Illumination

This article describes those factors that should be taken into account when the lighting of a house is being planned, if the lighting is to be considered adequate. Adequate lighting isn't synonymous with bright lighting but with sufficient lighting that's so placed as to avoid glare or distracting shadows.

The importance of good lighting can't be overestimated. About one in four individuals under 20 years of age has defective vision of some kind. One-half of those between 30 and 40 years of age have defective vision. Of those between 50 and 60, the proportion has increased to four out of five. Of those over 60 years of age, 19 out of 20 have defective vision.

It has not been shown to everyone’s satisfaction that poor lighting causes visual defects, but it's a fact that individuals who must use their eyes intensively are more likely to have visual defects than those who don't . E.g., over 80 % of all draftsmen have eye problems of some kind and less than 20 % of all laborers do. Whether or not these problems are caused by poor lighting, it's obvious that those who do have visual defects need good lighting if they are to see adequately, and in the long run this will include 95 % of those reaching old age.

ILLUMINATION AND THE EYE

How We See

When we concentrate our attention intently upon some object, we can see that object with great distinctness. This quality of acuity, or keeness of vision, is, however, limited to a relatively narrow cone, or central field, that extends outward from the eye at an included angle of about 1 degree (see Fig. 1). Within this central cone we can see whatever we are looking at in all its color and detail.

Beyond this central field, our acuity begins gradually to fall off. Although we are still aware of the color and shape of objects, we don't really look at objects outside of the central field, and they make a less vivid impression on our mind. This area of less-acute vision is called the surround. It extends about 30 degrees beyond the central field, forming a cone that has an included angle of about 60 degrees.

Fig. 1. Diagram of the visual field perceived by the human eye.

Beyond the surround is the area of peripheral vision. We don’t see the color or shape of objects within this area of vision at all. All we really notice are movements. The area of peripheral vision extends outward from the surround in an oval- shaped cone that has an included angle of about 160 degrees horizontally and about 120 degrees vertically.

Light Intensities and the Eye

The eye can respond to an enormous range of light intensities. A bright summer sky, for example, shines with an intensity as great as 10,000 foot-candles, and at the other extreme the light from a star may have an intensity of only 0.00003 foot-candle. The eye accommodates itself to this range of intensities mainly by altering the diameter of the pupil and , when the light is extremely bright, by squinting, which helps to reduce the amount of light that enters the eye. In addition, in extremely dim light, the retina will adapt itself over a period of time to the darkness and become sensitive to what light there is.

The eye can't take in too great a range of light intensities at the same time. If the range is too great, the pupil will contract until the eye can look at the brightest objects without discomfort, but this will leave the less-bright objects to shift for themselves. These objects will appear washed out and without detail. Light from objects that are too poorly lit may fail to register on the retina at all. This sort of thing happens, for example, on a very sunny day when objects that are directly in the sun appear brilliant and sharp while objects in the shade may disappear from view entirely.

Glare

Unwanted bright light that shines directly into the eyes or that's reflected from a bright source into the eyes interferes greatly with the acuity of vision. This interfering light is called glare.

Perhaps the most common example of extreme glare occurs when we are driving into the sun in the late afternoon, with the sun low in the sky and directly in our line of sight. The glare can make driving extremely hazardous for we can see nothing on the road ahead of us.

This sort of interference occurs indoors as well, if not in as extreme a form. The eyes will adjust themselves automatically to this glare. But when they do so, the object upon which our attention is fixed will appear less vivid to our sight than it should, and the eyes must strain to see this object accurately and in detail. If the strain continues for a period of time, the eye muscles tire. The entire body may even become fatigued, all because a light is shining in one’s eyes. Because of these physical consequences of glare, it's not too much to say that, in addition to having adequate illumination in the first place, the chief object in planning the lighting in a house is to prevent glare as much as possible.

