Guidance on Aviation Lighting Impact Assessment

This Guidance recommends that if an Aviation Lighting Impact Assessment is scoped into an EIA, it should include the following information (and take account of the following considerations):

Lighting Proposals and Mitigation 

Information to be provided:

• Plan showing the wind farm layout with numbered turbines and a corresponding schedule of visible lights (both nacelle and mid-tower lights where required).
• Information on proposed visible aviation lighting scheme, including a clear listing of all proposed mitigation measures.
• Information on any project-specific mitigation approvals that have been obtained or are still required from CAA.

Recommendations:

• Developers are encouraged to combine available mitigation measures into wind farm design to mitigate the lighting impacts as far as is reasonably practicable, whilst maintaining a proportionate approach. 

Night-time Aviation Lighting Impact Assessment

Information to be provided:

• Plan showing the extent of the night time Study Area.
• Assessment of night-time effects for each relevant receptor, in particular where qualities of wildness, darkness, remoteness and lack of man-made elements are features or characteristics of the landscape.
• An appraisal of all viewpoints within the study area should be included where there may be significant effects and can be informed by wireline visualisations. 
• Detailed assessment of, in most instances, two or three representative viewpoints accompanied by photomontage visualisations to adequately represent the key impacts. 
• Assessment of any likely significant cumulative effects from visible aviation lighting, where appropriate.

Recommendations:

• Ensure that both daytime and nighttime assessments are proportionate, appropriate, comprehensive, and transparent. 
• A proportionate approach, with a focus on significant effects, is required when scoping the relevant landscape and visual receptors and how they may be affected. It is worth noting that receptors and their sensitivity will not always be the same during the day and night.
• The assessment should take into account the baseline darkness/ artificial lighting characteristics and people’s likely use of different areas during darkness and low light (dusk/ dawn) conditions. 
• The extent of the lighting assessment study area for the LVIA should be informed by a hub height ZTV (to indicate the extents of aviation lighting visibility), and an understanding of the nature of the likely effects.  Experience suggests a study area of 10km - 20km should be sufficient for the night-time Aviation Lighting Impact Assessment, to ensure significant effects are captured, depending on the sensitivities. 
• Viewpoint assessment should provide commentary cross referenced to lighting wirelines and photomontages.

ZTV analysis of lighting

Information to be provided:

• A hub height Zone of Theoretical Visibility (ZTV) map which shows the areas from which the nacelle lights are theoretically visible. 
• A ZTV or comparative ZTV illustrating low-intensity mid-tower light visibility (e.g. nacelle light visibility vs nacelle and mid-tower height visibility) could be provided where these lights are required. 
• If ‘vertical directional intensity lighting mitigation’ is proposed, then the preparation of a lighting intensity ZTV can help to indicate where this mitigation may be effective and the different lighting intensities which may be experienced at different elevations relative to the lights. 
• Cumulative ZTVs may be useful in circumstances where more than one lit wind farm could affect key receptors. 

Recommendations:

• Any assumptions concerning the aviation light specification used to inform lighting intensity ZTVs should be provided, and assessors should take appropriate care not to over-estimate predicted reductions given that parameters can vary between different aviation lights and noting that it is unlikely that a particular aviation light model can be committed to with certainty at application stage.

Wireline visualisations

Information to be provided:

• Identification of turbine lighting on wireline visualisations. The turbine lighting may either be marked at the appropriate positions on the wirelines or provided as a separate note at the foot of the wirelines. This should apply to all the LVIA 53.5° (degree) wirelines (i.e. indication of visible lighting should not just be shown for the night-time photomontages).

Photomontage visualisations 

Information to be provided:

• Night-time photomontage visualisations from a proportionate number of representative viewpoints (two or three should be appropriate for most projects), selected based on sensitivity of potential receptors and frequency of visitors at night or dusk/dawn, and to be agreed with the Planning Authority (in consultation with NatureScot where appropriate).   
• Photomontage visualisations should illustrate the maximum case lighting intensity scenario (e.g. 200 cd where dimming of aviation lights proposed as embedded mitigation, and 2000 cd only where this mitigation is not proposed)

Recommendations:

• Whilst it is expected that two or three representative viewpoints will require a photomontage visualisation, an appraisal of all viewpoints where there may be significant effects should be included. 
• In some cases, there may be the need to select night-time assessment viewpoints based on the turbine lighting impacts as opposed to the day-time visual effects, therefore the viewpoints may vary from the suite of day-time assessment viewpoints. Edge of settlement locations are likely to be better lighting assessment viewpoints, compared with locations within towns/ villages (given the influence of existing street lighting, etc.). 
• Health and safety concerns exist over taking dusk/dawn photography, and associated assessment fieldwork from remote viewpoints. This should be a key consideration for all involved in agreeing the scope and undertaking aviation lighting photography/ assessment.  In some cases, a more accessible proxy viewpoint may be the proportionate, alternative safe location to capture viewpoint photography at dusk/night.  Moreover, sometimes the approach route to and from a remote hill summit is where receptors will experience a total absence of existing visible lights, and may be more representative than the hill summit /ridge (where receptors are perhaps less likely to camp).
• This Guidance advises against use of manipulated day-time photography due to the risk of misrepresenting the baseline (photographs should capture the baseline situation, i.e. any other lights in the view), however, there may be instances where the approach could be used for remote locations where it can be verified that no other sources of artificial light are present in the baseline.

