Marketing departments love numbers that they can quote in the sales literature. Car companies will give you fuel consumption figures and acceleration times. Monitor manufacturers quote bit depths and look up table resolutions. In this article I’ll try and explain what these numbers mean and why they matter.
Panel Colour Depth
All information in a computer is stored as bits. A single digit denoting a one or a zero. Colour is no different from any other kind of data and is stored and processed in bits. A 1-bit image (where each pixel is defined by just one bit) is black and white. Colour images usually have their colour depth defined by how many bits per colour channel there are. So a typical image may have 8-bits each for red, green and blue making a total of 24 bits for each pixel but you call it 8-bit and not 24-bit. Eight bits allows for 256 values (2 to the power 8). And if you cube 256 (for three channels) you get the 16.7 million colours that an 8-bit RGB image can theoretically contain.
An LCD panel is made up of pixels, and the possible colour values in each pixel are also defined in bits. Most LCD panels are quoted as being 8-bit. So they can display the 16.7 million colours that implies. However, some cheaper screens are actually 6-bit and make up the other 2 bits by using something called Frame Rate Control. Very simply FRC is a method of pixels flicking between two colour values very quickly to so that the user sees the illusion of a third colour value. It usually works pretty well for most purposes but if you are looking for a monitor for image editing you should choose one that’s true 8-bit.
FRC is also used to extend an 8-bit to support 10-bit display. 10-bit displays can display over a billion colours and is a bit of buzz word loved by marketeers in monitor companies but few users can or need to use 10-bit. For more info on 10-bit see this article. Most 10-bit LCD panels are 8-bit plus FRC but some of the more expensive ones are true 10-bit.
Graphics Card Look Up Tables
So, your image might be 8-bit and your LCD panel is probably 8-bit but to get from the image to the pixels on the panels the 256 values in each colour channel have to go through a series of look up tables. The first of these are on the graphics card of the computer. If you use graphics card controls to adjust the picture on your monitor then those adjustments are made in the graphics card look up table. Graphics card LUTs are typically 8-bit. Think of a look up table like a simple X, Y graph with 256 possible values along each axis. With X representing an input value and Y an output. A line from zero to 255 at 45 degrees would mean that each and every value input into the graphics card from the computer operating system and application software is output unchanged. But if you make any change in the graphic card controls then the line isn’t 45 degrees any more it will take on a curve so that, for example a value of 128 is mapped to 132, an so on.
The same is true if you use a standard monitor calibration solution. Adjustments are made in the graphics card look up tables to get the monitor to a specific tone curve or colour temperature. The problem with graphics card LUT adjustments is that they typically reduce the number of tones sent to the monitor. For example, if you want to make the white point of your monitor 6500K and the actual native white point colour temperature is 7000K then the blue channel needs to be reduced, so a curve will be put into the LUT and this could drop the top value from 255 down to 245 to reduce the intensity of the blue. So instead of having a possible 256 tones in the blue channel you now have only 246 (0 is a value). This may not seem like much of change but it cuts around 650,000 from the total of 16.7 million possible colours on the display. Good monitor calibration solutions allow you to minimise these changes in the graphics card LUT by letting you interactively measure and adjust the monitor using it’s own brightness, contrast and colour controls. And this brings us nicely to the next LUT in the chain.
The only physical adjustment you have on a LCD display is the brightness as this adjusts the intensity of the back light illumination. Contrast and colour controls all adjust the internal look up tables in the monitor software. Cheaper monitors have 8-bit LUTs just like the graphics card. So if you make radical adjustments then you can again radically reduce the amount of colours that can be displayed. However, better monitors have 14 or 16-bit LUTs (65,536 values on each axis and not 256) so that each of the original 256 input values can be adjusted but the output can still have 256 different values because of the high resolution of the look up table. So 128 could be mapped (very simplistically) to 128.5. I know it all sounds very complex but the practical result is that monitors with high bit look up tables can be adjusted more closely to calibration targets and achieve smoother rendition of colours and tones.
To confuse things even further some monitors have 3D look up tables. This means that instead of three individual X,Y graphs it converts the values using a three dimensional cube of red, green and blue. This makes it much easier to get good neutral values all the way up and down the tonal range and also improves the ability of the monitor to mimic other colour spaces. So high bit LUTs are better. 3D high bit LUTs best for accuracy and smoothness of colour reproduction.
Monitors that are advertised have having hardware calibration capability come with their own calibration software that can adjust the high bit, often 3D, LUTs in the actual monitor directly via a USB cable or other communication from computer to monitor. No adjustment is done in the graphics card LUT so you still have the full 16.7 million colours to play with and can get the best tonal rendition, smoothest gradations, best shadow and highlight detail and generally much more accurate colour. EIZO ColorEdge and NEC SpectraView screens all come with their own calibration software and have high bit LUTs, with the top models having 3D LUTs.
LUTs and bit depth do have an important influence on the quality of image on a monitor and they are one factor in how prices progressively go up from basic, to average to specialist computer monitors. Cheaper screens have low panel bit depths and lower resolution LUTs. Screens specifically designed for colour imaging will have higher bit depths and higher resolution, often 3D, LUTs.
As ever, if I have made things totally clear or you have any questions then please post a comment or get in touch.