when automotive displays, POS terminals and other devices are used outdoors, they often require brightness levels of 1,000–1,800nits. With TFT panels having a transmittance of only about 5%, the backlight must emit more than a watt of optical power. Simply adding more LED chips is no longer feasible: there is no room for extra LEDs, and the resulting power consumption and heat dissipation become serious problems. If you doubt this, think of a smartphone that gets hot after a long period of screen on; the same principle applies to car-mounted displays.

The working principle of backlight sources has been discussed in earlier notes, and improving LED light utilization is a research topic for optical engineers. This is why BEF (Brightness Enhancement Film) and DBEF (Dual Brightness Enhancement Film) have emerged.

BEF (Brightness Enhancement Film)

l Also called a prism film, BEF is a key component of the backlight module.

l It is an optical thin film with precise micro-structures that refract, reflect, and concentrate light.

l A single BEF layer can increase brightness by roughly 40%–60%.

l When two BEF layers are stacked with their prism directions orthogonal (90°), the brightness gain is even higher.

DBEF (Dual Brightness Enhancement Film)

l DBEF is a multilayer reflective polarizing enhancement film.

l It polarizes the backlight: the polarized light P passes through the LCD panel, while the opposite polarization P, which would normally be absorbed by the polarizer, is reflected back into the light source and converted into P. This recycling repeats, boosting efficiency.

l DBEF has two diffusing surfaces, enhancing LCD brightness and visual quality.

l Various configurations exist to meet specific application needs, and DBEF can be combined with BEF and ESR films.

DBEF product series

l DBEF-M: coated with a diffusion layer to prevent optical contact with other materials and to reduce moiré reflections when used together with BEF.

l BEF-D: bonded with diffusion sheets on both sides, increasing mechanical strength and thermal stability.

l DBEF-D200: same structure as BEF-D but only 200μm thick, suitable for space-constrained designs.

Practical notes

l Under identical electro-optical conditions, adding only DBEF to a backlight may actually lower measured brightness. DBEF must be used together with a polarizer to achieve a large brightness increase—typically 1.8× to 2×.

l Example: a backlight measured at 8,000cd/m2 can yield a module brightness of about 15,000cd/m2 when DBEF is applied.

l 3M’s DBEF is trusted for reliability, surpassing that of standard TFT components; information on domestic DBEF products is limited and welcomes contributions from backlight engineers.

l When installing DBEF, its transmission axis must be parallel to the transmission axis of the rear polarizer. This is why a backlight sample must be illuminated and adjusted with a FOG (optical debugging tool) before final assembly.

Why it’s called a “magic tool”

Because the mechanical structure and power consumption of the backlight cannot be changed easily, adding one (or more) enhancement films can meet the high brightness requirements of customers without increasing size, power draw, or heat. This optical solution is therefore regarded as a “magic” way to achieve brighter outdoor displays.