A scanning backlight is a technique used by LCD HDTV manufacturers to reduce motion blur, which LCD HDTVs are susceptible to, when displaying fast on-screen motion. Motion blur is not a problem with plasma TVs.

Motion blur can be described as the streaking of a rapidly moving object across the LCD display. It can be due to multiple causes, singularly or in plurally, including; slow liquid crystal response time, the sample-and-hold nature of LCD displays (retinal blurring), non-linear motion across the screen and frame rate conversions (pulldown).

An important factor for the human eye’s perception of continuous motion is for image frames to have breaks in between with an interruption of constant light. If this lacking the eye may interpret the motion as being blurred. Unfortunately, with LCD displays sample-and-hold technique, each frame being lit for the full time required for the frame rate frequency followed by the liquid crystals cycling quickly for the next frame which is illuminated immediately.

This lack of a sizeable break in the lighting is the reason LCD manufacturers use black frame insertion or backlight scanning to do it artificially. While back frame insertion is simple to envisage, have a look at the image below to appreciate the backlight scanning process. The image illustrates how the LCD backlighting is synchronised to more down the screen as the image is written to an LCD display panel a line at a time, from top to bottom. The resulting video allows better perception of on-screen motion by the human eye with less motion blur.

Scanning backlight technology is, or has, been used by LG, Toshiba, Samsung, Philips and Vizio. Often it is used in conjunction with frame interpolation (MEMC) to produce a 240Hz refresh rate. Frame interpolation using MEMC is used to achieve 120Hz and a scanning backlight is used to achieve a 240Hz effect. The image above shows this combination as used in Sony’s MotionFlowPRO. By using a combination of approaches to enhance the human eye’s perception of motion, one might expect it to be more effective than using one method alone. There is no know evidence known to back this as yet, however.

Resources

Philips brochure advertising Aptura backlighting

Dynamic-Scanning Backlighting Makes LCD TV Come Alive