Luminance, Grayscale and Desaturation

B&W Conversion Alternatives –

What’s the difference?

There are numerous ways to convert a color image to B&W (not counting special programs and Photoshop plug-ins such my favorite, Silver Efex Pro 2).  The first two that beginners encounter are –

  1. Desaturate the colors
  2. Convert the image to grayscale

A third technique produces what I’ll call a “luminosity” B&W. It can be done with any editing program that uses layers and has a Luminosity blending mode. The method (using Photoshop Elements) is shown in the following screen shot (click any image in this post to enlarge it) –

  • Make a copy of the original image
  • Place a white layer beneath it
  • Change the blending mode of the copy to Luminosity

The result derives from how Luminosity blending works.

Luminosity takes its hue & saturation values from the 2nd layer (the white layer below the luminosity blend layer) and its luminosity values from the top layer. Since the 2nd layer’s H & S value are zero (neutral) the result, as shown, is pure black and white (gray tones) whose lightness values match those of the original image. Voila – we’ve extracted the luminance channel.


Of course there are other B&W conversion approaches but these three represent the basics.
So what? Is one better than another? Can anyone tell the difference? Like most things – it depends. In this post I’ll try to explain and illustrate the differences among them. It’s up to you to decide – but it’s always nice to be able to make an informed decision.
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Appearance

Let’s first consider the visual difference among the three. In each 2×2 illustration that follows, the arrangement of images is –

  • Original………………..Luminance
  • Grayscale…………..Desaturation

Can you see any difference among the three B&W conversions in this next illustration (Alexei Jawlensky – Self Portrait)? Not easy is it? The shading to the right of the face is probably the most noticeable.

How about this next example? In this one the differences are obvious. The colors are Red/Green/Blue with values of 255/0/0, 0/255/0, and 0/0/255 respectively. (Please imagine that the right-most color is blue; it really was but there’s a color management issue using my screen capture program). Note the bottom right conversion, desaturation – it’s uniformly gray. What’s with that? Keep reading.

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How each conversion is computed in Photoshop (and elsewhere)

Are you surprised that a straight desaturation of the color image, as a means of B&W conversion, produced a uniform gray image? Once you know how desaturation is accomplished the mystery will be solved.
1. Desaturation B&W conversion is done by averaging the RGB component values, pixel by pixel. I told you that this image was pure RGB. This means that its color’s component R/G/B values are –

  • Red = 255/0/0
  • Green = 0/255/0
  • Blue = 0/0/255

To determine each color’s gray conversion value, compute the average of each component’s strength –

  • Red = (255+0+0)/3 = 85
  • Green = (0+255+0)/3 = 85
  • Blue = (0+0+255)/3 = 85

And, WOW, look at that! They’re all the same. They all convert to the same shade of gray. Maybe, just maybe – desaturation isn’t that great of a technique for B&W conversion (although at times you can get away with it; it depends on the image’s colors).
Now we know why the desaturation option gave the result that it did in our R/G/B panel.
2. How about  grayscale? How is that computed?
Grayscale attempts to take into account how our vision perceives color. We do not perceive all colors as being equally bright – green is seen as being brighter than red which is brighter than blue.

  • In order to convert colors to match the colors the way that we see them (more or less) we can’t simply average all three colors equally as in the desaturation method.
  • We must weight each color proportionally to how our eyes see them.
    • The accepted weightings are 30% of the red value, 59% of the green value, and 11% of the blue value  – and not equal weights as in desaturation.
    • The weighted average gives us the grayscale shown in the lower left of the above example.

Often, it is difficult to see a difference between the luminance and grayscale versions – I wanted to use the above RGB test panel example to assure you that there is one.
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Which conversion should you use?
As I said, it depends. On what? On which provides the result that YOU want. That’s all that matters.
Myself, I see no redeeming virtue in desaturation and would never use it. Between the other two I tend to prefer the luminosity technique. I’m still researching details on grayscale vs. luminance when it comes to human vision considerations – something for a later post.
Regardless, I do 100% of my B&W in Silver Efex Pro 2 and the preset conversions are nothing more than starting points in my use of SEP 2. (As an aside – the luminosity result is what Nik Silver Efex Pro 2 uses as its default neutral starting image – Preset 000. Take that for whatever it’s worth.)
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A few more examples – same layout as above.

  • Original………………..Luminance
  • Grayscale…………..Desaturation

I chose these for their use of strong harsh colors which give the best chance of showing conversion differences (other than my RGB test panel).


And one last example, this one using a “normal” image to illustrate that – depending on the colors – the difference among the various B&W conversion techniques may be hard to discern with a casual glance. As they say – the devil is in the details.

0 thoughts on “Luminance, Grayscale and Desaturation”

  1. Nice port!
    I wonder if you can help:
    If I’m accessing the colour contrast of a flat colour, let’s say red, what technique gives me the most accurate result, desaturation or grey scale? Completely agree grey scale gives a better result, but which one is truly realistic, I’m mostly concerned with a flat colour. At present desaturation is typically 10% darker than greyscale with the colour I’m using. I need to know which one is correct and therefore a fair test of colour contrast. Your point of view would be very interesting and useful. Thanks.

    • Chris,
      The answer to your quest(ion) is, as I said in the post – It depends. Most especially it depends on the mix of colors in a specific image. For some images your choice almost won’t matter as they’ll look similar or the same. For others, to your eye, one may be the winner – but never the same one.
      That said, I always start with Luminance which is the one that your did not mention. Give it a try and see what you think.
      Also, take a look at this post for my view of which B&W is best (or as the person who prompted the post asked me – Which conversion is the “real one”?).

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