Designing dark corridors for bats in a lighting scheme

A dark corridor is a linear feature that bats use to commute and forage, kept below a stated illuminance so that light-sensitive species will still use it. The phrase appears in hundreds of ecology reports as an aspiration. What an authority can actually condition is a number at a location, delivered by a specified luminaire, shown on a drawing. Bat sensitive lighting design is the exercise of converting the first into the second, and it needs the ecologist and the lighting designer in the same conversation before the layout is fixed rather than after.

The reference document is the joint guidance on bats and artificial lighting produced by the Bat Conservation Trust and the Institution of Lighting Professionals. It sets out the effects, the species sensitivities and the mitigation hierarchy, and it is the document a reviewer will hold you to.

Why artificial light affects commuting and foraging

Light affects bats in three ways that matter to a scheme. It delays or prevents emergence from a roost, shortening the foraging night. It causes avoidance of lit routes, so a commuting corridor can be severed without a metre of hedge being removed. And it changes the prey base, concentrating insects around lamps where fast, open-air species can exploit them and slower species cannot.

Sensitivity is not uniform. Horseshoe bats, Myotis species and brown long-eared bats are strongly light-avoiding. Pipistrelles, noctule and serotine are more tolerant and will feed around lit areas, which is why a site with abundant pipistrelle activity can still fail on the species that matter most.

That distinction should drive the assessment. A report that concludes lighting is acceptable because activity was high without saying which species produced it has not answered the question.

Identifying the features that must stay dark

The features to protect come from the survey work, not from a generic list. Roost access points and the first stretch of flight line away from them. Hedgerows, treelines, woodland edges, watercourses, ditches and green lanes that the activity data shows being used. Connections between the site and the wider landscape, particularly where a site sits between a roost and a foraging area.

The Preliminary Ecological Appraisal identifies candidate features, and activity transects and static detector data confirm which are actually used. Where a bat survey has recorded a roost, the flight line from the access point is the most sensitive receptor on the site and should be mapped before the road layout is drawn.

A plan showing the dark corridors as hatched zones, with their widths, is the drawing everything else keys back to.

Illuminance targets at sensitive features

Targets are expressed as maintained illuminance at the feature, both horizontal at ground level and vertical at the face of the vegetation, the latter usually being the harder one to meet.

The commonly applied working figure is a maximum of 1 lux at the dark corridor, with lower values, in the order of 0.5 lux, sought where the most light-sensitive species are present or where the feature serves a roost. The guidance is revised periodically and site circumstances vary, so treat those as starting points, confirm the target with the project ecologist, and record the agreed figure in the report so the condition can reference it.

Two practical points about measurement. Vertical illuminance at the vegetation face, taken at a stated height, is what the bats experience flying along the hedge, so it should be reported alongside the horizontal figure. And the target has to be met using maintained values with the correct maintenance factor rather than initial lamp output, because a scheme that only complies when the luminaires are new does not comply.

Spectrum, colour temperature and ultraviolet

Short wavelength light, blue-rich and ultraviolet, is the most disruptive: it attracts the most insects and produces the strongest avoidance response.

The design response is a warm spectrum with no ultraviolet output. Correlated colour temperature at or below 2700 kelvin is the usual requirement near sensitive features, with amber or phosphor-converted amber sources used where the ecological sensitivity is highest and colour rendering is not needed. White LED sources without ultraviolet emission are preferable to older discharge lamps for this reason alone.

Expect pushback from the highway authority, which will have its own adoption standards for colour temperature and uniformity on adopted roads. Resolving that conflict is easier at pre-application stage than at discharge of conditions, and the usual answer is a boundary treatment where the adopted street lighting stops short of the corridor.

Luminaire choice, shielding and mounting height

The controllable variables are few and they interact.

Position first, then optics, then accessories. Designers reach for shields because they cost little and arrive late, but a shielded luminaire close to a hedge still produces more vertical illuminance at the vegetation than an unshielded one placed further away. Where footpaths run alongside a corridor, low-level bollards with steep cut-off, or a rerouted path, usually beat any attempt to light the path from a column.

Building lighting deserves the same attention and rarely gets it. Spill from glazing, balcony lights, security floods and car park lighting can dominate the result on a residential scheme. Deal with window positions, obscured or shielded glazing near the corridor, and a prohibition on occupier-installed external lighting in the conditions.

Demonstrating the design in an isolux plan

The evidence a reviewer expects is a lighting drawing with isolux contours, produced from a calculation model, showing the 1 lux and 0.5 lux contours against the mapped dark corridors, with a schedule of luminaires listing type, optic, mounting height, tilt, colour temperature, output and any accessory.

Three checks catch most defective submissions. Whether the model includes building-mounted and car park lighting or only the street columns. Whether maintained rather than initial values were used. And whether the calculation grid extends across the corridor and beyond it rather than stopping at the site boundary.

Where the corridor is planted rather than existing, the Landscape Strategy has to deliver vegetation of a height and density that works, and the lighting drawing should show the corridor at establishment rather than at maturity.

Illustrative example. On a 40 dwelling scheme with a retained hedgerow along the northern boundary carrying lesser horseshoe activity, a workable package is: no columns within 15 metres of the hedge, the estate road realigned to the south, 3 metre bollards with rear shielding on the nearest footpath, 2700 kelvin throughout with amber sources on the northern branch, part-night dimming after midnight, and a 6 metre unlit planted buffer. The figures are illustrative, and the target figure should always come from the project ecologist.

Securing and monitoring the scheme by condition

A condition that requires lighting to be in accordance with an approved lighting scheme is only as good as the drawing it refers to. Name the drawing and revision, and include the luminaire schedule.

Ask for four things in the condition set: the approved scheme by drawing number, a post-installation verification survey carried out at night and submitted to the authority, retention of the scheme for the lifetime of the development with no additional external lighting without consent, and, where a roost is involved, a monitoring period with a remedial mechanism.

Works affecting a roost require a licence from Natural England and the survey work must be carried out by a suitably licensed ecologist, which is set out on the Protected Species Survey page. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and this is general guidance rather than legal advice.