Light refers to one small band of wavelengths within the much larger electromagnetic spectrum that our eyes are receptive to. This band lies between infra-red and ultra-violet and contains wavelengths from 400nm to 700nm. Radiation with wavelengths within this range is termed visible light and the band itself the visible spectrum as we simply cannot see wavelengths outside this band.
The Speed of Light
When light travels through a vacuum, there are not very many atoms or sub-atomic particles for it to collide with so it travels at what we term the ‘Speed of Light’ – which is a constant C with a value of 2.998x108 m/s. When referring to the speed of light as a constant, we should always really add the suffix ‘within a vacuum’, but people rarely do.
The reason we should is that light appears to travel at different speeds through different materials. This occurs because materials other than a vacuum contain molecules and sub-atomic particles which the light can collide with and bounce off. If each photon of light is continually bumping into things, it ends up taking a much more tortuous path through the material instead of travelling straight though. Generally, the higher the density of the material the more things there are in there to hit, which means the more tortuous the path and the slower the overall speed of light within it.
At a micro level, the speed of light between each collision is still the same as in a vacuum, so it is still constant. Its just that the extra distance travelled by each photon through the material is not something we can easily measure. However, the apparent speed of light in the material measured by timing the delay between entry and exit is a very useful property that we can use in many light calculations.
Transparency, Translucency and Opacity
Density is not the sole property that affects the passage of light, it also depends upon the molecular structure of the material. For example, the molecules in some materials such as water, glass and gemstones allow light to travel through without much scattering or absorption. This is because of the number of electrons in the atoms they contain and their energy states. They simply do not absorb photons with energy levels that equate to light in the visible spectrum. They do absorb photons with energy above and below this band, and some gemstones intrude into the visible spectrum at certain frequencies which give them a noticeable colour, but visible light is relatively unaffected and ends up traveling in virtually the same direction when it exits than when it entered the material. Such materials are termed transparent.
If there is a greater amount of scattering but still low absorption, light will still pass through but the direction each photon travels as it exits depends less upon the angle in which it entered. Such materials are termed translucent.
Finally, many materials have relatively high absorption such that all of the light energy is absorbed before it has a chance to pass all the way through. These materials are termed opaque.
It is important to note that many materials that we think of as opaque can actually be transparent if produced within a thin enough film. Similarly, if a transparent material is made much thicker than it normally occurs, it can turn translucent or even opaque.
Evolutionary Effects
Evolution suggests that the reason we see pure water as transparent is because our eyes originally evolved whilst living entirely in water. Water is very interesting in that there is a small band of radiation for which it has very weak absorption. This band includes the area that we refer to as the visible spectrum (see Wikipedia). Thus it would make sense to be most sensitive to this band as the low absorption means that it penetrates the deepest into the oceans, more than 220m for some frequencies, before being completely absorbed.
Reflectance, Transmittance and Absorptance
When light hits the boundary between two materials in which the apparent speed of light is different, some portion of the light will usually be reflected and the rest transmitted. The term ‘transmitted’ here simply means passes through the boundary and into the second material. In an opaque material, the transmitted light will be entirely absorbed before travelling very far. In a transparent material there will be some absoprtion, but there will also be a portion of the transmitted light that will pass right through.
It is important to properly understand the complexities of refraction, internal reflection and absorption as this will help you when specifying spectrally selective or solar control glasses with thin films or internal layering. For these concepts, see the Reflectance, Transmittance and Absorptance topics.
The Human Eye
Our vision depends solely on light. The eye is a complex organ whose role is essentially to convert light into sensory signals that can be interpreted within the brain.
The outer layer of the eye is called the sclera and has a transparent section at the front called the cornea. Light enters through the cornea, passing through a diaphragm called the iris, the coloured part of the eye that contains a variable-sized hole sometimes called the pupil. The crystalline lens behind the iris (coupled with the curved surface of the cornea) acts to focus light onto the inside back surface of the eye which contains the retina. This is a light-sensitive receiver which covers about two thirds of the internal surface of the eye. Cells in the retina are then connected directly to the brain via the optic nerve.
