Two elements that influence the effect of artificial light sources in a space are the color of the lighting (correlated color temperature) and its brightness (illuminance). The combination of these two greatly changes the psychological impression we experience.

It is well known that visually (through color), we psychologically feel a sense of temperature. ≪1≫ Regardless of the actual physical temperature of an object, colors in the red-orange spectrum give a sense of warmth, while blues and blue-violets evoke a feeling of cold. Lighting with a low (correlated) color temperature appears reddish and feels psychologically warm while lighting with a high (correlated) color temperature appears blueish-white and feels psychologically cold. It is important to note that the (correlated) color temperature value and the psychological sensation of warmth and coolness have an inverse relationship.

The diagram on the right shows the effects of (correlated) color temperature and illuminance on human psychology.

Lighting with a high (correlated) color temperature that appears white to blue-white can create a gloomy atmosphere when dim. However, if the illuminance is high, the same hue is energizing.

Conversely, lighting with a lower (correlated) color temperature that appears reddish at low illuminance can create a psychologically calm and soothing atmosphere. However, if the illuminance becomes too high, it can result in a harsh light.

Thus, when evaluated in terms of psychological comfort, the combination of (correlated) color temperature and illuminance can have opposite effects.

As mentioned above, lighting has a strong impact on human psychology, which is why different colors are used to achieve certain effects. For example, there are multiple types of fluorescent lamps available based on differences in correlated color temperature from 6500 K to 2700 K. (See figure right). ≪2≫

The principle of emission for fluorescent lamps involves generating ultraviolet radiation through a discharge between electrodes in a tube filled with mercury vapor, which then excites phosphors coated on the tube wall, producing visible light (fluorescence). (The ultraviolet components used for fluorescent excitation are cut off, so they do not leak outside the tube.) By selecting the type of phosphor, it's possible to alter the spectral distribution of fluorescent emission and produce fluorescent lamps in various colors.

While LEDs can be categorized similarly to fluorescent lamps, they also have unique classifications due to the nature of their manufacturing process. ≪3≫

While (correlated) color temperature is useful for representing the color of white light with a single value, it cannot adequately express color difference between multiple white lights using the difference in (correlated) color temperature values expressed in Kelvins (K). This is because the scale on the blackbody locus converges and becomes denser as the color temperature value increases.

For two white lights with a (correlated) color temperature difference ΔTCP = 100 K, for example, the color difference ΔE1 between 10,000 K and 10,100 K and color difference ΔE2 between 3,000 K and 3,100 K are quite different, with ΔE1 < ΔE2. This is because the relationship between temperature difference and color difference on the blackbody is not a simple proportional relationship.

An index called the reciprocal (correlated) color temperature was developed to express color differences between white lights with respect to (correlated) color temperature values. The reciprocal (correlated) color temperature is defined as one million times the inverse of the (correlated) color temperature (unit: mrd or mrk). ≪4≫

For example, the reciprocal color temperature of standard illuminant A with a color temperature T_C = 2,856 K is T_C^-1 = 10⁶ / 2856 = 350.1 mrd, and the reciprocal correlated color temperature of standard illuminant D₆₅ with a correlated color temperature
T_CP = 6,504 K is T_CP^-1 = 10⁶ / 6504 = 153.8 mrd. ≪5≫

The space in which reciprocal (correlated) color temperatures are displayed can generally be considered a uniform color difference space, and the relationship between the difference in reciprocal (correlated) color temperature ΔT_C^-1 (ΔT_CP^-1) and color difference ΔE becomes mostly independent of the absolute value of the (correlated) color temperature.

A reciprocal color temperature difference of approximately 5.5 mrd or less is considered indistinguishable to the human eye, regardless of the (correlated) color temperature value.

The concept of (correlated) color temperature is widely used for its convenience, but its application is limited to white light sources. When it comes to colored light sources like monochromatic red or green LEDs, although using two values (x, y) expresses the emission color objectively and accurately, it's not easy for people to associate those two numbers with the actual color. Dominant wavelength is a practical way to approximate the color with a single number.

