LIGHTING FUNDAMENTALS.pdf

LIGHTING FUNDAMENTALS Topics ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ Light and Architecture Light as Radiant Energy Color Temperature Fundamental Laws of Light The Eye Factors in Visual Acuity Luminance (Brightness) Exposure Time Quality of Light General Considerations of L...

LIGHTING FUNDAMENTALS Topics ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ Light and Architecture Light as Radiant Energy Color Temperature Fundamental Laws of Light The Eye Factors in Visual Acuity Luminance (Brightness) Exposure Time Quality of Light General Considerations of Lighting Quality Direct Glare Room Brightness Ratios Reflected Glare Diffuseness of Light Color Reaction to Color Terminology and Definition Footcandle Measurements Luminance (Brightness) Measurements Reflectance Measurements Luminous Intensity (Candlepower) Measurements Candlepower Distribution Curves Light and Architecture utilitarian lighting vs. architectural lighting ⚫ Competent lighting designer will make his utilitarian lighting complement the architecture, and the architectural lighting serve a utilitarian purpose, as well as an aesthetic one. ⚫ Light as Radiant Energy IF LIGHT IS CONSIDERED AS A WAVE, IT HAS A FREQUENCY AND A WAVE LENGTH. THIS SHOWS THE POSITION OF LIGHT IN THE WAVE SPECTRUM WITH RELATION TO OTHER WAVE PHENOMENA OF VARIOUS FREQUENCIES. VISIBLE LIGHT COMPRISES ONLY A VERY SMALL PART OF THE WAVE ENERGY SPECTRUM YET IT IS THIS ENERGY WHICH MAKES POSSIBLE OUR SIGHT. COLOR IS DETERMINED BY WAVELENGTH. Color Temperature THE COLOR OF LIGHT RADIATED IS RELATED TO ITS TEMPERATURE. BY DEVELOPING A BLACK BODY COLOR TEMPERA= TURE SCALE, WE CAN COMPARE THE COLOR OF A LIGHT SOURCE TO THIS SCALE & ASSIGN TO IT AN APPROXIMATE “COLOR TEMPERATURE, i.e., TEMPERATURE TO WHICH A BLACK BODY MUST BE HEATED TO RADIATE A LIGHT APPROXIMATING THE COLOR OF THE SOURCE IN QUESTION. TEMPERATURE IS MEASURED IN DEGREES KELVIN, WHICH IS A SCALE THAT HAS ITS ZERO POINT AT MINUS 456°F. THIS FIGURE SHOWS THE ASSIGNED COLOR TEMPERATURE OF SOME COMMON LIGHT SOURCES. COLOR TEMPERATURE OF LIGHT SOURCE IS AN INDICATION OF COLOR OF LIGHT PRODUCED & NOT OF ACTUAL SOURCE TEMPERATURE. Fundamental Law of Light LIGTH IS PREDICTABLE & FOLLOWS CERTAIN LAWS & EXHIBIT CERTAIN FIXED CHARACTERISTICS. When a composite light such as white falls on a surface other than black or white, selective absorption occurs. The component colors are absorbed in different proportions so that light reflected or transmitted is composed of a new combination of the same colors as had impinged on the surface. Similarly, when a white light passed through a piece of red glass emerges as a reddish light since the other components were absorbed in much greater proportion than the red. Fundamental Law of Light In general, transmission factors should be used only when referring to materials displaying non-selective absorption, e.g. clear glass. Figure shows the amount of absorption and reflection depends on the type of material and angle of light incidence, since light impinging upon a surface at small (grazing) angles tends to be reflected rather than absorbed or transmitted. Luminous transmittance of material is the capability of material to transmit incident light. Also known as transmittance, transmission factor or coefficient of transmission, is the ratio of total emitted light to total incident light. Similarly, the ratio of reflected to incident light is variously called reflectance, reflectance factor, & reflectance coefficient Fundamental Law of Light Figures show effect of the material finish on reflection. Fig. 26.4a: If reflection takes place on smooth surface such as polished glass or stone it is called specular reflection, where angle of incidence equals angle of reflection. Fig. 26.4b: If surface is rough, multiple reflections take place on the many small projections on the surface, and light is diffused. In diffuse reflection, incident light is spread in all directions by multiple reflections on the unpolished surface. Such surface appear equally bright from all viewing angles. Fig. 26.4c: Since the reflection factor is a measure of total light reflected, it does not depend on whether the reflection is specular or diffuse. Most materials exhibit a combination of specular and diffuse reflection. Such a surface will mirror the source while producing a bright background. Fundamental Law of Light TRANSMISSION CHARACTERISTICS DIFFUSE TRANSMISSION TAKES PLACE THROUGH ANY TRANSLUCENT SOURCE SUCH AS FROSTED GLASS, WHITE GLASS, MILKY PLEXIGLAS, TISSUE