Having insufficient light will also strain the eye muscles, especially when one is engaged in a task that requires concentration, such as reading, sewing, drawing, model building, watch repairing, and so on. The eyes of a young person—20 years old and under—adapt themselves very readily to poor light conditions, but this adaptability gradually disappears as one grows older. The pupils lose their facility for opening as wide as they once did, or for altering their diameter as rapidly as they once could. The muscles that control the shape of the eyeball, and thus allow the eye to focus on near and far objects, lose their strength, and the eyeball itself becomes less flexible with increasing age. This sort of physical degeneration is inevitable in all of us, but it can be compensated for in part by increasing the brightness of the object we are looking at. Figure 2 shows how much brighter an object must be lighted as we grow older in order to compensate for the increasing deterioration of the eyes.

Fig. 2. As a person ages, objects must be more brightly illuminated if they are to continue to be clearly seen. Bx brightness required at x years. B20 brightness required at 20 years.

THE MEASUREMENT OF LIGHT

Candlepower

The intensity of the light emitted from a source is measured in candlepower (or candela in the metric system of light measurement). Once there were actual candles made of wax, the residue of refined whale oil, and 1 candlepower was defined as the amount of light emitted from such a candle when it was consuming wax at the rate of 120 grains per hour. But times have changed and nowadays there are several different kinds of laboratory light sources, all more stable and reproducible than a wax candle, the outputs of which are defined as being equal to 1 candlepower.

The word candlepower by itself, strictly defined, means the intensity of the light emitted in a particular direction, not in every direction from the light source.

Candlepower by itself is thus a rather useless measure since what is usually wanted for practical purposes is a measure of the total light emitted from a source, not the amount of light emitted in one direction. And the total quantity of light emitted from a source is measured in lumens.

Lumen

And what is a lumen? Imagine that there is a point source of light that's emitting 1 candlepower of light in every direction. Imagine that this light source is located in the center of a transparent sphere that's 2 ft in diameter (see Fig. 3). There fore, every point on the inner surface of this sphere will be 1 ft from the light source, and all these points will be receiving an equal quantity of light. If we were now to mark off on the surface of this sphere an area of exactly 1 sq ft, then the total quantity of light falling on this 1-sq-ft area is defined as being equal to 1 lumen.

Fig. 3. The relationship between candelas, lumens, and footcandles. A uniform point source (luminous intensity or candlepower = 1 candela) is shown at the center of a sphere of 1 ft radius. It is assumed that the sphere is perfectly transparent. The illumination at any point on the sphere is 1 ft-c (1 lumen per sq ft). The solid angle subtended by the area A, B, C, D is 1 steradian. The flux density is, therefore, 1 lumen per steradian, which corresponds to a luminous intensity of 1 candela, as originally assumed. The sphere has a total area of 12.57 (4 r) sq ft, and there is a luminous flux of 1 lumen falling on each square foot. Thus, the source provides a total of 12.57 lumens.

Foot-candle

Lumens are used by the manufacturers of incandescent and fluorescent lamps as a measure of the total amount of light emitted from their products. Another very common measure of light is the footcandle, which is defined as the amount of light that's falling on a surface 1 ft square when that surface is 1 ft distant from a light source that's emitting 1 candlepower.

Note that footcandle and lumen measure light from different points of view. Lumen is a measure of light intensity from the point of view of the source. Footcandle is a measure of light intensity from the point of view of the object being illuminated. If, for example, a light source were to emit light with an intensity such that 100 lumens were to fall on a surface 1 sq ft in area, no matter how distant that surface is from the source, that surface would be illuminated by 100 footcandles of light.

