Colours of Objects and Colours of Illumination Dr David J. C. Briggs A presentation for the Colour Literacy Project’s Forum #8, "The interaction of colour and light", June 13, 2024 https://youtu.be/p0xJNvQQP-U Contents 00:00 Luminance and chromaticity 02:04 Colours of light 06:03 Colours of objects 09:10 Visual layering 12:36 Colour attributes and visual layering [0:00] I’d like to thank the Colour Literacy team for asking me to give this overview of two important foundational topics: the standard attributes of perceived colour and the concept of visual layering, which is central to our perception of colour. I’ll begin with two concepts that are essential if you want to understand what colour is a perception of. [00:27] Luminance is the intensity of the light from an area as visible to the human visual system, that is, the power of the visible wavelengths, weighted according to how visible light of each wavelength is to us. Variations in luminance appear to us as variations in brightness and are represented in this photograph on a finite scale of relative luminance, determined by the exposure setting I chose. Chromaticity is the overall balance of spectral or wavelength composition of a light, again as visible to the human visual system. Variations in chromaticity appear to us as variations in hue and saturation and are represented in the pixels of my photograph by varying proportions of red, green and blue lights. Our visual system does much more than just represent to us variations in luminance and chromaticity, however. Unconsciously, and seemingly instantly it accounts for these variations in terms of superimposed estimates of the intensity and chromaticity of the light falling on objects, and of the disposition of those objects to reflect different parts of the spectrum. We also perceive these unconscious estimates of spectral properties as colours, so that in the same area we can perceive colours relating to objects, colours relating to the light falling on objects, and colours relating to the light reaching our eyes from objects. [02:04] The detailed spectral composition of a light can be shown as a spectral power distribution or SPD. The SPD of noon daylight (that is, direct sunlight plus skylight) is not perfectly even, but has a slight bias or imbalance towards the short-wavelength half of the spectrum. Our visual system is attuned to perceive light with this somewhat uneven SPD as colourless or “white” light, and as our lenses slowly become brown with age it gradually adjusts such that we continue to see noon daylight as colourless. The hue of a coloured light is the way in which we perceive a direction of bias in its chromaticity relative to noon daylight. This principle goes back to Newton’s concept of the “centre of gravity” of the component ”rays” of a light in his famous colour circle, The pale blue colour of the skylight reflected from this white foam is the way in which we perceive a stronger bias towards short wavelengths than in noon daylight. The golden orange colour of the late afternoon sunlight reflected from this white building is the way in which we perceive a bias towards longer wavelengths than in noon daylight. Other hues of light, such as green and purple, are how we perceive other overall directions of bias in a circuit of directions towards long-, middle-, short- or long- and short-wavelengths. The reason why our visual system can’t detect spectral power distributions in all their detail, but only their overall balance called chromaticity, is because it relies on comparing the responses of just three cone cell types. The overall balance of spectral composition as detected by a mathematically defined “standard” human observer is represented on a chromaticity diagram such as the CIE 1931 x,y chromaticity diagram shown on the right here, the most familiar of several modern descendants of Newton’s circle. The three lights shown here differ physically but match to the standard observer and so plot at the same point W on this diagram. The colour of a light can be described in terms of the colour attributes of hue, brightness, and either saturation or colourfulness. The saturation of a light is perceived on a scale of decreasing white content, from white light through to coloured light with no whitishness, and is the way in which we perceive its spectral purity, or the amount of bias of its spectral composition relative to daylight, represented on a chromaticity diagram by relative distance from white light. Our blue skylight and golden orange afternoon sunlight are both fairly low in purity and so are perceived as fairly low in saturation. The brightness of a light is our perception of its intensity or luminance, and the colourfulness is simply how colourful the light appears, and depends on its combined saturation and brightness. The three purple lights in the middle diagram illustrate uniform saturation, but the brightest one exhibits the highest colourfulness. Since colourfulness is in effect saturation multiplied