For example, in the xy chromaticity diagram below, there is a color F at (0.32, 0.51). Drawing a straight line from the white point W (0.333, 0.333) towards F and extending it further, it will eventually intersect with the spectrum locus. Let's call this point S. This point S corresponds uniquely to the chromaticity of a monochromatic light of a certain wavelength, so this wavelength is taken as the dominant wavelength (λ_d) of color F. (In the diagram below, point S corresponds to the chromaticity point of 550 nm monochromatic light, so the dominant wavelength of color F would be 550 nm.) The colors of monochromatic light are familiar to everyone as they line up in the order of colors of a rainbow, and it is relatively easy to associate a wavelength with a color, so hearing the value of the dominant wavelength allows us to immediately imagine what color it represents. ≪6≫

We used the color F as an example earlier. Now consider color M at (0.44, 0.20). Extending a line from the white point W (0.333, 0.333) towards point M and beyond, it will hit not the spectrum locus but the purple boundary on the xy chromaticity diagram.

At the intersection point N with the purple boundary, there isn't a corresponding monochromatic light as with color F. In such cases, if we extend this line in the opposite direction, it hits the spectrum locus at point G. Since a certain wavelength of monochromatic light corresponds uniquely to point G, we take this wavelength as the complementary dominant wavelength (λ_c) of color M. In the diagram, point G corresponds to the chromaticity of 508 nm monochromatic light, so the complementary dominant wavelength of color M is 508 nm.

It's difficult to "directly" associate the value of the complementary dominant wavelength (λc = 508 nm) with color M, but by being aware of the relationship of complementary colors, it's possible to indirectly associate M. As you may understand from Chapter 25, since the xy chromaticity diagram is a linear space, colors that exist diametrically opposite around the white point W have a precise complementary relationship (when mixed in the appropriate ratio, they produce an achromatic color).

In the diagram above, with our understanding of complementary relationships, we know that the chromaticity point lies within the triangle region on the xy chromaticity diagram that uses the purple boundary as the base and the white point W as the apex – that is, within the range of purple to red-purple to red colors. Then, knowing that 508 nm monochromatic light is green, we can infer that the chromaticity point is in the red-purple (magenta) color spectrum.

As illustrated above, the dominant wavelength and complementary dominant wavelength can be said to correspond to hue, which is one of the three psychological attributes of color.

All the chromaticity points lying on (and extending beyond) the line that connects a color and the white point W have the same value of dominant wavelength (or complementary dominant wavelength). In other words, all colors on this line have the same hue.

As discussed in Chapter 23, chroma increases radially outward from the white point W on the xy chromaticity diagram. Exciting purity represents saturation using the relative ratio of the distance from W to the spectrum locus or purple boundary (WS or WN) in comparison to the distance from W to the chromaticity point (WF or WM).

Note that in the case of the complementary dominant wavelength, the line from the white point W to the color M was extended in the opposite direction, but for exciting purity, the intersection with the purple boundary point N and the white point W's distance is considered 100%.

Colors on the outer edge (spectrum locus and purple boundary) of the xy chromaticity diagram all have an exciting purity of 100% - the highest saturation, while the white point W has an exciting purity of 0% - the lowest saturation (achromatic).

As mentioned above, the definitions of dominant wavelength (or complementary dominant wavelength) are entirely different from those of color temperature (correlated color temperature). However, both are convenient ways to associate a color with a single value. Another thing they have in common is that an additional value can be paired to accurately denote the color — deviation from the blackbody locus (d_uv) for correlated color temperature (T_CP) notation, and exciting purity (p_e or p_ec) for dominant wavelength λ_d (complementary dominant wavelength λ_c) notation.

The difference between the two lies in their complementary applications: (correlated) color temperature only applies to the white regions near the blackbody locus, while (complementary) dominant wavelength is for the chromatic area. ≪7≫

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