The Law of Inverse Squares

Light, like all other forms of electromagnetic radiation, follows the law of inverse squares. That is, the intensity of the emitted light falls off sharply as the distance from the source increases. The relationship between footcandles, light intensity, and distance is defined by the following equation

Footcandle = candlepower / distance²

Going back to our definition of footcandle for a moment, we can see that 1 footcandle is equal to 1 candlepower falling on a surface 1 ft from the source, or

1 Footcandle = 1 candlepower/1 ft.²

Assume now that we have a source of light that's emitting 100 candlepower and that this light is 1 ft distant from a surface. The amount of light falling on that surface is, therefore

1 Footcandle = 100 candlepower/1 ft.² = 100

If we were to increase this distance to 10 ft, the intensity of the light falling on the surface would become

Footcandle = 100 candlepower / 10 ft.² = 1

That is, the amount of light striking the surface has decreased by 100 times even though the distance has increased by only 10 ft. Obviously, the closer a light is to a surface, the greater the illumination falling on that surface.

Brightness

Interesting as these facts no doubt are to the reader, the difficulty with footcandles and lumens is that we don't see an object because a certain amount of light happens to fall upon it but because a portion of that light is reflected from that object into our eyes. That is, a certain minimum amount of light must always be reflected from an object before we can see it. The object must have a certain amount of brightness, or what is more accurately called luminance.

To give an extreme example, if we were to enter a room illuminated by a 1000-lumen lamp, the walls and ceilings being painted a light color, we would see every object in the room, including themselves, with great distinctness. If, on the other hand, we were to enter the same room illuminated by the same 1000-lumen lamp, but with the walls and ceiling painted a dull, flat black, the furnishings in the room also being black, we would not see anything at all, or very little, because none, or almost none, of the light given off by the lamp would be reflected from the walls and furnishings into our eyes. All that we would really see would be the glare of the lamp itself.

Foot-lamberts

Brightness (or luminance) is measured in foot-lamberts, which is defined as the illumination falling on an object times the reflectance of the surface expressed as a % . That is,

Footlambert = (footcandle x reflectance)/100

All objects have a certain amount of reflectance; that's , they will reflect a certain percentage of the light striking them and absorb the remainder. No object ever reflects all the light striking it, which is to say that no object ever has 100 % reflectance. The percentage of light reflected by any particular surface will depend on the color and surface texture of that surface. Dark colors, for example, reflect much less light than light colors (see Table 1), and rough surfaces reflect much less light than smooth surfaces.

To return to our formula, assume that 70 footcandles of light are falling on a wall painted a light beige. Table 1 shows that this color reflects 70 % (i.e., 0.70) of the light falling on it. Therefore,

Footlambert = 70 footcandles x 0.70 = 49

The actual brightness of the wall is 49 footlamberts, not the 70 footcandles that we might assume if we were to take into account only the intensity of the light and its distance from the wall. Furthermore, if the wall were dirty or dusty, as is usually the case, even less light would be reflected from it. The wall, therefore, will appear even less bright than 49 footlamberts.

It should also be noted that if we were to measure the amount of light falling upon the wall surface using a lightmeter calibrated in footcandles, the meter would indicate 70 foot- candles when we pointed it toward the light source. But if we were then to point the meter directly at the wall, it would indicate only 49 footcandles—which is the amount of light striking the wall less the reflectance. As a practical matter, a light meter can be used to measure both footcandles and foot lamberts; it depends on where you point it.

Table 1. Brightness with which Different Color Expressed as a Percents Reflect Light Striking Them, Expressed as a Percent

INCANDESCENT LAMPS

When 115-volt electricity passes through the tungsten filament of the ordinary household incandescent lamp, the resistance offered by the filament to the current of electricity causes it to heat up until it's glowing white-hot. Visible light is then emitted by the filament. The filament can glow as hot as it does because the bulb has been evacuated of air (which contains oxygen, which will cause the filament to burn up). The bulb is instead filled with an inert gas such as argon, or it may be completely evacuated. The filament will give off a considerable amount of heat, an indication of an incandescent lamp’s basic inefficiency.

The electricity enters the lamp through a contact at the bottom of the screw-type base. The circuit is completed through the side of the base, which is insulated from the contact.

