L01 - Lighting Fundamentals (all KM)1. Introduction2. Fundamentals I - DefinitionsFundamental Definitions Calculation of Luminous Flux and Efficacy 3. Fundamentals II - Propagation of LightPropagation of Light Inverse and cosine laws of illumination Luminance and Luminous exitance Other illumination measures 4. Fundamentals III - The fundamentals of ColourColour and the visual process Colour measurement and quantification Light source colour performance Surface colour systems 5. Calculations I - Basic Illuminance calculationsPoint Source variations Line sources (aspect factors) area sources 6. Calculations II - Standard CalculationsLumen Calculation Glare Index Calculation 7. Calculations III - Lighting Design StrategyLighting Design Strategy 9. Human Factors I - Fundamental ConsiderationsColour Man’s response to light Brain Systems Eye and Vision Lighting Design The visual task 10. Human Factors II - Human Response to LightVisual acuity and threshold contrast Light and work Visual Performance Cells that specialize in seeing Colour Vision 11. Human Factors III - Perceptual Characteristics of the visual systemSize constancy and shape constancy Lightness constancy Colour Constancy L01(2). Fundamentals I - DefinitionsSpectral sensitivity of the eyeThe sensitivity of the eye is not uniform over the visible spectrum, but varies with wavelength. The spectral sensitivity is shown in the V(l) curve, with maximum sensitivity at 555nm. There are three types of vision; photopic, scotopic and mesopic, which correspond to how the eye reacts under different luminance levels Photopic visionThis is in effect when the surroundings have luminances of greater then 10 cdm-2. Vision is mediated entirely by the cone receptors, and is maximal in the blue-green end of the spectrum. Scoptopic visionThis comes into operation when the luminance is less than 10-2 cdm-2 and they eye has had time to become dark adapted (typically around 30 minutes). The spectral sensitivity curve for this, V ‘(l), is represented by a shift towards the blue end of the spectrum with maximal sensitivity at 507nm. Since this mode of vision is mediated by the rod detectors, you have equal sensitity over the entire vsual field, and it is quite lke you see things ‘out of the corner of your eye’. Unlike cones, rod detectors do not detect colour, and the scotopic view of the world is monochromatic. Mesopic visionIn the intermediate range, three effects can be noted, in addition to a general increase in luminance; · foveal detection becomes as easy as peripheral detection, and then easier · a sense of colour can be appreciated · the relative luminosity of different colours change; in particular the luminosity of the eds increase more strongly than the blues. The overall response of the eye lies some where between V(l) and V ’(l), moving from left to right, as the general luminance increases. Basic DefinitionsLuminous Flux,F; The total visible light energy emitted by a source per unit time. Unit - Lumens (lm) One lumen is by definition equal to 1/673W at the wavelength l=555nm where the eye has its maximum sensitivity. Luminous Intensity,I; The luminous flux from a source, F, in a specified direction inside a small solid angle,w.
Unit: lumen per steradian or candela (cd) Illuminance, E; The amount of luminous flux, F, incident per unit area of surface, A.
Unit: lumen pr square metre of lux (lx) Luminance, L; The intensity, Iu, per apparent unit area of the surface of the actual light source.
Unit: candela per square metre (cdm-2) Reflectance,r; The ratio of reflected flux to incident flux (either luminous or radiant)
DERIVATION ??? Perfect Diffuse Reflector This is a surface for which the luminance is independant of the angle of view. Light Output Ratio The ratio of the light output of a luminaire to the sum of the light outputs of the lamps it contains. Luminous Efficacy, h; For a light source with a total power input W watts the luminous efficacy, denoted by hV, is given by
where FV is the total light output of the source. Unit: lumen per Watt ( lmW-1) Typical Efficacys: GLS 9-20; TuHa 15-5; MCF 40-75; MBF 40-60; MBI 60-90; SOX 130-180; SON 95-130 L01(3). Fundamentals II - Propagation of LightMethods of light emission (L&L pp. 101)Light can be seen as either a wave or a particle. It is a form of electromagnetic radiation, and is generally involved in the transfer of energy between particles. There are many forms of energy transfer which emit light. Generally distinguished by the source of the input energy. Incandescence Solids and liquids emit visible radiation when hey are heated to temperatures about 1000K. The intensity increases and the appearance becomes whiter as the temperature increases. Electric Discharge When an electric current is passed through a gas the atoms and molecules emit radiation whose spectrum is characteristic of the elements present. Electroluminescence Light is generated when electric current is passed through certain solids such as semiconductor or phosphor materials. Photoluminescence Radiation at one wavelength is absorbed, usually by a solid, and re-emitted at a different wavelength. When the re-emitted radiation is visible the phenomenon may be termed either fluorescence or phosphorescence. Generation of radiation (L&L pp. 102)Quantum physics says that the outermost electron in a an atom may have one of discrete energies. Normally it resides in the state of lowest energy, or ground state. When it absorbs energy, it is exited, and jumps to a higher energy state. Eventually it is de-excited, and falls back to the ground state, emitting energy in the form of a photon. The frequency of the light is given by Planck’s equation;