PAPER, ETC. THIS DIFFUSING PRINCIPLE IS WIDELY EMPLOYED IN LIGHTING FIXTURES TO SPREAD LIGHT GENERATED BY THE BULB OR TUBE WITHIN THE FIXTURE. Fig. 26.6a: In nondiffuse transmission, light is refracted (bent) but emerges in same Beam as it enters. Clear materials, e.g. glass, water & certain plastics are examples. The transmission factor is 85% in this instance. Source of light is clearly visible through The transmitting medium. Fig. 26.6b: With diffuse transmission, source of light is not visible & in case of multiple Sources, diffusing surface will exhibit generally uniform brightness if spacing between Light sources does not exceed 1-1/2 times their distance from the material. VISION & LIGHT The Eye Figures show the structure of the eye and the parallel structure of a camera. Light enters through pupil, size of which controlled by iris, thereby controlling amount of light entering the eye. Lens focuses image on the retina from which optic nerve conveys visual message by electric impulse to the brain. The central portion of eye, near fovea, contains light-sensitive cells called “cones” that are responsible for ability to discriminate detail and give us sensation of color. A second type of cell called “rod” are extremely light sensitive but lack color sensitivity and detail discrimination, making “night vision” quite poor. They are highly motion sensitive, resulting in our being best able to detect movement when looking out of the “corner of the eye”. Factors in Visual Acuity 4 Basic Characteristics of Visual Task with which the eye is confronted: ⚫ Size ⚫ Brightness (luminance) ⚫ Contrast, and ⚫ Time exposure of object or area being viewed There are other minor considerations which affect visual acuity, such as pattern of the background, peripheral glare, pupil accommodation, and chromaticity, but these can generally be considered secondary. Size of Visual Object VISUAL ACUITY IS GENERALLY PROPORTIONAL TO PHYSICAL SIZE OF OBJECT BEING VIEWED GIVEN FIXED BRIGHTNESS, CONTRAST, & EXPOSURE TIME. SINCE ACTUAL PARAMETER IS NOT PHYSICAL SIZE BUT SUBTENDED VISUAL ANGLE, VISUAL ABILITY CAN BE INCREASED BY BRINGING THE OBJECT NEARER THE EYE. Luminance (Brightness) BRIGHTNESS AND LUMINANCE ARE ALMOST ENTIRELY INTERCHANGEABLE. THE INTENSITY OF CONES OF LIGHT DETERMINE & DESCRIBE THE PERCEIVED BRIGTHNESS OF THE OBJECT BEING VIEWED. If the surface reflectance of the object being viewed is uniform & the illumination is also uniform, the reflected rays of light will be equal in intensity & we will see an object of uniform brightness. If, however, as is generally the case, either the object or the illumination is nonuniform we will see an object of varied luminance. Luminance (Brightness) The human eye detect brightness over an astonishing range of more than 100 million to 1, the lowest levels being accomplished after an accommodation period, called adaptation. This period varies from 2 minutes for cone vision to up to 40 minutes for rod vision for dark adaptation, but much faster for both types for bright adaptation (going from dark to light) This table lists some typical brightnesses of every day visual task. Contrast THE BASIC VISUAL TASK ARE DETAIL DISCRIMINATION & DETECTION OF LOW CONTRAST. Reading of fine print is detail discrimination while examination of surface textures is detection of low contrast. HIGH CONTRAST IS HELPFUL IN DELINEATING OUTLINE, SIZE, AND DETAIL AS SHOWN HERE. Contrast HIGH BACKGROUND BRIGHTNESS MAKES THE OBJECT VIEWED LOOK DARKER AND THEREFORE ASSISTS IN OUTLINE DETAIL DISCRIMINATION. THIS FIGURE SHOWS A CHART OF ACTUAL VISUAL PERFORMANCE IN A HIGH CONTRAST (BLACK ON WHITE) SITUATION Ideally, the brightness of the task should be the same as that of the background, but ratios of up to 3 to 1 are acceptable in most circumstances. Exposure Time Registering a meaningful visual image is not an instantaneous process, but one that requires finite amount of time. Just as in photography a photo can be taken in dim light by using a longer exposure, so too the human eye can distinguish & discriminate fine details in poor light given enough time (and neglecting eye strain). This figure is a plot of seeing time versus illumination for a given visual task. Of course, time depends on the type of task, and different curves can be plotted for different tasks, but the principle of shorter time at higher illumination remains the same. This is particularly true when the object being viewed is not static, but in motion. Quantity of Light The amount of light falling on a surface is known as its