Lamp Efficiency

The amount of electrical energy consumed by the lamp is measured in watts. The efficiency of the lamp is measured by comparing the amount of light emitted (in lumens) to the amount of electricity consumed (in watts). That is,

Light output of lamp (lumens) / Electricity consumed (watts) = lumens per watt

If, for example, we have a 75-watt lamp with an output of 1150 lumens (the lumen rating of a lamp is marked on the box it's packed in), then

1170 lumens / 75 watts = 15.3 lumens per watt

Table 2. Output in Lumens per Watt and Average Life of Standard Incandescent Lamps

The first amps made by Edison in 1888 produced less than 2 lumens per watt, in the 100-watt size.

Table 2 shows the lumen-per-watt output of standard incandescent lamps of different sizes. Note that the efficiency of these lamps increases with wattage. A 25-watt lamp, for example, has an output of 9.5 lumens per watt, but a 150-watt lamp has an output of 18.5 lumens per watt. One could save on one’s electric bill by installing one 150-watt lamp instead of six 25-watt lamps, assuming one has a choice. The 150-watt lamp has nearly twice the light output of the six 25-watt lamps, but it uses only one-half as much electricity.

Table 2 also shows the variability in the lifetimes of lamps having different wattages. These figures are average lifetimes. Any one lamp may have a lifetime that varies ±20 % from these figures, as it's impractical to manufacture lamps to any closer tolerances.

Extended-Life Lamps

Several lamp manufacturers have extended-life lamps that are advertised to last two to three times as long as ordinary lamps. These lamps are made by increasing the diameter and length of the filaments. As long as the wattage remains the same, the temperature of the filaments is thereby reduced and they will last longer. One pays a higher price for the tamp, but this additional cost is more than made up for by not having to purchase one or two standard lamps as replacements.

There may be a hidden cost, however. Since the filaments operate at a reduced temperature, their lumen-per-watt output is 5 to 20 % less than the output of an ordinary lamp having the same rated wattage. That is, the extended-life lamps are somewhat dimmer. Whether this is important or not depends on the user. But if, in order to secure adequate illumination, it becomes necessary to purchase an extended- life lamp having a wattage greater than that of an ordinary bulb, then obviously the operating cost of the lamp will increase. The cost of the additional electricity consumed by the extended-life lamp over its lifetime may well exceed the money saved originally by not having purchased ordinary lamps.

Lamp Operation

Fluorescent lamps are much more complicated electrically than incandescent lamps. The basic construction and circuit of a preheat type of fluorescent lamp (which is the type of lamp most commonly used in dwellings) are shown in Fig. 4. A tungsten filament is located at each end of the lamp. Each filament is connected to the circuit through two pins that project from the lamp. The filaments are coated with a material that emits a stream of electrons whenever the filaments are heated. The inner surface of the glass tube is coated with a white phosphorescent powder that glows with light wherever it's struck by ultraviolet rays. The source of the rays is a small amount of mercury that's injected into the lamp during its manufacture, together with a much larger quantity of argon gas.

When the circuit is first energized, electricity flows to both filaments via a closed switch located in the circuit between the filaments. This switch, which is called a starter, operates automatically and is normally open when the lamp is inoperative. As the filaments heat up, the substance with which they are coated emits electrons, which travel at extremely high speeds through the lamp. The motion of these electrons excites the argon gas, which in turn excites the mercury.

Thereafter, whenever one of the electrons collides with an atom of the mercury, it causes the atom to emit a ray of ultraviolet light. Wherever this ray strikes the phosphor coating on the surface of the lamp, the phosphor glows with light for a moment.

By this time the ends of the fluorescent lamp will be glowing with light while the main portion of the glass tube is still dark. The starter now opens automatically, and it will remain open for as long as the circuit remains energized. The fact that the starter has opened forces the electrical energy to flow through the tube, which it can do since the mercury vapor within the tube is capable of conducting an electrical current. Once such an arc has been established between the two filaments, the entire tube begins to glow with light, and it will continue to do so as long as the circuit remains closed.

