Ski Goggles (blog)

Flat light and White-out, which colour goggle lens works best?

By Wayne (Fully certified Geek)

Ski goggles are worn for many reasons such as protection against impact, shielding against the weather, snow and wind rush, etc. Another common reason for donning a pair of goggles, as opposed to sunglasses, is that it’s difficult to see due to Flat-light or a White-out.

Before we look at why goggles help improve your vision in Flat-light or a White-out we need to know what these conditions are.

Flat-Light is a Diffusion of sunlight.

Cause:
Sunlight is both Scattered and Diffused by atmospheric particles e.g. water molecules, ice crystals, etc. plus ground laying snow causing light to be received from multiple directions. Commonly the effect is increased during a White-Out and/or later in the day when the sun drops towards the horizon, due to sunlight passing through the atmosphere for a greater distance.
Result:
Light is received from multiple directions with each light source producing overlapping shadows which cancel-out each other. This dulls the area and removes indicators such as tones and contrast, making it difficult to discern similarly coloured slope features. The loss of visual indicators of shape and edge detail results in objects and features seeming to blend into each other, producing a flat featureless vista. An effect of visual blending may be a loss of depth of field resulting in disorientation.

Fig 1.
Scattering refers to independent changes of direction.
Fig 2.
Diffusion (in this case) refers to compound changes;
a change is the result of a previous change


A white-Out is a Reduction and Scattering of sunlight.

Cause:
Sunlight is blocked, reduced and scattered by ice crystals in falling snow, wind-blown spin-drift, water droplets in low lying clouds or localised fog, etc. The remaining scattered light is merged and blended.

Result:
Due to a reduction in reflected light, visual references e.g. the horizon, terrain features, slope aspect, etc. are significantly reduced or completely blocked. This leads an inability to position yourself relative to the surroundings. In severe conditions, due to a loss of kinesthesia (ability to discern position and movement) confusion, loss of balance and an overall reduction in the ability to operate, due to the increased danger (real or perceived) this presents may be found.

Fig 3.
Blocking refers to light which is absorbed. Later this energy will be dissipated as heat
Fig 4.
Light is Reduced when some is reflected and some blocked / absorbed


The processes involved in the two conditions are different but both Flight-Light and a White-Out produce a higher proportion of blue light than there would be normally. You should note though that no “extra” blue has been added, but rather that all other colours have diminished relatively, so blue becomes more noticeable.

Snow is not white, it’s blue.
This fact is important when deciding on which goggles will help us during Flight-Light or a White-Out. Of course snow and water (droplets) aren’t bright blue as most of the light hitting it is in fact reflected (all colours of light combined create white), but it does absorb slightly more of the other colours more than blue.

Fig 5.
Spectrographic chart of the absorption of the sun’s energy by water/ice. The higher the line is the more energy is being absorbed. Notes: This chart assumes the snow/water is pure. The (slight) difference in absorption of ice and water are ignored here. Y Axis = Attenuation coefficient. This is a measure of the amount of energy lost (absorbed and scattered) as it passes through the snow/water. X Axis = Wavelength in nm’s (billionth of a meter).


As you can see the line is lowest at the 418nm mark, so the true colour of snow is violet and blue. The chart shows less blue than violet being absorbed however, as there is a greater amount of blue in the spectrum, this shifts the output colour to the right (in the chart). See appendix – why is snow blue and not violet?
   

As we have seen one result of both Flat-Light and a White-Out is that sunlight is reflected and scattered by either liquid (droplets) or solid water (snow). The reflected light continues on, hitting more H2O and becomes bluer after each interaction. As this process continues, possibly billions of times, the overall effect will be that an entire area is bathed in blue light. The more pronounced the cause (thicker cloud, deeper ice, more falling snow, etc) the more blue will be noticed.

Of course, normally, snow will appear to be white. This is simple due to light only being reflected from the top layers of the snow-pack which hasn’t been reflected enough times for any colour to overpower the others.

So, if your kids ever ask you why ice and the ocean are blue, you could explain it by saying “ sun light is made up of all the colours of the rainbow and when the sun shines onto ice or the sea all the different colours go in. The ice and water soak up all the colours except blue which is reflected and scattered about in all directions. Eventually this blue light comes back out and into your eyes. This is why ice and the sea are blue” – But don’t tell them this is also the reason the sky is blue, or you’ll look like an idiot when they learn the truth at school.

No lenses will increase the amount of available light, but some will optimise what there is. So we need to use goggles with lenses which will remove the excess blue which is overpowering all the other colours. This will bring the reflected light back (as far as possible) to normal and in doing so will improve the clarity of whatever it is we’re looking at.

To understand how this can be achieved we need to go (just a little) into the theory of colour itself.

There are two “types” of colours, Additive and Subtractive. Each type has three primary colours – that is a colour that cannot be made by adding different colours together.

Fig 6.
Subtractive colours are made by removing certain colours with, for example coloured filters or lenses, or by adding pigments such as ink or paint. A piece of white paper is white as all the colours in the spectrum are reflected and mixing together. If blue ink is added, you will see blue as it will absorb all other colours except blue. So you are “subtracting” from the spectrum.

Red is red as all colours, other than yellow and magenta have been absorbed. Yellow is yellow as all colours other than yellow have been absorbed. Adding all colours equally produces black.