where Q = difference in energy between the levels
v = frequency of light radiated Thermal radiationWhen a body is heated to a high temperature its constituent atoms become excited by numerous interactions between them and energy is radiated in a continuous spectrum. The continuous spectrum arises because the energy levels of the electrons in solids are broadened to the point of merging in a continuous band. Thermodynamics have defined a perfect black body or full radiator. By definition, a black body absorbs all energy falling upon it, provided it is held at a constant temperature. Only a few materials such as carbon black and platinum black approach this ideal in reality. Planck also derived a relationship for the spectral distribution of thermal radiation for a full radiator;
where DIAGRAM ??? The maximal radiant exitance increases with temperature, and the wavelength at maximum power is inversely proportional to the temperature T. This is known as Wiens Displacement Law;
This law corresponds to the observation that a heated body first glows red, then yellow, and the bluish. The total radiant exitance of a black body surface is found by integrating
where s is the Stefan-Boltzmann constant. Propagation of Light (L&L pp. 4-8)Light as a wave travel through vacuums at a constant velocity, approx. 3 x 108 ms-1. In a material medium e.g. air or glass, velocity of propagation is less than in a vacuum by a factor known as the Refractive Index.
RefractionLight passing through a smooth boundary surface into the second medium suffers a change of direction according to the following laws; · The incident ray, the refracted ray, and the perpendicular to the surface at the point of incidence all lie in one plane · If the incident ray is in a medium of refractive index n1 and makes an angle q1 with the perpendicular to the surface, and the refracted ray is in a medium of refractive index n2 and makes an angle q2 with the perpendicular, then;
where q1 and q2 lie on opposite sides of the perpendicular (Snell’s Law). When a ray passes from a high to a low refractive index material, such as
glass to air, a refracted ray exists only if q1 is less than the critical angle, which is equal
to If the ray is incident at an angle greater than the critical angle, no refracted ray is present and all of the incident light energy appears in the reflected ray; the term total reflection is used for this condition. ReflectionAt a bounding surface that is smooth compared with the wavelength of the incident light, specular reflection is said to occur. A single incident ray produces a single reflected ray and the following relations occur; · the incident ray, the reflected ray and the perpendicular to the bounding surface at the point of incidence all lie on one plane · the incident ray and the reflected ray make equal angles with the perpendicular and are on opposite sides of it. Reflection is the main way of directing light inside a luminaire. Absorption and ScatteringA light ray passing through a vacuum loses no energy, although the energy make become more spread out. In their progression through material media however, light usually loses energy through absorption and scattering effects. Absorption is caused by the conversion of light into some other form of energy, usually heat, but it could be changed into radiation of a different wavelenth (fluorescence), into electrical energy in a photocell, or chemical energy as in the photosynthesis that occurs in plants. MORE ??? Scattering occurs in non-homogeneous media and is caused by multiple refraction and reflection at numerous, randomly oriented, boundary surfaces within the media. Fog and cloud are examples of scattering conditions in air due to the presence of suspended water droplets. Scattering can be wavelength selective due to a contribution from diffracting light, and this can also impart colour onto a media. All material media scatter light to some extent because of the molecular structure of matter. Scattering by very small particles, such as molecules, is greater for the shorter wavelengths of light; the blue sky is accounted for in this way. Diffuse Reflectance and transmission; cosine diffusersWhen a light ray meets a surface which has irregularities comparable with or greater than the wavelength of incident light there is no longer a single reflected or refracted ray, but the light spreads out in all directions from the point of incidence, as in scattering. The light which returns to the medium from which the incident ray emerged is said to be diffusely reflected and the light which passes through into to the second medium is said to be diffusely refracted. In general the precise angular distribution of the reflected and tramitted light depends on the angle of incidence and of the nature of the surface. With very fine grained roughness the reflection may almost be specular at angles of incidence approaching 90o.