 1       This appendix sets out the key International and National Policy Context relating to visible aviation lighting. It does not seek to duplicate the details of key policy documents but provides a summary of relevant policy provisions.

The International Civil Aviation Organisation

2        The requirement to install aviation lighting on turbines is set at the international level by the International Civil Aviation Organisation (‘ICAO’), an agency of the United Nations set up to manage the Convention on International Civil Aviation, also known as the Chicago Convention.

3        ICAO created a set of Standards and Recommended Practices to ensure consistency of air operations across ICAO Member States. ICAO Standards are necessary for safety or regularity or air navigation, whilst Recommended Practices are desirable, but not compulsory.

4        The United Kingdom is a Contracting State to the Convention and is obliged to comply with ICAO Standards. Where that is not possible, they must file a “Difference” with ICAO. All Standards and Recommended Practices on the marking and lighting of obstacles are implemented in full in the UK.

5        Chapter 6 of Annex 14 to the Convention on International Civil Aviation contains the ICAO set of internationally agreed Standards and Recommended Practices relating to the marking and lighting of obstacles, including wind turbines. 

6        The International Electrotechnical Commission (IEC) offers a subset of the array of ICAO options for aviation lighting on wind turbines in its publication ‘IEC Wind energy generation systems - Part 29: Marking and lighting of wind turbines’ and provides a useful insight to the international context to aviation lighting for turbines. 

United Kingdom Air Law

7        Within the UK, The Air Navigation Order 2016 (‘ANO’), Article 222 and CAA publication CAP 764: “Policy and Guidelines on Wind Turbines” set out a legal requirement reflecting ICAO’s Recommendations on the lighting of obstacles of 150m or more.

8        Article 222(8) defines an “en route obstacle” as “any building, structure or erection, the height of which is 150 metres or more above ground level”.

9        Article 222(1) requires that “The person in charge of an en route obstacle must ensure that it is fitted with medium-intensity steady red lights positioned as close as possible to the top of the obstacle and at intermediate levels spaced so far as practicable equally between the top lights and ground level with an interval of not more than 52 metres.”

10       The “medium-intensity steady red lights” referred to in ANO Article 222 are defined in Table 6-1 of ICAO Annex 14 as a Medium-intensity Type C light, with a peak intensity of 2000 cd at night. 

11       The specifications for the vertical beam spread of Medium-intensity Type C lights are set out in Table 6-3 of ICAO Annex 14, reproduced below, and illustrated as Diagram 1 in Appendix 3 of this Guidance. In relation to the minimum requirements, ICAO provides that the Medium-intensity Type C light must have an average minimum intensity of 2000 cd at 0° elevation (i.e. horizontal), and 750 cd at -1° elevation. In addition, the light must have an intensity of at least 750 cd throughout a contiguous elevation sector – the “Vertical beam spread” - of at least 3°. These provisions are designed to provide warning to pilots who may be flying at low levels, and to avoid spurious warnings to pilots flying at safe levels above the obstacle.

12       There are no requirements for any specified minimum or maximum light intensity at elevation angles lower than -1°, or above +2°. It is therefore open to lighting manufacturers to design lights that have intensities as close as possible to zero at angles of elevation lower than -1°, and above +2°. This is an important consideration in relation to the mitigation of unnecessary light emissions, that is discussed further under the mitigation section of this Guidance.

13       Article 222(2) and (3) specifies when the lights must be illuminated:

• “(2) The person in charge of an en-route obstacle must, subject to paragraph (3), ensure that by night the lights required to be fitted by this article are displayed. 
• (3) In the event of the failure of any light which is required by this article to be displayed by night the person in charge must repair or replace the light as soon as reasonably practicable.”

1.       This appendix provides technical detail on the options to mitigate night-time effects from visible aviation lighting.

Current Mitigation Measures

Automatic Dimming of light intensity

2        The CAA’s Policy Statement permits automatic dimming of aviation lights from 2000 cd to one tenth of their minimum peak intensity, 200 cd, when the meteorological visibility is ‘good’ around the wind farm and measured as exceeding 5km. This mitigation can be very effective at ensuring that the aviation lights are only as noticeable as is required to maintain air safety. When dimming mitigation is applied, operation of the lights at 2000 cd intensity would only be triggered when any sensor at the wind farm (located on each of the lit turbines) measures sub-optimal visibility, whereas, in clear conditions, the lights should operate at a reduced intensity of approximately 200 cd. The dimming applies to all nacelle lights on a given wind farm, therefore all receptors would experience this reduction in intensity. 

