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Introduction to Gamma Curves in LED pixel tapes application

When it comes to Digital RGB/ RGBW Pixel LED tapes, the sheer volume in the variety might be more confusing than helpful that you do not know which IC pixel led tape is more suitable for you. You would always get datasheets of the pixel IC chips to check their exact specs. You would realized that there always have some values like 8 bit, 16 bit. Some datasheets even have Gamma curve of 1.0, Gamma curve of 2.2, etc. Today let's discuss this Gamma curve in LED pixel tapes application. 

1. Understanding Gamma Correction Concept

Gamma is an important but seldom understood characteristic of virtually all digital imaging systems. It defines the relationship between a pixel's numerical value and its actual luminance. Without this gamma, shades captured by digital cameras wouldn't appear as they did to our eyes (on a standard monitor). It's also referred to as gamma correction, gamma encoding,but these all refer to a similar concept. Understanding how gamma works can improve one's exposure technique, in addition to helping one make the most of image editing.

2. Understanding Gamma Curve function

  • 1. Our eyes do not perceive light the way cameras do. With a digital camera, when twice the number of photons hit the sensor, it receives twice the signal (a "linear" relationship). Anyway that's not how our eyes work. Instead, we perceive twice the light as being only a fraction brighter — and increasingly so for higher light intensities (a "nonlinear" relationship).
  • Compared to the camera, we are much more sensitive to changes in dark tones than we are to similar changes in bright tones. There's a biological reason for this peculiarity: it enables our vision to operate over a broader range of luminance. Otherwise the typical range in brightness we encounter outdoors would be too overwhelming.
  • But how does all of this relate to gamma? In this case, gamma is what translates between our eye's light sensitivity and that of the camera. When a digital image is saved, it's therefore "gamma encoded" — so that twice the value in a file more closely corresponds to what we would perceive as being twice as bright.

  • 2. Gamma encoded images store tones more efficiently. Since gamma encoding redistributes tonal levels closer to how our eyes perceive them, a shortage of bits are needed to describe a given tonal range. Otherwise, an excess of bits would be devoted to describe the brighter tones that humans cannot differentiate (where the camera is relatively more sensitive), and too few bits would be left to describe the shadow values that humans are sensitive to (where the camera is relatively less sensitive):

3. Understanding Gamma correcion curve in LED Pixel tapes application

The gamma curve in the LED pixel industry, is in fact the "grey scale/brightness curve". It defines the correspondence between grey scale and luminance. The luminance is converted to the brightness that the human eye actually perceives.

As we discussed above, the human eye's perception of luminance is non-linear, in order to harmonise the mapping between luminance and subjective grey scale perception of the two values, the Gamma correction curve must be used to correct the LED luminance, so that the human eye can obtain a linear visual effect.

The Gamma correction curve value on the LED pixel display is as follows, the smaller the Gamma value, the lower the image contrast, the character white; Gamma value is larger the higher the image contrast, the character more three-dimensional sense. From the curve, Gamma1.0 does not correct the display data, Gamma 2.2 will be able to meet the visual needs of the human eye. 

Currently the LED IC pixel IC chips on the market generally do not have a built-in Gamma correction curve (i.e. GAMMA 1.0) or use data fitting to obtain a segmented Gamma correction curve (GAMMA X). The standard Gamma correction curve allows for a soft display with undistorted colour overlap, but this requires a large data storage unit inside the chip and 16bits of PWM to display the grey scale. The large data storage unit will lead to higher chip costs, while the internal high frequency clock designed to support the 16bits PWM display grey scale also significantly increases chip power consumption, which is the reason why LED pixel IC chipss are generally reluctant to build in standard GAMMA curves.

In order to achieve Gamma correction results while avoiding the complex technical requirements and data storage burden, some LED driver solutions choose the segmented Gamma correction curve mode. The segmented Gamma X correction curve is a good low-cost solution, but it can lead to problems such as loss of grey scale and uneven colour overload, resulting in a distorted display, and is not suitable for LED landscape projects where the display is increasingly demanding.

Here's a chart to compare the three gamma curve:

 Item No Gamma Correction segmented Gamma correction  Standard Gamma Correction
Gamma curve  1.0 Gamma X  2.0 / 2.2 / 2.5 / adjustable
Color Resolution 8 bits 10 bits  16 bits
Gray level  256 1024  65536
PWM Rate normally<2KHZ <2KHZ normally ≥2KHZ
 Example  UCS512C4, TM512AC4 LPD8806  TM512AC, Hi512D, UCS512G6, UCS8903, UCS8904

From this sheet, we can simply say the 16 bits IC chip have the standard Gamma curve of 1.0/2.0/2.2/2.5 adjustable like Hi512D / Hi512E series on 24V DMX512 5-in-1 RGBTW led pixel tape. Some LED IC pixel chips have fixed Gamma curve value like TM512AC is 2.2

Please note: some customers might wondering if there was any relationships with the color resolution and the channel qty? For example, the TM512AC and UCS512C4 have same pin definition, just TM512AC is 16 bits, UCS512C4 is 8 bits. When we make the DMX512 RGBW led strips with these two IC chips, the TM512AC DMX512 RGBW led pixel tapes no need double channel qty than the UCS512C4 version. They are NO any relationships. 

If you have any info that we should be revised, tell us freely.

Best regards from Art LED Team