Uniform Diffuser In order to allow for simple calculations to be made, the concept of a uniform or cosine diffuser is often used. A uniform diffuser is one in which the reflected light distribution is independant of the angle of the incident light, and the intensity of the reflected light in a direction making an angle q with the perpendicular to the surface is proportional cos q. This is also known as a Lambertian Reflector. No real surface completely satisfies the conditions for a uniform diffuser, but some surfaces make a good approximation, for example a layer of magnesium oxide powder. Inverse square and cosine laws of illumination (L&L pp. 15)
We consider a point source, S, illuminating a plane surface, P. We know the illuminance on a small area dA, illuminated by a luminous flux dF is
Similarly, the luminous Intensity I is given by
where dw is the angle subtended by the element dA at the source. The illuminance produced at a point source at a distance r from a plane is obtained by first eliminating dF from the above two equations to give;
and since
substituting for dw gives
This expresses both the inverse square and cosine laws of illumination from a point source. Luminance(L&L pp.15)Luminance is related to the sensation of brightness, although the two are not equal. The concept of luminance can be applied to any surface which is emitting or reflecting light, and can be generalized to include any imaginary surface in space through which light is passing, e.g. a portion of the sky. It can be shown that the luminance of any such surface in a non-emitting, non-absorbing, and non-scattering medium is constant along a ray passing through the surface. In less formal terms, this corresponds to the observation that in clear air the brightness of a surface does not depend on its distance from the observer. A uniform diffuse source is one which has the same luminance over its entire surface for all viewing directions. An element of area A emitting a total flux F in all directions (i.e. over a solid angle of 2p sr) has a luminous exitance given by
For a unform diffuse source M=pL and
Other Illumination Measures(L&L pp. 84-86)Illumination VectorThe expression for the the illuminance E on a plane surface produced
by a point source, If the illumination vector at a point is 0, it does not mean no light is falling on the point, merely that the illuminances on both side of a plane placed there are equal. There are other non-planar measures of illuminance which can be useful. Mean Scalar IlluminanceMean spherical illuminance is the mean illuminance over an indefinitely small sphere placed at the point of interest and, because the measure is independant of direction, it is sometimes called the scalar illuminance. Consider a point source of luminous intensity I at a distance d from
a small sphere of radius r. The sphere intercepts the same luminous
flux as would a disc of radius r, which is therefore
For a uniform diffuser, this becomes
where w is the solid angle subtended by the whole source. The vector-scalar ratio relates the direct and diffuse components of light falling on an object. Evidence shows that a ratio of 1:1.2 to 1:1.8 gives a pleasing effect. Mean Cylindrical IlluminanceMean Cylindrical Illuminance id the mean illuminance over an indefinitely small cylinder placed at the point of interest, usually with the axis of the cylinder vertical. In contrast with mean spherical illuminance, mean cylindrical illuminance depends on the direction of the light. The calculation is similar to that of scalar illuminance, except that he
flux intercepted by the cylinder depends on the angle y between the axis of the cylinder and the incident rays. Then
if the radius of the cylinder is r, and the height l, its projected
area in the direction of the incident light is
L01(4). Fundamentals III - The fundamentals of ColourColour and the visual process(Measuring Light pp. 4)The eye responds to different wavelengths of light by the sensation of colour. A light source emitting energy with equal power throughout the visible spectrum would appear to be white. No such source is known, but the concept is used to define equal-energy white. It is useful to not, that due to the spectral sensitivity of the eye, although the radiant energy is constant with wavelength, the luminance of the source is mostly due to light emitted in the 520nm to 600nm band ( the green and yellow parts of the spectrum). A near approximation to equal-energy white is given by sunlight, and certain artificial light sources. The colour temperature of a source is the temperature at which a heated black body would produce light giving the same colour sensation as the source. North-light can reach a colour temperature as high as 25 000 K. A pure spectral colour is the most intensely coloured source of light available, i.e. it is fully saturated. When we mix a saturated colour with white light, we say it is de-saturated. Therefore, we could create any colour by mixing a pure spectral colour with a given amount of white light. (The exception to this would be the purples, which are not spectral and are a mix of red and blue). Broadly speaking, to match a colour, we simply need to record a wavelength of spectral colour along with the degree of saturation involved. This is of often done, and can be adequate for many measurement purposes. In fact, any colour can be matched my a mixture of three primaries (i.e. any primary cannot be matched by a combination of the other two), but on occasion we need to use desaturation (i.e. negative mixing of one colour), as in yellow/green - 540nm. (Grassman 1854) We should try and relate this to the physiology of the eye; We have three types of cones, b,g and r. These have maximum sensitivity in the short, medium and long wavelengths respectively. We believe that we have two colour channels arranged in antagonistic pairs, i.e. one channel measures Red + Green , another measures Blue + Yellow. This is called Opponent Colour Theory (Hering). Below is a simplified representation of the colour system in the brain.