3        The images below illustrate a light manufactured by CEL (Contarnex Europe Ltd. (CEL) - CEL-Ml-ACWGAM Medium-intensity Red 2000cd Light. 230VAC - LED Aircraft Warning Light Technical specification) operating at 2000 cd and 200 cd respectively.

4        The dimming to 10% of minimum peak intensity applies to all elevation angles of light. Visibility is defined in this case by the measurements made by an automated ‘forward-scatter’ sensor, which projects an infra-red beam. The sensor's receiver measures the light scattered by moisture particles in the air and calculates the visibility range from that. The transmitter and receiver are less than a metre apart, normally located on the turbine nacelle, and are therefore measuring / calculating visibility at that wind turbine/sensor. 

5        The scenario of a sensor measuring ‘poor’ visibility way beyond the wind farm, when the lights themselves are in clear sky, cannot therefore arise. The measurement is entirely local to the site of the sensor. One visibility sensor is sufficient to control the dimming for the entire wind farm, although for an exceptionally large wind farm, there may be several sensors positioned around its periphery. If one sensor identifies poor visibility it will switch all of the lighting to a poor visibility (2000 cd) setting. 

6        Modelling has predicted that, at the visibility threshold where the switchover occurs, and assuming uniform surrounding visibility, a 2000 cd light (in those poor visibility conditions), would be perceived as less bright than a 200 cd light (in good visibility) at the point where an observer reaches approximately 5km away from the source. If visibility conditions are poorer than the threshold at which the switchover occurs, then you would need to be closer than 5km from the source for the same to be true. Therefore, 2000 cd lights would normally only be seen near to their fullest intensity in closer views (i.e. somewhat less than 5km), or perhaps due to a local patch of poor visibility (e.g. localised mist or rain) at the sensor. If the sensor detects poor visibility, then all the nacelle lights will switch to 2000 cd even if, for example, there are better weather conditions at a more distant periphery of a large farm. 

7        In infrequent circumstances nacelle lights may still be perceived at close to 2000 candela intensity, even where automatic dimming mitigation is included. As noted, this may occur where, for example, the viewer is close to the turbine light during poor weather, or where patchy cloud on the far side of a wind farm results in 2000 candela intensity lighting remaining at full intensity and visible from the near side. However, when there is good visibility, 200 candela intensity will normally be triggered (if dimming mitigation is employed) and it is therefore considered reasonable to assess and portray this intensity of lighting when considering the maximum case effect (also noting that baseline photomontage images are captured during good visibility conditions). 

8        The assessment should also acknowledge that, when triggered, 2000 cd lights will normally be perceived at less than full brightness, and that triggering of the lights to 200 cd will predominate. In terms of the frequency of dimming to 200 cd, submitted lighting studies suggest that nacelle lights will typically sit above the cloud base on several hundred occasions per month but that, when not in cloud, dimming mitigation will typically be triggered for around 90% to 95% of the time (but can vary depending on site specific circumstances). 

9        In reality, 2000 cd branded lights are actually brighter in the middle of the beam (at +1 degree) than 2000 cd, as it is difficult to meet the average intensity requirement without exceeding it at some point in the beam spread. A modern light may for example be 2500 cd in the centre of the beam, and some older lights may be even brighter. For the sake of relative simplicity, however, and noting that there is also a degree of inaccuracy inherent when illustrating lighting, ‘2000 cd’ is referred to as the worst case (or 200 cd where dimming is proposed). 

Vertical directional intensity

10       Vertical directional intensity mitigation (sometimes called ‘narrow vertical beam spread’ or ‘angle intensity mitigation’), comprises the use of an aviation light design that allows for reduction in brightness when viewed from certain elevations above and below the horizontal plane. If the proposed vertical directional intensity lights fulfil the minimum requirements of the ICAO Regulations at Annex 14, Table 6-3 then there is no need for developers to seek project-specific approval from CAA for this form of mitigation.

11       Aviation lighting on wind turbines is required to operate at the minimum intensities defined in ICAO, across a relatively narrow (3°) vertical beam spread, projecting horizontally from the nacelle light, between -1° to +2°, as explained in ICAO Table 6-3 included in Appendix 2 and illustrated in Figure 4 below.

12       Beyond these parameters there is no requirement for light to be seen. Some manufacturers in the UK lighting industry are attempting to achieve a position whereby extraneous light is minimised or curtailed through the design of modern LED fittings, although at present none have been able to design a light that fully cuts out light emissions above +2° and below -1° vertical elevation. Instead, lighting design companies are endeavouring to refine LED lights to minimise the intensity of extraneous light above and below the minimum requirements.

