CRTs are analog devices controlled by the varying voltages in the signal. There are also two controls for each of the three "beams" coming off the electron gun: bias and gain. When the three bias controls and gain controls are lumped together, you have contrast and brightness. All of these together control the floor and ceiling of the amplification (black and white luminance) as well as how quickly the luminance of the display increases from black to white. These are the controls most users know.
LCDs are digital devices and are a completely different animal. When run digitally, there is no bias or gain and in some instances...no contrast adjustment. The only variation is the intensity of the backlight. This is how LCDs connected to a computer through a DVI or ADC cable will operate. Unfortunately, digital interfaces for displays did not exist when LCDs were first introduced, so manufacturers tried to graft on analog controls since an analog signal was being used. This has caused mixed results. In most cases, setting brightness and/or contrast too high on an LCD with an analog connection will introduce clipping of the lighter tones.
Our goal is to achieve the highest contrast ratio and luminance without introducing clipping. We are measuring the luminance of a white patch and a very light gray patch and checking to see if there is an appropriate difference between them. We have found that some displays (usually laptops) will not produce a large enough luminance difference between the two patches regardless of the contrast setting. You can do this test visually. The alternating patches will be displayed continuously. If you can see them change with contrast all the way up, you can proceed. If not, turn the contrast down until you can see the difference and then proceed.
LCDs respond rather slowly to contrast and brightness adjustments and may take some time to stabilize. Since they change over time, this can cause the luminance of two sequential measurements to be greater or smaller, thus causing the indicator to move. That's why we recommend waiting for the indicator to stabilize. When the indicator stabilizes, the display probably has as well.