The visual sensation has three attributes, Brightness, Hue and Saturation. · Brightness is affected by all cone types, as well as input from the rods. · Hue depends on the value of the two colour channels; · Reddish = C1 +ve · Greenish = C1 -ve · Bluish = C2 - C3 +ve · Yellowish = C2 - C3 -ve · Saturation depends on the strength of C1 and C2 - C3. Colour measurement and quantificationBecause the human eye cannot separate the component colours of a light source, it is possible to produce the sensation of colour by any number of colour mixtures. For example, white can be made by mixing two spectral complementary colours. Grassman postulated that light of any colour can be matched by the combination of not more than three suitable primary colours. He also stated the principle of additivity: that lights producing the same visual effect separately produce the same effect in mixtures. Wright studied these hypotheses, and showed they were correct. It is interesting to note that at except for the wavelengths corresponding to the three primaries used, a negative amount of one of the primaries needed to be used. In colour matching we adopt the following algebraic convention; the result of a match is written as C(C) º R(R) + G(G) + B(B) where C is the amount of colour (C) and R, G and B are the amounts of the primaries and are known as the tristimulus values. If suitable units are used for R, G and B, then the amount of (C) is given by C = R + G + B Often we want to only consider the quality of the light, and ignore its intensity. Then we write (C) = r(R) + g(G) + b(B) where This equation for (C) is known as a unit trichromatic equation, since r + g + b = 1. Graphical Expression of Colour
This figure shows Wright’s primaries plotted in a different way. All colours that can be matched with additive mixtures of the three primaries lie inside the triangle RGB. It can be seen that all the spectral colours lie outside this triangle, and the line joining them is called the spectral locus. We can see that the spectral locus is everywhere convex, therefore there exist no three primaries which can match all spectral colours without first desaturating them. A line can be constructed any point on the locus to the centroid of the triangle to represent the locus of a colour that is being progressively desaturated. If such a line is continued further, the point where it meets the locus represents the colour that is complementary to the first. The CIE colour systemWe can see that any system for numerical representation of colour must take into account · the energy distribution of the source with respect to wavelength · the varying sensitivity of the viewer’s eye with respect to wavelength · the absorption of light of different wavelengths by a coloured surface The CIE system was agreed upon in 1931. It drew upon the work of Wright and Guild, who had produced closely similar results. Wright and Gould had enlisted the help of a number of people with normal eyesight, whose characteristics were averaged to produce the CIE standard observer, on which the system was based. Imaginary Primary stimuliWright had defined the spectrum locus in terms of three primaries. Since no three spectral primaries were capable of representing all primaries without using negative coefficients, three imaginary primaries (X), (Y) and (Z) were used. In Wrights diagram, these would be outside the spectral locus, which would then be completely enclosed by a triangle XYZ. (C) º x(X) + y(Y) +z(Z); x + y + z = 1 We see that we can uniquely determine any colour simply by two variables x, y since these determine z. This allows us to map the colour space into two dimensions; This can be seen in the CIE Chromaticity chart. DIAGRAM ??? As with Wrights three primaries, the CIE tristimulus values can be reduced to the coefficients x, y and z, where The CIE was concerned with making colour calculations as easy as possible. Therefore the line joining (0,1) and (1,0) follows the spectral locus in the region where it is almost straight. Thus, spectral colours in the region 550nm to 780nm all have values of z equal to or near 0. Also, the units in which (X), (Y) and (Z) are expressed are such that an equal amount of each is required to give a match with equal-energy white. This white is therefore represented by (x,y,z) equal to (0.333, 0.333, 0.333). Also, the luminance of a coloured light is directly proportional to its Y tristimulus value only. This is because the CIE stimuli (X) and (Z) both lie on the alchyne or line of zero luminance. Uniform chromaticity scalesThere is one serious disadvantage with the X-Y chromaticity chart - MacAdam(1942) and Wright(1941,1943) were carrying out experiments on the size of ‘noticeable differences’ in colour in terms of x and y. MacAdam gives results for the size of ‘10 minimum perceptible colour differences’. It is seen that the size of the mpcd varies with position on the (x,y) chromaticity diagram, and the direction with which the difference occurs -- an ellipse represents equal visual steps in colour away from the centre in any direction. This means that the (x,y) diagram is not uniform, and this is a disadvantage when measuring colour differences and specifying colour tolerances. Various projections of the (x,y) diagram have been developed in order to find a more uniform system, but none have been completely satisfactory, since they have been developed upon linear transformations of the data. In 1960, the CIE recommended at transform by MacAdams(1937) which improved the uniformity so that the near the full radiator locus at 6500K the m.p.c.d. locus is almost a circle. Light source colour performanceThe centroid of the colour triangle represents the colour of an equal-energy stimulus, a white light whose energy is constant for all visible wavelengths. Since this does not exist, it is not possible to see an object illuminated by it. Therefore we must define some standard illuminants so that we specify the appearance of surfaces under them. In 1931, the CIE defined three standard illuminants, A, B and C. They were all based on the light emitted