Machine Design

Light Filters

Many displays are difficult to read when viewed in sunlight. Fortunately, optical filters offer a low-cost solution to this problem.

To specify a filter, the lighting conditions a display will be used in and how the eye responds to these conditions must be known. While different types of displays (for instance, CRTs and LEDs) generally have vastly different design considerations, there are common points. In general, a filter should maximize the contrast between display and background. In high ambient light, both display brightness and color are important.

Color is important because the eye is more sensitive to some colors than others. Therefore, even though two light sources may have the same radiant flux, one may appear brighter than another. Eye sensitivity peaks at a wavelength of about 555 nm, which is a yellow-green color. As a result, it is difficult to produce high contrast for green displays through filtering.

Color contrast is usually described by two quantities: chromatic distance and chrominance index. These characteristics are determined with a chromaticity diagram. Data points for both the background color and the color of the emitted light are plotted on the diagram. The theoretical chromatic distance between the points is then calculated with an equation that takes the mixing of emitted and reflected light into account.

The smallest discernible color difference is a dimensionless number called threshold chrominance, which has a value of about 0.00384. However, to easily differentiate between two colors, a difference of about 0.027 is recommended. This value is called the unitary color difference.

Chrominance index (IDC) is the ratio of the display's chrominance difference to the unitary color difference. Knowledge of a display's IDC is important for two reasons. First, an IDC of one implies that the display's color difference is just large enough to be easily recognized. Secondly, the IDC, in combination with a similar index for brightness, is a measure of visibility in high ambient-light conditions.

Luminance contrast compares the display brightness with its background. For LEDs, display brightness includes the ambient light reflected off the element itself. For a filtered LED display, a contrast ratio, called the luminance contrast ratio Cr can be defined as:

where LvS = sterance (intensity/unit area) of the illuminated element through the filter, (Lv)off = sterance of the light reflected off the element through the filter, LvB = sterance of the light reflected off the background through the filter, and L->vF-> = sterance of light reflected off the filter.

As Cr approaches one, the display becomes increasingly difficult to read because the reflected ambient light washes out the display's light. The smallest Cr the eye can see is about 1.05. For monochrome displays, a minimum of 1.4 is recommended. However, because the eye responds to light intensity logarithmically, a quantity called the luminance difference, EL has been defined as EL = log Cr.

The EL for comfortable viewing (0.15) is called the unitary EL. The EL for the minimum luminance contrast ratio (0.021) is the threshold EL and is about seven times less than the unitary EL. A comparison of a display's EL and unitary EL, called the luminance index (IDL), is often made. The IDL, like the chrominance index, is a measure of how easily the display can be seen. An IDL of one is the minimum for comfortable viewing.

An overall measure of a display's sunlight viewability is its discrimination index (ID), which is the square root of the sum of the squares of IDL and IDC. A three-dimensional representation of ID can be gained by using photocolorimetric space. Photocolorimetric space is defined in the horizontal plane by the chromaticity system (chromaticity diagram). The luminance index, IDL, forms the vertical axis. To determine ID, the display's background and emitted color intensities are plotted. ID is the distance between these two points.

Reflected light: Reflected light usually has both diffuse and specular components. Diffuse reflections are scattered light that appears equally bright regardless of viewing angle. The uniform intensity results from a nearly Lambertian radiation pattern. For specular reflections, light is reflected at an angle equal to its angle of incidence.

The effect of reflected light is important for two reasons. First, if reflectance is not taken into account, artificially high display contrast ratios may be calculated. In addition, reflections decrease the actual chromatic distance between the background and illuminated elements. One way to diminish specular reflections is with a polarizing filter.

A widely used filter is a circular polarizer which consists of a linear polarizer and a quarter-wave plate. The filter is oriented so that sunlight first strikes the polarizer and then passes through the quarter-wave plate. The quarter-wave plate circularly polarizes the light in one direction. When the light reflects off the display, its direction of polarization is reversed; that is, right-circularly polarized light becomes left-circularly polarized light. Then, when the light passes through the quarter-wave plate, it again becomes linearly polarized, but at 90° to the linear polarizer. As a result, the polarizer absorbs the light.

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