To complicate this description a bit, a pulse of high-voltage electricity is required to initiate the arc in the first place, but this pulse of high voltage must be very brief, otherwise the lamp will be irreparably damaged. Furthermore, even if this high-voltage pulse is brought back quickly to its original 115- volt value, the current flowing through the lamp thereafter must be closely controlled; otherwise the filaments will burn out.

FLUORESCENT LAMPS

The solution to both these difficulties is to insert a choke coil, or reactor coil, in the circuit, as shown in Fig. 4. A choke coil is widely used in AC electrical and electronic circuits. In a fluorescent-lamp circuit it's called a ballast, presumably because it acts as a kind of weight, a resistance, that holds down the amount of current that can flow through the circuit. The ballast has two properties that make fluorescent lighting practical: (1) it resists any change in the direction of the current flowing through its windings, and (2) whenever the current flowing through it's cutoff, a momentary surge of high-voltage electricity is induced in its windings, which is, of course, transmitted throughout the circuit.

Fig. 4. (A) The fluorescent phosphor coating on the inside of the tube is activated by electrical energy passing through the tube. Light is thereupon given off. The starter in standard starter-type fixtures permits preheating of the electrodes in the ends of the tube to make it easier to start. The ballast limits the current to keep the tube functioning properly. The channel holds ballast and wiring and spaces the lamp holders. (B) shows the flow of current in a fluorescent lamp circuit at the instant the lamp is turned on. (C) shows the flow of current after the lamp has started. The starter has opened, and current can't flow through it now.

Now then, when the starter opens, such a surge of high- voltage electricity is induced in the ballast windings. The ballast is so designed that the amount of voltage induced is sufficient to force an arc between the filaments but not so high as to damage the lamp. And once the arc has been established, the ballast thereafter acts as a resistance in the circuit, limiting the amount of current that passes back and forth within the lamp.

Other Types of Fluorescent Lamp

Since the introduction of the preheat type of fluorescent lamp in 1938, two other methods of initiating the arc have been commercially developed. Both eliminate the need for a separate starter. Both are also used much more in commercial and industrial installations than in domestic installations.

Instant-start lamps do, as a matter of fact, start instantly. The circuit and ballast are so designed that the moment the circuit is closed there is a momentary surge of 450- to 600-volt electricity through the circuit, which produces the arc within the lamp. Thereafter, the ballast will maintain the current at the proper level for continuous operation.

Rapid-start lamps will light up in a fraction of a second, not far behind instant-start lamps. In a typical rapid-start lamp the filaments are heated by a special 3V2-volt winding in the ballast, in addition to the heat provided by the arc. This reduces the voltage required for starting to 250 to 400 volts. Otherwise, both instant-start and rapid-start lamps operate continuously in the same way that preheat lamps do.

Fluorescent versus Incandescent Lighting

The advantages of fluorescent lighting over incandescent lighting are considerable. First, fluorescent lamps are much more efficient than incandescent lamps (see Table 3). The preheat type of lamp, for example, produces from 58 to 75 lumens per watt compared with the 14 to 23 lumens-per-watt output of the ordinary household incandescent lamp. The preheat type of lamp has a lifetime from five to ten times greater than that of an incandescent lamp. A lifetime of 18,000 hours isn't unusual; it depends on the way the lamp is operated. The greater efficiency of fluorescent lamps also shows up in lower operating temperatures compared to incandescent lamps. Finally, their color apart, fluorescent lamps produce a visually more comfortable light than incandescent lamps because the light is emitted along the length of the tube, which results in a more diffused, shadow-less illumination. Fluorescent lamps also have much less surface brightness—they produce much less glare—since the light output is distributed over a very large area compared to the small surface area of an incandescent lamp.

