The look-up table (LUT) is a key factor in an LCD monitor's ability to display tonal grades and transitions. While these are a step beyond what average users consider when choosing products, they have a significant impact on colour reproduction. Users are well-advised to understand these factors, especially when choosing an LCD monitor for colour-intensive applications like photo retouching or design work.
EIZO says that a LUT is a table containing the results of calculations. It converts the mathematical colour display capability values for each pixel into a colour palette or colour map to determine the colours and intensity values with which they can appear on screen simultaneously for a particular image.
When a system needs to process a standardised calculation, improvements in performance can be achieved by having it look up a LUT value instead of performing the calculation.
In the context of LCD monitors, the term LUT refers to a component that calculates input signals from the PC (at eight bits per RGB colour) and maps them to output signals suited to the LCD monitor (also at eight bits per RGB colour).
An inexpensive LCD monitor will employ an LUT table with eight bits per RGB colour while an LCD monitor designed for colour reproduction applications will incorporate an LUT with more than eight bits (i.e., 10 or 12 bits) per RGB colour and employ internal calculations at 10 or more bits to map input signals to output signals.
So how does a LUT with more than eight bits improve display quality? Well results show significantly smoother tonal transitions and improvements in hue divergence by improving the gamma curve of each RGB colour in the output. A 12-bit LUT, for example, generates approximately 16.77 million optimal colours from roughly 68 billion, improving colour reproduction and gradation beyond even a 10-bit LUT.
Next, let's look at calculations for multi-gradation of an eight-bit per RGB colour input signal at 10 or more bits per RGB color within an LCD monitor. Even if we use a 10- or 12-bit LUT, calculating multi-gradation at 14 or 16 bits will result in even more precise final tonal transitions. The need for 16-bit precision when the final output is only eight bits may not be obvious, but particularly when we seek to depict subtle differences between colours at a low-gradation gamut (shadow gamut), the precision of internal calculations is extremely important. In essence, the higher the number of bits used in the internal calculations, the closer the gamma curve in a low-gradation gamut to the theoretical curve.
A look at the current range of LCD monitors shows that even in lower-cost categories, growing numbers of products offer 10-bit LUTs. Nevertheless, only products at the top of their class have bit counts greater than LUT bit counts. In particular, models that process colours to the most stringent requirements, using 12-bit LUTs and 14- or 16-bit internal calculations, are ideal for colour management use, targeting applications that require high-performance color.
Visual comparisons of a monitor employing an eight-bit LUT and eight-bit internal calculation with one incorporating a LUT of 10 or more bits and internal calculations of 10 or more bits shows unexpected differences. Since the latter class of products tends to feature high-performance ICs for image control, differences in picture quality are likely to be even more apparent to a discerning viewer than for entry-level products associated with inconsistent performance. When we examine the grayscale chart, models with higher bit-count LUTs and internal calculations tend to produce smoother tonal transitions and better representation of tones in shadow areas. Such products have almost no tone jumps or hue divergence and offer stable contrast in which lightness and darkness in gradation are depicted naturally. For all these reasons, it's recommend that a product with at least a 10-bit LUT is used and this is not just for applications that require high-fidelity colour reproduction, but for ordinary PC users seeking better picture quality, too.
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