Introduction to Lighting

Transcription

1 Introduction to Lighting This first section introduces some general background information and terminology. This document is not intended as an exhaustive study of this subject, but as an introduction to some of the essential concepts on which subsequent topics are based. Contents: Lighting Basics Lighting Quantities Visual Functions Colour in Radiance Materials in Radiance Daylight Factor Sky Component/Vertical Sky Component Glare Page 1 of 14

2 Lighting Basics When considering light which is visible electromagnetic radiation we are concerned on the one hand with energy and on the other with a sensation obtained through the eye two principally dissimilar things. The human eye is an extremely sensitive and complex sense organ. A large proportion of the brains function is concerned with vision and perception. In lighting simulation we are not trying to model the way the eye and brain work but rather the way a more simple device operates the camera, hence the term photo-realistic images. Lighting is an art and a science. The lighting quality of a space may be judged on a number of quantitative and qualitative criteria. It is important that these criteria are not seen in isolation to one another, for each is dependent on and influenced by each of the others. The principal quantitative criteria are: the lighting level luminance (or brightness) distribution in the field of view 5.the freedom from disturbing glare The principal qualitative criteria are: 6.the light colour, colour appearance and colour rendering 7.light directionality and shadows Page 2 of 14

3 Lighting Quantities For the quantitative measurement of light, a special set of concepts and units has been adopted that bear no direct relationship to those used in other domains of physical science. This is in contrast with measuring practice in other wavelength regions of the electromagnetic spectrum which is generally based on the familiar concepts of energy and power, and therefore use the SI units of joules and watts. The principal reason for this is that a lighting unit must not only take into account the energy content of the radiation but also the spectral distribution of the sensitivity of the human eye, which varies greatly with wavelength. Luminous flux is the total amount of light radiated by a light source per second. A more familiar term would be light output". It is expressed in lumens (lm). Illuminance is the quantitative expression for the luminous flux incident on unit area of a surface. A more familiar term would be lighting level. Illuminance is expressed in lux (lx), one lux equals one lumen per square metre (lm/m²). [other units are metrecandle, phot, nox] In Imperial units the unit is the foot-candle which equals lumen per square foot (lm/ft²). Luminous intensity is the luminous flux radiated by a light source in a specific direction. Luminous intensity is expressed in candelas (cd). Luminance is the quantitative expression for the amount of light reflected by a surface in a specific direction. A more familiar word is brightness, although this term must, strictly speaking, be reserved to Page 3 of 14

4 describe the subjective impression of luminance on the eye. The luminance of a surface is determined by the illuminance on the surface in question and its reflective properties. Luminance is expressed in candelas per square metre (cd/m²), referred to as the nit. [other units are lambert, stilb, apostilb, blondel, skot]. In Imperial units the unit is the foot-lambert, which is candelas per square foot (cd/ft²) Luminous efficacy is the ratio between luminous flux and power dissipation, and is expressed in lumens per watt (lm/w). Each lamp type has a different luminous efficacy. Page 4 of 14

5 Examples of Illuminance Summer, under a cloudless sky, in the open Summer, under a cloudless sky, under a tree In the open, under a heavily overcast sky Indoors by the window, shaded, clear day Indoors away from the window Full moon, in the open, on a clear night 100,000 Lux 10,000 Lux 5,000 Lux 2,000 Lux 300 Lux 0.25 Lux The target illuminance for an interior space depends on the specific visual tasks carried out in the space and can be anything from hundreds to thousands of Lux. 100 Luxinteriors used occasionally with visual tasks confined to movement and for only limited perception of detail. 200 Luxinteriors occupied for long periods, or for visual tasks requiring some perception of detail. 500 Luxmoderately difficult visual tasks, perhaps involving colour judgement Lux very difficult visual tasks Lux exceptionally difficult visual tasks [see CIBSE, Code for Interior Lighting for more details] Page 5 of 14

6 Visual Functions This section contains definitions of some of the functions the eye can perform, without attempting to provide too much detail. Superficially the eye resembles a camera in so far that it has a lens, which throws an image onto the light sensitive back surface, which is called the retina. Focussing is not achieved by altering the distance between lens and retina (as with a camera) but by changing the shape of the lens. In the retina there are two types of receptor, rods, which are highly lightsensitive and are principally responsible for detection of shape and movement, and cones, which are less sensitive to light, but can distinguish colours. Accommodation Accommodation is the ability of the eye to focus on objects at varying distances from the eye. This is achieved by changing the focal length of the lens of the eye using the ciliary muscles. This ability varies with age of the individual and state of tiredness and also with the luminance of the visual scene. Adaptation This is the mechanism by which the eye changes its sensitivity to lighting levels. Adaptation from normal lighting levels to dark conditions can take up to 10 minutes, adaptation from dark to light is more rapid. Page 6 of 14

7 Convergence Almost invariably, we focus both our eyes on the same target. When that target is distant the lines of sight are in parallel. However when we look at a nearby object our lines of sight intersect at the target. Convergence allows the eyes to rotate inward so that both eyes focus on this object. Visual Acuity The ability to differentiate between closely spaced visual stimuli. This can vary from person to person and also is strongly linked with the background luminance and observation time. Contrast Detection Most of the visual information we receive is the result of luminous variations in the field of view. We call this contrast. Contrast can take two forms, which mostly occur together, contrast in colour and contrast in luminance. Glare There are two forms of glare, discomfort glare and disability glare. Discomfort glare is a sensation of annoyance or pain, probably as the result of frequent changes in pupil size caused by excessive brightness contrasts. Disability glare is the result of interference in the visual process, there are two sub-categories of disability glare, veiling glare and adaptive glare. This subject is in more detail in subsequent chapters. Colour in Radiance In common with most computer based interfaces which use images as a method of displaying information, Radiance uses the RGB colour model. This is the technology of the TV screen and computer monitor, beams of electrons are fired at a screen composed of 3 different phosphors, Red, Green and Blue. [known as the additive primary Page 7 of 14