illumination,and is a measure of light flux density. One lumen of flux incident uniformly on 1 sq. ft of area will cause an illumination level of 1 footcandle. footcandles (fc) = lumens Area Attempts have been made to relate the factors of visual acuity to actual seeing tasks in order to establish brightness requirements and thereby illumination levels for various seeing tasks so that practical design can be made of the laboratory experimental data. Dr. H. R. Blackwell experiments resulted in the determination of contrast requirements for various accuracies of sightings, which was then related to actual practical seeing task by means of an empirical “field factor” and a device known as a Field Task Simulator. The chart shows the resultant footcandle illumination figures for the tasks. QUALITY OF LIGHTING General Considerations of Lighting Quality Quality of lighting describes the luminance ratios, diffusion, uniformity and chromaticity of the lighting. Since uncomfortable brightness ratios, where background luminance exceeds object luminance, are commonly referred to as glare, the quality of lighting system is also a description of the visual comfort and visual adequacy of the system. When discomfort glare is caused by light sources in the Field of vision it is known as direct glare. When the glare is caused by reflection of a light source In a viewed surface it is know as reflected glare or “veiling reflection”. Direct Glare Generally, direct glare can be controlled by reducing source brightness and size, positioning sources Outside the direct line of view and raising background brightnesses. A common technique is to use Diffuse high reflectance paints or materials in light colors on upper walls and ceilings. Evaluation of a direct glare situation is quite literally in the eye of the beholder, and even a well-designed Installation may not please all the occupants of the place. Room Brightness Ratios The IES recommendations for luminance ratios to achieve a comfortable environment are tabulated in this table. To achieve these luminance ratios it is obviously necessary to carefully control the reflectances of the major surfaces in a room. Room Brightness Ratios The recommended reflectances for a typical schoolroom and office are shown graphically in these figures (26.19 and 26.20). Room Brightness Ratios The marked difference between an installation with proper reflectances and one with excessive brightness ratios caused by the low surroundings reflectances is shown in Figures 26.21 and 26.22. CONTRAST IS DESIRABLE IN THE OBJECT OF VIEW BUT UNDESIRABLE IN THE FIELD OF VIEW. Reflected Glare THE PROBLEM OF REFLECTED GLARE IS MORE COMPLEX THAN THAT OF DIRECT GLARE IN THAT WE HAVE THE ADDITIONAL CONSIDERATION OF THE NATURE OF OBJECT BEING VIEWED. Many refer to reflected glare when dealing with specular (polished or mirror) surfaces and to veiling reflections in dull or semi matte finish surfaces, which always exhibit some degree of specularity. In all cases, the result is a distinct loss of contrast due to the veiling of the image by the reflection of the light source. THE SOURCE OF REFLECTED GLARE ARE BRIGHTNESSES WITHIN THE GEOMETRY OF REFLECTED VISION AS SHOWN IN THIS FIGURE. Reflected Glare Since reflected glare is, as the name states, caused by reflection, the most effective means of reducing visual impairment is simply to avoid the possibility of reflection. This can be done by arranging the geometry. Unfortunately, this is only totally effective when a single luminaire is involved and when placement of the viewer is completely adjustable – a rare combination. Since reflected brightness which causes loss of contrast is proportional to the luminaire brightness, glare may be reduced by reducing luminaire brightness which, if we are to maintain illumination, means the use of more or larger low brightness luminaires. Diffuseness of Light The quality of lighting is affected by the direction from which the light emanates, or its diffuseness. DIFFUSION is a function of the number of directions from which light impinges on a particular point and the relative intensities, and is therefore measurable by the shadows cast. PERFECT DIFFUSION, rarely obtainable, would have equal intensities of light impinging form all directions, therefore yielding no shadows. Diffusion is generally judged by the depth and sharpness of shadows. A room with welldiffused illumination resulting from multiple sources and high room surface reflectances, yields soft multiple shadows which do not obscure the visual task. Diffuseness of