LCD Panels – Backlights & Brightness

For the video display developer LCD panels are available in many sizes and resolutions, they are also available with many choices of maximum brightness. The following considers the topic of LCD panel brightness, the choices, the methods for adjusting brightness and some brightness adjustment scenarios.

LCD panels are generally rated as to their maximum brightness level which is expressed in Nit. In terms of a practical understanding the following is a rough guide:

  • Laptop panels are typically up to 250 nit
  • Desktop monitors and TVs are typically up to around 350 nit
  • Digital signage displays are typically 450 to 500 nit
  • Outdoor displays range from a low end of 700 nit to typically 1,000 or 1,500nit and up with 2,000~2,500nit and even up to 5,000nit seen with some models.

Note: Some documentation refers to CD/m2 (Candela per square meter), CD/m2 and Nit are equivalent.

Adjusting the Backlight

Virtually all LCD panels have a LED backlight these days, these are powered by an LED driver board. Brightness control via the driver board will be by one of two methods:

  • PWM (Pulse Width Modulation): This varies the duty cycle of the backlight “on time” – it is predominant in modern LCD panel LED backlight designs to enable support for digital brightness controls.
  • Analog – Uses a simple variable analog voltage to adjust brightness. Older technology using a potentiometer to control.

One of the advantages of LED for the backlight is the range of adjustment that is possible, however it is important to note that the range varies significantly from model to model. Some industrial panels can be turned to very low light levels making them suitable for use in special environments such as at night. Lower cost panels limit the range of brightness to what might be required for typical usage, whereas panels with full range dimming from full off to full on require more complex backlight drivers.

Why adjust the backlight brightness:

  • Environment: Day bright/cloudy or night.
  • Usage preference: A display may be set to bright for presentations but if working on it close up, ie a touch screen, better to turn it down.
  • Backlight lifetime: Many LCD panels have a backlight lifetime rating of 50,000 hours (typically measured to half brightness), this can be extended by running the LED backlight at a lower brightness level.

Active backlight & local dimming: This is a circuit within the LCD panel that adjusts the backlight brightness according to the image. It may provide local dimming, so the LED’s of the backlight are adjusted to match the content of the area around them, these backlight LED’s will be much lower resolution than the image itself.

Digital View provides…

  • Backlight adjustment: Most Digital View controllers offer both PWM and Analog as a method for adjusting the backlight brightness level.
  • DPMS (Display Power Management System): The backlight will be automatically turned off after a period if there is no valid video signal being received.

Ability to control the LED backlight brightness through:

  • Buttons to access the OSD: Momentary control
  • IR remote
  • RS-232: Command set
  • Ethernet: RS-232 command set. Ethernet connection is built in to all SVX models and available to other models via the IP-60 add-on board.

Autobright+: A compact accessory add-on board designed to work with Digital View LCD controllers to enable automatic backlight adjustment according to ambient lighting conditions.

The Digital View Autobrite+ board supports the following settings:

  • Multiple ambient light thresholds, at which the backlight brightness is adjusted.
  • Frequency of sampling the lighting conditions,
  • A delay period before executing backlight adjustment,

A connected sensor (included in the kit) monitors ambient light levels at 10 second intervals, the Autobrite+ board uses serial (RS-232) commands to adjust the backlight via the controller.

Digital View typically provides a standard settings board that allows for fine tuning of the ambient light level via an onboard pot. Other configurations are possible on a custom basis.

Scenarios

Day / Night:

  • Digital Signage: Brightness adjusted according to an ambient light sensor (Autobrite+), an external timer, an instruction from a media player.
  • Transport & outdoor monitors: Triggered by ambient light sensors (Autobrite+), or switching to headlights, or manually.
  • Color, color temperature etc: In addition to adjusting the brightness other settings may be adjusted as well. For example the color temperature or for example a switch to green monochrome for night vision.

Room / Office: Presentation display(s) installed in the room

  • Auto on/off if lights on/off or if motion detected. Ambient light sensor (Autobrite+) or motion detector.
  • Auto-dim/bright if wanting to focus on another display. This might be triggered by a room controller.
  • Auto-dim if lights dimmed for a projector. This might be triggered by a command from a room sensor or automatically by an ambient light sensor (Autobrite+).

brightness

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