Fig 7.
Additive colours are formed by adding different colours of light together, e.g. if you shine a green and blue light, in equal amounts, onto a screen you will create the secondary colour of cyan.

Red is red as it emits red light. Yellow is yellow as there is an equal amount of red and green light. Adding all colours equally produces white. We see something we seeing light of various colours reflected from it. As stated additive colour are formed by mixed light, so the colours we see are “additive”.

Fig 8.
For example, leafs on a tree are green as they have absorbed all other colours more than green, which is reflected. The bark of a tree is brown as it has absorbed all colours more than brown, which is reflected.

Fig 10.
As we have seen there are three Additive primary colours, Red, Green and Blue. These, when mixed together in equal amounts, make what are known as the secondary colours (Triangles)


Fig 11.
If primary colours are mixed in unequal quantities, for example 75% to 25% more colours, known as Tertiary colours are created.

In this graphic Circles are Tertiary colours. It is possible by mixing the R, G and B primary colours with different amounts of each to create up to 16,777,216 different colours.


The position of the colour on the wheel is important.
The colours that are opposite each other on the wheel are known as Complementary colours, for example Red and Cyan.

To recap: the light we see from reflected objects is additive. To remove the extra blue light caused by Flat-Light and a White-Out we need something that will subtract this excess and return the view to as normal as possible. In fact we need something that will absorb blue whilst allowing other colours to reflect.

Fig 12.
It is relatively simply to understand that a fully red object will selectively absorb all colours “except” red, which will be reflected, a green object will reflect green, and a blue object will reflect blue. Don’t forget that any object will selectively absorb its complementary colour – that is the colour directly opposite it on the wheel in Fig. 10. For example, on the wheel red is opposite cyan which is a mixture of blue and green, so these colours are absorbed and whatever is not absorbed (red) is reflected.
Fig 13.
It takes slightly more thought to work out why a cyan object will selectively absorb all red wavelengths and reflect green and blue. It must be remembered that cyan is actually a mixture of blue and green, so these are the colours which will be reflected and all others (such as red) will be selectively absorbed.


THIS IS HOW SKI GOGGLE LENSES WORK

Fig 14.
The lens is actually a clear material coloured with pigment which contains coloured particles (objects) and these selectively absorb colours. Any colour that is not absorbed will be transmitted through the lens.


If we were simply to look at the physics of light absorption by snow we would expect it to have a colour of somewhere between violet and blue:


In reality the colour of snow is much lighter: The appendix explains how and why this colour perception changes.



When choosing the colour of our lens we need the complementary colour of what we actually see, not what a spectrographic analysis (see Fig. 5) says it should be.

We need a lens that is complementary to somewhere around light blue. As “light-blue” is non-specific we need to use a system known as a Split-Complementary Colour – this uses a broader range of colours as it includes the Secondary and Tertiary colours either side of the main colour. As the actual light reflected from snow contains mainly white light we need to lighten the blue towards the cyan and the Split-Complementary Colour of this will (in most cases) be our ideal lens colour.

Fig 15.




So the ideal lens for Flat light and a White-out is…..(drum roll please)






I have to add just a little about Visible Light Transmission (VTL)

Of course if we had a lens that was completely coloured we wouldn’t be able to see anything at all. The amount of light transmitted though a lens is shown as the VLT, this is a percentage of the available light. The higher the percentage, the more light is allowed through, so a lens with a VLT of 94% will allow through most of the sunlight as it only blocks 6%. A lens with a VLT of 60% will block 40%, etc.

I have selected these Oakley O Frame goggles simply as they were the first image produced by an internet search. The Oakley website gives the following information: Lens colour = Persimmon & VTL = 55

Using this information we can find out which colours will be absorbed and what percentage of the remaining will be transmitted through the lens.

Lens Colour Persimmon
(or to you and me) RGB = 255 / 99 / 77
So we can calculate that the blocked colours
will be RGB = 0 / 156 / 178
   
These RGB ratios and VTL tell us that these lenses would work well in Flat-Light or a White-Out.





Appendix – why is snow blue, and not violet?

Snow is blue for the same reason the sky is blue. I have to add (before anyone else does) that the processes are different, but this doesn’t alter the fact that the reason (we see blue) is the same.

The simplest explanation would be that the sky is blue as violet and blue light scatters more off the gas in the air than the other colours, and snow is blue as water absorbs less violet and blue than other colours. This still doesn’t explain why they are blue and not violet: the answer is that we see with our brain, not our eyes.

At the back of our eyes are cells called Rods which are sensitive to hues (light dark, etc) and Cones which are sensitive to colour, these cells send signals to our brain which we interpret as colour.

Each of the Cones respond to a broad range of overlapping colours although they have a peak response at either Yellow, Green or (approx.) Blue. As they respond to a range of colours some colours cause the same signal to be sent to the brain. As an example if a yellow light is shone into our eyes our brain will receive the same signal as that for an equal mixture of red and green light and we will see yellow in both cases.

So our brain can be confused by the colour of the light our eyes receive. This is just what happens with the sky and snow. Our eyes receive Violet and blue light plus white (all the other colours mixed together) and the signal sent to our brain is the same as that of light blue.