Focal plane for green (510nm) light Focal plane for red (650nm) light
Focal plane for blue (475nm) light
Varying refraction by wavelength
Figure 2.5: Schematic showing varying focal planes for different wavelengths. Imagep.
2.2.3
Chromatic Aberration
As described din the previous section, light gets focused through a lens, and the focal plane depends on the lens and image plane (see further down for detailed equations). However, this is a slight simplification as not all light gets focused equally by a lens. The refractive index of an optical medium depends on the wavelength of the light, and thus light of an object in different wavelengths gets focused at different points (see Figure 2.5 for a schematic overview. This kind of chromatic aberration is called axial chromatic aberration, as the focus point varies along the optical axis. This is opposed to transverse chromatic aberration in which the image planes are focused equally, but transposed, that is, the images are not perfectly aligned.
2.3
Basics of Visual Perception
Vision provides us with information about the space around us: the spatial relation of objects to ourselves as well as information like colour
Pupil Iris Lens Cornea Anterior chamber (aqueous humour) Ciliary muscle Retina Fovea
Figure 2.6:Schematic diagram of the human eye, showing the different parts of the lens system. Author: Rhcastilhos,p
and texture that allows us to identify objects.
To see the eyes act as sensors that gather external information: they mea- sure the incoming light. This information is then further processed by the visual system to create an internal representation of the environment. The following sections will give an overview of the structure and properties of the eye and the processing of the visual signals, especially with regard to colour and depth information.
2.3.1
The Eye(s)
The eyes are the sensors that gather the information the light provides. The properties of human vision are directly tied to the physical structure of the eyes, for example, colour vision through light-sensitive cells in the retina and depth sensing through the disparity information the eyes provide. At its core, a single eye can be described as an optical lens system and a sensor.
2.3. Basics of Visual Perception
2.3.1.1 The Eyes’ Lens System — Optical Properties
Light enters the eye through the pupil and gets focused by its cornea and lens on the retina. While the cornea accounts for most of the eyes focusing power, the lens is variable and can change its shape to focus at different distances. This process is called accommodation (Wang and Ciuffreda, 2006). The eye also exhibits a limited depth of field that is blurriness away from the plane of focus, which is affected by the size of the pupil opening (Wang and Ciuffreda, 2006).
On its way to the retina, the light has to pass through optical media, for example, the cornea, lens but also the aqueous humour (see Figure 2.6 for a schematic overview). All of these layers can affect the light, and introduce optical aberrations and distortions, for example, visible shadows through floaters (Murakami et al., 1983) or colour fringes at contrast boundaries due to chromatic aberration (Thibos et al., 1992; Atchison and Smith, 2005)).
2.3.1.2 Chromatic Aberration in the Eye
CA is of special interest for visual perception since it could affect the accu- racy of vision (Hartridge, 1918), colour perception (Fry and Somers, 1974) and accommodation behaviour (Charman and Tucker, 1978). Atchison and Smith (2005); Campbell et al. (1999); Howarth et al. (1988); Ogboso and Bedell (1987) thus have investigated the optical properties of the human eye and measured the refractive indices of the eye and estimate the resulting CA through models and simulations.
Chromatic aberration is closely related to defocus blur. CA in the eye is dependent on the accommodation state and thus can contain information about distance. Sanson et al. (2012) have shown that CA can play a role in the perceptual system in jumping spiders. For humans there are optical illusions based on colours that induce the perception of depth and Winn et al. (1995) propose that these could be based on CA-based depth perception. In addition to the perceptual evidence, a computer vision approach from Garcia et al. (2000) uses CA to extract depth information from still images, which shows that CA does indeed contain information about depth.
0 0.2 0.4 0.6 0.8 1.0 400 450 500 550 600 650 700
S M
L
Wavelength (nm)
Nor
malised c
one r
esponse
Figure 2.7:Normalised human cone response curves for short (S), medium (M) and long (L) cones. Each cone has varying sensitivity to different wavelengths and creates a different response. Author: Vanessa Ezekowitz ,cb
2.3.1.3 The Eyes’ Sensor – The Retina
The retina is the sensor that gathers the incoming light for further processing in the visual system. It is made up of a variety of light-sensitive cells, typically rods and three types of cones that are each sensitive to specific spectra of light (see Figure 2.7 for a sensitivity diagram of the cones) (Stockman et al., 1993). Rods serve to see in low light levels while the three types of cones allow us to differentiate between the different wavelength of incoming light at normal light levels, enabling the perception of colour.
Rods and cones are not uniformly distributed throughout the retina. Rods are mostly found in the periphery of the retina, while cones are most dense in thefovea. Further out in the periphery the density of rods reduces and image information is, therefore, coarser (Curcio et al., 1987). There also exists an area in the retina which does not have any rod or cones at all: the blind spot. While no information is available there, the perceptual
2.3. Basics of Visual Perception
Figure 2.8:This diagram shows the relationship between the cone responses and the derived (opponent) colour components. The L, M and S signals get converted into a achromatic luminance signal (A) and two opponent colour signals (red- green R-G and yellow-blue Y-B). The sign of the contributions of each cones (positive or negative) are noted at the corresponding edges.
system fills in information that falls into this area (Cumming and Friend, 1980).
After the rods convert the incoming light into a signal consisting of three components (long, medium and short, LMS) the next step of processing in the retina happens by converting the LMS signal into two opponent colours and a brightness signal (Figure 2.8). The opponent colours are derived from the L-M cones, resulting in a red-green signal and the L-M-S cones resulting in a yellow-blue signal. These encoded signals are then further processed in the remaining part of the visual system (Fairchild, 2013).
2.3.2
The Eye’s Movement
Eye movement generally consists of phases of relative stillness called fixationwhere the gaze is maintained at a specific location inside of the visual field andsaccadeswhich consist of rapid eye movement between the fixation locations. In addition to these two modes of operation, the eyes also have the ability to track a moving object in the visual field, which results in an eye movement pattern calledsmooth pursuit.
During a fixation the area of interest is projected onto the retina, resulting in a clear image with the highest perceptible resolution. Even while maintaining a fixation, the eye is not completely still and some movements (<12 arcminutesCollewijn and Kowler (2008)) called microsaccades occur. During a fixation, the visual system takes in the sensory information that will result in visual percepts.
Switching the location of a fixation happens through a saccade. Saccades are characterised by a fast ballistic eye movement. The target of the saccade is determined before it starts, and the trajectory cannot be changed while it is underway. During a saccade, the eye is effectively blind. This property is especially of interest for gaze-contingent displays, that try to hide changes from the observer, since changes performed before a fixation starts will be imperceptible to the observer.