13       Vertical directional intensity mitigation comprises the use of aviation lights that are designed to operate close to the minimum required intensities (as discussed above). For these types of aviation light, with more sophisticated bulbs, minimisation of extraneous light is included as an integral part of the design of the aviation light and the actual emissions vary (above the minimum requirements) with different manufacturer’s specifications of lights. Not all aviation lights include this mitigation and few, if any are in operation to date (2024) on wind turbines in Scotland. For lights where this mitigation is included, some, but not all manufacturers will provide a data sheet with the performance parameters of their light demonstrating the narrowed vertical beam spread. The CAA has confirmed that a physical cap on the top and/or bottom of the light to guarantee this mitigation would not be acceptable. However, as discussed above, as long the aviation light design complies with the ICAO minimum intensity standards (discussed above), they require no additional agreement from the CAA.

14       Vertical directional intensity mitigation could be considered when determining the worst-case brightness that will be seen from particular viewpoints, and a ‘lighting intensity ZTV’ could help illustrate this. However, any assessment would need to be appropriately cautious about the assumed reductions in brightness. Unless developers are willing to commit to a particular light model and specification at application stage and are willing to accept a planning condition to that effect, assessments should be cautious about relying on specific intensity parameters experienced at different angles of elevation, that may not be capable of being secured at time of installation. 

15       One option is for the assessment to be based on a cautious worst-case (i.e. least-best) assumption over intensity reductions (taking account of the range of ‘directional’ aviation light models that might realistically be fitted). If the assessment adopts this approach, the basis of assumed intensity reductions should be explained, and there should be a willingness to accept a planning condition that commits to fitting ‘vertical directional intensity’ lights that comply with that assessment (or, ideally, that do even better at reducing intensity). 

16       Alternatively, the assessments could be based on worst case assumptions of 2000 cd (or 200 cd where dimming is proposed), but separately indicate how vertical directional intensity might mitigate effects through an assumed light fitting.  If this is the case, the general expectation should still be to include vertical directional intensity mitigation as part of the submitted lighting scheme when seeking to discharge planning conditions. 

17       ‘Vertical directional intensity’ lighting can be an effective form of mitigation, especially for those receptors located at lower elevations relative to the turbine nacelles, but can be less effective for elevated or more distant receptors from where greater intensities may be seen. A lighting intensity ZTV can be helpful to illustrate the potential mitigation from an assumed light fitting, to demonstrate how light emissions experienced by particular receptors may reduce relative to the lighting source. 

18       This Guidance recommends against seeking to portray the resultant light intensity reductions in photomontages due to the challenge of achieving accurate representation (and committing to a specific bulb type at application stage).

19       The specifications for the vertical beam spread of Medium-intensity Type C lights are set out in Table 6-3 of ICAO Annex 14, reproduced in Appendix 2. 

20       It can be seen from the table that there are no requirements for any specified minimum or maximum light intensity at elevation angles lower than -1°. It is therefore open to lighting manufacturers to design lights that have intensities as close as possible to zero at angles of elevation lower than -1°. This is an important consideration in relation to the mitigation of unnecessary light emissions.

21       In reality, aviation light manufacturers have not been able to design a lamp that is precisely confined to such a narrow beam width, although state of the art light fittings are getting much better at eliminating unnecessary light emissions below -1° and above +2°. The photograph below illustrates an aviation light fitting lying flat on a table, projecting a narrow beam of light onto the ceiling.  It would normally be positioned upright on a turbine nacelle (projecting horizontally) but shows how light can be controlled within tight parameters.

22       In practice, even the best lamps that are currently available cannot eliminate all unwanted light, but the perceived intensities outside the 3° band are capable of being substantially reduced through careful choice of light technology. Alongside the use of a dimming function as described above, it is now possible to achieve light emissions of a much lower intensity, especially at negative angles which are important in terms of how people positioned below wind turbine nacelles may perceive the light.

23       The table below presents an indication of how light emissions can be substantially reduced through modern technology and illustrates the results for both 2000 and 200 cd operation, for one manufacturer’s light fitting. 

Reduced Lighting Scheme

24       The Air Navigation Order (‘ANO’) makes provision for lighting other than in accordance with international recommendations. Derogations for reduced lighting schemes can be made on the basis of an application to CAA supported by a ‘special aeronautical study’. This study is generally carried out by a specialist aviation consultant. The study involves the identification of airspace users specific to the local area, and consults with them on how they operate within the airspace and on their views on a proposed reduced lighting scheme. This consultation allows pilots using the airspace to provide feedback on the proposed scheme, and to request the inclusion of any specific measures that will benefit them, such as infrared lighting.

25       The reduced lighting scheme is designed specifically for each proposed wind farm and can offer valuable mitigation of lighting effects.

26       Depending on circumstances, a reduced lighting scheme can propose that some or all of the lights are replaced with infra-red, rather than visible lights. Mid-tower lights can be replaced with infra-red or removed altogether. which is beneficial in that commercially available ‘32 cd lights’ will often exceed 32 cd in reality (up to 70 cd between -30deg and +40deg) and this means they may actually appear brighter than well-designed nacelle lights which incorporate ‘vertical directional intensity mitigation’ (discussed above). 

