January 19th, 2021

Yellow light vs white light in fog

One general view about yellow vs white light in fog is based on the theory that scattering, by anything at all, is always greater at the short-wavelength end of the visible spectrum than at the long end. It must be true because the nineteenth-century British scientist Lord Rayleigh showed this in his paper of 1871 on the dispersion of electromagnetic radiation. This explained, amongst other things, why the sky is blue. This is because when the pure white light from the sun passes through the gases and extremely small particles in the atmosphere it’s scattered. Blue, and violet, light is dispersed the most because they travel as much shorter waves. So, the sky looks blue.

You’d assume therefore to get the greatest penetration of light through fog, you should use the longest wavelength possible. Red, being the longest, is obviously unsuitable however because it is used for traffic stoplights. So, you compromise and use yellow light instead.

This view is flawed though when it comes to light penetrating through fog. Rayleigh scattering – as it’s known – applies only to ‘scattering’ particles that are smaller than the wavelength of light and at wavelengths far from absorption. Fog droplets are huge compared with the wavelengths of visible light. This means that the scattering of light by fog is essentially wavelength independent. See Reference Articles 1 below.

We don’t need to consult scientific papers to understand this is true though. Just look at cars on the road at night. Designers of vehicle headlights have known for a long time that there is no magic colour that gives greater fog penetration. That’s why most headlights are white and why, for example, EU regulations require all new vehicles to have white lights. See Reference Articles 2 below.

So, for light penetration and perception in fog, the colour of light is unimportant. Yellow light. White light. There’s no difference.

Reference Articles 1

Bohren C. F, & Fraser A. (1985) – Colours of the Sky in The Physics Teacher pp 267-272. And Bohren, C. F. & Huffman, D. R. (1998) – Particles Small Compared with the Wavelength, in Absorption and Scattering of Light by Small Particles, Wiley-VCH Verlag GmbH, Weinheim, Germany.

Reference Articles 2

Nelson, J.H. (1938) – Optics of headlights in The Journal of Scientific Instruments Vol. XV, pp. 317-322. Also see the more recent Schreuder, D. A. (1976) – White or yellow light for vehicle head-lamps? In the Dutch Institute for Road Safety Research SWOV.

Commercial Director & Co-Founder

As an entrepreneur, Yuli has worked across sectors as diverse as Finance, Oil & Gas, Music, Real Estate and Electronics. His passion in business is challenging the status quo, disrupting markets, building first-class teams, and solving complex challenges with creative solutions.

Yuli trained in Finance and Economics in London, with postgraduate studies in Law (LLM) and Engineering (MEng) in Scotland and Australia. He’s also been appointed as an Export Champion by the UK Government’s Department of International Trade.

Recent Blog Posts

Sep 1st, 2020

Seven apron floodlighting lessons learned over the years

As LED lighting pioneers, we’ve been working with aviation clients, from major international hubs to regional airports to military facilities, for over a decade. And we’re masters at making LED lighting solutions for airport aprons.

Here are just some of the lessons we’ve learned over the last ten years.

Lesson One: Apron lighting must be a design-led solution

Most people working on the airfield side of an airport will be familiar with Airfield Ground Lighting (AGL). They’re very standard products. It doesn’t matter who you buy them from, things like their output, size and optics will be almost the same.

But with LED floodlighting that’s not the case. You can’t assume a 600W floodlight from one supplier will behave in the same way as a 600W floodlight from another. They can in fact perform very differently.

This means you can’t approach a potential supplier saying ‘We’ve had a design from a supplier using 123 pieces of 400W floodlights. Can you give us a quote for the same number so we can use that as your competitive bid?’.

Unfortunately, you can’t just do that as floodlights will vary depending on their photometry and efficacy – the efficiency of useful light on the ground. So, you could have a 400W floodlight, with the right optics, performing better much than a more ‘powerful’ 600W one.

Another important factor to consider is how well products will fare over time. LED lighting will degrade, just like sodium or metal halide lighting – at a much slower rate though. But you still need to know by how much and over what time period. Again, this will be different for each manufacture and will certainly depend on the environment in which they operate

So, a manufacturer’s design and quote for a project must be based on their actual products. If they’re not, you can’t make any meaningful comparison.

The lesson here, in summary, is don’t try to compare ‘apples with oranges’.

Lesson Two: Test, measure and verify

Let’s move on to the next phase of a project.

You’ve chosen a supplier. Their design works – on paper. You know it meets or exceeds your needs. And it’s on budget. You give it the green light and it gets completed. You may think that’s it, job done.

But it’s not.

You’ve got to check that what was promised in the design phase is true to real life and it really complies with or exceeds what’s required by your local aviation authority.

