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Supersize Your TV for $300: Build Your Own XGA Projector!

Gray Theory: Brightness, Contrast, Color Temperature

At this point, let's take a look at some technical details of LCDs, including the most important parameters: contrast, brightness and color temperature. The contrast value defines the ratio between the darkest and the lightest signal level that a projector can display. For example: if 2 ANSI lumens of ambient light remain in a completely black area with a maximum luminous efficiency of 1,500 ANSI lumens, the contrast ratio is 750:1 (1500/2). In general, the higher the contrast ratio, the better the display quality.

This one number is not enough to determine quality, however. Essential to an exact reproduction on the screen is a high range of color values between the extremes of black and white. If this is not the case, the result is a faint and murky picture despite a high contrast ratio. This issue particularly plagues certain projectors in the low price range (up to $1,000), yet manufacturers still insist on citing high contrast values on their packaging.

If the brochure is to be believed, our Liesegang projector has a luminous efficiency of 3,500 ANSI lumens. This brightness is sufficient for screens measuring up to 10 feet from corner to corner. Black, however, appears as a light gray on screens this size.

And what about the lamp brightness? A luminous efficiency of 3,000 to 4,000 ANSI lumens should form the basis for a homemade projector. Against a suitable background and with good illumination, this means that screens of up to 8 to 10 feet can be projected. On the other hand, at higher luminous efficiencies, bright scenes on the wall will reflect a lot of light into the room.

Color temperature is also an important factor. A value of 6500K is standard for video films. However, the light source is decisive in determining the color spectrum that a projector is able to achieve. Values between 5500K and 6000K are acceptable; anything above 7000K and the image becomes heavily overdrawn. Metal halide and xenon lamps produce a superb color spectrum, and are in fact a prerequisite for natural color display.

  • Hi,
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    Reply
  • just looking into this idea and was just wondering what the implications would be to have the lcd screen supported away from the glass of the projector thus aiding the cooling of the screen ?? Say a 2 to 3cm gap? would this affect the picture quality or would the screen still need the same amount of cooling as it does on the screen. As you can tell by the questions i havn't made one but seriously thinking about it. also ... !! woulld there be an option of other video inputs to the screen ie .. Phono jack rather than laptop / pc?
    Cheers for the info if you respond.
    Reply
  • chinmayj007
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    Reply