Light as a stimulus for vision

Transcription

1 Light as a stimulus for vision The physics of light: Light is considered both as a propagating electromagnetic wave and as a stream of individual particles (photons). In Vision Science, both of these aspects are important. Light as rays help us trace the path of light through optical systems. Light as waves help us understand refraction, diffraction and interference phenomena. Wave front measurement is a common tool for describing the optics of the eye. Light as photons helps us understand the detection of light by photoreceptors and its conversion to electrical activity. Electromagnetic spectrum

2 Radiant Energy (Electromagnetic) Spectrum Nanometer (10-9 m): most common units for wavelength Solar Radiation Spectrum UV c b a

3 Black Body Radiators Hot objects like a stove, an incandescent bulb, or the sun give off light in a characteristic way. At 700 F they start to glow a dull red, as they heat more they get bright red, then white, then bluish. The surface of the sun is about 10,340 F (6000 K) Black body spectra vs. Temp Color appearance Spectral radiance of some artificial lights The spectrum of a light source depends on how energy is converted to photons. Incandescence produces a broad, continuous spectrum that depends on temperature. Fluorescence produces a peaked, discontinuous spectrum that depends on the chemicals being electrically excited. Lasers produce extremely narrow spectra.

4 Absorption of short wavelengths in the eye Radiometric and photometric units Radiometric units measure the total energy of light from a source, or falling on a surface. Photometric units are similar, but are compensated by the human spectral luminous efficiency curves Vλ or V λ." Retinal illuminance T Troland = cd/m 2 x pupil area mm 2

5 Luminous Efficiency Functions scotopic (rod vision) photopic (cone vision) Spectral luminous efficiency of rod and cone based vision photopic & scoptopic spectral sensitivity functions: CIE 1951 scotopic luminous efficiency and CIE 1964 wide field (10 ) photopic luminous efficiency, relative to peak photopic sensitivity on log vertical scale; relative peak sensitivities from Kaiser & Boynton Human Color Vision (1996)

6 Sun s surface at noon Tungsten filament White paper in sunlight Comfortable Reading White paper in moonlight White paper in starlight Absolute Threshold Scotopic photopic Luminance of everyday scenes Our eye are able to adjust to a very large range of luminance levels in the environment, spanning more than ten orders of magnitude from dim to dazzling. Units are approximate cd/m 2 Adapted from Vision and Visual Perception, C.H. Graham, Ed. Photons and Trolands compared Recall that the Troland is a photometric unit of retinal illumination, cd/m 2 * pupil area. For a pupil size of 3.5 mm, a surface with luminance of 1 cd/m 2 will produce 10 Trolands on the retina. This table from Kaiser & Boynton Human Color Vision 1996 converts the Trolands into photons, assuming white light. (cones)

7 Energy of one photon The energy of a photon depends on its frequency. Higher frequency, shorter wavelength photons have higher energy. Energy and frequency are related through Plank s constant: E = h * ν Where E is energy in joules ν (Greek letter Nu ) is frequency in cycles / second (aka Hertz) And h is Planck s constant = joule - seconds. Alternatively we can write E = h * c / λ" Where c is the speed of light in a vacuum 3 x 10 8 meters / second And λ (Greek letter lambda ) is the wavelength of light in meters Example: What is the energy of one quantum of 500 nanometer light? E = x * 3 x 10 8 / 500 x 10-9 = about 4 x joules Photons from the sun Solar energy reaching the Earth s surface is about 1000 Watts per square meter on a sunny day. 1 Watt = 1 joule/sec. Based on the previous calculation, the number of photons falling on one square meter of Earth each second is about Or 10,000,000,000,000,000,000,000 Or 10 thousand billion billion The pupil is around 1 * 10-5 m 2, so photons per second enter your pupil from the sun when you look up at the sky. Compare this to Niagara Falls: Each second, there are 5 x drops of water that pass over the falls. So the photons entering your eye from the sun is like 2 million Niagaras!

8 How light interacts with a medium How light interacts with the eye Scattered light forming haze Focused light forming image Courtesy of Dr. Ray Applegate RAA

9 Light speed and wavelength change with different media The index of refraction n describes the ratio of light speed in vacuum compared to a given medium, such as glass. The frequency of light and the energy of the photons do not change as light passes from one medium to another, but wavelength does. Refraction at a surface Refraction occurs when light is transmitted from one medium to another with a different index. The change in direction depends on factors such as index change and wavelength.

10 Diffraction of light at an aperture Diffraction occurs where light hits the edges of an aperture, such as the pupil of the eye. Diffraction limits the ability of the eye to focus points sharply. Diffraction of light at an aperture The spread of a focused point is called the Airy disk. In order to resolve two points, they must be separated so that their peaks are distinct.

11 Mie and Rayleigh scattering Particles smaller than the wavelength of light scatter by diffraction, in proportion to 1/λ 4 (Rayleigh) Larger particles scatter by reflection primarily (Mie). Mie and Rayleigh scattering Rayleigh scattering is strongly wavelength dependent (1/λ 4 ) and gives us the blue color of the sky. Mie scattering is not strongly wavelength dependent and produces the almost white glare around the sun when a lot of particulate material is present in the air. It also gives us the white light from mist and fog. Mie scattering in the eye produces haze from cataracts and other particles clouding the ocular media. The direction of scatter depends somewhat on the size of particles.