27       It may also be possible in some exceptional cases for the requirement for any visible aviation lighting to be avoided, subject to site specific conditions and informed by an Aeronautical Study, as was the case at the consented Bhlaraidh Extension Wind Farm. 

28       Where wind farms are expanded through extensions, it can offer an opportunity to rationalise lighting across a number of different turbine clusters to mitigate the incremental effects from multiple lighting schemes. However, the potential timing of operation and decommissioning of individual schemes within a cluster may need to be considered to ensure adequate lighting provision at different stages. For those projects which are larger in scale or part of larger clusters, a reduced lighting scheme has the potential to significantly reduce the impacts of the required lighting. Therefore, consideration of this should occur during the design phase of the project, to build in mitigation at the earliest opportunity.

29       Given the reduced lighting scheme is based on a specific proposal (fixed layout) and the level of resource involved in engaging in consultations for both stakeholders and the CAA, a reduced lighting scheme can take time to agree. Therefore, engagement on this issue should be undertaken at an early stage in the EIA process, noting that formal agreement with the CAA cannot helpfully take place until a fixed wind turbine layout has been achieved. If agreement has not been achieved prior to completion of the EIAR, then the worst case scenario (without a reduced lighting scheme) should be assessed. An optional additional assessment could be included assuming the reduced lighting scheme has been submitted to the CAA for approval. 

30       The results of the special aeronautical study and the proposed reduced lighting scheme are then submitted to the CAA for their review and feedback. This feedback may result in further consultation with specific aviation stakeholders to allay any concerns raised by the CAA, or with the scheme being approved or rejected.

Emerging Mitigation Measures

Aircraft detection lighting systems (ADLS)

31       Two types of aircraft detection lighting systems have been considered by the renewables industry in recent years, and these are based around primary and secondary radar. This is the fastest evolving area of potential mitigation of night-time lighting effects.

32       Primary surveillance radar systems transmit pulses of radio energy and detect the reflected energy from any object picked up by the transmissions. This type of system does not require any technology installed in the aircraft. Investigations into primary radar systems within wind farms has generally found it to be a very expensive and technically limited solution, which has not gained support widely in the industry. 

33       Secondary surveillance radar systems require the installation of transponders within aircraft. The pulses transmitted by the secondary system are received by the transponder in the aircraft which responds and the sensor in the wind farm detects the approaching aircraft. The sensor system communicates with the lighting system, and lights are switched on / off depending on the aircraft’s location and proximity to the wind farm. The lighting system would respond by only switching on the visible aviation lighting when an aircraft enters a specified airspace ‘box’ around the wind farm. This solution is less expensive than primary radar, but for this to occur across the UK as a whole would rely on a change in UK Air Law to require mandatory carriage of transponders on all civil aviation aircraft. 

34       Other technical means of delivering ADLS are currently being explored for their potential in meeting the requirements. Recently, discussion has focused on Transponder Activated Lighting (‘TAL’) solutions, which are widely in use in other parts of Europe. ScottishPower Renewables is currently (June 2024) exploring with the CAA the potential to authorise a localised ‘airspace change’ to create a Transponder Mandatory Zone (TMZ) across a small part of southwest Scotland. This trial is progressing in 2024 with a view to testing and developing a safety case for TMZ and TAL. It is hoped that this trial will help to inform the implementation of an effective TMZ and TAL strategy in the UK.   

35       Data for this type of proximity activated lighting systems at operational wind farms in Germany and Austria suggests that, where these are not located close to airfields and low flying routes that are routinely used at night, activation times for lights will be between zero and 0.1% of night-time. Activation can atypically be higher (up to c.7%), where the wind farm is located close to airfields and low flying routes routinely used at night. Therefore, in most circumstances, a TAL system could represent an extremely effective form of lighting mitigation and could mean that a night-time Aviation Lighting Impact Assessment of visible lighting can be scoped out of EIA. While TAL offers tangible hope of a solution to addressing significant effects from visible aviation lighting it will be for developers and consultees to review its deployment when it becomes a viable option in the UK. Its use may not be justified or necessary on all wind farm sites.

36       Noting the potential effectiveness of this form of mitigation, both in ensuring air safety and substantially mitigating adverse impacts from lighting, applicants have in some recent cases been willing to engage a suitably worded planning condition requiring them to review aviation lighting provision at a later date, in the event that TAL becomes a practical option post consent.

1        This appendix provides recommendations on supporting figures and visualisations which should accompany an Aviation Lighting Impact Assessment.

Relevant Current Guidance

2        This Guidance does not seek to duplicate current industry best practice guidance for the preparation of Zone of Theoretical Visibility (ZTV) mapping, wireline visualisations or day-time photomontage visualisations for wind energy proposals. Reference is made where relevant to the following published guidance:

• NatureScot (SNH) (2017) Visual representation of wind farms guidance – Version 2.2
• Landscape Institute (LI) (2019) Technical Guidance Note (TGN) 06/19: Visual Representation of development proposals
• NatureScot (2024) Pre-application guidance for onshore wind farms

3        As understanding and experience of the effects of aviation lighting evolves, it is anticipated that so too will visualisation techniques to illustrate potential effects. 

