The retinal image

The retina contains three classes of cells, known as L, M and S cones, that are responsible for colour vision and that are maximally sensitive to long-, medium-, and short-wavelength light in the visible spectrum. The relationship between the spectral reflectance i of a surface and the excitations e, (where i e L, M, S ) of the cones that occur when the snrface is viewed in a light source defined by a spectral power distribution E is often expressed by the eqnation 

It is important to understand that the results from colour-matching experiments, such as those used to determine the CIE colour-matching functions, can be nsed to predict when two stimuli will look the same in certain circnmstances, bnt caimot be used alone to tell us what the stimuli will look like (Hurlbert, 1991). 

One of the limitations of the original CIE system of colorimetry is that a colom stimulus is considered in spatial isolation whereas evidence will be presented in this chapter to show that colom vision is inseparable from spatial vision. Whereas Equation 4. 1 encapsulates the main colour properties of the cells in the retina we need also to be aware of the spatial properties of retinal processing. For example, it can be considered that at the level of the retina the visual system captures three separate images of scenes that are viewed, one image for each of the cone classes. There are differences, however, in the spatial properties of the cells between the cone classes. The L and M cones are very much more numerous that the S cones. Indeed, it has been demonstrated (Williams et al., 

The colonr sensitivity of the cone classes, the physical spacing of the cones within the retina, and the effects of chromatic aberration at the lens all contribute to what we call the spatiochromatic properties of the visual system. Whereas the CIE colour-matching functions describe the colour properties of the visual system reasonably adequately, if we wish to be able to analyse the colour-appearance properties of colour images or scenes then we need models that can incorporate the spatiochromatic properties of the visual system. In the next section some colour-appearance phenomena are described that demonstrate the intrinsic link between colour and spatial properties. 

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Perceptual constancy is one of the hallmarks of the field of perception, for it strongly indicates that visual perception involves more than the direct registration of the retinal image in the brain. Without perceptual constancy the world would be perceived as a booming blur of chaotic confusion in which the sizes, shapes, and colors of objects would be constantly shifting. Thus it can be seen that perceptual constancy serves an important adaptive fimction. 

Eye composition was examined in several interesting papers. The water content in bovine lenses was determined by Zink et al. [129] Lopez-Gil et al. [130] measured the retinal image quality in the human eye as a function of the accommodation. The nerve fiber layer of an isolated rat retina was studied by Knighton and Huang [131]... 

Stephen Westland continues the topic of human perception of colour in Chapter 4. He describes the retinal image and the effects of the surroundings on the perception and appearance of colour, especially the phenomena of colour constancy and colour contrast. The importance of white as a reference point in the field of human vision and its role in adaptation is presented. The development of colour appearance models is outhned and the relationship of these uniform colour space models to their use in the formation of digital image... 

Eye fundus imaging allows the viewing of the small vessels of the cardiovascular system. It is the only non-invasive and direct observation of the cardiovascular system and has number of applications [1-4], One important feature in the retinal image is the retinal vessel diameter which is an important measure of the cardiovascular health of the person. For instance, the change in the vessel diameter may provide an early indicator of the risk of stroke incidence and mortality [5]. 

A 2D-matched Gaussian kernel filtering is herein considered for detecting vessels [10]. This method involves the convolution of a set of Gaussian kernels with the retinal image under investigation, where a generic kernel may be expressed as... 

It can be noted that better values of Accuracy are obtained with the proposed method, when compared with the methods developed in [14] and in [16], and lower values of clip limit p for all the retinal images under investigation have been used. 

Closely related methods have been used in a number of studies. Chen et al. (2001) used the technique to investigate possible age-related variations in MP spatial distribution. Bour et al. (2002) used a film-based retinal camera to obtain retinal images at 480 and 540 nm, which they converted to... 

Figure 2.2 The retina consists of three layers. Retinal ganglion cells are located at the top followed by a layer of bipolar cells and receptors at the bottom. Light has to pass through the top two layers to reach the light-sensitive sensors. Information then travels upward from the receptors to the bipolar cells and on to the retinal ganglion cells. Information is also exchanged laterally through amacrine and horizontal cells. (Retina illustration from LifeART Collection Images 1989-2001 by Lippincott Williams Wilkins, used by permission from SmartDraw.com.

Hayasaka et al. [157] reported the determination of the fatty acid distribution in mouse retina by using AgNPs in nano-PALDI-IMS. The sections were sliced to a thickness of 10 pm and sprayed with AgNPs or DHB matrix solution at 50 mg/mL in 70 % methanol/0.1%TFA. The mouse retinal sections were analyzed at a high spatial resolution with a scan pitch of 10 pm. The MS images showed the distribution of palmitic acid, linoleic acid, oleic acid, stearic acid, eicosapentaenoic acid (EPA), arachidonic acid, and docosahexaenoic acid (DHA). 

Figure I. Structure of a ciliated photoreceptor cell in vertebrates. (A) Scheme of a rod photoreceptor cell. (B) Transmission electronmicroscopy image of a part of a mouse rod photoreceptor cell. The apical extensions of cells of the retinal pigment epithelium (RPE) evolve the tips of photoreceptors light-sensitive outer segments (OS). The OS is linked via a connecting cilium (CC) to an inner segment (IS) which bears the basal body complex (BB) in its apical region. Synaptic terminals (S) link the photoreceptor cell and the 2nd-order neurons, bipolar and horizontal cells. N = nucleus in B, arrow point to axonemal microtubules projecting into the OS. Bar in B = 0.2 p,m...

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