By Marcus Wilson 26/08/2020

One of the top challenges for physics in the modern era, along with Climate Change and explaining Dark Energy, has to be fixing the problem of bad light*.

(I’m talking cricket – what else?)

It’s a quintessentially English problem. It’s not raining, the pitch is perfectly playable, the spectators (COVID-19 notwithstanding) are enjoying themselves, but the umpires bring the teams off because they deem the conditions unplayable. So the second test between England and Pakistan manages only a day’s play out of five – partly because of rain but also because of Bad Light.

It’s frustrating, but at a professional level it really is a safety issue as much as one of fairness. I’ve done my share of amateur 20-over-a-side cricket games squashed into that short period between finishing work and sunset, and, late in summer particularly, the last few overs become hairy. It’s no fun to watch a bowler run towards you, deliver the ball, and then fail to see it. (“Was that the ball that just fizzed past my outside edge?”). None of the bowlers I’ve ever faced could really be called quick by professional standards, but they were scary enough in poor light conditions. It’s not just the batsmen that are troubled. Imagine standing at square-leg and seeing the batsman shape a pull shot in your direction, and having no idea where the ball has gone. Remember this is all at very amateur level. Professional cricketers bowl faster and batsmen can hit the ball much much harder.

So why is it hard to see in ‘bad light’? What is ‘bad light’ anyway?

A familiar measure of vision is ‘visual acuity’. Basically, it’s a measure of the smallest angular separation you can discern. The larger the visual acuity, the smaller the letters you can read on a chart a given distance away. This is what the ‘Snellen Chart’ (the classic vision chart with letters of different sizes) does in a simple way – the higher your visual acuity, the smaller the letters you can read on the chart, or the further away you can be to read a given row. ‘Good’ vision, or 6/6 vision (or 20/20 if you are from the US, with 6 metres about equal to 20 feet) is given a visual acuity of 1.0. Higher numbers (e.g. 6/3 = 2) mean excellent vision; you can discern at 6 metres letters that someone with 6/6 vision could discern at 3 metres.  Lower numbers (e.g. 6/12) are poorer performance.

Now, visual acuity depends on the background light level. You’ll experience this when you are reading during the late afternoon. As the light fades, you find it more and more difficult to read the text in front of you. Eventually you are prompted to get up and switch on a light, and suddenly, you can see what you are reading again. Visual acuity also depends on age, which is why Grandpa is always wanting to have the lights on in his house when you think it’s bright enough without them. The following graph, from the article by Kalloniatis and Luu, here, shows visual acuity (1.0 is equivalent to 6/6 or 20/20 vision – higher numbers are ‘better’) as a function of background lighting level (‘luminance’).

Luminance is a measure of how bright the background is – basically how much visible light is emitted from a background. Formally, it’s the power carried by the light, weighted by the spectral response of the eye, per unit area of background, per solid-angle. That’s a mouthful, but basically it’s a linear measure of how much visible light is being emitted by a piece of your background. In the case of an English summer day, that’s the cloudy sky.  Note the logarithmic scale on the x-axis of the above graph – one unit to the left is a reduction in luminance by a factor 10. The eye is amazing in its ability to function in a huge range of different lighting levels. But as the light level reduces, the acuity reduces too.  That means there will come a point when it’s easy to miss seeing that cricket ball, particularly if you on the boundary away from the action.

Another feature to think about is the contrast between the thing you are looking for and the background. Visual acuity depends on contrast too. Think about that Snellen Chart, but instead of having black letters on white, have grey letters on white. Not so easy to read. Now reduce that contrast and have very light grey letters on white. You are going to need to get closer to read it – in other words visual acuity has reduced.

Basically, this is why floodlit cricket is played with a white ball. The light reflecting off the white ball gives the ball a high contrast with the much darker background (the black sightscreen or the distant spectators). Using a red ball wouldn’t be so good – the red ball reflects less light and appears in less contrast against a dark background. Moreover, just turning the floodlights on when ‘bad light stops play’ means that the batsmen sees a fairly dark ball against a fairly dark sightscreen (the white sightscreen not reflecting much light because it isn’t illuminated) which doesn’t really work. So ‘just turning the floodlights on’ isn’t really an option. In day-night tests, a pink ball gives a better option for visibility, though its aerodynamics and wear are a bit different to both the red and the white options.

All this means, then, is that “Bad Light Stopped Play” is a challenge that’s not likely to go away, unless one just resorts to using pink balls.

*No, I don’t really think that this challenge is as important as Climate Change, in case you are worried about my priorities.

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