GUIDE TO DISPLAY TECHNOLOGIES
Sensors serve a variety of functions in medical equipment, such as monitoring patient vital signs like blood pressure and the oxygen content of blood, regulating the flow of drugs to a patient, measuring a patient's body temperature, or noninvasively creating images of internal organs. Medical equipment would be useless if the information from the sensors could not be read and used by medical professionals or home users. Electronic displays provide the vital link between the equipment and the human beings involved allowing the appropriate action to be taken. In a medical environment, a quick and accurate reading of the display is critical to ensuring that the best course of action is taken as quickly as possible. When a piece of equipment malfunctions or does not operate as designed, an error occurs. However, errors can also occur if the data from the equipment are misread.
Glare and reflection increase the time required for reading and can lead to errors in reading information on a display. Glare occurs when light from the surrounding environment reflects off the display screen. Reflection comprises two optical phenomena: specular reflection and diffuse reflection. Specular reflection produces a virtual image (i.e., like a mirror) of the source. Diffuse reflection scatters light out of the specular direction.
Diffuse reflection can be separated into Lambertian and haze reflection. Lambertian is the sum of reflections in all directions, while haze reflection is the intermediate state between specular and Lambertian. The luminance of haze is proportional to the illuminance from the source, while specular reflection is proportional to the luminance of the source.
Illuminance, measured in units of lux, characterizes the amount of visible light striking an object. Conversely, luminance is the amount of light generated from a source, such as a display, and is typically measured in candelas or nits. Ambient light can lead to luminance in the form of glare when it strikes a display and is reflected, thus washing out the display. This occurs because light reflected from the display surface increases the luminance of the dark state, thereby reducing contrast, or the ratio of the white state to the dark state reflectance, and reducing the eye's ability to easily distinguish between light and dark.
Since it acts as a source of light itself, the display can cause glare. If the display luminance is not properly adjusted, reflections from sunlight or room lighting can seriously impair a display's readability. Optimally, a display should be the same brightness as the light surrounding it, and the surrounding light should be darker than the whitest white on the display. In addition to causing glare, if a display's intensity is set too high, its life will be decreased.
Figure 1. Automatic display backlight brightness control.
(click to enlarge)
In liquid-crystal display (LCD) monitors, lamp life depends strongly on the current, which is directly proportional to display intensity. Decreasing display intensity when ambient lighting is lower increases the life of a display. Since ambient lighting conditions constantly change, display lighting must be able to change automatically with ambient lighting changes (see Figure 1). The advent of digital display technologies such as LCDs has made possible greater intensity control and color calibration of monitors.
Ambient-light sensing provides the ability to automatically control the brightness and optimize a display's characteristics under various lighting conditions to reduce eyestrain and maximize visual performance. Ambient-light sensing requires measuring environmental light as the eye sees the light. The most common device for measuring light intensity is a silicon photodiode. Photodiodes have a broadband response, making them sensitive to both visible light and near-infrared (IR) radiation, and they cannot distinguish between the two. Sunlight and artificial light contain both near-IR radiation and visible light. Therefore, a single photodiode will not accurately measure light the way the human eye perceives it, since a broadband photodiode cannot separate the visible light from the near-IR in the measurement.
Combining a visible-light-blocking photodiode with a broadband photodiode on a single complementary metal-oxide semiconductor (CMOS) integrated circuit can solve the problem of accurately measuring light as the human eye perceives it. This combination allows two simultaneous measurements to be taken: one with visible light plus near-IR radiation, and one with only near-IR radiation. The output data from the two channels can be input to a microprocessor where illuminance (i.e., ambient-light level) in lux is derived using an integrating conversion technique that requires less than 200 bytes of code.
This approach has the added benefit of removing the effects on the measurement of flicker in fluorescent lights. Although it is too fast for the human eye to see, a 120-Hz flicker is present in all fluorescent lighting. The integrating conversion technique eliminates the effect of fluorescent flicker, thereby increasing the stability of the measurement. Devices are available that can measure light levels ranging from 0.1 to 40,000 lx. No external circuitry is required for signal conditioning, which also saves PCB real estate. A variety of package options are commercially available with sizes as small as 1.25 X 1.75 mm.
Color sensors allow manufacturers to improve display quality for equipment such as MRIs, where accuracy of information depends on true color reproduction on the monitor. RGB-backlight control requires accurate measurement of the relative intensities of red, green, and blue LED backlights, in addition to matching the overall intensity of the display to ambient lighting conditions. The light sensor must also be able to sense what type of ambient light is present, since different light affects the appearance of the color on the display differently. Red colors need to be intensified in fluorescent light to offset the blue cast that is rendered. Conversely, blue colors should be intensified in incandescent light to offset the red-yellow cast. Color sensors can also be used to calibrate the monitor for true color rendering.
Displays, therefore, provide a vital link between medical equipment and patients. However, to maximize efficiency and accuracy, display characteristics must be adjusted to the ambient lighting conditions. As a result, ambient-light sensing is a necessary part of optimizing the displays used in medical equipment.