Light Pollution Abatement Site Calgary Centre Royal Astronomical Society of Canada

The Eye and Nighttime Lighting

At night, the human eye can discern details in a scene if the range in brightness varies by less than a factor of about 10. Our ability to see at night depends strongly on the distribution of light sources, the range in their brightness and their colours.

Good nighttime lighting works with the physiology of the eye, not against it. To design effective nighttime lighting, it is important to know about:

• the distribution of the two type of light receptor cells in the eye (cones and rods),
• the ability of the iris and the rod and cone cells to compensate for changes in illumination levels,
• the spectral sensitivity of the two types of receptor cells in the eye,
• the ability of the eye's lens to form clear images due to defects such as astigmatism, chromatic aberration, and clouding with age.

Cones, Rods and Glare

The human retina has a central fovea centralis, where cone cell density is about 125,000 per square millimetre. Cone cells are responsible for our colour vision (photopic vision) and provide a high-resolution view of the world. Outward from the fovea, cone cell density falls off and rod cells dominate. Rod cells provide us with a lower resolution view of the world and only in grey and white. This is our peripheral vision. Rod cells are, however, very efficient in low light and give us our night vision (scotopic vision). During twilight, and under moonlight and good street lighting, we use both rods and cones, or a mixture of photopic and scotopic vision; this is called mesopic vision.

The distribution of light-receptive cells in our eyes dictates what part of a scene we perceive in high-resolution colour and what parts are merely "ghosted-in" shades of greys.

The fovea is only about 0.25 mm across and corresponds to an angular dimension of about 1 degree (twice the diameter of the Moon). It is centred in macula lutea, which has a 4 degree size. The rods occur in greatest abundance some 20 degrees from the fovea. What this translates to is a drop off in visual acuity from the fovea to the periphery of the retina; at 10 degrees from the fovea, acuity is only 25 % and at 40 degrees, it's just 3 %!

Let's look at an example of how this affects us in everyday life. In the front row at a movie theatre, movie-goers need to constantly shift their gaze to take in all parts of the screen. With a very large screen they may even need to turn their heads. Now picture what it's like in the back row; the entire screen may be comfortably viewed without shifting your gaze (but details may be hard to see since they are now very small in angular size, that is, they look small to you). The same movie at home on a small television is viewed almost entirely on the fovea and macula of the retina. Your peripheral vision only reminds you that you need a larger television or that you should have seen this movie on the big screen where your peripheral vision helps create the illusion that you are part of the scene.

How effective is our peripheral vision in filling in a scene? We can estimate this by comparing we view with photographs of the same scene taken with a standard 35 mm camera. "Normal" lenses on a camera of this format have a focal length of 40 to 55 mm, with 50 mm being the most popular focal length. Shorter focal lengths such as 28 mm are deemed to be wide-angle lenses and capture a view that is felt to be wider than normal human vision. Telephoto lenses with longer focal lengths magnify a portion of a typical view.

A 50 mm lens translates to a photograph that captures a view approximately 30 degrees tall by about 45 degrees wide (in what would be called landscape mode). This represents 15 degrees above and below and slightly more than 20 degrees on either side of the central portion of the photograph. Held on its side, such a camera takes a picture 45 degrees tall by 30 degrees wide (so-called portrait format). Such portrait format photos strike viewers as unusual, encompassing a greater vertical extent than normally taken in by the eye. This tells us that the imaging part of our peripheral vision gets routinely used up to about 30 to 40 degrees in vertical extent (15 to 20 degrees above and below the central part of the scene); beyond that, our peripheral vision likely contributes more with motion stimuli than imaging.

The movie example above shows us that we depend on both our fovea and peripheral vision to create our view of the world. While driving, the acute colour vision is used to decipher street signs, traffic lights and the colour of vehicles. Imaging peripheral vision fills in the scene to tell us about pedestrians and vehicles in adjacent lanes. Peripheral vision also alerts us to motion such as incoming snowballs or the small child darting out between parked cars.

Now, where do you want to put a light source such as a streetlight relative to the scene we wish to illuminate? Remember, we wish to minimize the negative impact that glare has on night vision, but we still want the roadway lit. The discussion above suggests that placing the light source in the driver's peripheral vision will minimize the negative effects, especially if we get it out of the imaging peripheral vision and thus more than about 20 degrees above the roadway.

How can we accomplish our goal of more than 20 degrees when distant streetlights appear to be shorter because of perspective? Won't they appear in the middle of our view and ruin our plans? Here is where good design comes into play. The semi-cutoff light fixture that for so many years was the standard on city streets shines a significant portion of its light output sideways and even upwards - distant views of these streetlights dominated what drivers could see, and obscured foreground detail with glare. New full cutoff fixtures put out no light above the horizontal and lighting engineers strive to keep most of the light below about 20 degrees from the horizontal. This means, that in order for a driver to see the bright light bulb while concentrating on the roadway, the fixture must be at least 20 degrees above the driver's gaze and hence out of the imaging peripheral vision. Perfect!

The Eye and Changing Illumination Levels

The sensitivity of human vision varies over a brightness range of about 1,000,000. The eye has three mechanisms to compensate for changing illumination levels:

• firstly, the iris changes the size of the pupil to let in more or less light depending on available light,
• secondly, the eye switches from cone cells when there is abundant light to rods which are more effective under dim conditions,
• thirdly, the rod cells change their sensitivity to light according to illumination levels (dark adaptation).

Changing the aperture of the pupil occurs 1 or 2 seconds and there is little negative impact on vision while driving. This method for accommodating illumination changes works well under relatively bright conditions, but only compensates for brightness changes by less than a factor of 100, depending on the person. In total darkness, the pupil's diameter can expand to as much as 8 mm in young people. As the eye ages, the maximum aperture shrinks, attaining only about 5 mm in most middle-aged individuals. This means that the ability for people's eyes to adjust to changes in brightness lessens with age and that the ability to see faint objects also lessens with age.

