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Bright LCDs

Bright LCDs

Mar 13th 2020


One of the most popular questions engineers working on new product design ask is how to increase the brightness of an LCD.

Their display needs to be brighter to compete with the sun, to be seen from a long distance, or to alert people in case of an emergency.

Before we go through each display type and how to increase its brightness, we need to explain a few key terms: Nits, half-life, sunlight readability, and hot spots.

Nits:

One nit is the amount of light generated by one candle. The brightness of an LCD is measured in nits.

Half-Life:

This is the amount of time (in hours) before the backlight is ‘half’ as bright as when it was first turned on. This is not the amount of time for the backlight to burn out. Most LED backlights have a half-life of 50K to 90K hours depending on how they are driven.

Sunlight Readability:

The backlight needs to be bright enough to overpower the sun (good luck on that one). This requires between 700 to 1K Nits.

Hot Spot:

No, this hotspot is not the popular place to meet new and exciting people.

LCD hot spots occur when the LED sits too close to the top polarizer and creates spots of high brightness and places of darkness.

Most of the time, hot spots can be eliminated, or greatly reduced, with the correct polarizer. 

Two Types of Display Technology:

1. Monochrome Displays:

Monochrome modules provide one color background and one-color icon/segment/letter.

The three types of monochrome LCDs are: Segment, Character, and Graphic Displays.

Method 1: Increasing Current:

Decreasing the current limiting resistor increases the LED backlight’s brightness. The lower the resistor value, the more current passing through the LED. The more current, the shorter its half-life.

This is not a recommended solution since the backlight could grow dim within the first year.

Also, the more current, the faster your battery will drain.

Method 2: Transmissive Polarizer

A polarizer sits behind the LCD’s glass. A polarizer can reflect ambient light or allow the backlight to pass through depending on the type. There are three types of polarizers. Transmissive polarizers produce the brightest light.

Transmissive:

Transmissive provides the brightest backlight, but the backlight must be on to make the display readable.

Reflective:

Reflective is like a mirror and best for sunlight readability. However, a backlight cannot be placed behind it because the polarizer will block the light from coming through.

It is possible to build this as a side-lit (also called edge-lit) display by placing LEDs above the polarizer and around the perimeter of the LCD. This works great for smaller displays but can produce hot spots in larger sized displays.

Transflective:

Transflective polarizers allow light from the backlight to pass through, while also reflecting some ambient light. Displays with transflective polarizers can have their backlight turned off during the daytime to decrease power consumption and it will still be readable.

Method 3: PWM (Pulse Width Modulation)

This is a great option. The backlight is brighter and battery life won’t suffer. Here is a good article on how PWM works. 

Method 4: Adding More LEDs

The additional charge to add LEDs is low and the one-time tooling fee (NRE) is low.

More LEDs create an even glow but draw more current and shorten battery life.

2. Color Displays:

Most color displays are TFT (Thin Film Transistor) or OLED (Organic Light Emitting Diode). We will only cover TFT in this article.

TFTs can generate between 64K and 64M unique colors. Depending on their interface, they can display videos, characters, icons etc. The majority of TFTs are transmissive, when means the backlight must be on to be readable.

Most TFTs have a standard brightness between 250 to 500 Nits but can be increased with modifications.

The best option to increase brightness is to add LEDs. The display will be brighter, but it will shorten battery life.


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