8 colours, as against the subtractive primary colours, Yellow, Magenta and Cyan, which are used in printing] Varying levels of these three colours are mixed to give the impression of all possible colours from Black to White. It is possible to visualizing the 3 colours as the X, Y and Z axes in conventional 3D space. The volume bounded by the minimum and maximum values of each colour forms a cube. The origin (0,0,0) represents Black and the diagonally opposite corner (1,1,1) represents White. [the scale is sometimes defined as 0.0 to 1.0 in real numbers and sometimes as 0 to 255 in integer numbers] Thus any colour can be represented by the co-ordinate location within the cube i.e. the amount of each primary colour. Black = 0.0, 0.0, 0.0 White = 1.0, 1.0, 1.0 Red = 1.0, 0.0, 0.0 Green = 0.0, 1.0, 0.0 Blue = 0.0, 0.0, 1.0 Yellow = 1.0, 1.0, 0.0 Magenta = 1.0, 0.0, 1.0 Cyan = 0.0, 1.0, 1.0 Page 8 of 14

9 Materials in Radiance In Radiance we can define different material properties for the various objects of our models. These different materials have different ways of manipulating the rays of light that interact with them, based on the physics of light. We have reflection, transmission, and/or refraction depending on the type of material. In this version of the <Virtual Environment> we have limited the material types to the most commonly occurring materials found in buildings. In future versions of the <Virtual Environment> additional material types may be made available. Plastic has a colour associated with diffusely reflected light, but the specular component is uncoloured, most materials fall into this category. Define the R, G, B reflectance values and the specularity and roughness. [The name plastic should not be interpreted as referring to plastic objects]. The reflectance values have the range 0.0 to 1.0 (although 0.0 and 1.0 do not occur in nature) [sometimes for a given surface a single reflectance value is given this probably refers to the average hemispherical reflectance, where you are only interested in illuminance this single value can be used for each of the R, G and B values]. Specularity also has the range 0.0 to 1.0, 0.0 for a perfectly diffuse surface and 1.0 for a perfect mirror. In reality plastic materials are generally not very reflective and the specularity value is usually in Page 9 of 14

10 the range Roughness, with the same limits, refers to how the surface scatters what light is reflected, 0.0 meaning perfectly smooth. Plastic materials generally have a roughness in the range Metal is the same as plastic except that the specular component is coloured by the material. Define the R, G, B reflectance values and the specularity and roughness. Specularity and roughness have the same theoretical limits as given above. However, metal materials are reflective and the usual range for specularity is , and for roughness a range of Glass is used to model transparent materials. Define the R, G, B transmissivity values. [glass is a special case of dielectric with a refractive index fixed at 1.52 and all that needs to be defined is the transmission at normal incidence ]. The properties of glass are commonly defined in terms of the transmittance (by glazing manufacturers), to convert to transmissivity use the following equation - transmissivity = (sqrt(a+4*sq(b*tn))-c)/(d*tn) where Tn = transmittance, a = , b = , c = , d = We have also found it impossible to get RGB data from glazing Page 10 of 14

11 manufacturers, who will only quote a single transmittance value (even for tinted glass). We suggest for illuminance images this single value is used for each of the R, G and B values. For luminance images make minor adjustments to the relevant colour e.g. for a green glass increase the G value and decrease the R and B. The following materials have recently been added and will be discussed more fully in a separate document. Trans is used to model a translucent surface. It takes the same parameters as plastic plus the transmission factor and a transmitted specularity value. Dielectric is a transparent material that refracts and reflects light (such as water or crystal). Define the R, G, B transmissivity values, the refraction index and the Hartmann constant. Page 11 of 14

12 Daylight Factor The ratio of the illuminance at a point on a given plane within an interior due to the light received directly and indirectly from a sky of assumed or known luminance distribution, to that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct sunlight is excluded from both values of illuminance (i.e. CIE Overcast Sky). E = illuminance on unobstructed plane e = illuminance at point in interior Daylight Factor = e/e (often expressed as a percentage) Illuminance is measured in LUX Page 12 of 14

13 Sky Component/Vertical Sky Component The ratio of the illuminance at a point on a given plane within an interior due to the light received directly from a sky of assumed or known luminance distribution, to that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct sunlight is excluded from both values of illuminance (i.e. CIE Overcast Sky). Note: this is the same as the Daylight Factor except the indirect component has been removed. E = illuminance on unobstructed plane e = illuminance at point in interior Sky Component = e/e (often expressed as a percentage) Vertical Sky Component = v/e Page 13 of 14

14 Glare Glare is caused by either or both the following : 1. excessive luminance values in the field of view 2. too high luminance contrasts Windows can have a high luminance compared with other luminances in a room. This gives a strong contrast from inside to outside, potentially causing glare. In Radiance simulations we may offset this by providing some internal background lighting. The strongest luminance source is the Sun, and if this in the field of view then glare is inevitable. The default glare threshold is calculated by the program to be 7 times the average luminance level, if required the user can specify an alternative value. We normally give the CIE Glare Index and the GUTH Visual Comfort Probability (% of people who are satisfied) as measures of glare. These values are calculated at fixed angles to the left and right of the centre of focus (usually at 10 degree intervals from 60 to +60 degrees), see figure below. Page 14 of 14