Lighting | Elizabeth line service channels. The aim was to facilitate ease of installation and future maintenance over the design life of the equipment. Indirect lighting within the concourses, escalator tunnels and platforms emphasises the interior public areas rather than drawing attention to the luminaires themselves. The light grey, matt-textured, glass-fibre reinforced concrete (GFRC) cladding of the sub-surface environment is illuminated with indirect light, which creates a sense of space and an elevated ceiling for passengers, staff and other users. There is a clear distinction between the use of ambient and accent lighting. The former provides general illumination and the overall uniformity required for safe passage without creating harsh shadows. The accent lighting draws attention to specific areas andtask surfaces, such as help points, signage, escalator landings and other identified visual tasks. Accent- and task-lighting elements offer enhanced levels of local illuminance in accordance with London Underground standards, and the perceptual difference in illuminance values according to BS EN 12665:2018 Light and lighting. Basic terms and criteria for specifying lighting requirements. Assisting wayfinding The lighting design was used to influence intuitive wayfinding by the passengers and to assist their decision-making, whether when moving from concourse to platform or disembarking from a train and seeking anexit from the station. Thresholds, entrances and exits were clearly identified, and signage elements and passenger information highlighted. Conceptually, the different public spaces in sub-surface stations were classified as either wayfinding or transit spaces with the following characteristics. Wayfinding spaces: Multiple activities, such as reading signage, customer information displays, and identifying entrances and exits Multiple routes and decisionmaking required Gathering space Larger architectural volume. Transit areas: Single-activity space D irect movement along a fixed route defined by architecture Transitory space Smaller, linear architectural volume. Escalator tunnels The lighting scheme for the escalator tunnel was developed as two discrete elements. Functional lighting for the escalator steps was initially provided by the uniformly illuminated side panels. During the design and build phase of the project once the concept design stage and the clients requirements had been set this was replaced by visually continuous escalator skirting lighting, which gives the required levels of task illuminance on the escalator itself. This task lighting is coupled with indirect lighting of the vaulted escalator tunnel, using deck-recessed linear LED uplights. The escalator soffit is a key surface because it is directly in the sightline of passengers approaching the top of the escalators from each station ticket hall. The design intent was to create a welcoming introduction to the sub-surface environment by illuminating the vault overhead with a soft-edged wash of cool white light. The escalator deck uplights will create a broad upward light distribution to the vaulted escalator tunnel, a soft-edged light distribution with graduated coverage across the vault, and a darklight appearance to the luminaire itself. This is created by the black cross-blade louvre design, whichensures the luminaire does not create discomfort glare to passengers approaching and using theescalators. A suspended illuminated sign at the top and bottom of the escalator gives additional task lighting at the locations where passengers step onto and alight from the escalators. The low-brightness appearance of the luminaires will ensure a balanced lit environment, with the focus on the illuminated cladding surfaces and wayfinding elements of the design. Intuitive wayfinding is promoted by using visibly different colour temperatures for the lighting within adjacent spaces. Cool white (5,000K) is used for transition spaces, while warm white (3,000K) is applied within the wayfinding areas. Vertical illuminance and visual comfort Illuminance measurements are normally taken on the horizontal plane at ground level or on a specified task surface. Relevant standards and guidelines are primarily based on these criteria. However, humans process vertical surfaces within the visual field more easily and more often than horizontal ones. So, good vertical illuminance has been prioritised to improve facial recognition of fellow passengers, enhance the legibility of signs and information displays, and allow for the better detection of potential obstacles and hazards. The use of indirect lighting and of luminaires with large, diffuse, luminous surfaces creates a visually comfortable environment with fewer shadows and veiling reflections on materials and finishes. 12 December 2018 www.cibsejournal.com CIBSE Dec18 pp10-13 Supp Crossrail.indd 12 23/11/2018 12:54