Reflection and contrast

	Overview   |   Screen orientation   |   Contrast   |   Contrast-enhancement filters   |   Other contrast-enhancement devices   |   Contrast Specification   |   Character size   |   Character size measurement

Contrast-enhancement filters
The two luminances used in determining contrast in an actual workplace are:

l. The reflected room light from the screen.

2. That reflected room light plus the light generated by the phosphor.

Generally, the reflected room light would be too large a value if a clear (transparent) glass screen were used on the CRT. The darkest gray achievable would simply be too light a gray for good contrast. The reason a movie theater is darkened is that the "black" level of the movie image can only be as black as the reflected room light allows. Since a workplace cannot be darkened if paper documents and a CRT image are to be used, the screen is made of glass that is tinted. It is characterized by its transmittance, T.

In the case where the screen is opaque, T = 0. In the case where the screen is completely transparent, T = 1. When the value of T is explicitly stated, the two luminances become:

l. T² x the reflected room light from the screen.

2. T² x the reflected room light from the screen + T x the light from the phosphor.

In a first example, namely in the clear glass case, where T = 1, and where the light from the phosphor is assumed to be 200 cd/m², and where the room light reflected from the screen is 75 cd/m², the result is: (75 + 200) cm/m².

In a second example, where everything is the same except that the glass transmissivity (T) = 0.5, the new value for light reflected from the screen is 0.25 x 75 cd/m² = 18.75 cd/m². The new value for the second luminance (i.e., where the phosphor light is assumed to be 200 cd/m²) thus becomes:

18.75 + 0.5 x 200 = 118.75 cd/m²

The filter, in that second case, improves the contrast ratio from about 3.6:1 to about 6.3:1. In most office illumination environments, this built-in property is sufficient. The user should exercise good judgment, placing the VDT perpendicular to windows, avoiding specular reflections by adjusting the tilt of the screen and using task lighting on paper documents if necessary.

Some display terminals use additional filters or contrast improving techniques over the filter already in the screen. They may use additional neutral density filters or selective filters that reduce some wavelengths more than others, or use some wavelength mechanical baffling device to absorb the ambient light.

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