Here’s an example why. Istanbul Airport asked 10 major lighting suppliers to pitch for an upgrade to the airport’s lighting. As part of the pitch process, they asked each supplier to set up two test poles to demonstrate they could reach the compliance levels shown in their designs. 80% didn’t pass the test. The Airport Authority was very relieved they ran the test!

And you need to go on testing and verifying your lighting regularly, with a documented testing methodology, to make sure it stays compliant.

As an example, Frankfurt Airport had a new system installed during the winter. After live testing, it was confirmed it matched the light levels needed. In fact, the lighting exceeded what was required, as a ‘buffer’ had been allowed for in the design. Six months later, in the summer, when they ran the tests again, the levels had fallen by 20%. They were still within the required levels though because of the ‘buffer’. The airport however was obviously concerned why they’d fallen and what would happen in the coming months and years. When they tested again in the winter, the lighting was back to its original levels. It was the ambient temperature change between the winter and the summer that was having an effect. By factoring this into their testing methodology they were able to make true comparisons going forwards, test after test.

The lesson here? Test, measure, and verify regularly to make sure you stay compliant with regulations. And going a bit above and beyond what’s required in the standards is never a bad idea…

Lesson Three: Optics rule

Glare is the enemy of all airports. It primarily affects pilots and can cause not just discomfort but also temporary blindness, which in turn can lead to accidents. So, from a health and safety standpoint reducing glare is exceedingly high on an airport’s agenda.

This is where the optics used come into play.

In virtually all the projects we’ve done, we’ve used our proprietary asymmetric optics. Why? Simply because they deliver a low glare output spread over a large angle compared to symmetric optics. We also make sure that they have a full cut-off above the horizontal plane to further reduce the chance of any glare issues.

The result is lower glare, not just for pilots but for ground staff too.

This lesson? Put your supplier under the spotlight when it comes to glare and get a guarantee it won’t be a problem for you.

Lesson Four: Pay cheap – pay twice

When it comes to virtually any project, engineers want to use premium products to give the best possible outcome. Finance departments, however, are usually focused on the price. And this can cause problems.

We’ve heard many ‘horror stories’ where the budget has been the primary reason for choosing a particular supplier’s solution.

In one case, because the luminaire’s heat dissipation wasn’t up to the job, the LEDs burnt themselves out so much, they actually fell off their circuit board.

In another, cheap floodlights were used and within six months nearly all of them had failed. They were replaced under warranty. But within another six months, they’d failed again. To solve the problem once and for all, the airport had to approve a new budget for a more expensive, more resilient solution.

So, remember these three things:

  • Apron floodlighting is part of your critical infrastructure. Don’t let budget be your controlling factor when it comes to it. The cost of having a cheaper system can be quickly outweighed by the costs of continued maintenance etc. And just imagine that you had an accident on an apron – if your lighting wasn’t up to prescribed standards you could end up facing huge legal costs.
  • Premium products will almost always outlast low-quality ones. And premium product providers usually will be around for a lot longer than their competitors. Go cheap and you may find that, when problems start to happen later down the line, the company you went for isn’t around to put things right.
  • Contractors are always looking to make as big a margin as they can. They try to reduce costs by taking what’s in the design specification and replacing certain products with cheaper ones. Reducing their cost increases your risk. Don’t let them. Make sure they stick to the specifications to meet your needs.

Compromising on quality can cost you a lot more, in the long run, is the lesson to take out here.

Lesson Five: Remote vs integrated drivers

This is a question we get asked a lot. ‘Is it better to have your drivers in the luminaires or a separate box?’

There are positives and negatives on both sides. Depending on where your airport is based can influence your choice too. In the UK, Italy, and Germany integrated drivers are more common. In France and the US remote drivers are mainly used.

So, make sure your supplier has both options available to meet what you want.

One thing to remember though is voltage spikes are killers when it comes to LED lighting. That’s why surge arrestors are so important. Make sure your supplier’s remote or integrated solution takes voltage spikes into consideration and can protect your lighting.

This lesson is simple, talk to your supplier about which will work best for you and how they’ll protect you from voltage spikes.

Lesson Six: Control and intelligence

Everyone thinks they need controls. But do you really need them? Before you add a control system to your project specification you need to:

  • Decide who will be in charge of the control systems. Will they need to be trained to handle them? Will you need to have 24/7 cover in case something goes wrong?
  • Look at what intelligence they can give you. Is it really that useful to your operation?

Control systems come at a cost. There’s no point in having them if they’re just going to turn your lighting on and off.

The lesson here? If you’re not going to see any additional benefits from having them, don’t be convinced by your supplier to buy them.

Lesson Seven: Yellow vs white? Which is best.

Another question we get asked quite often is, ‘For fog dissipation is yellow or white light the best?’.

Most people think yellow light is. But in truth, they perform exactly the same as each other.

