The baseline and receptor section of a lighting assessment
Reviewers reject lighting assessments for the zone or the receptor list, not the modelling. Here is how to get both right and what outputs to expect.
A lighting assessment baseline receptors section decides whether the rest of the document means anything, because every obtrusive light limit is expressed relative to an environmental zone and measured at a named receptor. Get the zone wrong and the limits are wrong. Miss a receptor and the assessment is silent about the window that generates the complaint. Reviewers who reject lighting assessments almost always reject them for one of those two reasons rather than for a modelling error.
The structure comes from the Institution of Lighting Professionals guidance on reducing obtrusive light, which sets limits by zone for sky glow, light intrusion into windows, luminaire intensity and the luminance of illuminated surfaces, with separate values before and after a curfew.
Assigning the environmental zone
The zones run from the darkest, covering intrinsically dark landscapes and protected dark sky locations, through rural and village settings and outer suburban areas, to town centres and finally city centres with high levels of night-time activity.
Three rules keep the assignment defensible. Agree the zone with the local planning authority in writing before the assessment is written, because the guidance leaves the designation to the authority and a disagreement discovered at consultation stage costs a revision. Assign the zone of the receptor, not of the development, which matters on a site at the edge of a settlement where the light source sits in a brighter zone than the dwellings it affects. And where zones adjoin, apply the more sensitive of the two to receptors in the transition.
The common error is assigning a zone from the character of the development. A distribution warehouse on the edge of a village is not in a town centre zone because it is an industrial building. The village is the receiving environment.
Where a dark sky designation applies, say so explicitly and identify the designating body, because the limits in the darkest zones are close to absolute and the design consequences are substantial.
Measuring the existing night-time condition
A baseline that consists of the sentence "the site is currently unlit" is not a baseline. Measure, and report the measurement conditions.
At a minimum, record vertical illuminance at each identified receptor window plane with the proposed lighting absent, taken after dark and after any local curfew, with the instrument, its calibration date, the measurement height and the orientation stated. On sites where surface brightness matters, add luminance readings of the surfaces that will be lit. In the darkest zones, sky brightness readings expressed in the conventional astronomical units add useful context.
Conditions matter as much as numbers. Note the date and time, cloud cover, moon phase and whether any existing lighting on or near the site was operating, since a reading taken under full cloud in a lit urban setting will be higher than one taken on a clear moonless night, and a baseline inflated by circumstance flatters the later comparison.
Record existing lighting installations too. A scheme replacing older floodlights with modern luminaires may reduce obtrusive light even while increasing illuminance on the task area, and that argument only works if the existing installation was surveyed.
Identifying receptors, including the non-human ones
Receptor identification is a drawing exercise before it is a calculation. Plot every habitable room window with a view of the site, by storey and orientation, and include upper floors, which are frequently omitted and are often worse affected because they see the luminaire directly.
Then add the rest.
Private gardens and shared amenity space, where intrusion is assessed on the horizontal as well as at the window plane. Highway users, where glare and threshold increment apply rather than intrusion. Footpaths and bridleways. Bat commuting routes, foraging areas and roost access points, which have their own guidance and their own limits, and where the acceptable figure can be close to zero. Other nocturnal species and hedgerow corridors. Astronomical and dark sky sites. And aviation, where an aerodrome or helicopter landing site lies nearby.
Ecology is where most assessments are thinnest. Coordinate with the Preliminary Ecological Appraisal and, where roosts are present, with the bat survey, so that the lighting design responds to identified commuting routes rather than to a general statement that bats may be present. The joint guidance from the lighting institution and the bat conservation sector is the reference a reviewer will expect to see cited.
Pre-curfew and post-curfew limits
Every limit in the guidance has two values, one applying before the curfew and a markedly lower one after it. In the darkest zones the post-curfew figure for light intrusion into windows is zero, which means the design has to switch off or be fully shielded rather than merely dimmed.
