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.