by brightness, we can also describe saturation as the colourfulness of a light relative to its brightness, as in the definition on the left. [06:03] Colours of light-reflecting objects can be described in terms of the colour attributes of hue, lightness and chroma, as in the Munsell system. The lightness (or greyscale value) of an object can be defined as its nearest match on a scale between black and white, or as the brightness of its appearance compared to a similarly illuminated white object. The chroma of an object can be defined simply as the strength of its colour, or the amount of difference from the grey of the same lightness, or as the colourfulness of its appearance at a given level of illumination. The lightness of an object is our perception of its physical property of reflectance, the proportion of light of all wavelengths that the object reflects. The hue and chroma of an object constitute our perception of its physical property of spectral reflectance, that is, the proportion of the light of each wavelength that the object reflects. However, as with the spectral composition of light, our visual system can only detect an object’s overall spectral reflectance, “overall” again meaning at the level of its long-, middle- and short-wavelength components. The hue is our perception of the direction of “overall” bias of the spectral reflectance, and the chroma is our perception of the absolute amount of this bias. A Munsell notation of 10R 6/14 means a Munsell hue of 10R (our perception of a certain direction of overall bias towards the long-wavelength part of the spectral reflectance), a high Munsell chroma of 14 (our perception of the high amount of this bias, shown by the strongly plateaued shape of the spectral reflectance curve) and a moderate lightness of Munsell value 6 (our perception of a moderate reflectance of about 30%). Like all Munsell chips, this orange chip is manufactured to a colorimetric specification, xyY, of the light it reflects in daylight. The small x and y refer to the chromaticity or balance of wavelengths of this light, telling us that it exhibits a specific orange hue at high saturation, and the capital Y of 0.3 tells us that it has 30% of the power of the light falling on the chip. Strictly speaking the perceived colour of 10R 6/14 only corresponds to this colorimetric specification for a CIE standard observer and a specific daylight illuminant called Illuminant C. But to an extent we tend to perceive this colour-exhibited-in-daylight through considerable variations in illumination as the colour belonging to the object, in a phenomenon I’ll describe now as visual layering. [09:10] I said just before that the lightness of an object is our perception of the proportion of the light falling on it that it reflects. I wonder if you’ve ever thought about how remarkable it is that we have a perception of lightness. If we needed to place a perfect white reflector beside an object in order to judge its lightness, it wouldn’t be nearly so remarkable, we could just compare the brightness of the light reflected directly. But the fact that we routinely perceive objects as exhibiting a particular lightness without needing to do this shows that our visual system automatically presents us with an estimate of the amount of light falling on objects, without having any direct way of measuring this amount. To do this, in the words of Frederick Kingdom, our visual system “decomposes the visual input into light and material”. Without requiring any conscious intellectual activity from us, our visual system seemingly instantly arrives at an estimate of the combination of illumination and reflectance responsible for the light reaching our eyes from each object that we see. And it does this so automatically and consistently that it seems to us that our eyes simply detect colours of objects directly, and it doesn’t occur to us how extraordinary this perception really is! In this animation based on Kingdom’s Figure 1, notice how the appearance of the image on the left involves two superimposed perceptions, of reflectance and of illumination. Our perceptions of colours of objects are always accompanied by a superimposed perception of illumination in this way. We perceive these unconscious estimates of the spectral properties of the object and of the illumination as a colour belonging to each, respectively called an object colour (here a uniform middle grey) and an illumination colour (here a white light of varying brightness). We perceive objects as maintaining a relatively stable object colour belonging to them under large variations in intensity of the same illumination and to a lesser to much lesser extent under illumination of varying spectral composition. This relative stability of object colour, called colour constancy, is the way in which we perceive the relatively consistent estimates of overall spectral reflectance that our visual system can arrive at. Colour constancy breaks down severely if the illumination is strongly