Lighting : LIGHTING Apr-May 2018
20 LIGHTING MAGAZINE | April/May 2018 April/May 2018 | LIGHTING MAGAZINE 21 TECHNICAL FEATURE Daylight: the essence of human-centric lighting Warren Julian Humans and their predecessors evolved in a daylit world. Our visual system evolved to make use of that source, so much so, that we cannot see at night and we are afraid of the dark. The interiors that give us the greatest pleasure are those that make use of daylight, even when they use supplementary electric lighting. Commercial and industrial buildings, since the invention of electric lighting, unless they have very high ceilings, large windows and plenty of rooflights are mostly borderline in terms of visual comfort, visual amenity and what our daylight-evolved brains are seeking. The current fad of human-centric electric lighting is unlikely to improve commercial and industrial interiors if it ignores daylight’s essential contribution. Electric-only lighting is essential at night and under overcast conditions but, at best, it will be a feeble emulation of the real thing, so sought after by building occupants. This paper looks at some aspects of daylight and its benefits for human wellbeing. DAYLIGHT Daylight is the light produced by the sun and sky, that is, it is the combination of skylight and sunlight. Daylight is an important illuminant, often ignored by lighting designers. People prefer to work in daylit interiors over those without daylight. SUNLIGHT Sunlight is the source of the earth’s energy and sustains all life. As a light source, it produces very high beam illuminances (up to 100,000 lux normal to the direction of the sun) and it provides the light for the secondary source – the sky. Sunlight has a luminous efficacy, at the earth’s surface of about 100lmW-1 . The sun appears as a black body with a “surface” temperature around 6000K. The passage of the sunlight through the atmosphere results in some selective absorption and scatter, reducing the correlated colour temperature. While the sun appears small from the earth, it is much larger than the earth, producing an almost parallel beam at the earth. The sun subtends an angle of about 0.5° from the earth, so the shadows produced by the sun have a large umbra (the “pure” shadow) and almost no penumbra (the “soft” shadow edges produced by non-parallel light). Sunlight, therefore produces very strong shadows. The earth travels around the sun in an elliptical orbit of about 150 million kilometres in about 365.25 days. The earth rotates on its axis in 24 hours. The earth’s axis is tilted about 23.5° to the plane (the ecliptic) in which it orbits the sun, giving rise the seasonal variation in day-length (that is, hours of daylight). Day-length and other solar quantities measured on earth (for instance, horizontal illuminance) vary sinusoidally throughout the year. IfQ=A+Bsinθ,whereQisthe quantity (say, day-length), then θ can be thought of as the year divided over 360°, commencing on the vernal (spring) equinox (around 22 September in the southern hemisphere), A as a fixed value (say, 12 hours of day-length) and B as the maximum amplitude of the variable quantity. A little thought will show that B is a function of the latitude (distance from the equator): B will, for day-length, be zero at the equator and equal to A at the poles. Within the tropics (between the equator and latitudes ±±23.5°) the position (bearing) of the sun at noon can be in either the northern or southern parts of the sky. Outside the tropics, the noon bearing will always be in the north in the southern hemisphere and vice versa. This means that outside the tropics there are preferred building orientations for solar access (and shading). Since the earth rotates in 24 hours, the sun traverses the sky at 15° per hour. This is a constant angular velocity, so if there are more than 12 hours of day- length, as there are between the vernal and autumnal equinoxes, the sun must rise south of east and set south of west in the southern hemisphere (and vice versa in the northern). Between the autumnal and equinoxes vernal, the day-length will be less than 12 hours, with the sun always in the northern part of the sky. The apparent motion of the sun across the sky is a function of time- of-year and time-of-day but can be calculated with great precision. The flux arriving on a surface varies with season and time. Whilst the dynamic characteristic of sunlight is one of its pleasures, its movement and varying flux make it a difficult source for design. And, of course, on average, it is absent for half of each day. However, lighting design can make use of sunlight, particularly reflected sunlight from the ground and other surfaces. Use of beam sunlight requires filtering-out the infrared and (easier) the ultraviolet. Computer-aided design methods exist for calculating the illuminances produced by both beam and reflected sunlight. Sunlight illuminates West Mitten Butte, with some light from the sky. Excellent modelling results. The objects appear brighter than the sky. (http:// upload.wikimedia. org/wikipedia/ commons/2/22/ West_Mitten_Butte_ in_Monument_ Valley.jpg) Commercial and industrial buildings, since the invention of electric lighting, unless they have very high ceilings, large windows and plenty of rooflights are mostly borderline in terms of visual comfort, visual amenity and what our daylight-evolved brains are seeking.
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