by a gas-filled tungsten-filament lamp operating at a colour temperature of 2856K. This was in fact Illuminant A, and B and C were derived from it by using filters with B representing noon sunlight (4874K) and C representing average daylight (6774K). Since B and C were found deficient in the near-UV range, which has now assuming importance in the colorimetry of fluorescent sources, another range was introduced, the D range. D65 represents daylight with a colour temperature of 6500K. The CIBSE guide introduces standard words to descirbe source of different correlated colour temperatures, · below 3300K – Warm · 3300 – 5300K – Intermediate · above 5300K – Cold We also need a standard for ‘whiteness’, i.e. that of a perfect diffuse reflector. In 1931, CIE adopted magnesium oxide as the standard for colour measurement. Now this has been updated to relate all colorimetry to the perfect diffuser instead. Surface colour systems (L&L pp 59-60)The Munsell System of colour notationMany colour atlases and colour solids have been developed based on different methods of arranging solids. The best known is that developed by Munsell (Nickerson 1976) in which the central vertical axis, called the Value (lightness) scale ranges from white at the top to black at the bottom with nine intermediate values of grey. The saturated colours are arranged on the equator of the solid in 10 Hues, B, BG, G, GY, Y, YR, R, RP, P and PB, each of which is divided into 10 steps. The saturation, or Chroma, of the colour increases as it moves away from the central axis. A colour is specified by a combination of three numbers, for example 5YR 6/8 where 5YR is the Hue, the Value is 6 and the Chroma is 8. In practice the munsell solid is not a sphere, since it is not possible to produce saturated greens and blues with lightness as high as for yellows and reds. The complete system is approximately 1,000 samples which are intended to be viewed under daylight. Munsell’s idea was that the visual difference between adjacent samples at any position should be the same, but this was not achieved. The system has the advantage that it is well-known internationally and hence provides for a means of communicaton between users. Natural Colour SystemThis was a colour circle with four defined points R, Y, G, B. Also B/W scale was defined. For example, a purple would be classified as R40B, i.e. 60% red to 40% blue. This was used extensively by ICI. FINISH !!! BS 5252: Framework for colour co-ordination for building purposes.The colour system has about 240 values,and are arranged systematically using the concepts of hue, greyness and weight. Greyfulness gives an estimate of the colourfulness of the sample, and the amount of grey decreases as the saturation of the colour increases. The system also has various finishes. Colour Rendering (CIBSE pp. 23-24)The ability of a light source to render colours of surfaces accurately can be conveniently quantified by the CIE general colour rendering index. This index is based on the accuracy with which a set of test colours is reproduced by the lamp of interest relative to a test lamp, perfect agreement being given a score of 100. The CIE index has some limitations, but is the most widely accepted measure of the colour rendering properties of light sources.
L01(5). Calculations I - Basic Illuminance calculationsPoint Source variations(CIBSE pp. 242)There are three applications for the inverse square and cosine law · The general case, already considered · illuminance on a horizontal surface · illuminance on a vertical surface In each case the luminous intensity I(q) in candelas at the angle of elevation (q) is required. It can be found from the luminous intensity distribution of the luminaire. Line sources (aspect factors) (L&L pp.81-83)The inverse square law of illumination only strictly holds for point source. In practice, it can be used for large diffusing sources if the distance to the point being illuminated is more than five times the largest dimension of the source. For shorter distances, the source may be split into smaller elemnts and the combination from each of these to the illuminance at a point summed, either by calculus or numerical integration. CIBSE TM11 goes into details about aspect factors, their calculation and use.
The formula for calculating Illuminance at a point P due to a line source AB, length l, which is at a normal height d from the plane of interest, and the plane containing A, P and B is at an angle q to the vertical, with the points A and B at angles a1 and a2 respectively to P in that plane, is;
For perpendicular planes, we use the perpendicular aspect factor function af[a].
Area sources(CIBSE pp. 246)We can really only consider the basic case of a uniform area source with
cosine distribution, giving the illuminance directly below one of the corners. To
obtain the illuminance at a point that is not directly beneath one corner,
we must add or subtract contributions from four imaginary sources, each with
a corner over the point to obtain the resultant. Let Ap be the
peak luminous intensity (we assume Then
where
L01(6).Calculations II - Standard CalculationsLumen Method calculationLuminaires in Regular Arrays[1]
where · E = average Illuminance (lx) · N = no. of luminaires · n = no. of lamps in each luminaire · F = flux from one bare lamp · UF = utilization factor · MF = maintenance factor, allowing for effects of dirt and depreciation · Af = area of working plane or floor Utilization FactorThe Utilization Factor(UF) is the proportion of light flux emitted by the lamps which reaches the working plane. Luminaire manufacturers issues tables of utilization factors for various combinations of Room Index and surface reflectances. The Room Index for a rectangular room l x w where hm is the height of luminaires above the working plane, is given by;
Maintenance Factor
where · LLMF = Lamp Lumen Maintenance Factor · LSF = Lamp Survival Factor · LMF = Luminaire Maintenance Factor · RSMF = Room Surface Maintenance Factor UniformityThe CIBSE code recommends that uniformity over task areas should not be less than 0.8.