coloured or very dim, when we might have a quite different perception of the colour of the object, although at a certain point we might more naturally say that we can’t see the colour of the object clearly, and want to examine the object is daylight to judge what we think of as its seemingly inherent colour. [12:36] I’ll conclude by showing how the six attributes of perceived colour I discussed apply to our superimposed colour perceptions of this image. We perceive a cube exhibiting the same high chroma orange object colour on all three faces, as if it were painted all over with the same paint that we might judge to have a Munsell notation of about 10R 6/14, resting on a floor of uniformly black and white tiles, of Munsell values around 2 and 9.5 respectively. If these were actual materials we could expect that they would each have a different spectral reflectance that might be variations on the ones shown here. But at the same time, areas A to B to C on the cube appear progressively brighter and more colourful. These differences in brightness and colourfulness describe the colour of the light reaching our eyes from different areas of the cube. Similarly, although we perceive the lighter-coloured areas of the floor as being white things, that is as having a uniform white colour of high lightness belonging to them, the corresponding areas send light of a wide range of intensities to the eye, perceived as a wide range of brightnesses. So, to describe the colour appearance of this scene we need five different colour attributes: hue, lightness, chroma, brightness and colourfulness. Because objects exhibit relatively consistent hue, lightness and chroma under varying illumination, we perceive these colour attributes as belonging to the objects and thus to constitute the object colour. The variations in brightness and colourfulness, on the other hand, are perceived as being imposed by the illumination and not as belonging to the objects. Now notice that the light from areas A to B to C increases in colourfulness in step with the brightness. This means that it stays the same saturation. This uniform saturation is our perception of light of uniform chromaticity or spectral balance, as results when a uniform object reflects the same light at varying intensities. Since objects exhibit the same saturation under widely varying intensities of illumination, saturation can also be perceived as belonging to the object and thus as another attribute of object colour alongside lightness and chroma. As always, our perception of object colours is accompanied by a perception of illumination. Illumination can vary in hue and saturation or colourfulness, but in this image we perceive achromatic or “white light” illumination, varying only in brightness. Notice that once again we experience these perceptions of object colour and illumination colour superimposed in the same area, as if the colours perceived as belonging to the tiles and cube are seen through the illumination. Although these object colours are perceptions created by our visual system, we have the illusion that they are physically located in the objects themselves, even when, as here, the objects don’t physically exist. For anyone wanting more detail than I could provide in this brief overview, here are two longer presentations and my two-part AIC Journal paper, all from last year. Illustrated texts of these two presentations are available on my academia.edu page where they’ll be joined by this presentation tomorrow (https://nassydney.academia.edu/DavidBriggs ). Briggs, DJC (2023). The Elements of Colour. Part 1: Colour Perceptions, Colour Stimuli, and Colour Measurement. Part 2: The Attributes of Perceived Colour. Journal of the International Colour Association, Volume 33, Special Issue: Contributions of the Colour Literacy Team (editor Robert Hirschler): https://aic-color.org/journal-issues Briggs, DJC (2023). Colours of Objects and Colours of Light. Keynote address, Colour Society of Australia National Conference, Colour Sense and Sensibility, Perth, October 12-15, 2023, https://www.youtube.com/watch?v=ii9dWIG9nOY Briggs, DJC (2023). The Elements of Colour. Invited webinar for the Design Institute of Australia, May 18, 2023. https://www.youtube.com/watch?v=AtrRpGb-7FU NOTES 1Exported from Spectrashop by Robin Myers Imaging https://www.rmimaging.com/spectrashop.html 2Shamey 3Kelber R (2023), Springer Encyclopedia of Color Science and Technology (2023, p. 719) et al. (2017), Phil. Trans. R. Soc. B 372: 20160065. 4 Artist’s Helper by Robert Burrage https://www.artistshelper.com/p/home.html 5Exported from https://fluxometer.com/ 6Commission Internationale de l´Eclairage e-ILV https://cie.co.at/e-ilv 7From Spectrashop (1, above), extrapolated one chroma step from curves for the Munsell matte edition, which lacks 10R 6/14. 8 Kingdom, FA (2008), Perceiving light versus material. Vision Res. 2008 Sep ;48(20):2090105. 9MacLeod, University. RB (1932), An experimental investigation of brightness constancy. Columbia