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holyeyeolo
holyeyeolo
Participating Frequently
July 10, 2018
Question

Equal Luminosity

  • July 10, 2018
  • 3 replies
  • 3573 views

Hi,

I'm trying to find a way to choose equal Luminosity colors. From what I understand Luminosity is the PERCEIVED Brightness by a human viewer, and is thus different from the Lightness of sRGB workspaces. My question is, is it also different from the Lightness of LAB? Explanation of luminosity in Photoshop  and Re: Luminosity/Color blend modes - what do they really do?  give different answers. If I open a file in Color Mode - Lab Color, then in the color picker I pick 2 different colors in the Lab color selection WITHOUT altering the L, would that give me two colors with similar Luminosity? Two colors with similar perceived Brightness to the standard human eye?

Also, is desaturating them a good way to check that? Wouldn't that mean that desaturatin a color in Lab mode different from desaturating a color in RGB mode?

Thank you

    This topic has been closed for replies.

    3 replies

    G
    G_Hoffmann
    Inspiring
    July 11, 2018

    Continued about colors with equal brightness:

    Munsell colors were made by the inventor for equal visual brightness for the same "Value",

    which is the numerical Munsell parameter for lightness / luminance / brightness.

    http://docs-hoffmann.de/munsell15052009.pdf

    It's surprising that the Lab lightness is almost constant for the same Value. The Lab numbers

    are shown on reference pages for the color tables.

    If sorting by Lab doesn't show similarly sorted brightness, then sorting by Value wouldn't be

    very different. Still an enigma...

    Best regards --Gernot Hoffmann

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    July 10, 2018

    Actually I think Luminosity in Photoshop is identical to the L channel in Lab.

    That makes sense, since Lab is always present in the background as a Profile Connection Space (PCS) in all color management operations. There's always a Lab value behind any RGB value, and it's always readily available.

    This explains itself:

    holyeyeolo
    holyeyeoloAuthor
    Participating Frequently
    July 10, 2018

    That's what I wanted to verify. Since in this thread Re: Luminosity/Color blend modes - what do they really do? (from 2008 mind you) see comments 2 and 6, the opposite is stated - Luminosity and L* of Lab are different. While in your comment from 2016 Explanation of luminosity in Photoshop  they are the same. So as of 7/10/18 Luminosity and Lightness of Lab in Photoshop are the same. Is that correct?

    I got really confused because in other softwares the Luminosity functions have no connection to the meaning of the word. They are like HLS workspace or something... I downloaded Photoshop Trial ONLY to check CIELAB out.

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    July 10, 2018

    You can verify for yourself. Open two copies of the same image. Convert one to Lab and desaturate.

    In the RGB version, you need to keep gamma constant in order to not get false results. Grayscale is displayed according to your working gray - and what you want to do is display a luminosity mask which is grayscale.

    So to keep this constant, set your working gray to gamma 2.2, and convert the file to Adobe RGB which also has a straight gamma 2.2 tone response curve (sRGB has a custom TRC, and ProPhoto 1.8).

    With this set, make a luminosity selection, ctrl + 2, and convert to mask by adding an adjustment layer. Now alt-click the mask icon to display it as a grayscale image.

    These two should now match, if my theory holds. I haven't done extensive testing, but so far it seems to hold up.

    c.pfaffenbichler
    Community Expert
    c.pfaffenbichlerCommunity Expert
    Community Expert
    July 10, 2018

    Also, is desaturating them a good way to check that?

    Well, what do you mean by that?

    Desaturating with the Hue/Saturation Adjustment in RGB averages the three Channels, so that would not give you a proper representation of Luminance as evidenced in Lab.

    holyeyeolo
    holyeyeoloAuthor
    Participating Frequently
    July 10, 2018

    Not in RGB, in Lab mode. If I desaturate two colors In RGB color mode and they give the same gray, then they both have the same Brightness. If I desaturate two colors in Lab color mode and they give me the same gray, then they both have the same Luminance. If I desaturate two colors in RGB mode that have the same L in the Lab color selector, then they won't necessarily give me the same Brightness. Is that all correct? 

    G
    G_Hoffmann
    Inspiring
    July 10, 2018

    Nobody in color science knows, how Photoshop´s luminosity is defined.

    Therefore I don't refer to this expression.

    R.W.G.Hunt says in his great book Measuring Colour:

    "3.2. The Y tristimulus value

    If the Y tristimulus value is evaluated on an absolute basis as Y (with index L),

    in candelas per square metre, for example, it represents the luminance of the colour.

    This provides the basis for a correlation with the perceptual attribute of brightness.

    As has already been explained, the correlation is complicated by the effect of the

    viewing condiditions, by the non-linear relationship between brightness and

    luminance, and by the partial dependance of brightness and colourfulness."

    (bold by me)

    Y is a linear combination of R, G and B in any linear RGB color space, that is to say

    without any gamma encoding.

    L* in CIE 1976 (CIELab) is called CIE 1976 lightness or simply Lab lightness.

    The relation between Y and L* is nonlinear but monotonic: a series of patches

    with increasing values Y has as well increasing values L* (and patches with identical

    values Y have as well identical values L*).

    But alas, such a series doesn't show monotonously increasing brightness (visual

    appearance) if the colors differ considerably by saturation.

    Open page 10 in this doc

    http://docs-hoffmann.de/gray10012001.pdf

    in Photoshop in mode LAB (!).

    Each image is built in index color with 256 colors in the palette. The palette diagram

    shows these colors ordered by values of Y or  L* (which delivers the same order).

    That is, the Lab values are monotonously increasing, but the visual brightness seems

    to jump. This is mainly a result of the so-called Helmholtz-Kohlrausch effect: more

    saturated colors appear brighter.

    (The order is almost monotonous, there are small deviations because of various

    roundings in the calculations, please don't worry.)

    The result: the bold text as cited above – there is no simple recipy for colors with

    equal perceived (visual) brightness.

    Best regards  --Gernot Hoffmann

    P.S.:

    This doc

    http://docs-hoffmann.de/palette30082003.pdf

    shows as well, that the attempt to create "balanced palettes" with colors of equal

    brightness based on equal values L* failed partly.

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