CPD PROGRAMME | LIGHTING education applications, positioned in front of the light source to prevent glare and, in many cases, enhance directional control. Common lighting luminaires in general lighting applications typically employ opalescent (opal) or frosted materials as diffusers. These are low-cost, but robust, smooth-surfaced, polymer materials, such as polycarbonate or acrylic (otherwise known as PMMA a highly transparent thermoplastic polymer), which are manufactured to provide good light transmission while also delivering physical and aesthetic benefits. Opalescent plastics are created by added pigmentation to the clear base polymer to mask the light source. Frosted light diffusers are manufactured by using a blend of two polymers that have different refractive indices, thereby providing diffusion of the light source.7 Electroluminescence creates light from the LED package, which will typically include one or more semiconductor chips mounted on a heat-conducting material, encapsulated in a protective material that may incorporate a primary optical lens. These are typically grouped together to make up an LED light source, and will produce highly directional, high-intensity light. The optics that are required to provide the best performance from such an electronic lamp are likely to be different from those of more traditional technologies, but still have a primary need to Figure 1: Example of acrylic luminaire diffuser designed to eliminate LED hot spots (Source: Covestro AG) Figure 2: Luminaire with opalescent diffuser (Source: Tamlite) SELECTED LUMINAIRE PERFORMANCE TERMINOLOGY Luminous intensity measures the illuminating power of a luminaire in a specific direction, measured in candelas (cd, lumens per steradian, lm.sr-1), normally presented in tabular form. Polar curves also provide an opportunity to illustrate luminous intensity and distribution on a single chart typically shown as two curves, indicating the two major axes of the luminaire (as employed in Figure 3 and Figure 5) Disability glare impairs the ability to see detail without necessarily causing discomfort, and can be produced directly or by reection Discomfort glare causes discomfort without necessarily impairing the vision of objects, and can be produced directly or by reection Unified glare rating (UGR) is an index used to evaluate discomfort glare in interior workplaces. The equation used to evaluate UGR is from CIE 117.5 The calculation method determines a logarithm of the ratio of the luminaire luminance that can be seen by the observer, to the observers relative, angular position, and the background luminance as seen by the observer. A representative calculation is typically undertaken for a proposed installation (or tabular approximations employed) to ensure that the UGR is no greater than 19. Although some manufacturers market UGR19 luminaires, the true UGR cannot be properly evaluated without details of the application. The CIBSE SLL Code for Lighting3 provides examples of how to calculate the UGR, but the actual calculation may be more readily undertaken using software such as Dialux.6 As noted by CIBSE TM40,1 UGR has been developed for conventional offices and is not suited to all environments. Lambertian distribution (as described by Johann Heinrich Lambert in the 18th century) is where emitted radiance is independent of the direction of the observer, and the luminous intensity varies with the cosine of the angle between the normal to the surface and the observer. (An example is the distribution illustrated in Figure 3.) reduce glare. Traditional opalescent diffusers are unsuitable for LED technology, as they are unlikely to mask bright spots from the individual LED emitters, and offer insufficient directional control. Initially, frosted polymers were often employed, because their ability to mask the LED was good; however, this was at the expense of light output. Specialised acrylic diffusers such as those illustrated in the examples in Figure 1 and Figure 2 are now widely available with various performance characteristics, and provide a near Lambertian distribution of light (see boxout, Selected luminaire performance terminology). The choice for optical control in LEDs for such environments is limited principally to two options an opalescent diffuser (such as in Figure 2), or one that contains lenses in a micro-prismatic diffuser (often abbreviated to microprism diffuser). Simple opal diffusers will effectively scatter the light by refracting it as the light passes through the opal diffuser to produce a Lambertian distribution (as shown in the polar diagram in Figure 3), which ensures a more even, dispersed light distribution, enhancing uniformity and reducing shadowing. An opal diffuser would, for example, be a reasonable application for a flow area in a work building, providing a wide spread of light so people can see clearly as they move through the communal space. LED luminaires employing microprism diffusers have seen increased application in architectural lighting where the prisms (lenses) are designed to control the emitted light into a specific distribution. The pattern of the light distribution can be finely adjusted through the specific design of the prismatic elements, which need to be manufactured from good-quality material that will not discolour or distort over time. Such diffusers are usually made of acrylic, polycarbonate or silicone resin, depending on the qualities required. This microprism diffuser increases the downward proportion of light, reducing horizontal emission and the opportunity for unwanted glare, so can be used to achieve improved values of UGR. A multicell optic, such as that shown in Figure 4, can provide optimum spacing, but also maintain a controlled lower angle to satisfy UGR needs. The recessed light source also reduces discomfort glare, as there is no direct illumination experienced by occupants in adjacent workplaces. The aesthetic appeal of such luminaires may be debatable for some; however, they can provide benefits in terms of occupant wellbeing, space utilisation and energy savings. The polar curve for an example multicell luminaire employing a microprism diffuser is shown in Figure 5. A luminaire with correctly directed light is likely to have a greater overall efficiency in application compared with a fitting with an opalescent diffuser. Office and educational working environments have evolved swiftly in recent years, as they shift away from plain, light-coloured walls to more complex surface 54 March 2021 www.cibsejournal.com CIBSE March 21 pp53-56 CPD 176.indd 54 19/02/2021 17:50