The following is the order in which entering light is affected by the various components:
The Cornea is the transparent curved front surface of the eye that serves mainly as the outer protective layer. It is here that most of the refraction occurs. The refractive index of the cornea is approximately 1.376 which is comparable to that of glass or clear plastic.
The Iris is the coloured part of the eye. It works in the same way as a camera aperture, automatically adjusting the size of the pupil to control the amount of light entering the eye based on external conditions.
The Pupil is the hole in the iris through which the light enters. It appears black because there is virtually no light reflected from it.
The Lens acts to make small corrections for focusing the image of objects at different distances onto the retina. It is flexible and can change its shape and therefore its focal length. This control is exercised by the ciliary muscles.
The Retina is the coating of the interior surface at the back of the eye. It consists of an array of receptors called rods and cones which convert the light energy into electrical signals which are transmitted to the via the optic nerve. It essentially plays the role of the film in a camera.
The Macula, or fovea, is a small spot at the centre of the focus area in the retina where the cones are very closely packed and the best colour discrimination and sharpest image are formed.
In a normal relaxed eye, parallel light rays produce a clear image on the retina. For objects closer than infinity, the ciliary muscles act to stretch out the crystalline lens to bring that object into focus.
Physiological Processes
The retina is the beginning of the nervous system which transports visual information to the brain. It contains around 100,000,000 nerve endings that, because of their shape, are called rods and cones. Rods outnumber cones by a ratio of approximately 10:1 and are spread out relatively evenly over the entire retina except for a small area centred on the optical axis called the Macula. This area contains a large concentration of closely packed cone cells, with almost no rods at its centre.
Rods are very sensitive to light levels but are unable to distinguish between different frequencies. Cones, on the other hand, are not as sensitive to light levels but can clearly distinguish different frequencies. In normal lighting levels, the rod cells are inactive with the cones cells being receptive to colour in quite fine detail - commonly called photopic vision. In very low light levels, however, the rods are the only receptive cells, hence we see only in shades of grey - a process called scotopic vision. As light levels change, there is a small band where both rods and cones are active - called mesopic vision.
The persistence of vision of cones is much longer than that of rods, around 1/20th of a second. This is why flicker is much easier to detect out of the corners of the eye rather than directly in front.
Another interesting phenomenon resulting from the difference between rods and cones can be observed on a dark night when viewing stars. It is generally not possible to see very faint stars if you look directly at them. However, if you focus on a nearby brighter star, you will be able to make out many more fainter stars which will disappear as soon as you look directly at them. This is because the macular, the focus point of the eye, contains very few rods, almost none at its very centre.
Visual Sensation
The eye does not respond equally to all frequencies of light. In fact it varies quite substantially. The following graph shows the three characteristic peaks of sensitivity within the red/orange, green and blue frequency bands.
This non-linear response is not normally a problem as the eye is not a precise optical instrument able to accurately measure light levels. In fact, it is a very flexible and forgiving instrument able to adapt to an extremely wide range of conditions. Thus for most people, it is actually very difficult to consciously discern the frequency content of a light signal, other than to simply detect dominant frequencies as well as the presence/absence of certain ranges.
Visual Acuity
Visual acuity is defined as a measure of the ability of the eye to distinguish subtle detail. Four factors major affect visual acuity:
Size
Size is the most generally recognised and accepted factor in seeing. Visual acuity is dependent upon the size of an object, which affects the size of the image on the retina. The important aspect of size is not the physical size of the object, but the visual angle that the object subtends at the eye. When an object is brought closer to the eye, we are actually increasing the visual angle, thus making the object clearer.