4        Through the collaborative working to develop this Guidance, the Working Group has agreed that any further guidance on aviation lighting visualisations will be published in due course as an update or addendum to the NatureScot (2017) Visual representation of wind farms guidance – Version 2.2. Any such update will be progressed by NatureScot and include consultation with industry and relevant stakeholders.

Zone of Theoretical Visibility (ZTV) Mapping

5        Zone of Theoretical Visibility (ZTV) mapping is commonly used in LVIA to illustrate the potential visibility of wind turbines. Generated using a specialised software package, often within wind farm design or GIS software, ZTV outputs are familiar and well understood by assessors, consultees, decision-makers, and members of the public.

6        The use of software programmes such as Geographical Information System (GIS) can assist the assessment process by helping to identify locations from where any aviation lighting may be seen. For proposals where visible aviation lighting is required, Aviation Lighting ZTVs generated to specific fixed heights where lights may usually be located, for example on the turbine nacelle or mid-point of the tower, have become an invaluable tool in informing the extent of the lighting assessment study area for an LVIA; the identification of potential receptors; and the consideration of potential mitigation measures. Comparative ZTVs illustrating low-intensity mid-tower light visibility (e.g. nacelle light visibility vs nacelle and mid-tower height visibility) may also be helpful in some instances where both medium-intensity (2000 cd / 200 cd) and low-intensity (32cd) lights are required.

Aviation Lighting Intensity ZTVs

7        With adequate technical information about the type of light fittings that may be used, it is also possible to produce useful Aviation Lighting Intensity ZTVs showing the likely intensity of light at different elevations relative to the lights, illustrating the vertical directional intensity mitigation which is embedded in the design of many modern aviation lights.

8        When generating Aviation Lighting Intensity ZTVs, calculations are based on the technical specification of aviation safety lights. The specification and claimed performance of such lights can vary, and any assumptions made as to the accuracy and certainty of this should be referred to with caution when informing the preparation of visual representation materials, including both ZTV mapping and photomontage visualisations.

9        Cumulative lighting ZTVs illustrating aviation lighting may be useful in some circumstances where more than one lit wind farm is likely to affect key receptors. These can be helpful in understanding the combined and individual effects from a number of wind farms with visible aviation lighting. These may include, including consideration of both existing (operational) and/or proposed (consented/subject to applications) sites with visible aviation lighting. If these are undertaken, then this should include the nacelle lights only, otherwise the ZTVs can become overly complex. 

Visualisation of Aviation Lighting

10       Visualisation can be a helpful tool to the assessor, consultees and decision maker and good quality, reliable, accurate, visualisations are supported in this Guidance. LI Guidance Note (TGN 06/19) highlights that for technical visualisations which form part of a professional LVIA, ‘It is critical that these visualisations are accurate, objective and unbiased. They should allow competent authorities to understand the likely effects of the proposals on the character of an area and on views from specific points.’ (para 1.2.2, page 1). 

11       This Guidance does not seek to replace the prescriptive guidance to produce wireline and photomontage visualisations set out in NatureScot (SNH) (2017) Visual representation of wind farms guidance – Version 2.2 but instead provides guidance on how the visual representation of visible aviation lighting may be best approached in support of a night-time Aviation Lighting Impact Assessment within an LVIA, in an appropriate and proportionate way.

12       Although there is widespread familiarity with wind farm visualisations and associated preparatory methodologies for day-time photomontage images, the preparation of photomontages illustrating lighting (including aviation lighting) has some notable limitations. 

13       Photomontage visualisations prepared for representative viewpoint locations are helpful in providing an indication of the likely locations and approximate visibility of aviation lights from different parts of the study area, but they are essentially artist’s impressions of the light emission and do not carry the same assurance that can be drawn from a day-time photomontage of a wind turbine. This limitation should be noted.

14       Unlike the use of photomontages to illustrate day-time effects arising from wind turbines, night-time digital or printed visualisations of aviation lighting cannot reasonably achieve the same degree of accuracy in what they illustrate, not least because an individual’s eyes perceive light at night differently. This is acknowledged by this Guidance and should be clearly acknowledged in any assessment.

Limitations of Visualisations

15       There are several key aspects that influence what we can see at night, and how it compares to photographic evidence. The main ones are how dark-adapted our eyes are and the contrast between any individual light when compared to the surroundings. It is harder to see a light against a bright background or foreground simply because our eyes have limited contrast, and this is especially true in overall darker conditions. A camera has separate issues regarding the dynamic range it can acquire but typically these are not as limiting at night as the eye’s limitations. This is one of the reasons that photography at twilight may show presence of light sources that our eyes simply cannot detect.   

