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Frequently Asked Questions: Glint and Glare

Our technical experts at Metrica are able to undertake Glint and Glare Assessments to identify potential impacts on light-sensitive receptors due to solar PV developments, from single-dwelling roof-mounted panels, to large-scale NSIP projects. Our concise, pragmatic approach ensures any potential significant impacts are identified, whilst remaining cost-effective for development of all scales.

We have compiled some of the most commonly asked questions in order to help you understand Glint and Glare effects. If you have a question that isn’t answered below, please don’t hesitate to get in contact.

What is Glint and Glare?

Solar Glint and Glare occurs is when sunlight is reflected from a surface. The Federal Aviation Administration’s (FAA) Technical Guidance for Evaluating Selected Solar Technologies on Airports[1] provides the following definitions of Glint and Glare:

  • Glint – a momentary flash of bright light
  • Glare – a continuous source of bright light

Glint and Glare effects can present a hazard to light-sensitive receptors as they can result in an after-image (a visual illusion where an image persists after exposure to the original image has ceased). This can be a hazard for moving receptors such as road, rail or air traffic. Glare effects can also affect nearby communities though impacts on residential amenity.


How is Glint and Glare Assessed?

A Glint and Glare Assessment is an investigation into the expected glint and glare hazard of a given development. The assessment calculates the times and duration of of glint and glare effects, based upon:

  • The reflector’s position, tilt angle and azimuth (the direction the reflector is facing, relative to north).
  • The position of nearby light-sensitive receptors. This can include individual point receptors such as houses, or moving receptors just as road traffic, or aircraft following a specific flight path.
  • The sun’s path across the sky (elevation and azimuth).
  • Local topography, including reflector and receptor heights above sea level.

The assessment considers whether glint and glare effects could be dangerous for road / rail / air traffic or constitute an unreasonable adverse impact for neighbours. Where necessary, the assessment can also contain recommendations for mitigation such as additional screening, the use of anti-glint/glare solar panels or modifications to the reflectors tilt or azimuth angle.

What are the limits for Glint and Glare effects?

As mentioned previously, the primary guidance in assessing the likely effects of Glint and Glare (on aviation infrastructure) comes from the FAA in the United States. Their document, Technical Guidance for Evaluating Selected Solar Technologies on Airports is accepted internationally as the most detailed methodology for assessing Glint and Glare effects. Guidelines also exist in the UK, produced by the Civil Aviation Authority (CAA)[2], however there are no specific limits in either the FAA or CAA guidance to determine the significance of Glint and Glare effects.

Notwithstanding this, limits are defined in several other international guidelines. For example, in Germany, the Light-Guideline (Licht-Leitlinie)[3] issued by the Ministry of Environment defines glare a “significant nuisance” at durations longer than 30 minutes per day and /or 30 hours per year. However, predicted effects for longer durations are not necessarily significant, and should be considered on a case-by-case basis. Other factors to be considered include the likelihood of cloud cover at the developing’s location, and the intensity of the predicted glare, which decreases as the viewing angle toward the reflector increases.

[2] CAA (2010) Interim CAA Guidance – Solar Photovoltaic Systems
[3] Guideline for the Ministry of Agriculture, Environment and Climate Protection for the measurement and assessment of light Immissions, 2014.

Who Needs a Glint and Glare Assessment?

Typically, glint and glare assessments are required by Local Planning Authorities when considering applications for solar developments, to ensure that there will be no significant impacts following construction. However, it should be noted that a Glint and Glare assessment may not always be requested. It is therefore worthwhile undertaking a glint and glare assessment as early as possible during the design stage to avoid any issues post-construction which may require expensive changes to the panel array and / or landscaping.

Please get in touch at to discuss how we can support you in the assessment of Glint and Glare effects.

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Leave a comment

  1. I have been trying to establish what if any quantification forms part of ‘glint and glare’ assessments when applied to solar PV installations. There must surely be a distance up to which a visual receptor can be said to experience a significant effect from reflected sunlight from a planar surface, and a distance beyond which any such effect will be so small within the overall field of vision compared with the size of the sun in the sky they will invariably be looking towards, that it can be said to be insignificant.
    Nobody however seems to address such an obvious issue, which means that planning authorities continue to ask for glint and glare assessments even when the identified visual receptors are over 1km from the solar panels.
    Can you clarify what a reasonable cut off point is?

    1. Hi Richard, thanks for the message. We have responded to you directly by email.