First, it’s not quite correct to say that the user changes brightness and contrast of the monitor with Brightness and Contrast settings, respectively, since a question arises – what brightness is adjusted and at the expense of what the contrast is changed. As I mentioned above, the brightness L of a pixel can be ideally described by the formula: L = B + x*C, where B is a value proportional to the Brightness setting of the monitor, C is a value proportional to the Contrast setting of the monitor and x is the signal the monitor receives from the computer (x=0 for black color and x=maximum for white color). It follows then that the Contrast setting is adjusting the brightness of white color (to be exact, of all shades of grays, but black color remains intact), while the Brightness setting affects both black and white colors at once.
In most monitors the Brightness control is realized through changing the intensity of the backlighting – this is logical enough. The employed cold-cathode fluorescent lamps allow doing this in two ways: by adjusting the discharge current in the lamp (this provides rather a narrow adjustment range since the charge loses stability after the current is far from the nominal) or by a pulse-width modulation of the power of the lamp at a rather small frequency (it’s small considering the physics of the lamp’s charge, but big enough for the eye not to notice it; this frequency is usually from 200 to 500Hz in practice). Pulse-width modulation is a widespread method of controlling voltages and currents. Its key point is such: depending on the desired voltage, the width of the applied impulses is controlled, while their frequency and amplitude remain the same. The average voltage is proportional to this width. The control process is illustrated by the oscillograms below:
This signal was taken directly from the monitor’s screen with the help of a photo-sensor, not from the lamps’ power circuits, so the impulses had been already smoothed out by the afterglow of the lamps’ phosphors, and it’s clearly visible in the animated picture how the average brightness grows up. The distances between the peaks remain the same as the brightness is changing, so we evidently deal with nothing else but pulse-width modulation.
Brightness can also be controlled with the matrix – when the user chooses a higher brightness setting, the monitor adds a definite constant to the signal sent to the matrix. Alas, the contrast ratio degenerates with this method – the backlight lamps are always working at the power necessary to provide the maximum possible brightness, so when working at a small brightness, even if the added constant equals zero, the monitor will produce a higher level of black than a model that controls brightness with the backlight lamps. Suppose the brightness of black is described by the formula Lb=L*nb, where L is the brightness of backlighting, and nb is the pass-through coefficient of the closed pixel.