Display Differences

General discussion:

http://sromagazine.biz/mag/killer_projected_images/

High ambient light screen: http://www.screeninnovations.com/ check out the Mirage.

Sony Black Screen: http://cache.gizmodo.com/archives/im…lackscreen.jpg and http://www.gizmodo.com/archives/bett…een-016964.php

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There is a range of brightness that extends from absolute black to retina searing light that no display will be able to recreate, certainly not the monitor you are viewing right now. But for graphical purposes we will use a gradient as shown in Figure 1 to illustrate this range. There is no particular values for the various light levels, just imagine it spanning total darkness to the brightest light imaginable.


Figure 1

Now all displays are not equal in the ability to reproduce this range either. CRTs and plasmas will tend to reproduce the darker end of the range and LCDs will tend to reproduce the brighter end of this range. Generally, front projectors will have a somewhat reduced range. The following figure 2 shows the relative ranges that CRTs, plasmas, LCDs and front projectors generally reproduce. CRTs and plasmas are represented by the red outline, LCD panels by the green outline and front projectors by the blue outline. Even though there are not any particular levels associated with these areas, visualize this as the typical relative output.


Figure 2

As can be seen in figure 2 the CRTs and plasmas extend to the darker area, whereas the LCD panels extend further into the bright area. Front projectors will have a reduced range, but bear in mind the room will have a great deal of influence on how much.

IRE is a measurement of a video signal that defines the range of the video signal where 0 IRE would represent black and 100 IRE represents white. Every display will have different actual amount of light output for the video signal range depending on the display characteristics. Ideally, a display with a 0 IRE signal input would not emit any light and with a signal input of 100 IRE would be as bright as anything known to man. Of course no display will be able to reproduce that ideal range.

Contrast ratio simply stated is the ratio of the light output of a display when at full brilliance signal (100 IRE) and complete black signal (0 IRE) as generally measured by some sort of meter. As shown in figure 2 the different display types can have the same contrast ratios, like the CRTs, plasmas and LCD panels, but also have very different abilities to reproduce pictures. Just looking at figure 2 you can see why the CRTs and plasma displays are said to have better black response and LCD panels are better for brightly lit areas. Also figure 2 shows how front projectors generally will have a reduced range due partly by the maximum light that can be output over very large screens and room lighting or reflected light.

Both the brightness and contrast controls will elongate the range to some degree. The contrast control, more properly called the white level control, will cause the light areas of the picture to lighten as the control is increased. But it will also cause the black areas of the picture to lighten as well, just at a lesser amount. This is illustrated by the areas outlined in magenta in figure 3 where the lower bars represent increasing the control.


Figure 3

The brightness control, more properly called the black level control, will cause the blacks to darken as the control is decreased. But it also will cause the upper end of the range to darken as well, just at a lesser amount. This is illustrated by the areas outlined in magenta in figure 4 where the lower bars represent decreasing the control.


Figure 4

As has been shown, the two controls will interact with each other to some degree and it is necessary to go back and forth between them to get the best overall results.

Now it should be noted that although I have presented the operation of the brightness and contrast controls in the manner that technical people would like you to think of the way the controls function, in the end they will both seem to brighten or darken the picture. You just need to bear in mind that the brightness control will mainly operate in the dark areas and the contrast control will mainly operate in the light areas. Getting them both set properly will allow both the best reproduction your display will allow.

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When a front projector is used as the display there are other factors that have to do with the room that will affect the range of light output thus changing the contrast ratio. Two of these are ambient (or reflective) light and distance of the projector to the screen. These effects are shown in figure 5.


Figure 5

The top half of figure 5 represents the effect of increasing ambient or reflective lighting in the room. As you can see both the darker areas and the lighter areas of the picture will suffer. The reason is pretty simple in that the darkest area can not be any darker than the light reflected off of the projector when the projector is off. As light in the room increases the darker areas of the picture will become lighter and some detail will be lost. Additionally there is a point with the lighter areas of the picture where the room light will overwhelm the light output from the projector and those ares will suffer. That is why light control is very important for front projectors.

Another issue that is many times overlooked by front projector installs is the distance of the projector from the screen. Light output from the projector is constant, but the amount reflected from the screen diminishes as the distance increases. The reason for this is the same amout of lumens have to be spread out over a larger area, thus reducing the overall light reflected in any given area of the screen. Notice that the darker areas of the picture will not be affected as far as the darkness because that is a function of the room lighting, but the lighter areas of the picture will suffer. The net result is a picture that seems to be washed out and does not have the snap of direct view displays.

Some LCD panels will also have a backlight control. Basically LCD panels work by blocking the light supplied by the backlight. Unfortunately the LCD panel can not fully block the light. That’s why LCDs have poorer black response than CRTs or plasmas. By lowering the light output of the backlight less light has to be blocked and the black response will be improved. Of course the brightness in the lighter areas of the picture will also be reduced as well. Generally, backlight is set lower for dimly lit rooms and set higher for brightly lit rooms. The effect of the backlight control is shown in figure 6. As the control is increased the spectrum shifts lighter.


Figure 6

One thing I have noticed is that the black sent by broadcasters is often lighter than the darkest black my LCD display will make. I notice this when the station is sending out a 16:9 picture in a 4:3 frame. The letterbox bars are lighter than the pillerbox bars my display adds.

REV. 1

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