LCD Projection or High Temperature Poly-Silicon (HTPS)

Technology Overview & Description

HTPS is an active matrix transmissive LCD. It's advantage over LCDs using other systems is that it is smaller, has higher resolution and higher contrast, and can embed drivers.

A Thin Film Transistor (TFT) display is used for the light valves. Light from an ultrahigh pressure mercury (or other) lamp is split into red, green, and blue using a special mirror called a dichroic mirror, which passes light with a certain wavelength while reflecting a specific wavelength. After the image is created by LCDs for each color it is rejoined with a prism and projected. Light usability is better in a 3-Light Valve System than a 1-Light Valve System as you can achieve greater color depth and contrast. This transmission method is utilized in both LCD front projection and rear projection systems.

LCD projection technology continues to dominate the low-end projector market and was almost exclusive to the office projector market until last year. The portability and brightness of LCD projectors have made them a popular choice for portable presentations. The lightest LCD projectors weigh-in at less than 5 lbs.

What's Next

In HTPS microdisplays, the aperture ratio is defined as the ratio between the bright pixel area and the pixel area that is blocked by the transistor required to drive each pixel. The aperture ratio must be maintained in order to preserve display brightness. Due to technical difficultly of shrinking the intra-pixel transistors, HTPS pixels cannot be made smaller without lowering brightness and reducing picture quality. A lower aperture ratio worsens the pronounced "screen door effect" often seen with LCD technology. Adding more pixels without shrinking each pixel maintains the aperture ratio, resulting in an enlarged panel and added cost.

Newer LCD projectors include special optics enhancers like micro-lens array that minimize pixelization known as the "screen door effect." and have contrast ratios as high as 1500:1, though as we have found during our measurements, contrast ratio numbers are largely just marketing gimmicks (we try to provide fairly accurate "real-world" measurements on any displays we review). Look for increased contrast ratios and better black levels with higher generation LCDs. A lot of companies are behind this technology so it won't be sitting idle.

LCD Projection Advantages

• Excellent color reproduction
• Relatively inexpensive
• Lightweight
• Only 12-16" depth required for RPTVs
• Newer units sporting better contrast levels and deeper blacks

LCD Projection Disadvantages

• Perceived "screen door" effect common in lower cost front projectors
• Lamp life (2000-3000 hours typical)

 

Sony KDF-70XBR950 LCD Rear-Projection Television (left); Yamaha DPX-500 LCD Projector (right)