The disadvantages of fluorescent lighting are: (1) the initial installation is much more expensive than for incandescent lighting; (2) a fluorescent lamp can't operate below 50°F, unless it's used with a special circuit designed to operate at low temperatures; (3) the ballast may produce a low hum that can be distracting in a very quiet room; and (4) fluorescent lamps will go Out if there is a pronounced drop in voltage while incandescent lamps will continue to operate, merely dimming until the voltage picks up again.

The Color of Fluorescent Lamps

But the main disadvantage of fluorescent lighting for most people has to do with the color of the light emitted. The phosphors used in most fluorescent lamps radiate energy chiefly in the blue and green portions of the color spectrum. That is, the illumination tends to emphasize the cooler-looking blues, greens, and light-yellows in a room, and the warmer browns, reds, and oranges appear dull and faded. Fluorescent lighting is especially unflattering to the complexion; it tends to make people look pale, if not sallow. Incandescent lamps, on the other hand, radiate more in the red end of the color spectrum. Incandescent lighting tends to emphasize the browns, reds, and oranges at the expense of the blues and greens. The complexion has a healthier, more ruddy look under incandescent lighting.

There are seven different whites available in fluorescent lamps now, each of which is different from the others. This may not be obvious when the lamps are viewed at separate times, but if the same multicolored object is observed under these different whites in a showroom, one after the other, the color changes can be striking.

The practical consequence is that one can be very deceived about the colors one is selecting for one’s home. Paint, fabric, and wallpaper colors for a home that's to be illuminated by incandescent lamps are usually chosen in a shop illuminated by fluorescent lamps. But the greens and blues that looked so brilliant in the shop turn dull and quiet in the home, and apparently dull-looking reds and browns brighten up consider ably. A brown selected in a shop for its look of quiet richness may prove to be rather garish-looking in the home. The obvious rule to follow is always to selector match colors under the same kind of lighting that exists in the home.

The Efficiency and Output of Fluorescent Lamps

In principle, the voltage level is irrelevant for the efficient operation of a fluorescent lamp. The lamp will work as well in a 230-volt circuit as in a 115-volt circuit. However, the ballast used with the lamp can be affected by the voltage level, and , since ballasts and lamps must be very closely matched, lamps should in fact be operated only at their designated voltages.

The wattage rating marked on fluorescent lamps (see Table 3) is a bit deceptive in the sense that the electricity consumed by the ballast isn't included in the wattage rating of the lamp, even though the lamp will not operate at all without its ballast. In the smaller size lamps, the ballasts require an additional 30 % over the wattage consumed by the lamps, whatever that wattage is. In larger size lamps, the ballasts require an additional 10 % of the wattage consumed by the lamps. On the average, then, the power required by the ballast will increase the total power consumption of the circuit by 10 to 15 % .

Table 3. Characteristics of Typical Fluorescent Lamps: Total lumens and lumens per watt are for cool-white lamps after 100 hr of use. The average during the useful life of the lamps will be about 15% less. The figures are based on watts consumed by the lamps, not including power consumed by the ballasts.

Although the lumens-per-watt ratings of incandescent lamps are arrived at in a straightforward way, the lumens-per-watt ratings of fluorescent lamps are a more arbitrary figure. For one thing, the light output depends on the color of the lamp.

Furthermore, the light output of a fluorescent lamp isn't the same over its entire lifetime. A fluorescent lamp does not produce a steady output of light until it has been in use for about 100 hr. Thereafter the light output of the lamp begins to drop slowly and steadily over its lifetime until by the end of its life its lumens-per-watt output may be only 70 % of its output after its first 100 hr of operation. As a practical matter, therefore, over the entire lifetime of a lamp, its average lumens-per-watt output will be only about 85 % of its published rating.

The lumens-per-watt output of any given lamp also depends on its size and type. As with incandescent lamps, larger fluorescent lamps have a higher lumens-per-watt output than smaller lamps.

