An essential parameter of LED screens is an ability to display a certain number of colors. All visible colors result from color mixing of basic colors that make up a pixel, typically, Red (R), Green (G) and Blue (B). Apparently, the more colors a screen may render, the higher the image quality, provided that the colors are natural and color transitions are smooth.
Initially, the image is formed on a computer and image quality is evaluated on a PC monitor. The image on a LED screen must be as close to the initial image as possible. A current de-facto standard is a 24-bit color coding (TrueColor) where the brightness value of each channel is presented as a 8-bit number. Thus, ideally, a high quality LED screen should render at least 224colors (or more than 16 million).
We already analysed how the image is generated on an LED screen with the help of PWM technique. The more logical levels are supported by PWM on a given screen, the higher is the image quality.
PWM method establishes a linear dependence between current (average value) and logical level of brightness. Thus, PWM with N levels ensures that the real brightness of LEDs for all these levels will change linearly. In other words, the brightness of an LED with PWM at level 1 will be exactly twice lower that at level 2 and 256 times lower than at level 256.
N logical levels in PWM corresponds to N brightness levels of LEDs on screen and the actual brightness directly depends on the input level. However human eye perceives brightness in a non-linear fashion. The empiric psychophysical Weber-Fechner Law postulates the logarithmic function as a method for human perception of light intensity.
At lower intensity human eye will notice even the insignificant changes in brightness; at higher intensity similar insignificant changes in brightness will go unnoticed. A much bigger change will be needed to register with the eye. Let us imagine all possible brightness levels N (with N being no less than 100) as a horizontal line with 100 segments. The eye will easily perceive adjacent segments of brightness at the beginning of the line, then in the center the difference between adjacent segments will not be evident, and at the end of the line – not noticeable at all. In reality this means that out of the total number of N levels we can select a much smaller number of levels M that will be perceived linearly.
It’s interesting to learn that the image generation on LCD monitors takes this quirk of human perception into account. In CRT-monitors this is a direct consequence of image formation method; in LCD-monitors a hardware gamma correction is utilized. All this results in a relatively linear function that describes the subjective perception and the coded logical level of brightness. Thus, the initial image file has 256 brightness levels (for TrueColor) in each channel that are perceived in a linear fashion.
If PWM with 256 logical levels is used to display an initial TrueColor image, we will notice visible distortions. On dark sections of the image we will see sharp borders between brightness segments, on bright sections all brightness levels will merge. Distortions will affect the colors, especially where a smooth color gradient is essential, e.g. on a picture of a human face. This happens because the initial image uses 256 non-linear levels which are converted on a screen in a linear levels – unsuitable for human eye perception.
For TrueColor image to be rendered on an LED screen with minimum distortions, the logical brightness levels have to be corrected. This can be achieved by increasing the number of logical levels on a PWM-generated image. Then, out of the much larger number of levels it will be easier to select 256 levels that can be coded for a linear perception. This selection of 256 levels out of a much larger number is called “a gamma correction” or “a choice of palette”. The more brightness levels can be generated via PWM, the easier it will be to make a correction so that colors and brightness are perceived in a “proper” linear format.
At present the best LED screens offer PWM with 216 logical brightness levels. This is more than enough to select the necessary 256 levels to display accurate TrueColor image. It should be noted that an LED screen can display 256*256*256 colors, and not 216*216*216; it means that color is still coded by a 24-number, not a 48-bit. The artificially expanded color palette is only necessary to select the minimum necessary number of colors to ensure natural color and brightness perception.
Each channel may be coded in 8, 10, 12, 14–bit or maximum 16-bit brightness. On lower channel codes the correction will naturally be less effective. On the other hand, an LED screen with smaller number of logical brightness levels will be cheaper while accurate TrueColor rendition on screen is not always necessary. For example, LED signage with running letters or informational digital signs do not need huge number of colors.
Wider color range is preferable for several other reasons:
First, it allows to adjust LED screen brightness depending on the time of the day, season of the year, peculiarities of screen site. Human eye adapts flexibly to general lighting conditions; therefore, screen brightness on a sunny day and during the night should be drastically different. Moreover, if a LED screen is installed on the background of the sky, it’s adjustment parameters must differ from a similar screen standing in front of a building or hanging on the wall.
Secondly, it allows for flexible adjustment of white balance. Palettes can be selected independently on each color channel; thus, we can add blue or reduce red in a target image. This is essential for both the initial screen settings and later adjustments that are necessary due to aging of LEDs (LEDs losing brightness with age) or LED lens losing opacity.
Thirdly, it allows some color correction in individual LED screen modules. This may be necessary when some modules fail and are replaced by spares: in this case LED parameters in new and adjacent modules will be different and need tuning.
This function is essential for the control system, too. It is not enough to make 16-bit PWM; manufacturers must have the option of flexible brightness adjustments in the hardware and software of the control system. Otherwise, this important application will have no practical use.