This does non refer to wall-to-wall working plane. Consider diversity of illuminance out side task area. Diversity in the core area[2] of a working plane should not exceed 5:1. Spacing to Mounting Height ratio (SHR)
For an asymmetric luminaire, one figure, SHRMAX, is quoted. SHRMAX should not be exceeded if uniformity is to be acceptable for general illuminance. For a di-symmetric luminaire, two figures, SHRMAX and SHRMAX TR are quoted. Spacing is subjected to the following three constraints; · axial spacing must not exceed SHRMAX · transverse spacing must not exceed SHRMAX TR · (axial spacing x transverse spacing) must not exceed (SHRMAX)2 Glare Index Calculation(CIBSE pp 172, 200)The CIBSE glare index system for the evaluation of discomfort glare is described in CIBSE TM10. The glare index system can be applied to a wide range of conventional luminaires, but it does have some limitations. It cannot be applied to large area light sources such as luminous ceilings, since the basic formula in the invalid. It cannot be applied to coffered ceilings and similar large cut-off luminaires. Also it may underestimate the discomfort glare for some ceilings-mounted luminaires, especially those in which the luminaire intensity distribution is such that the luminance of ceiling adjacent to the luminaire is greater than the luminance of the luminaire itself. Glare indices are specified for particular building types. These are set three apart, since this is the minimum difference being necessary for a change in discomfort glare sensation to occur. Basic Glare Index Formula
where · Lb = Luminance of background as seen by observer · Lj = luminance of luminaire j as seen by the observer · j = solid angle of luminaire j as seen by the observer · pj = position index of luminaire n as seen by observer This method normally requires a computer program, but has the advantage that the formula can be applied to individual, randomly spaced or regular arrays of luminaires for any specified direction. The second method, calculating glare indexes based on photometric data provided by the manufacturer, is sufficiently accurate for most purposes, and is easy to use. Uncorrected Glare Indices, with correction factorsThis method is based upon the following assumptions; · The luminaires are at a SHR of 1.0 · The luminaires are at a height of 2.0m above eye level · The total light output of the lamps in the luminaire is 1000 lumens · the observer is located at the mid-point of a wall, with horizontal line-of-sight towards the centre of the opposite wall · the eye level is taken as 1.2m above floor level.
Uncorrected Glare Indices are tabulated according to room dimensions and reflectances. We specify the room dimension in terms of multiples of the mounting height above eye level. The x direction is always perpendicular to the line of sight, and the y direction is always parallel to the line of sight. The worst glare conditions will occur for viewing from the centre of wither the long wall or the short wall. Interchanging x and y will allow for this. When the glare index has been found (interpolation may be needed) it must be corrected for · mounting height above 1.2m eye level · total lamp luminous flux per luminaire if this differs from 1000 lumens · extra correction terms if the published UGI table covers a variety of luminaire sizes or lamp types These correction terms are added, or subtracted, from the initial glare index to give the final glare index of the installation. The height correction and total luminous flux correction may be calculated as follows; · Height
correction term - · Total
lamp luminous flux correction term - L01(7). Calculations III - Lighting Design Strategy (CIBSE pp.178-181)Overview· Daylight + Sunlight · window types - how fabric of building is penetrated · fixed and flexible controls · glazing · Lamps · Light Fittings · Control · Adaptation - adaptation sequence throughout the building · Glare Design ObjectivesSafety· What hazards need to be seen clearly? · What form of emergency lighting is needed? · Is a stroboscopic effect likely? Task Requirements· What tasks are likely to be performed in the interior, what planes do they occupy? · What aspects of lighting are important to the performance of the task? · Are optical aids necessary? Appearance· What impression is the lighting required to create? · Light pattern? · Colour pattern? ConstraintsStatutory· Are there any statutory requirements which are relevant to the lighting installation? Financial· What is the budget available? · What is the relative importance of capital and running costs including maintenance? Physical· Is a hostile or hazardous environment present? · Are high or low ambient temperatures likely to occur? · Is noise from control gear likely to be a problem? · Are mounting positions restricted, and is there a limit on luminaire size? Planning ChecklistDaylight and Electric Light· What is the relationship between these forms of lighting? · Is it possible or desirable to provide a control system to match the electric lighting to the daylight? Solar glare and heat gain· Are the windows designed to limit the effects of solar glare and heat gain on the occupants of the building? · Are the window walls of suitable reflectances? · Ensure diffuse ‘cool light’ used to maximum effect (not sunlight) Choice of electric light system· Design in terms of sub-systems · Is general, local or localized lighting most suitable for the task? · Does obstruction make some form of local lighting necessary? Choice of lamp and luminaire· Does the light source have the required · lumen output · luminous efficacy · colour properties · lumen maintenance · life · run-up and re-strike properties? · Is the type of luminaire appropriate? · Task/local luminaires · accent and display luminaires · uplight luminaires · wall-washing luminaires · Downlight luminaires · Is air handling heat recovery appropriate? · Does it have a suitable appearance? · Will create the required lit effect? · Is reliable photometric data available? Maintenance· Has a maintenance schedule been agreed? · Has a realistic Maintenance factor been estimated? · Can the equipment be easily maintained? · Will replacement parts be available? Control systems· Is a dimming facility desirable? · Type of control needed; · localized manual switching · time switching · daylight linking · on/off · top up · occupancy linking · ultrasonic/infra-red · Have manual override switching systems been supplied, and are they intuitive in respect to the sub-system layout? Detailed Planning· Integration of electric lighting with daylighting · Avoidance of unnecessary high levels of electric lighting · Designed Maintained Illuminance · Task size and contrast · Task duration · Error risk · use of high efficacy equipment Statement of assumptionsWhen submitting a design proposal to a client, it will usually be necessary to supply the following information to a client; · the design specification i.e. · type of lighting system · designed maintained illuminance · illuminance variation · maintenance program · glare index · lamp colour properties · wall-to-task illuminance ratio · ceiling-to-task illuminance ratio · equipment used · lamps · luminaires · control systems · equipment layout · costs · lighting conditions which will be achieved if the maintenance program is adhered to · the calculations and measurement tolerances which apply to these values · power density and operating efficacy of the installation · all assumptions made in the design
Table 1 -Assumptions to be made explicit when describing the lighting conditions produced by a proposed general lighting scheme L01(10). Human Factors II - Human Response to LightHuman Response to LightHumans respond to light in many different ways; PhotobiologicalExposure to sunlight is necessary for our health - it enables production of healthy bone growth in children. But too much exposure can also be harmful. Our skin goes brown in to much sun, a pigmentation response to too much sunlight. Erythema, or reddening of the skin, is a reaction to the exposure by Ultra-violet light of wavelength shorter than about 320nm. UV light is categorized by wavelength; · UVA 315-400nm · UVB 280-315nm · UVC 100-280nm UVB is about 103 times more potent than UVA, while UVC is mostly absorbed in the atmosphere. The tissue of the eyes is particularly sensitive to UV-B radiation. McKinley & Whitlock, NRPB looked at the effect of white/ warm white/ cool white fluorescent tubes, and their UV output. They showed that 2,000 hours at 500 lux produced a dose of 5 MEDs. A MED (Minimum Erythemal Dose) is about 300 Js-1. In the UK, we get about 40 to 400 MEDs due to sunlight. This sort of level gives risk factors for 40-44 yr old for basal cell carcinoma at 1:2 500 000 and for malignant melanoma at 1:500 000. OcularEach eye has a field of view of about 1600 with overlapping fields of view. The two eyes work together to provide stereoscopic vision. Our eyes are in continual movement (called saccadic motion). The small ocular tremors set the retinal image in oscillation and the neural network is tuned to detect the consequential local changes in luminance. We are therefore more responsive to sharp changes in luminance than luminance itself. The eye is moved by extra-ocular muscles; weakening in these muscles causes deterioration in vision. The general term for deterioration in any of these muscles is Heterophoria, which covers; · Exophoria – looking out · Esophoria – looking in · Hypophoria – one eye looking up · Hyperphoria – one eye looking down · Cyclophoria – eye tilted The principle refracting surface of the eye is the cornea; the lens changes the focal length, known as accommodation. The pupil size is controlled by · Retinal illumination · Spectral quality of light · Field (of luminance) size · Age of observer · Novelty of the stimulus Subjective· Performance· Visual acuity and threshold contrastVisual Acuity is the capacity for discriminating between objects that are very close together. Quantitatively, it can be expressed by the reciprocal of the angular separation in minutes of arc between two objects which are just separable by the eye. The expression which is more commonly used for visual acuity is the ratio of the distance at which the individual can read a line on a standard opticians chart divided by the distance at which a person of normal sight can read it, e.g. 6/12. Charts such as the Snellen Chart (a standard optician’s chart) and Landolt Rings are used to measure and do experiments involving visual acuity. The luminance contrast C between a background task, such as a printed dot of luminance Lt and its background Lb, is defined by the expression
If its contrast is too small, the detail will be invisible. The threshold
contrast Light and workMany factors come into play, both from the lighting and the work itself in determining how difficult a visual task is; · Light · Illuminance on task · Colour of light · directional properties · spatial distribution · Work · size (visual acuity) · contrast · colour · complexity · form + texture · duration and direction of movement Visual Performance (L&L pp. 29)Much work has been done on visual performance, and its relationship with luminance, contrast and illuminance. The earliest law, Weber’s Law, asserts that the minimum perceptible
