LCD Hot Spots Explained
A picture is worth a thousand words, so here is a picture of a LCD with hot spots.
LCD hot spots produce an uneven flow of LCD lighting and occur when areas of the LCD are very bright (hot), making other areas dim (cold). This only happens on monochrome displays such as character displays and custom segment technology.
These nasty, annoying hot spots occur when the LED is too:
- Bright (over driven/too much current)
- Close to the edge of the glass (edge-lit/ aka side-lit)
- Close to the rear diffuser. (back-lit)
This tends to occur more with white LED’s than any other color.
Hot spots were not an issue until LEDs replaced older EL (used on smaller displays) and CCFL (used on larger displays) technology.
These older technologies produced an even flow of light, eliminating the need for a diffuser. But now all displays use LEDs and care must be taken when designing and building a custom LCD module.
Note: El and CCFL backlights fell by the wayside due to their short half-life of 3K hours compared to the LEDs half-life of 70K hours.
Half-life: Is the amount of time for the display to become half as bright as when it was first turned on.
Two ways to avoid hot spots:
Don’t modify the current limiting resistor for the LED backlight (the lower the resistance, the brighter the backlight and the shorter the half-life).
Add more LEDs and reduce their brightness to produce an even flow of light.