16       Evidence suggests that contrast affects most people in a similar way. Dark adaptation, by comparison, is something that varies greatly between individuals, with the main differences being due to diminishing dark response with age. In partly lit surroundings, dark adaptation never becomes complete. In unlit surroundings it can but requires from a few minutes up to half an hour to do so. Our eyes also develop greater blue sensitivity at night as they become dark adapted. A camera by comparison can be set to give a variety of colour responses (by variation of what is known as the white balance – i.e. what setting makes a sheet of white paper appear white under ambient lighting).  As a result of these differences, twilight or night-time photographic images often bear a limited resemblance to what a person might see in the same circumstances. In addition, the perspective changes dramatically as the sky darkens. What may be visible in full dark may be completely invisible in twilight. There is no one simple visualisation technique that can capture the full effect.

17       The final aspect to consider in what a visualisation should represent, is whether it is as expected. In perfectly clear weather, with excellent visibility, to a person, the same light located twice the distance away would appear a quarter as bright as the nearer light. Any atmospheric opacity due to mist, dust, pollen etc. will make the more distant light fainter than this, but as a rule of thumb it is a good first guide. Another way to express this is to say how far away an aviation warning light at 200 cd would be compared to other more common red lights. The simplest of these to envisage in most locales is a car’s rear brake lights since these are similarly red in colour. These vary slightly in intensity but are typically about 80 cd.

18       In recent times methods and techniques to illustrate how such lights may appear to the naked eye have been developed, often including the capture of baseline photography taken in low light conditions (just before sunrise/ or after sunset) to prepare photomontage visual representations in a similar format to those depicting day-time views of wind turbines. 

19       Whilst multiple software packages and methods are available for use when modelling visible aviation lighting, it is important to note that digital visualisations prepared in such software cannot replicate the additional variable influence which distance (between the light and the viewpoint/observer) or atmospheric attenuation from aerosols or other influences can have on the observed illuminance (brightness or brilliance) of the lights. However, it is understood that the additional influence of these factors would typically lead to a further decrease in the perceived brightness or brilliance. 

20       Because of these limitations, it is recommended that visualisations produced in support of a night-time Aviation Lighting Impact Assessment should be caveated as being only a reasonable indicative illustration of the lighting effects. Visualisations should, as far as practicable, seek to represent the reasonable maximum case effects likely to arise so that these can be considered as part of the Environmental Impact Assessment (EIA) process (where such assessment is necessary). Where dimming mitigation is agreed with the CAA, photomontages need only illustrate 200 cd illumination.

21       This Guidance does not suggest any deviation from this approach, and an update or addendum to the NatureScot (2017) Visual representation of wind farms guidance – Version 2.2 may provide further advice in relation to the preparation of such images.

Capturing Baseline Photography

22       When capturing baseline photography at dawn/dusk/night-time at viewpoints, it is important to consider receptor activity as well as baseline lighting conditions. In some instances, it may be the existing presence of other sources of artificial lighting (i.e. other lights in the view) that is important in planning fieldwork. 

23       In other situations, capturing the complete absence of natural lighting may create challenges to assessors, where the landform of the site, or other key landmarks and features, are important to the night-time experience.

24       It is important to capture a reasonable balance between visibility of the underlying landform and the apparent brightness of any artificial lights, as both should be visible in the image. It is important that photography represent the levels of darkness as seen by the naked eye at the time and the camera settings used do not make the image appear artificially brighter or darker than it is in reality.

25       Recent NatureScot guidance for undertaking night-time photography has suggested it should normally be carried out 30 minutes after sunset (or before sunrise), around the end of civil twilight (or beginning, if at dawn), when ICAO/ CAA regulations require the lights to be switched on (or off at dawn). However, the timing of photography will vary substantially with the seasons and is also heavily influenced by weather and viewing conditions, which can sometimes result in narrow windows of opportunity to obtain suitable baseline imagery. Advanced planning of fieldwork with reference to weather forecasts, sunset/sunrise times and knowledge of the location in relation to visibility of the horizon where the sun will set/rise can be important. Setting the camera up in daylight can often be advantageous. Unlike in day-time conditions, there is often a very limited window when appropriate night-time baseline photography can be captured, so if weather conditions are not favourable during this limited window (i.e. mist drifting onto the site for instance) then opportunities may be missed. This can become particularly challenging for remote viewpoints.

26       In practical terms, taking photographs after sunset rather than before sunrise is easier because it allows the photographer more time to safely set up and adjust the equipment and camera in preparation. On average, 30 minutes after sunset is a time when the shape of landform can still be discerned, and the night sky will still not be at its darkest, but importantly there will likely be a distinction between the sky and the land, or bodies of water. 

27       It is recommended that multiple ranges of photography several minutes apart are captured (e.g. either side of the suggested 30 minutes after sunset or before sunrise) because the particular location (e.g. view direction relative to the sunset or sunrise horizon) and ambient lighting conditions on any given day can affect what might be the best timing for capturing appropriate baseline photography. It is recommended that observations on the prevailing weather conditions are noted when capturing the baseline photography (e.g. clear sky, partly cloudy etc.) and referenced when preparing the visualisations, as this can affect the perception of indirect background lighting.