Luminance
Luminance is an objective physical quantity that a lot of people often confuse with brightness. It is important, however, for the lighting professional to maintain a distinction between the two. The critical difference lies in the fact that brightness is a subjective evaluation or interpretation of the amount of light reaching the visual system. Luminance, on the other hand, is dependent on the amount of light striking a surface and the amount of light being reflected back to the eye. Surfaces with low reflective values require more light than those with high reflective qualities if the luminance is to be equal.
The visual system sees luminance (the energy reflected from surfaces), not illumination (the light arriving at an object). Therefore, calculation procedures based upon the light reaching a surface will give little indication of how well a task can be seen. Seeing is a function of the light leaving a surface (luminance). The eye receives bundles of light from objects of different luminance in space and must accommodate to these luminance differences by an averaging process.
Comfort and visibility are dependent upon the luminance patterns within the visual field. Comfort is dependent on the variation in luminances in the visual field (luminance ratios). Visibility is affected by the adaptation state, quality of luminance patterns, and clutter within the visual field. Clutter can create an overload of the visual system due to excessive luminance ratios.
Contrast
Contrast is the basic seeing mechanism in vision. Contrast threshold is a measure of the ability of an observer to distinguish a minimum difference in luminance between two areas a given percentage of the time. The contrast threshold is expressed as a fraction.
If the surrounding luminance is much greater than the target luminance, the task contrast will be lost and the target will go into silhouette. If surrounding luminance is much less than target luminance, the task contrast may not be affected by excessive luminance but may create discomfort, and hence a reduction in visibility. To see surface detail, that is to have good surface discrimination, the luminance ratios in the visual field must be controlled. Maximum visual efficiency occurs when the surrounding luminance, has a ratio to target luminance that ranges from 0.1 to 1.0. The highest visibility occurs when the object is brighter than the background (white print on black paper).
Time
Time is the fourth factor that affects visual acuity. Seeing is not instantaneous. A time lag exists in the electrochemical processing of the retinal signal that reaches the brain. As the level of background luminance increases, the time required to interpret details will decrease. Just as the camera requires a longer exposure time in dim light than in bright light, so does the eye. The eye can distinguish and discriminate details at low luminance levels if given enough time.
The lighting designer usually has little or no control over size or time. If these two variables are fixed, the ability to see surface detail is dependent on task contrast and background luminance.
Measuring the Visual Process
Contrast
In full daylight, the eye can easily perceive differences in luminance of around 1%. Under poor lighting conditions however, differences of up to 10% may be perceived as equal. Contrast is therefore a variable, but very important aspect of any lighting design.
Visual Acuity
Sharpness of vision, or visual acuity, can be measured as the size of the smallest detail the eye can perceive at a given distance. This size is usually expressed as a visual angle (a) in minutes, with visual acuity being the reciprocal of this angle. Visual angle increases with increasing illuminance, subject to the law of diminishing returns. Typical values would be 0.25 (a = 4’) at 100 lux, 0.5 (a = 2’) at 300 lux, 1.0 (a = 1’) at around 2000 lux.
Visual Performance
The concept of visual performance is quantified as the number of characters perceived in unit time, or as the time taken to accurately see a character. Many experiences have proved that performance improves with increasing illumination. This means that the time taken to perform complex visual tasks decreases with improved illuminance.
Visual Efficiency
Given the geometry of the head and face, as well as the internal mechanisms of the eye, the field of view is usually only a 120° cone. There is some additional vision straight down, usually only an additional 10°, as shown in the diagram immediately below.
Useful References
Visible Light and the Eye’s Response
http://www.physicsclassroom.com/class/light/Lesson-2/Visible-Light-and-the-Eye-s-ResponseWebVision - The Organisation of the Retina and Visual System
An excellent in-depth look at the anatomy of the eye and retina from the John Moran Eye Center at the University of Utah.
http://webvision.med.utah.edu/The Human Eye
A very comprehensive online biology textbook by Dr. John W. Kimball of Harvard University.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.htmlThe Joy of Visual Perception
A useful web book covering aspects of visual sensitivity, acuity and adaption.
http://www.yorku.ca/eye/thejoy.htm
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