Optics Lesson 5: Color Optics
Recommended Article : 【Physics】 Physics Index
3. CIE Standard Colorimetric System
1. Munsell Color System
Figure. 1. Munsell Color System
⑴ Overview
① Devised by the American painter Albert H. Munsell in 1905
② Modified by the Optical Society of America’s Colorimetry Committee in 1943 and widely used internationally
⑵ Three elements of color
① Hue: Differentiated color categories
② Chroma: Clarity, purity, and intensity of a color, also referred to as saturation
③ Lightness or brightness: The degree of lightness or darkness of a color
⑶ Color representation
Figure. 2. Color representation in the Munsell Color System [Footnote: 2]
① Based on the primary colors of red, yellow, green, blue, and purple, with additional colors such as orange, lime, turquoise, teal, and violet, totaling 10 colors
② Each color is divided into 10 steps, resulting in 100 colors. The 5th step represents the primary color
③ Significance: Achieving objective scientific representation of color through the three attributes of hue, lightness, and chroma
④ Limitation 1: Gaps between colors are not strictly defined, especially for blue
⑤ Limitation 2: Variation in chroma positions makes color mixing difficult
⑷ Lightness representation
① Assigned values from 0 to 10: 0 represents black, and 10 represents white
② For achromatic colors, “N” is prefixed, followed by an appropriate value
⑸ Chroma representation
① Centered on the achromatic axis, numbered from 0, increasing horizontally with higher saturation
2. Ostwald Color System
⑴ Devised by the German chemist Friedrich W. Ostwald
⑵ Color representation
① All colors are represented using the sum of white amount (W), black amount (B), and pure color amount (C), normalized to 100
② Using yellow, red, blue, and green as primary colors, with orange, violet, magenta, and cyan in between, divided into 24 equidistant complementary colors
③ Advantages: Developed with consideration of color and human physiological aspects
④ Disadvantages: Color names are difficult to understand in the representation. Lack of clear relationships between colors hampers practical use
⑶ Lightness and chroma representation: Not separately represented in the Ostwald color system
3. CIE Standard Colorimetric System
⑴ Established by the Commission Internationale de l’Éclairage (CIE) in 1931
⑵ A scientific color system reflecting both physical light quantities and human perceptual qualities
⑶ Advantage 1: Can express source illuminants’ color unlike Munsell or Ostwald systems that are limited to object colors
⑷ Advantage 2: Independent of human perception
4. CIE RGB Color System
⑴ Color matching experiments: Conducted actively in the early 20th century using three primary colors
① 1st: RGB primary wavelengths defined as 700.0, 546.1, and 435.8 nm
② 2nd: White screen divided into reference and test surfaces
③ 3rd: Illumination of reference surface with three independent brightness-adjustable primary lights
④ 4th: Illumination of test surface with specific test light wavelength, adjusting reference surface’s primary lights to match brightness
⑤ 5th: Color matching: Standard observer judges if mixed color matches test color
⑥ 6th: Averaging results from multiple observers: Called color-matching functions
⑦ Example 1: Mixing approximately 80 W of R primary light and 0.4 W of G primary light creates the same color as 1 W of orange light at 600 nm
⑧ Example 2: Mixing R primary light, G primary light, and B primary light in ratios 243.9 W:4.697 W:3.505 W produces white
⑵ Tristimulus curves: Defined to satisfy the following
Figure. 3. Tristimulus curves in terms of primary units [Footnote: 3]
① E: Spectrum entering the observer’s eye
② R, G, B: Primary lights
③ Drawback: Contains negative values, leading to inconveniences
① Method 1: gray = (R + G + B) / 3
② Method 2: gray = 0.2989 R + 0.5870 G + 0.1140 B
5. CIE XYZ Color System
⑴ Tristimulus curves: Defined as follows
Figure. 4. Tristimulus curves in the CIE XYZ Color System
① S(λ): Spectral radiance based on wavelength λ
② I(λ): Spectral reflectance based on wavelength λ, ranging from 0% to 100%
③ E(λ): Spectrum entering the observer’s eye
④ K: Scaling Factor, ranging from 1 to 100
⑤ Generally, integration is performed for visible wavelengths λ ∈ [380, 780]
⑵ Experimental process
Figure. 5. Experimental process in the CIE XYZ Color System
⑶ CIE Color Coordinates (x, y, z) are normalized versions of X, Y, Z
⑷ Relationship between RGB tristimulus and XYZ tristimulus
① Matrix transformation
② (Comment) The relationship between RGB and XYZ tristimulus seems non-linear; thus, the above formula might not be suitable
③ Instead, refer to the online converter at http://www.easyrgb.com/en/convert.php#inputFORM
6. CIE Lab* Color System
⑴ XYZ → Lab* transformation
⑵ Lab* → XYZ transformation
⑶ (Xn, Yn, Zn) are normalization values for CIE XYZ with respect to standard white, such as (98.072, 100, 118.225)
7. YUV Color System
⑴ RGB to YUV
① Y = 0.299 R + 0.587 G + 0.114 B
② U’ = (B - Y) × 0.565
③ V’ = (R - Y) × 0.713
⑵ YUV to gray
① gray = Y
8. HSV Color System
⑴ HSV to RGB
① When 0 ≤ H < 360, 0 ≤ S ≤ 1, 0 ≤ V ≤ 1,
② C = V × S, X = C × (1 - ( H / 60° ) mod 2 - 1 ), m = V - C
③ R = (R’ + m) × 55, G = (G’ + m) × 255, B = (B’ + m) × 255
9. Luv* Color System
Input : 2020.05.26 09:27