Why? Fog droplets are on average, much smaller than cloud droplets. But they are still huge compared to the wavelengths of visible light. So the scattering of such light by fog is essentially wavelength independent. Car manufactures have known this for years and that’s why they don’t use yellow fog lights any longer.

This last lesson learned is a simple one. Don’t worry about it.

That’s it your seven lessons learned! Fill in your details below to watch the 7 Lesson Learned webinar recording

Yuli Grig, Commercial Director & Co-Founder, Midstream Lighting

As an entrepreneur, Yuli has worked across sectors as diverse as Finance, Oil & Gas, Music, Real Estate and Electronics. His passion in business is challenging the status quo, disrupting markets, building first-class teams, and solving complex challenges with creative solutions.

Yuli trained in Finance and Economics in London, with postgraduate studies in Law (LLM) and Engineering (MEng) in Scotland and Australia. He’s also been appointed as an Export Champion by the UK Government’s Department of International Trade.

Recent Aviation Blogs

April 26th, 2020

Lighting the way for military aircrafts

In this latest ‘Midstream Lighting Sectors’ blog, we talk to Midstream’s Co-Founder Yuli Grig about our military aviation experience.


We didn’t have any contacts in military aviation, and they don’t particularly have an online route you can contact them through. So, it was quite hard for us to break into this market at first. Then, we met some design consultants at a trade show who were working for the Royal Air Force. They were so impressed with our products and application knowledge, they invited us to make a presentation at a large RAF airbase. We were in!

Since then, we’ve been growing our contacts and winning projects by word of mouth.


Typically they follow similar rules. But the military has its own interpretation of the International Civil Aviation Organisation (ICAO) standards. And in the UK, we have to follow the Military Aviation Authority’s Manual of Aerodrome Design & Safeguarding (MADS).

The layout of a military airfield, however, is inherently different from a civil aviation one. They’re built for military aircraft – fighter jets and bombers, not holiday airbuses. There’s also typically no terminal building, no air bridge, and no passenger buses. So, the lighting design must take things like this into account. This usually means we have to use lighting solutions designed for stadia rather than airfields.

Give us a feel for some of your most recent work?

I can’t say too much but recently we’ve completed, or are working on:

Three projects in the UK.
One for the US Airforce in Italy.
And, one each in Spain, Germany, Qatar, and Greece.

What type of specific difficulties or problems are there with military projects?

Quite often, when bases are shared with the US Dept of Defence, there can be some debate about which standards apply.

Military bases also need to illuminate a large surface area with a few remotely located masts. This means we’ve got to use a lot of very high-power projectors on each mast. And that can present a big challenge when working on retrofit projects.

Another issue we can face is that military sites have usually been in operation for decades. Over that time military needs, equipment and aircrafts have changed. But, field layouts and the infrastructure haven’t. So, we need to solve today’s problems with masts that were designed many years ago.

Why is military lighting so important?

Like any site, lighting is critical to providing a safe working environment. As aircraft arrive from missions or training or are being refuelled and inspected, the level of lighting must be 100% right. And the lights used must remain in good working order throughout their design life. Just as with commercial airports, there’s no room for failure.

That’s not all. The lighting must be able to be controlled for operations where pilots arrive using night vision devices – a unique need for military airfields.

What product do we use usually recommend and why?

All our projects are design-led, and each airfield layout needs the right combination of lighting products. So, we don’t have a standard product we recommend. Our experienced Lighting Design and Engineering teams work together to deliver perfect lighting solutions for each specific airfield. Most of the military projects we’ve competed though have incorporated our high-power Modus R1200 sports floodlight to some extent.

What are the technical advantages of using this product on military sites?

The Modus R1200 sports floodlight lets us place a lot of quality light far away from where it’s based. And its range of proprietary optics and visors also means we can meet a projects specific needs when others can’t.

What does the military looking for when they appoint a lighting solutions company?

For the military, the two key considerations, when commissioning any project, are complying with standards, and reliability. Both of which are at the heart of all we do. They’re used to all their equipment being of the highest order and highly reliable. Careers have been made or broken depending on choices made. And commissioning an airfield’s lighting system is no different.

We deliver an added benefit too. Our experience of designing military airfield sites means we can value engineer existing projects to deliver real capital and operational savings.

Can you tell us about a particular project – the challenge faced and our solution?

As you’ll appreciate, all our military projects are all confidential and highly sensitive. So, we can’t give any details away. I wish we could, to show you how we’ve successfully tackled some of our toughest challenges ever. But, as the old joke goes, ‘If I told you that…’.

What advice you would give to a military airfield looking to upgrade their lighting?

I’d take the same approach as when you’re buying any military equipment. It’s got to be top quality, made for purpose and tested in the field. Military airfield lighting needs specialist knowledge and equipment. So, make sure you come to the experts. Us.

Midstream Lighting