The curfew time is not fixed by the guidance. It is set by the local planning authority, commonly at 23:00, and it has to be stated in the assessment because the whole post-curfew analysis depends on it. Confirm it with the authority alongside the zone.
| Limit | What it controls |
|---|---|
| Upward light ratio | Sky glow, expressed as the proportion of luminaire flux emitted above the horizontal |
| Vertical illuminance at the window plane | Light intrusion into habitable rooms, with a much lower post-curfew value |
| Luminaire intensity towards the receptor | Source glare from the luminaire seen directly |
| Average surface luminance | Brightness of a lit building or other structure |
| Threshold increment | Disability glare for road users |
Take the actual figures from the current edition of the guidance rather than from a previous assessment, since the values and the zone descriptions have been revised.
The outputs a reviewer should expect
An isolux or spill contour plot over a site plan, at the relevant height, for the pre-curfew and post-curfew operating conditions separately. A table of predicted vertical illuminance at every identified window plane against the applicable limit. Luminaire intensity towards each receptor. Upward light ratio for the installation. Threshold increment where a road is affected. A luminaire schedule with photometric file references, mounting heights, aiming angles, output and correlated colour temperature. And a statement of the modelling assumptions, including maintenance factor and surface reflectances.
A Lighting Assessment lacking the luminaire schedule cannot be checked, because the prediction is only as good as the photometric data behind it. Ask for the file references and the aiming angles, not just the product names.
Mitigation through luminaire selection and controls
The order of effectiveness is fairly consistent.
- Provide only the illuminance the task needs, since over-lighting is the root of most obtrusive light.
- Select full cut-off luminaires with flat glazing and no upward component, mounted level rather than tilted.
- Set mounting heights and positions so the light falls where it is wanted, accepting that lower mounting needs more units but spills less.
- Add shields, louvres or baffles on the receptor side, specified per luminaire rather than as a general note.
- Choose a warmer correlated colour temperature, and a substantially warmer one near bat habitat.
- Control the installation with time clocks, part-night dimming and presence detection, so the post-curfew condition is delivered automatically rather than by someone remembering.
Controls are what deliver the post-curfew limits, and they are the element most often omitted from the condition. Offer a control strategy with the application, specifying what switches off, what dims and to what level, and at what time.
Reporting against the limits
Report receptor by receptor, not as a summary. For each one, state the zone, the receptor type and location, the baseline measurement, the predicted pre-curfew and post-curfew values, the applicable limit and the margin. Where a limit is exceeded, say so and set out the mitigation rather than adjusting the receptor position until the number works.
One honest limitation worth stating in any assessment. Predictions carry uncertainty from photometric tolerance, surface reflectance assumptions and installation accuracy, and a result within a few per cent of a limit should be treated as at the limit rather than as compliant. Where the scheme also involves a Landscape and Visual Impact Assessment, the night-time visual effect of the lit development should be addressed there and cross-referenced here rather than covered twice with different assumptions.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified lighting professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. A Lighting Assessment is work councils expect a suitably qualified author to carry out, so professional review is strongly advised.
Browse the report types on PlanScope to see how the lighting, ecology and landscape documents fit together.
Frequently asked questions
Who decides the environmental zone?
The local planning authority. The Institution of Lighting Professionals guidance leaves the designation to the authority, so agree it in writing before the assessment is written. Assign the zone of the receptor rather than of the development, and where zones adjoin, apply the more sensitive of the two to receptors in the transition.
Is a statement that the site is unlit an acceptable baseline?
No. Measure vertical illuminance at each identified receptor window plane with the proposed lighting absent, after dark and after any curfew, stating the instrument, its calibration date, the measurement height and orientation. Record cloud cover, moon phase and whether existing lighting was operating, since those conditions change the reading.
Which receptors are most often missed?
Upper floor windows, which frequently see the luminaire directly and are worse affected than ground floor rooms, and ecological receptors. Bat commuting routes, foraging areas and roost access points have their own guidance and their own limits, and the acceptable figure can be close to zero. Gardens, footpaths and road users also need separate treatment.
What is the curfew and who sets it?
It is the time after which lower obtrusive light limits apply, commonly 23:00, and it is set by the local planning authority rather than by the guidance. The assessment has to state it, because the entire post-curfew analysis depends on it. In the darkest zones the post-curfew limit for window intrusion is zero.
What makes a lighting assessment uncheckable?
A missing luminaire schedule. The prediction is only as good as the photometric data behind it, so a reviewer needs file references, mounting heights, aiming angles, output and correlated colour temperature, plus the maintenance factor and surface reflectances assumed. Product names alone do not allow the model to be verified.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.