Worked Example: Museum Lighting Design with Emergency Lighting — The Jodrell Bank First Light Pavilion

The First Light Pavilion at Jodrell Bank Observatory in Cheshire, England, opened in 2022 as a dramatic earth-sheltered museum celebrating the history of radio astronomy. Designed by Hassell Studio, the building features sweeping curved concrete walls and a domed ceiling that houses one of the largest planetarium-style projections in the world. It is an architectural triumph — but its commissioning revealed significant problems with the lighting design that cost £400,000 in remedial works and delayed the opening by three months.

The emergency lighting calculations had been performed using the standard lumen method, which assumes flat horizontal working planes and regular luminaire spacing on flat ceilings. In reality, the curved interior surfaces created deep shadow zones and uneven light distribution that the calculations did not predict. Several areas along curved escape corridors fell below the minimum 1 lux emergency illuminance required by BS 5266-1. The main gallery lighting also suffered from excessive glare on display screens and interactive exhibits, because the luminaire selection had not accounted for the way the curved ceiling profile redirected light at unexpected angles.

This example demonstrates the standard lumen method for a museum gallery space — the same method that was applied at Jodrell Bank. Understanding both the power and the limitations of this method is essential for any lighting designer. For spaces with complex geometry, the lumen method provides a starting point, but point-by-point calculation (or full 3D modelling software such as DIALux or Relux) is required for accurate results.

Design the general lighting system for a rectangular museum gallery space, then verify emergency lighting compliance.

Per BS EN 12464-1:2021, Table 5.34 (Museums and art galleries):

For the main gallery, we design for 300 lux maintained illuminance on the working plane. Interactive display areas will use supplementary task lighting to reach 500 lux.

The room index (RI or k) characterises the room’s proportions and determines how effectively the luminaires illuminate the working plane. Per BS EN 12464-1, Annex A:

RI = (L × W) / (H_m × (L + W)) — (Eq. 1)

Where L = 40 m (length), W = 20 m (width), H_m = 5.15 m (mounting height above working plane):

RI = (40 × 20) / (5.15 × (40 + 20))

RI = 800 / (5.15 × 60)

RI = 800 / 309

RI = 2.59

An RI of 2.59 indicates a relatively efficient room geometry — a large, well-proportioned space where light from the luminaires reaches the working plane efficiently. Narrow corridors (RI < 1.0) are much less efficient because more light is absorbed by the walls before reaching the working plane.

The utilisation factor (UF) is the ratio of lumens reaching the working plane to the total lumens emitted by the luminaires. It is read from the luminaire manufacturer’s UF table, indexed by room index and surface reflectances.

For our recessed LED panel with reflectances C:0.7 / W:0.5 / F:0.2 and RI = 2.59, interpolating from the manufacturer’s UF table:

Interpolating at RI = 2.59:

UF = 0.67 + (0.59/0.50) × (0.71 − 0.67) = 0.67 + 0.018 × 4 — (Eq. 2, linear interpolation)

UF = 0.677

The maintenance factor (MF) accounts for the reduction in light output over time due to lamp depreciation, luminaire dirt accumulation, and room surface degradation. Per BS EN 12464-1, Clause 4.2.2 and CIE 97:2005:

MF = LLMF × LMF × RSMF — (Eq. 3)

Where:

• LLMF (Lamp Lumen Maintenance Factor): LED at 50,000 hours L80 = 0.80
• LMF (Luminaire Maintenance Factor): clean environment, 3-year cleaning cycle = 0.90
• RSMF (Room Surface Maintenance Factor): museum (clean, maintained) = 0.95

MF = 0.80 × 0.90 × 0.95

MF = 0.684

The maintained illuminance must be achieved at the end of the maintenance interval, when all depreciation factors are at their worst. The initial illuminance will be approximately 300 / 0.684 = 439 lux, dropping to 300 lux at the end of the cycle.

The lumen method calculates the total number of luminaires required to achieve the maintained illuminance:

N = (E_m × A) / (Φ × UF × MF) — (Eq. 4)

Where E_m = 300 lux, A = 40 × 20 = 800 m², Φ = 4,200 lm per luminaire, UF = 0.677, MF = 0.684:

N = (300 × 800) / (4,200 × 0.677 × 0.684)

N = 240,000 / (4,200 × 0.463)

N = 240,000 / 1,944.6

N = 123.4 → use 126 luminaires (round up to a grid-friendly number)

Arrange in a 14 × 9 grid (126 luminaires). Spacing: 40/14 = 2.86 m along length, 20/9 = 2.22 m along width.

The spacing-to-height ratio (SHR) must not exceed the luminaire’s maximum SHR to maintain uniformity. For the recessed LED panel, the manufacturer’s maximum SHR is typically 1.5.

SHR_length = spacing / H_m = 2.86 / 5.15 = 0.56 — (Eq. 5)

SHR_width = spacing / H_m = 2.22 / 5.15 = 0.43

Both are well below the maximum SHR of 1.5, so the uniformity criterion U_o ≥ 0.6 will be met comfortably.

