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Module 6: Liquid Crystal Thermography
Lecture 36: HSI model
Imaging using colors
Tristimulus Theory of Color Perception
The HSI Model
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Understanding Color Imaging using the HSI Model in Liquid Crystal Thermography, Slides of Mechanical Engineering
The use of the hsi (hue, saturation, intensity) model for color imaging in liquid crystal thermography. It explains how the human visual system perceives color and the role of the hsi model in simplifying image analysis. The document also covers the tristimulus theory of color perception and the relationship between rgb and hsi color spaces.
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Module 6: Liquid Crystal Thermography
Lecture 36: HSI model
Imaging using colors
Tristimulus Theory of Color Perception
The HSI Model
Module 6: Liquid Crystal Thermography
Lecture 36: HSI model
Imaging using colors
The human visual system can distinguish thousands of varying color shades and intensities but only around 100 shades of gray. Therefore, in an image, a great deal of extra information is contained in the form of color. The extra information can be used to simplify image analysis such as object identification and extraction. Color sensation scattered from liquid crystals covering a surface is generated by a number of factors. These include the orientation of the crystals over the surface, the spectral characteristics of light used for illumination, and the spectral response of the color-sensing detector which could be a human eye or an imaging sensor in a color camera. The procedure for recording color images from a surface covered with liquid crystals using a color camera connected to a computer is discussed here.
Module 6: Liquid Crystal Thermography
Lecture 36: HSI model
The HSI Model
In the HSI model, color is specified by the three quantities namely,
hue, saturation and intensity.
Hue
In the visible spectrum, hue directly corresponds to the dominant wavelength of color.
Saturation
Saturation refers to the degree to which a color deviates from a neutral grey of equal intensity. It is also identified as pastel and vividness. Saturation may also be defined as color's purity or the amount of white contained in a specific color (Camci, 1992). When highly desaturated, any color of the spectrum should approach the standard white color. The analogy here is to white noise when the signal strength is progressively reduced.
Intensity
Intensity of a color refers to its relative brightness in the color mixture. It represents spectral energy at the specific wavelength arriving at the sensor from all directions.
The HSI model can be represented in terms of the color-space by defining a three dimensional cylindrical coordinate system and a subspace as shown in Figure 6.1. The hue distribution (H) is represented as an angle varying from 0 o^ to 360 o^. Saturation (S) corresponds to the radius, varying from 0 to 1. Intensity (I) varies along the Z-axis with 0 being black and 1 being white. Adjusting hue will vary the color from red at
0 o^ through green at 120 o^ , blue at 240 o^ , and back to red at 360 o^.
Figure 6.1 Double cone model of HSI color space; Color triangle is suitable for
color matching.
In camera measurements, the individual values of R , G and B would vary over 0 to 255 (8-bit resolution).
Often, intensity is expressed as a fraction between 0 and unity by dividing the value derived as above by
- Similarly, hue can be normalized by dividing the value derived above by 360 (degrees).
In experiments involving temperature measurements, the primary quantity of interest is hue (H). Other quantities (S and I) need not be computed since they show small variation with temperature. Hence, the major advantage of using hue as a sensitizer of temperature is the one-to-one relationship obtained (as opposed to 3-to-1 in the RGB data).
Module 6: Liquid Crystal Thermography
Lecture 36: HSI model
Contd...
The HSI ( Hue , Saturation, Intensity) model describes color in terms of how it is perceived by the human eye. It is what an artist refers to as pigment; it is what we think of as color - yellow, orange, cyan and magenta are examples of different hues. An artist usually starts with a highly saturated (pure), and intense (bright) pigment. Some white is then added to reduce its saturation and some black to reduce its intensity. Red and Pink are two different saturations of the same hue, namely Red. The main advantages of HSI model is that, it is useful while comparing two colors, or for changing a color from one to another. For example, changing a value from Cyan to Magenta is more easily accomplished in an HSI model; only the H value needs to be changed. Making the same change in an RGB view is less intuitive; since we must know the correct amounts of Red, Green and Blue needed to create Magenta. In short, the RGB model is suited for image color generation, whereas the HSI model is suited for image color description. Owing to these inherent advantages, hue is best suited to establish a unique relation between hue and temperature in the pertinent LCT experiments. Therefore, though the colors of the thermochromic liquid crystals (TLCs) are observed by a data acquisition system that senses RGB, the R-, G-, and B-data are not used to calibrate the TLCs. Instead. the RGB color space is first transformed to the HSI color space. Invariably, temperature turns out to be a monotonic function of the hue. Thus, a calibration curve is immediately obtainable. The calibration data is mostly insensitive to image intensity and external illumination.