If brightness levels change sufficiently so that cone cells no longer register the faint light, then rod cells take over. Here, changed illumination levels lead to a change from colour vision to black and white vision, which can be confusing.

It takes rod cells up to 2 hours for total dark adaptation, when they can respond to a single photon, though most of the change occurs over the first 20 to 30 minutes. Cone cells reach total adaptation in about 10 minutes, but, since they have limited sensitivity, they are not as important under extremely dark conditions. Dark adaptation in rod cells occurs because of the slow buildup of the light-sensitive chemical rhodopsin (also known as visual purple) under dark conditions. When we drive past bright light sources (e.g. illuminated structures, signs or car headlights) the changes in illumination take place over periods of a few seconds or less. This means that brightness changes are occurring on a time scale too quick for the eye to compensate with dark adaptation. For this reason, it is important that general illumination levels at night not vary strongly. Typically, the worst situations occur near gas bars and car dealerships who are using bright lights as advertising, rather than illumination for safety.

Since the eye has an effective ability to discern details in a scene if the range in brightness (the contrast) is about 10 to 1 or less, it is important that when driving, important details such as the roadway or parked cars not be masked by bright, glary light bulbs. If unshielded or poorly-aimed lights are in view of drivers, then the lights set the upper brightness range - things 10 times less bright like potholes, trees or pedestrians become featureless. The following are examples where such situations occur.

• Oncoming traffic on a 2 lane country road at night - there's twice as much light on the road between the 2 vehicles, but the glare of oncoming headlights ensures that less detail is seen by either driver than with just their own set of headlights.
• Driving towards the rising or setting Sun. Judicious use of the vehicle's sun visor works here.
• Driving towards bright twilight. Even with headlights and streetlights on, many people comment how difficult it is to see under these conditions and claim to be "night blind*." Here again, proper use of the sun visor can help.
• Driving near overly-lit car dealerships. Phone calls to your alderman may work here.

Shielded, full cutoff and properly aimed lights are generally not part of the driver's view, so the 10:1 brightness range occurs entirely on the ground between cars, trees, pedestrians, etc.

*True night blindness occurs when a deficiency of vitamin A leads to an incomplete buildup of rhodopsin in the eye.

Spectral Sensitivity of Cones and Rods

There are three types of cones in the eye, each is most sensitive to a different part of the spectrum of light. Together, these three cell types give us colour vision.

Rod cells, on the other hand, interpret light purely by brightness level and not by the colour of the photon, giving us a black and white view of the world. It turns out, however, that rod cells are more sensitive to certain parts of the spectrum than to others. Their peak sensitivity is to blue-green photons (507nm); they are almost blind to deep red colours.

For this reason, astronomers commonly use red flashlights during their nighttime work. The red is bright enough for the red-sensitive cones to register it. As long as it is not too bright, it will not affect dark-adaptation and will not impair the dark-adapted rods from seeing in almost total darkness once the flashlight is turned off.

We see these ideas applied by automobile manufacturers with their speedometer lights and colours. Some manufacturers colour their speedometers yellowish green thinking that drivers' will keep dashboard illumination levels low at night so as to not impair their night vision. Such drivers will be calling on their rod cells to help interpret the speedometer. Other manufacturers feel that red speedometers will allow drivers' cone cells to register speedometer information, while allowing the rod cells to remain dark-adapted for driving purposes. Both ideas have merit.

How does this affect the choice of streetlights? Under streetlighting we are typically using mesopic vision, that is a combination of rods and cones. We therefore need light that is most easily detected by mesopic vision, i.e.light that has a strong component in the green-blue part of the spectrum. To avoid light pollution, the total light output should be minimised. The most energy-efficient luminaires for this purpose are the HPS (High Pressure Sodium) ones, whose output is mostly in the orange/yellow and blue parts of the spectrum. If they are properly shielded, so that light is only emitted 20 deg or more below the horizontal, then we can achieve optimum lighting with minimum glare and pollution.

Defects of the Eye

Defects of the eye that can be mitigated by proper nighttime lighting include general cloudiness of the lens and cataracts. Both of these types of defects are more common in older people and both cause bright light sources to scatter light across the retina, obscuring fainter details. By the age of 70, people are twice as afflicted by this problem than when they were younger. By 80, three times as much. Again the technological fix is simple - use shielded light fixtures so that bright light sources do not exist in the driver's field of view.

At night, our eye is most sensitive to greenish-blue wavelengths of light, even though our rod cells interpret these photons in black and white. Our cornea and lens are also optimised for these same wavelengths. In the daytime, our pupil is too small for this effect to be noticeable, but at night, with the pupil widening to 5 mm or more, chromatic aberration becomes important. In a nutshell, our lens not only focuses light, it acts like a small prism, splitting the light into a rainbow. If the eye is focused for yellow light, blue and violet light is focused in front of the retina. This can be seen by the "fuzziness" of blue lights at night. Watch for this effect around police emergency lights, blue or purple Christmas lights (especially outdoors where you are far away from the lights) and blue lit advertising and lit building signs. Similarly, if the eye is focused for yellow light, red light is focused behind the retina. People with hypermetropia (long-sightedness) or presbyopia (a decreased focal range) therefore find it difficult to focus red light.

To minimise the negative effects of chromatic aberration, streetlights and other outdoor lighting should emit most efficiently in the yellow region of the spectrum. Of the choices of lights for nighttime lighting, mercury vapour lighting puts out the most light in the blue and violet part of the spectrum. Both low and high pressure sodium lights are optimal. Other light types fall in between these cases in terms of minimising chromatic aberration effects.