12 Reflection at a surface Reflection occurs when light passes from one index of refraction to another. The amount reflected depends on factors such as the index change, the angle of incidence, the wavelength, and the polarization angle of the light. Absorption of photons Once light reaches the photoreceptors of the eye, it is absorbed by a particular molecule called a photopigment. The energy of the photon causes the molecule to change shape (isomerize) and this starts a cascade of activity that results in electrical signals through the nervous system. Two important characteristics of this photon absorption are: 1) There is an inherent randomness to the process. When and where a photon gets absorbed is a matter of probability, following a Poisson statistical distribution. 2) Once a photon is absorbed, the photoreceptor responds the same way, regardless of the wavelength. This is the Principle of Univariance.

13 Light exposure damage Intense light exposure can damage the eye through three principle mechanisms: Thermal Damage: proteins in the eye are denatured by tissue heating, especially in the retinal pigment epithelium (RPE) which absorbs light over a broad spectrum. Photochemical Damage: particular pigments, such as photoreceptor photopigments, or lipofuscin in the RPE, are altered by absorbed light resulting in damaging byproducts such as free radicals. ThermoAcoustic Damage: extremely intense pulses of light can produce vibrations that damage tissue. As light intensity increases, usually either thermal or photochemical damage will occur first. With very brief pulses of light, thermoacoustic damage might be the first to occur. To determine safe levels, we make sure none of the three will occur! Example of damage from laser exposure A 15 year old bought a laser pointer through the internet. He stood in front of a mirror and looked right into it. The laser was 50 X more powerful than most laser pointers. NEJM september 9, 2010

14 Maximum Permissible Exposures (MPEs) MPEs refer to the highest intensity of light exposure that is considered safe: that is, a level that is not likely to cause damage to the eye. MPEs are established through animal experiments and through accidental damage in humans. The MPE for a particular situation is 1/10th the intensity of light that would produce damage in half the cases. MPE values are published by the American National Standards Institute in their document ANSI Z Other organizations have similar standards. Calculating MPEs is not simple Determining light exposure safety limits is complicated. It depends on wavelength (nm), power (Watts), exposure time (sec), area of retina exposed (mm 2 ), pupil size (mm 2 ), eye motion (deg/sec). We have to answer: How much energy in Joules was delivered to one retinal location? Does the wavelength(s) used produce thermal or photochemical damage first? If it is thermal damage, was the exposure long enough that some of the heat could dissipate away? A significant problem is that some eyes may be more susceptible to damage than others, for example in retinal disease.

15 Laser Classes and Safety Lasers are sorted into Classes based on their potential to do damage. (The old system used Roman numerals and A/B designations. The classes shown here are the new system.) Class 1 lasers do not exceed MPE level even for very long exposures. Class 1M is a special case: they are safe as sold, but could be hazardous if optics were used to focus the beam to a point. Class 2 lasers are visible ( nm) lasers that do not exceed MPE level for exposure durations less than 0.25 sec. The assumption here is that you will blink or look away (aversion reflex). Power is less than 1 mwatt. A Class 2M laser is safe unless optics are used to focus it down. Class 3R lasers (firearm sights, laser pointers) have power less than 5 mwatt and generally won t do damage unless they are focused. Class 3B lasers have power from 5 to 500 mwatt and will damage the eye if viewed directly even with short exposures. Diffusely reflected light from the spot they make is not hazardous. Class 4 lasers have power over 500 mwatt, will burn the skin, start fires, etc. Surgical lasers are in this category. Comparing direct vs. indirect view of a laser The damage calculations assume that all the laser light goes into the eye. When you look at a laser dot on the wall, only a very small fraction of the light goes into your eye. The screen scatters light from laser dot in all directions At a distance of 7 meters, the light has spread out into a hemisphere that is 308 square meters in area a 4 mm pupil has an area of square meters so pupil gets just 1 part in 12,254,000 of the light!

16 Effects of time and wavelength on exposure damage From Delori et al. JOSA 2007 Apparent Brightness is not a measure of safety! The curves show the luminance of light at the threshold for damage across the visible spectrum. The heavy curve is for 300 seconds continuous exposure, the lighter curve is for 30 seconds exposure. The vertical axis plots log Trolands, a photometric unit. Damaging light at 800 nm will appear 10,000 times less bright than equally damaging light at 550 nm. Notice that the curves are different in the blue, but the same in the red. In the blue, damage is photochemical and accumulates over longer time periods.

17 The longer the flash lasts, the lower the power must be to avoid damage. The vertical axis here is radiometric, showing power in Watts entering the pupil for a 2 arc minute spot at 488 nm. For brief flashes thermal damage is more likely. Time and damage threshold do not trade off perfectly, because heat can dissipate away if exposures are long. For very long exposures (e.g seconds) photochemical damage becomes more likely.