The edge spacing (distance from the wall to the first row of luminaires) should be half the inter-luminaire spacing:

Edge spacing (length) = 2.86 / 2 = 1.43 m

Edge spacing (width) = 2.22 / 2 = 1.11 m

Verification: 2 × 1.43 + 13 × 2.86 = 2.86 + 37.18 = 40.04 m ≈ 40 m ✓

The Unified Glare Rating quantifies the discomfort glare experienced by an observer. Per CIE 117:1995 and BS EN 12464-1, Clause 4.5:

UGR = 8 × log₁₀((0.25 / L_b) × Σ(L² × ω / p²)) — (Eq. 6)

Where L_b = background luminance, L = luminance of each luminaire in the observer’s field of view, ω = solid angle subtended by the luminaire, and p = Guth position index.

In practice, UGR is calculated by the luminaire manufacturer and published in a UGR table. For our recessed LED panel in a room with RI = 2.59, reflectances C:0.7/W:0.5/F:0.2, looking along the length of the room:

UGR = 19.2 (from manufacturer’s tabulated data)

19.2 < 22 (limit for museums) — ✓ PASS

Per BS 5266-1:2016, Clause 5.4, emergency lighting must provide:

• Escape routes: minimum 1 lux on the centre line of the route, 0.5 lux minimum on centre band
• Open areas (> 60 m²): minimum 0.5 lux at floor level (anti-panic lighting)
• High-risk task areas: minimum 10% of normal illuminance (but not less than 15 lux)

For the 800 m² gallery as an open area, we need 0.5 lux minimum (anti-panic). Using self-contained emergency LED luminaires with 200 lm maintained output, 3-hour duration:

N_em = (E_em × A) / (Φ_em × UF_em × MF_em) — (Eq. 7)

Where E_em = 0.5 lux, A = 800 m², Φ_em = 200 lm, UF_em = 0.45 (lower for emergency fittings), MF_em = 0.80 (3-year maintenance):

N_em = (0.5 × 800) / (200 × 0.45 × 0.80)

N_em = 400 / 72

N_em = 5.6 → use 6 emergency luminaires (minimum for anti-panic)

However, uniformity for emergency lighting requires maximum spacing. For a mounting height of 6 m and an emergency luminaire with a wide distribution, maximum spacing per BS 5266-7 (EN 1838), Clause 4.3 is approximately 4 × mounting height = 24 m.

For 40 m × 20 m: use a 3 × 3 grid (9 emergency luminaires at approximately 13 m × 10 m spacing) to ensure adequate coverage with margin. The additional 3 luminaires provide the safety margin needed for uniformity compliance.

The LENI value quantifies the annual energy consumption of the lighting system per unit floor area, per BS EN 15193-1:2017:

LENI = (W_L + W_P) / A — (Eq. 8)

Where W_L = annual energy for lighting (kWh), W_P = annual energy for parasitic loads (controls, emergency charging), A = floor area.

Annual lighting energy:

W_L = P_total × t_D × F_D × F_O / 1000 — (Eq. 9)

Where P_total = 126 × 40 = 5,040 W (total installed power), t_D = 3,000 hours/year (museum operating hours), F_D = 0.90 (daylight dependency factor — minimal, interior gallery), F_O = 0.95 (occupancy factor — gallery is occupied most of operating hours):

W_L = 5,040 × 3,000 × 0.90 × 0.95 / 1,000 = 12,927 kWh/year

LENI = 12,927 / 800 = 16.2 kWh/m²/year

For comparison, the BS EN 15193-1 benchmark for museums is approximately 25 kWh/m²/year. Our LED-based design at 16.2 kWh/m²/year is well below the benchmark. ✓ PASS

Design: 126 recessed LED panels (14 × 9 grid) plus 9 self-contained emergency luminaires (3 × 3 grid). The lumen method confirms the design meets all BS EN 12464-1 requirements for a rectangular gallery. Total installed lighting power density of 6.3 W/m² is efficient for a museum application requiring CRI > 90.

The Jodrell Bank First Light Pavilion lighting problems arose because the design calculations assumed standard flat geometry. To avoid similar issues:

• Use 3D lighting simulation software for non-rectangular spaces — the lumen method is valid only for rectangular rooms with flat ceilings; spaces with curved, domed, or irregular geometry require point-by-point calculation in software such as DIALux, Relux, or AGi32, which model actual surface geometry and luminaire photometry
• Perform glare analysis from multiple viewing positions — standard UGR tables assume horizontal viewing in a rectangular room; in a museum where visitors look in all directions and curved surfaces create specular reflections, rendered false-colour luminance maps from multiple observer positions are essential
• Mock up critical areas before full installation — for prestige projects, build a full-scale mock-up of a representative section and measure actual illuminance, uniformity, and glare before committing to the complete installation
• Commission emergency lighting with actual measurements — never rely on calculation alone for emergency lighting compliance; BS 5266-1 requires verification by measurement after installation, and this is doubly important for complex geometries where calculation predictions are unreliable
• Specify luminaires with appropriate beam control for curved ceilings — wide-beam panels designed for flat ceilings will scatter light unpredictably when mounted on curves; asymmetric or adjustable luminaires allow the designer to direct light where it is needed