SPONSOR CPD PROGRAMME LUX Manufacturer of theYear 2018 Continuing professional development (CPD) is the regular maintenance, improvement and broadening of your knowledge and skills, to maintain professional competence. It is a requirement of CIBSE and other professional bodies. This Journal CPD programme can be used to meet your CPD requirements. Study the module and answer the questions on the final page. Each successfully completed module is equivalent to 1.5 hours of CPD. Modules are also available at www.cibsejournal.com/cpd Lighting the way for occupant wellbeing This module explores how the human eye processes light and the impact of artificial lighting on wellbeing The significant role that the built environment can have in improving health, wellbeing, comfort and cognitive performance has prompted the current revision to the CIBSE Technical Memorandum 40 Health Issues and Wellbeing in Building Services. A key element of ensuring occupant wellbeing is to provide an appropriate visual environment. This article will consider the parameters sensed by the human eye and the impact of artificial lighting that contributes to the wellbeing ofoccupants. The visual receptors in the human eye comprise of around 100 million rods and five million less sensitive, but faster response cones in the retina (as illustrated, in part, in Figure 1). The densely packed cones provide photopic vision for light at high intensities. Mesopic vision incorporates the range of light levels where signals from both rods and cones contribute to vision across a factor of 1,000 in luminance from the lower threshold of the cones up to the point where the rods are saturated with light (see Figure 2). This includes the levels that are often experienced in working environments, and where the spectral composition has a significant impact on the relative strengths of rod and cone signals. In dim light, scotopic vision takes over, integrating the light received by the rods as peak visual sensitivity shifts towards the blue end of the visible spectrum. Vision for night activity is likely to be in the mesopic range, with the peak being somewhere between yellow-green and blue-green.1 The intrinsically photo-sensitive retinal ganglion cells (ipRGCs) are located in the first layer of the retina but are not linked to vision. Alongside other inputs, the signals from the ipRGCs affect the master circadian clock the part of the brain known as the suprachiasmatic nucleus so influencing alertness and preparation for rest. The response of the ipRGC receptors, and therefore the impact of light on physiological processes, is strongly influenced by the spectral composition of light their peak sensitivity is in the blue area of the light spectrum Humans have evolved so as to be able to visually interpret objects and their surroundings at lighting levels ranging from in the order of 100,000 lux, in SEEING THE LIGHT The candela, cd, measures the visual effect of optical radiation and is unique in being the only SI base unit that is related to human perception. Originally based on a standard candle a candela is now defined by the luminous power emitted by radiation of frequency 540 x 1012 hertz (the green-yellow region of the spectrum, where human vision is most receptive) that has a radiant intensity of 1/683 watts per steradian, sr. (The surface area of a sphere with radius r is 4r2, and so, as a sphere is 4sr, the surface area of 1sr is r2 or around 8% of a spheres surface area.) For a particular lamp, the luminous intensity, measured in cd, will be dependent on the position of the viewer. The lumen, lm (also cdsr), is the luminous flux emitted within one steradian by a point source having a uniform luminous intensity of 1cd. Although a lamp will have many positiondependent values of candela, it will have only one lumen output, which will depend on the type of source and spectrum of light and will be the sum of all measured values of individual values of (cd x sr) around the sphere. Illuminance on a plane is represented by lux, lx (also lmm-2). So, for example, typically for task areas in offices, this would be either 300lux for mainly screen-based activity or 500lux for mainly paper-based work.6 www.cibsejournal.com June 2019 25 CIBSE Jun19 pp25-28 CPD 147 Supp v4.indd 25 24/05/2019 18:19