The Lifetimes of Fluorescent Lamps

The lifetimes of fluorescent lamps are also highly variable. A lamp’s lifetime will depend directly on the number of times the lamp is turned on and off. The more times this happens within a given period, the shorter the lamp’s lifetime. E.g., if a lamp is turned on and off every five minutes, its lifetime may be no more than 500 hr. If the lamp is turned on and left on, its lifetime may be as much as 30,000 hr. The reason for this enormous variability is the chemical coating on the filaments. A small but definite amount of this material is consumed every time the lamp is started; this is one reason that the initial voltage surge must be controlled. Once this material has been entirely consumed, the lamp will no longer operate and must be replaced.

Given these circumstances, it's difficult for manufacturers to establish typical lifetime for their products, since they have no idea how any given lamp will be used. If, however, they arbitrarily assume that the typical lamp operates for 3 hr every time it's turned on, then it's possible to assume what the probable lifetime of the lamp will be. Under these conditions it's possible to say that the average lifetime of the typical 20-watt lamp is about 6000 hr, and the lifetime of the typical 40-watt lamp is about 12,000 hr.

Types and Sizes of Fluorescent Lamps

Table 3 shows the range of fluorescent lamps available, their wattage ratings, sizes, and lumen outputs. In general, the wattage rating and size of a fluorescent lamp go together. Since each type and size of fluorescent lamp requires a ballast especially designed for it (as well as a specific starter for preheat types of lamps), the fixture-plus-lamp must be considered the basic functional unit. One can't interchange fluorescent lamps in fixtures in order to increase or decrease the light output of the lamp, as one can do with incandescent lamps, because the lamps are designed not to be interchangeable; nor can one interchange preheat lamps with either instant-start or rapid-start lamps (except for a 40-watt 48-in.-long lamp called a preheat/rapid-start), nor, indeed, can one interchange instant-start with rapid-start lamps. When fixtures for different lamps are the same length, the fixtures are designed with bases that make it impossible to install the wrong lamp in them.

DESIGNING LIGHTING SYSTEMS

Table 4 shows the general illumination levels recommended for the different rooms in a house. In working areas such as kitchens, laundries, workshops, and , sometimes, garages, this kind of general room illumination is necessary for safety and efficiency as well as to prevent eye strain.

Table 4. Footcandles Recommended for General Domestic Use (Also shows lighting levels in lux)

*General lighting in these areas need not be uniform. Source: Illumination Engineering Society.

But a general room illumination is less important in the living quarters of a house where illumination is usually necessary only for conversation, watching television, eating, and getting ready for bed. In the living quarters, the decorative aspects of home illumination take precedence and , often enough, only enough light is present to keep from tripping over the cat. More intense illumination must, however, also be available for reading, sewing, writing letters, doing homework, and performing similar sight-intensive tasks. Providing adequate illumination for these tasks is perhaps the most important part of home-lighting design, and it should be done before the general room illumination. One can always fill in the dark areas of a room with whatever illumination is thought necessary.

When planning the illumination for light-intensive activities, there are three visual zones, or areas, that must be taken into account (see Fig. 5). These are (1) the area of the task itself—the book or magazine one is reading, the piece of cloth one is sewing, and so on, (2) the area immediately surrounding the task, and (3) the overall room area.

Fig. 5. Seeing zones and brightness (i.e., luminance> ratios for visual tasks (Illumination Engineering Society).

Table 5. Illumination Levels Recommended by the Illumination Engineering Society for Different Seeing Tasks

Table 5 shows the amount of light required, in footcandles, to perform different kinds of tasks. The table shows the mini mum footcandle requirements, that's , the amount of light that should actually fall on the task after the factors that we have discussed in the previous section of this article have been taken into account. This illumination is usually provided by floor and table lamps or, with modern decor, perhaps by pole- mounted spotlights or recessed ceiling fixtures.

