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While reading on the Lab color space, I stumbled onto the Wikipedia page for the Helmholtz–Kohlrausch effect. I can honestly say that, while all the words in that page make sense individually, I cannot form an image in my mind of what this effect is :(

So, would anyone here have an example or simple illustration of the Helmholtz–Kohlrausch effect?

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up vote 7 down vote accepted

I found this explanation:

CIE lightness is a psychophysical scale based on colourimetric measurements, and may not quite coincide with lightness as experienced by an observer, even under ideal conditions. In particular, certain colours have a tendency to look lighter than a grey of the same CIE lightness, an effect known as the Helmholtz–Kohlrausch Effect (Figure 8.1). In Photoshop, this can result in some small surprises when converting images to greyscale mode, which translates all colours to a grey of the same value of CIE lightness.

There is also an image explaining this, take a look at figure 8.1 in

enter image description here

Figure 8.1. Helmholtz–Kohlrausch Effect. A: Various colours on a grey background, all measuring L = 50 in Photoshop. B: same image converted to greyscale mode.

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A simpler example is comparing a blue led to a blue light bulb with the exact same color and luminance. The h-k effect says that the led will look brighter. This surprises us since we are accustomed to light source with the same color and measured luminance appear to have the same brightness. We just have to realize that brightness is not luminance. It has been said that luminance is a photometric parameter whereas brightness is a colorimetric parameter.

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I have to say I don't get this. Can you explain why a blue bulb and blue led give different results? – e100 Nov 19 '12 at 10:18
Something to do with the LED having a narrower spectrum than the light bulb? – endolith Feb 26 '13 at 3:38

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