28       It is relevant to note that the intensity of aviation lighting will not be perceived to its fullest until astronomical twilight has passed. Visualisations produced using photography taken 30 minutes after sunset should carry a caveat to this effect.

29       It may be necessary in some situations and times of the year, to capture baseline photography at a much later period after sunset. During British summer time (March to October) when day length is at its longest, and in more northern latitudes, discretion should be used to determine the best approach to the timing used for capturing baseline photography, and this Guidance accepts that experienced practitioners should have scope to adopt an appropriate project specific approach.

Health and Safety and Proportionality

30       Health and safety should be an important consideration when commissioning or undertaking dusk/dawn photography and associated fieldwork, particularly from remote viewpoints. This Guidance strongly recommends that obtaining such photography should be undertaken by competent and experienced individuals, who should anticipate potentially spending considerable time at night-time viewpoint locations and should therefore be suitably prepared to do so safely. 

31       Assessors and consultees/stakeholders should take the practical and health and safety considerations in account when identifying and agreeing night-time viewpoint locations to best represent effects. This should not preclude hilltop viewpoints from being chosen, where they may provide an essential insight to the lighting effects, but an appropriate and proportionate approach should be taken to minimise risk to the assessors.

32       In some cases, a more accessible proxy viewpoint may be appropriate, for example where similar views from the periphery of a Wild Land Area (WLA) may be equally representative of those experienced from more inaccessible remote interior, or where the approach to a hill summit located at a lower elevation may provide opportunity to view aviation lights at closer distance and comparable relative lighting intensity.

Types of Visualisations

33       As outlined in the limitations above, the preparation of realistic and accurate visualisations depicting lights seen during hours of low light and/or darkness in either digital or printed format is challenging. LI Guidance Note (TGN 06/19) states that ‘Technical visualisations can take a variety of generally 'static' forms, including: annotated photographs, wirelines, photomontages, and 3D simulations.’ (para 1.2.5, page 1).

34       As for day-time visualisations of wind turbines, a range of different mediums and presentation formats exist to illustrate potential visible aviation lighting to provide the viewer with a fair and reasonable representation of what would be likely to be seen and portraying the proposal in scale with its surroundings. 

35       This Guidance recommends that in most instances both wireline visualisations and photomontage visualisations are prepared to accompany an assessment, illustrating the location and likely visual appearance of aviation lighting, and the influence of any mitigation proposed which is capable of being reasonably illustrated.  It cautions against trying to illustrate directional lighting mitigation, unless the developer is willing to commit to a specific lighting unit by way of a planning condition.

Wireline visualisations

36       If only certain turbines are to be lit, for example due to an agreed reduced lighting scheme, 53.5° (degree) wireline visualisations should include indication of the individual turbines that will be visibly lit. These should be included for all assessment viewpoints as annotations on the 53.5° wireline or a simple note at the foot of the wireline, simply illustrating the location of nacelle and/or tower mounted visible aviation lighting.

Photomontage visualisations

37       Night-time photomontage visualisations from a proportionate number of representative viewpoints (typically two or three), selected based on the identified sensitivity of potential receptors and frequency of visitors at night or dawn/sunset, and to be agreed with the Planning Authority (in consultation with NatureScot where appropriate). Photomontage visualisations may be prepared using:

• Dusk/night-time photography – baseline photography is typically best captured on average approximately 30 minutes after sunset (or before sunrise), around the end of civil twilight (or beginning, if at dawn), in order to capture definition of the landscape and the presence of other baseline aviation lighting.
• Manipulated day-time photography – there may be instances where the approach could be used for remote locations where no other sources of artificial light may be present in the baseline.  Using digital software, baseline day-time photography captured in accordance with appropriate guidance may be manipulated to create a representation of the baseline dusk/nighttime scene but does not represent the realistic baseline at this time. It is advised that this approach may be most appropriate for remote locations and should be agreed with stakeholders in advance. 

38       It is recommended that 53.5° photomontage visualisations, and potential cumulative visualisations, should illustrate:

• Medium-intensity turbine nacelle lighting at its minimum illumination – i.e. 200 cd where automatic dimming mitigation (to 10% of maximum intensity) is proposed.
• Medium-intensity turbine nacelle lighting at its maximum illumination – i.e. 2000 cd only where automatic dimming mitigation is not proposed.
• Low-intensity mid-tower lighting – i.e. 32 cd lights unless agreed otherwise with the CAA as part of a reduced lighting scheme.

39       Whilst simulating visible aviation lighting in a 3D virtual representation is possible and may help supplement understanding of how these types of lights may appear to those less familiar with their appearance, this Guidance recommends that static 2D digital images provide the most appropriate visual representation to accompany an Aviation Lighting Impact Assessment at this current time. As such, this Guidance focuses on providing recommendations on the types and format of images to be prepared to accompany an assessment.

Abbreviations and terms used in this Guidance