The area immediately surrounding the task (the top of a desk, say) must also be adequately illuminated; otherwise the person performing the task will be unpleasantly aware of any excessive difference in brightness every time he glances up or is distracted from his work. This difference in brightness will eventually result in eye fatigue. For optimum visual comfort, therefore, the brightness of the surfaces immediately surrounding the task should be at least as bright as the task area itself or, at a minimum, one-third as bright.

For the same reason, the brightness of the general area within the person’s view should never be more than five times brighter than the task area nor less than one-fifth as bright. The greater the concentration required, and the longer the period of concentration, the more closely should both the area immediately surrounding the task and the general room area approach the task area in brightness.

Illuminating the Task Area

As we mentioned, the task area is usually illuminated by a table or floor lamp or by some kind of specialized fixture. The manufacturers of these lamps and fixtures usually publish a graph or table of some kind that shows how light is distributed from their product. Figure 6, for example, is a candlepower distribution curve, which is typical of the kind of information that's available from manufacturers. The graph shows the amount of light, in candlepower, that's emitted in various directions from a particular lamp-fixture combination. In order to use the graph, it's necessary to know the distance of the task from the light source and to convert candlepower into footcandles. This can be done using the formula already described

Footcandles = candlepower / distance²

Actually, the best and simplest method o determining the level of illumination received from any fb is to use a light meter calibrated in footcandles. (A photographic light meter calibrated in f-stops is satisfactory if the meter measures incident light and if the manufacturer can provide a conversion table.) One need then only place the meter on the task area, pointing the photoelectric cell toward the light source to obtain the footcandles directly. If there isn’t sufficient light, one can then change the wattage of the lamp or change the position of the fixture, if this is possible, until the desired level of illumination has been obtained. One must, of course, still take into account the probable depreciation in the light output of the lamp-and-fixture combination.

Lamp Placement and Glare

Just as important as the brightness level of the task area is the direction from which the light is shining. When one is working at a desk or table, especially, it's very common to place the lamp directly in front of or directly above the task in order to maximize the illumination. The amount of light falling on the task will certainly be at a maximum, but so is the probability that some of this light will bounce up from the task area directly into one’s eyes. That is, this placement of the lamp will result in reflected glare. This is particularly true if one is reading a book or magazine printed on smooth, white paper, or if the top of the desk is polished.

A light source should always be positioned in relation to the task so that the light falling on the task will not be reflected into one’s eyes. An incandescent desk lamp, for example, should be placed to one side of the table, and the shade should be low enough that the incandescent lamp itself can't possibly shine directly into the eyes. A fluorescent lamp fixture mounted on a flexible support should be positioned rather close to the reader’s head so that the light emitted from the fixture will bounce off the task and away from the eyes. At a worktable illuminated by an overhead ceiling fixture, the best position for the fixture is above the front edge of the worktable. This position provides maximum illumination, and it also reduces the possibility that light will be reflected back into the eyes.

General Room Illumination

Having determined what concentrated sources of light are necessary in a room, one can then plan the general lighting. This planning consists of tying the concentrated sources together with fixtures that provide general illumination. The basic idea is to avoid or prevent having too great a contrast in brightness between the different parts of the room. As already mentioned, the general lighting level shouldn't be more than five times as bright or less than one-fifth as bright as the light provided for the performance of particular tasks. The same principle should be followed when lighting adjacent rooms, as between a living room and a hallway, say.

Stairways in particular should be well lighted, which means that the stairs should be at least one-fifth as bright as the adjacent hallways or rooms. The walls along a stairway should be painted a light color to increase their apparent brightness. Under no circumstances should a light be placed at the bottom of a flight of stairs in such a way that the bulb is directly visible to anyone descending the stairs.

Fig. 6. A candlepower distribution curve for a ceiling-mounted lighting fixture (Or luminaire) (Illumination Engineering Society). Luminaire with horizontal and vertical coordinates; lighting distribution demonstrated visually; candlepower distribution curve for luminaire.

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