Light Some directional lighting is often introduced as an adjunct to diffuse general lighting to lend interest by producing shadows and high brightness variations. It is what creates shape and is precisely the characteristic best used to influence architectural space and form. Small exposed incandescent lamps, a brightly lighted, rough-textured wall, and pendant fixtures with pierced reflectors are some of the techniques used to create visual interest. Color The color of the illuminant (light) and correspondingly the coloration of the objects within a space constitute an important facet of the lighting quality and should not be considered separately. The 3 characteristics which define a particular coloration are hue, brilliance and saturation. Hue is that attribute by which we recognize & describe colors as red, yellow, green, etc. Brilliance or value is the difference between resultant colors of the same hue so arranged. Saturation is an indication of the vividness of hue or the difference of the color from gray. Among the systems of color classification is the MUNSELL Color System herein shown. Brilliance Is referred to as “value” and saturation as “chroma”; thus a color is defined by hue, value, and chroma. Brilliance (value) of a pigment or coloration is related to its reflectance to white light. The higher the value or brilliance, the higher the reflectance factor as might be predicted when one considers that white and black are the poles of brilliance. Chroma or saturation may be thought of as either the difference from gray or the purity of the color. Spectral colors have 100 percent purity and therefore maximum chroma. When white is added to pigment, it produces a tint; adding black produces a shade. When pigment are mixed, we create color by subtractive process. Reaction to Color Light of a particular hue (other than white) is rarely used for general illumination except to create a special atmosphere. When a space is lighted with colored light, the eye adapts bya phenomena known as “color constancy” so that it can recognize colors of objects despite spectral quality of the illuminant. There is an observed relationship between the color of the light in a space and the range of acceptable levels of illumination. The curve in this chart indicates that cool illuminant color is desirable at high levels and warm at low levels, corresponding to blue sky and the light of a fire respectively. Other psychological effects of color are the coolness of blues and greens and the warmth of reds and yellows. Similarly, red and yellow are “advancing” colors while blue and green are receding colors. LIGHT: CHARACTERISTICS & MEASUREMENTS Terminology and Definition ⚫ ⚫ ⚫ ⚫ Candela (candlepower) abb. cd., is unit of luminous intensity. Lumen, abb. lm, is unit of luminous flux or quantity of light. Footcandle, abb. fc, is unit used to measure density of luminous flux and equal to lumens per square feet. Footlambert (fl), unit of luminance or brightness. Is luminance of surface reflecting or transmitting 1 lm per sq. ft, in the direction being viewed. Footcandle Measurements Measurements of illumination levels are most commonly made with one or another of the available portable footcandle meters, some shown here. Luminance (Brightness) Measurements For luminous source, the cell of the meter is placed directly against the surface; the reading in footcandles is the brightness in footlamberts. (Because footlamberts = lumens per area and incident lumens per area = footcandles. Reflectance Measurements It is often desirable to know the reflection of a given surface since brightnesses can be readily computed (fL = fc x RF). Two methods of measuring reflections are shown here. Luminous Intensity (Candlepower) Measurements Luminous intensity (candlepower) cannot be measured directly but must be computed from its illumination effects. Since candlepower is not uniform in all directions from anything but an ideal point source, the average of the candlepowers in all directions is used. Candlepower Distribution Curves IF WE PLOT ON POLAR COORDINATE AXES RESULTANT CANDLEPOWER FIGURES AT VARIOUS ANGLES, WE OBTAIN WHAT IS CALLED A CANDLE POWER DISTRIBUTION CURVE. THE STEPS IN MAKING SUCH CURVE ARE SHOWN IN THIS FIGURE. Candlepower Distribution Curves IN MAKING CANDLEPOWER DISTRIBUTION CURVES OF NONSYMMETRICAL SOURCE SUCH AS A FLUORESCENT LUMINAIRE, IT IS NECESSARY TO CHOOSE SPECIFIC PLANES IN WHICH THE VALUE WILL BE TAKEN. IT IS STANDARD PRACTICE TO CHOOSE 3 PLANES – A PERPENDICULAR, PARALLEL, AND 45° PLANE. End of Presentation