmic function of the light intensity incident on the eye.", https://people.cs.umass.edu/~elm/Teaching/ppt/691a/CV%20UNIT%20Light/691A_UNIT_Light_1.ppt.pdf https://people.cs.umass.edu/~elm/Teaching/ppt/691a/CV%20UNIT... [2] https://spectrum.ieee.org/podcast/biomedical/bionics/does-the-brain-work-logarithmically https://spectrum.ieee.org/podcast/biomedical/bionics/does-th... [3] https://learn.sparkfun.com/tutorials/light/visible-light https://learn.sparkfun.com/tutorials/light/visible-light
- jacobolus 8y ago
  Human vision is non-linear, but it is not logarithmic. What it does have is several different mechanisms of adaptation which allow it to respond to very dim or very bright contexts. Some of these adaptations take 30 minutes or more. But what we are talking about here is the wavelength-responses of a particular type of cone cell. Individual cone cells are activated when some photon excites a little protein and causes a chemical reaction. You can read about the details here https://en.wikipedia.org/wiki/Opsin https://en.wikipedia.org/wiki/Opsin The spectral sensitivity charts we are talking about here show more or less a summary of how likely that signaling process is to happen for a particular type of cone cell for light coming in at a particular wavelength. To figure out the eye’s overall cone response to some particular light source, you can more or less take the spectrum of the light source, multiply by the spectral sensitivity of the type of cone cell, and then integrate. Notice that that part of our process is linear in terms of the charts we are talking about. The non-linear part has to do with various types of adaptation which attenuate or discount those cone sensitivities more or less uniformly across wavelengths (for each cone). This doesn’t suddenly make the left part of the long-wavelength cone response more relevant.
  - blincoln 8y ago
    Let's assume you're correct, and that the more-or-less logarithmic aspect to human vision is inconsequential here, and also that the hump in absorption around 410nm by the red cones is also irrelevant. How would you explain the human perception of violet light as a purple colour, then?