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Vision_Notes_2 - II Visual Stimuli A Light and light...

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28 II. Visual Stimuli A. Light and light sources Light is a form of electromagnetic radiation. For this course it will be adequate to think of light as traveling waves that are emitted and absorbed in discrete amounts (quanta) of energy referred to as photons . Figure 2.1 illustrates, at a frozen moment in time, a light wave that is traveling from Figure 2.1 left to right. The light source has been on for some time, so the wave is extended over space. The distance from one peak in the wave to the next is call the wavelength , λ . Light waves are always in motion, and the speed that the wave peaks move is called the speed of light, c. The number of peaks that sweep past a point in space per unit time is known as the frequency , ν . Because wavelength is the distance between peaks, and frequency is how many peaks go by in a second, the speed of light is the product of the wavelength and the frequency: λν = c (2.1) The speed of light is constant in a vacuum, independent of the wavelength or the speed at which the light source is moving. However, the speed of light is lower if it is passing through some medium such as air, water, or glass. 1. Visible light The visible range of wavelengths is from about 350 to 750 nanometers (nm), but higher or lower wavelengths can be seen if they are intense enough. The range of
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29 wavelengths (and frequencies) visible to humans, and the relation of visible light to other forms of electromagnetic radiation is shown in Figure 2.2. Figure 2.2 2. Definition of power The power of the light wave (energy per unit time in watts) is the integral of the square of the wave height (instantaneous amplitude, a(t)) at each point in time divided by the length of time (T) over which the wave is integrated: 2 0 1 ( ) T e P a t dt T = (2.2) 3. Coherent and incoherent light If the light wave is sinusoidal (a single wavelength), as in Figure 2.1, then the light is said to be monochromatic. Perfectly monochromatic light (e.g., laser light) is called coherent light. However, most light is composed of a continuum of sinusoidal waves of different wavelengths added together, and hence is incoherent light. In fact, even the "monochromatic" light used in most vision experiments consists of a range of wavelengths and hence is incoherent. For incoherent light sources, power is additive (that is, the power of two lights mixed together is the sum of their individual powers).
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30 4. Spectral power distribution The typical way of describing a light is to show its total power and its spectral power distribution, P( λ ), which is the amount of power per unit wavelength as function of wavelength. Figure 2.3 shows the spectral power distributions of ten different phases of daylight. Figure 2.3 5. Photons The energy in light waves is emitted and absorbed in discrete amounts called photons.
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