Monday, November 1, 2004
System Design: The Struggle For Sunlight Readability
Almost every avionics designer encounters that one moment when a whole project seems destined to fall off the design cliff because of sunlight readability issues. The indicators can't be clearly seen. The display washes out. The legends disappear in reflected glare. And because display imagery is non-discernible, user feedback is terrible.
Everybody struggles with the problem, and all available solutions are difficult. So if you are a designer having trouble, you're not alone.
Brightly lit environments have a profound physiological effect on a person's vision. High-intensity light can easily overload the eye, so the iris closes to reduce light input. Intense light also creates some dynamic range and contrast loss. Add to this visual situation a pair of sunglasses -- what pilot doesn't wear them? And you find your equipment is being viewed by eyes that are far less able to detect or discern detail than if they were in a comfortable office environment.
This is why so many designs go wrong on the bench. The conditions in the cockpit are simply too different to make objective judgments.
The problem is a matter of physics. The sun is a powerful, broadband, all-angle optical emitter, and all surfaces reflect, pass and absorb some optical energy. The front surface of your system has a glare coefficient that generally is expressed as a percentage of the front ambient light returned to an observer. (A mirror approaches 100 percent glare coefficient, and anti-reflective surfaces are under 1 percent.)
If the reflected light is greater than your display's emitted light, the display is washed out in sunlight. The display still is there, but the visual contrast has dropped to zero, so your important message is now unreadable.
Another problem is unwanted background illumination, which is common with text-style light-emitting diode (LED) displays. The display's inner surface itself illuminates from incoming sunlight, and the result is a low-contrast display, even if front surface glare is not a serious issue. This problem can be difficult to control, and some displays have many visible traces, making the problem much worse. Yellow/amber displays suffer most from this problem, and blue and deep green displays suffer least, due to a combination of contrast, filter and sunlight factors.
Small legend indicators and illuminated switches often have small text areas. And even if their background illumination is strong, little light energy escapes from the transilluminated, or backlit, text. This internal light has to compete with significant sunlight reflection from panel and matte switch surfaces. LED illumination, if too far back from the front surface, also could cause a problem, in which the exiting light will be too parallel in alignment and not provide good side-angle visibility.
Sunlight is a powerful heater, too. It is loaded with long infrared (IR) radiation and raises the temperature of all panel surfaces and closely mounted components. This heat rapidly drops the emission levels of devices like LEDs. Their emitted light therefore is seriously reduced, typically by as much as 30 percent from the heat effect, alone -- a frustrating situation.
What is the best way to attack visibility problems? First, you have to understand that reflected solar glare is your worst enemy and accept that some surface treatment will be required to overcome it. Second, you need to design for maximum contrast in displays and panel markings. And finally, you have to launch enough optical energy to overcome the ambient light from the sun and the consequent reduction of the eye's sensitivity.
The overwhelming majority of off-the-shelf LED indicators are not suitable in a daylight environment. A study of many indicators shows terrible optical energy ratings, often 2 to 10 millicandellas (mcds) in a diffused indicator. Although a display may seem bright on a workbench, this level of optical energy simply isn't sufficient to be useful in a cockpit.
LED bar graphs typically are only 2 mcds per segment, and many 7-segment or -pixel display LED displays are fewer than 2 mcds per segment. Generally, for a surface to be readable in sunlight, 50 to 200 mcds are required, depending on surface treatment and object size. More optical energy is never wasted.
A good, easily recreated bench standard for sunlight does not exist. However, a useful simulation is to take a common flexible bench task lamp with a 60-watt, daylight-rated incandescent lamp, and shine it on your device from 1 foot (0.3 m) away. Move the lamp to find the worst angle (above the device would be appropriate, since sunlight usually comes from that direction). Create the most glare and you have a fairly good simulation of daylight, representing most locations. To simulate sunlight in Phoenix or Dallas on a hot summer day, use two 60-watt lamps, one on the left and right sides. This also simulates heat loading quite well, so you get two useful tests in one.
Assuming you see your display vanish during this test, what can you do? Many firms produce high-efficiency anti-glare (HEA) coatings, usually on glass. Consumer products with liquid crystal displays (LCDs), from laptops to cell phones, have this filter bonded to the display to improve contrast and sunlight visibility. A typical vendor is OCLI, and you can see data on HEA material by visiting www.ocli.com/products/hea_products.html. This type of glass coating can reduce the glare coefficient to as low as 0.3 percent, which is essential for large electronic flight instrument systems (EFIS) and moving map displays.
If the display light is equal to the reflected solar glare at 3 percent reflection, the display will be invisible. If you add a display filter and reduce the solar glare to 1 percent, the display will appear three times brighter. Reducing glare via an HEA filter to 0.3 percent makes the display appear 10 times brighter. It improves contrast by the same ratio, a tremendous achievement. All too often the sunlight readability solution is not brighter displays, but simply better filters.
Plastic is found to be more suitable than glass for many cockpit surfaces. For rear-illuminated overlays, matte-finish polycarbonate or Lexan often can be a successful low-cost panel treatment, and both are easily fabricated as a transilluminated overlay. Since the matte surface will create some inescapable glare, this technique requires bright LEDs or lamps.
Most bright LEDs provide a narrow angle of illumination--often as small as 7.5 degrees--which would not be suitable as a panel indicator. Your system will need to fit into that target 50- to 200-mcd energy range with a wide-angle (up to 100-degree) diffused indicator, but without driving the LED with too much current. That can be hard to source and achieve.
Your system must be well within the LED's temperature curve at your drive current, so that you are not pushed into the self-defeating, reduced emission area. Flood illuminators, or LED arrays, for legends need the highest possible emission, typically 200 mcds and up, as the area is so large.
These are just a few of the ways to make sure your avionics project doesn't fall off the design cliff. In my next column, we will discuss the use of filters, polarizers and display coatings, as well as backlighting for liquid crystal displays.
Walter Shawlee 2 may be reached by e-mail at firstname.lastname@example.org.