luminance step difference The minimum detectable contrast Smith & Rea(79) did an experiment with subjects comparing two sets of numbers. They varied the contrast by using black and grey paper, and varied the luminance in the range of 1 to 1000 cdm-2. They showed that high contrast tasks saturated quickly, and that contrast may be more important than luminance. Weston(??) carried out tests using Landolt rings looking at varying the Illuminance, contrast and task size. He showed that increasing the illuminance usually stopped producing increases in task performance and that larger improvements in visual performance could be achieved by changing task size or contrast than by increasing illuminance. In summary, it is not possible to make a visually difficult task reach the same level of performance as a visually easy task simply by increasing the illuminance. Cells that specialize in seeingThe retinal surface contains two types of cells which are photosensitive, the rods and the cones. The rods provide for scotopic vision, used at low light level, but is monochromatic. The cones, which are centred around the foveal region provide for colour vision. The eye adapts to different luminance levels in three stages; · Rapid – neural mechanism · Medium – adjustment of pupil size · Slow – photochemical process · Rods take 7-8 minutes · Cones take 2 minutes The Aging eyeOn aging, there is a gradual degradation in the visual system, known as senile maccular degradation; · Cornea – loss in transparency, rise in scattering which can lead to glare · Pupil – decreases in size with age · Lens – continues to grow, and becomes compacted along with an increase in fluid · Ability to focus decreases rapidly, due to · Hardening of lens · Lack of flexibility · Yellowing of lens leads to problems in colour vision · Retina – there is a falling cone density, with a fall of 50% between the ages of 20 and 80 Overall, the retina of a 60 year-old will receive only 30% of the light of a 20 year-old retina To combat eye diseases in an installation · Selectively increase lighting levels · Control glare · Increase contrast by varying lightness and colour L01(11). Human Factors III - Perceptual Characteristics of the visual systemMany experiments have been done to probe the visual system. They can be broken down into three types; · Visual system as “detector” · Contrast experiments fall into this category · Lots of experiments have been done, but they tend not to have much relation to the real world, since they analyze only one or two variables at a time · Visual system as “organizer” · Visual system as “interpreter” ConstancyThere is a common theme in the result of all the experiments – the visual system seeks stability; · Perceptual constancy · Size – we ‘understand’ the size of objects, unless visual cues, such as binocular vision, motion parallax and overlapping of objects break down · Shape – if we rotate a circular plate to see it as an ellipse, we still see it as being circular · Lightness – Direct and Indirect path – each ganglion has a receptive field of about 1mm · Colour – Land Mondrian; apply LW, MW and SW light in ratios. Conclusions; · The colour of a patch does not correspond to the colour of the predominant l reflected from it · When the patch is part of a multi-coloured scene, there is no obvious relationship between l composition of the light, and the colour · The colour of the patch in the normal viewing conditions is determined not only from l composition of the light reflected from it, but also by the l composition of the light reflected from the surroundings · No simple and obvious relation with either The receptive field is the area over which light stimulation can influence a ganglion(Kuffler, 1950s). The increase in rate of firing of the receptive field is the luminance in the centre is about 2% higher than the surround. Edge detection is the main method of luminance detection in real life – luminance differences more important than luminance values. The same arguments apply to colour too. ColourCurrent thinking is that colour is a comparison, which takes part as a 2 stage process; · The brain compares the reflectance of the different surfaces for long, medium and short wavebands to produce a lightness record. · It compares three lightness records of the scene to generate colour There seems to be three types of colour channel – Blue/Yellow, Red/Green and Black/White (Hering) Brain Function(Zeki)· Knowledge acquisition · Invariant properties of objects and surfaces · Functional specification · Reflectance, colour, form and motion can be mapped to different areas of the brain · Integration · Interaction · Communication and feedback · Plasticity [1] Regular Array has a precise meaning in this context. The luminaires are arranged in a rectangular arrangement on the ceiling with spacing S between the luminaires, and spacing S/2 to the walls. [2] The Core Area is the area of the working plane having its boundary at a distance 0.5m from the walls of the room. |
= Planck’s
Constant (6.6626176 x 10-34 Js)
is the
spectral radiant exitance (Wm-3)
.
shows
that E can be regarded as the component of a vector of magnitude
. The
component is in the direction of the perpendicular into the plane on which
the value of E is required, with vector being directed away from the
source towards the plane. If there are many sources, we can think of them
producing a illumination vector which is the combined into a resultant vector,
which can be used to calculate the illuminance on a plane.
. The
mean illuminace, ES on the surface is hence given by
. If I is
the intensity of the source, and d its distance rom the cylinder,
the flux intercepted by the cylinder is 
. This
divided by th surface area of the cylinder gives th mean cylindrical illuminance
; 
; 
; 
; 
where I(
). 
;
; 
where
H is the height above eye level(m).
where F is
the luminous flux per lamp(lumens) and n is the number of lamps per
luminaire
,
is the value of luminance contrast when the task is just visible.
is proportional
to
, i.e.
the threshold contrast is independent of 