How do we choose and calibrate a display system to work well in a given environment?

An excellent place to start is to consider the factors that are recognised as important in obtaining a high-quality image.

The Imaging Science Foundation (ISF) defines the hierarchy of image quality parameters as:
• Contrast ratio (dynamic range);
• Colourful (colour saturation);
• Lifelike colours (correct colours);
• Sharp details (resolution).

Note that the contrast ratio, or ‘dynamic range’, of the display is the most important parameter. It is also the parameter most likely to be affected by the ambient environment.

The contrast ratio is the relationship between the black level and the brightest white level. Typically this is quoted as an on/off ratio where the black level is measured with an all-black screen and the white level is measured with a separate white image.

This intra-scene (or scene to scene) ratio is actually not important.

The main factor is the in-scene contrast ratio, or the variation from dark to bright. It is something that can be determined through a single image. This is can also be expressed by the ANSI contrast ratio.

ANSI ratios are typically much lower than the on/off contrast ratio and are rarely quoted.

Further, the ANSI ratio of a system greatly depends on the environment and, as you will soon see, it is the role of the system designer and calibrator to maximise the in-scene ratio.

The contrast ratio can be improved by increasing the brightness of the image or by reducing the brightness of the black level.

Rather than being concerned with the measured image brightness, what matters are the perceived levels and contrast ratio. To work this out we need to consider how the human eye perceives image brightness.

The human eye has a logarithmic response, with a gamma of about 2.3. Put simply, we are far more sensitive to changes in black or dark details than bright.

This is analogous to human hearing, which is very sensitive to volume changes for quiet sounds yet very large changes in measured volume are required for a noticeable change at loud levels.

As a consequence, our efforts to improve the black level will be far more effective.

In a projection system, the black level is ultimately determined by the environment. A projector cannot project black.

With the high performance of modern projectors it is typically the environment that limits the black level. This is especially the case when considering the all-important in-scene contrast ratio.

In an image with bright and dark areas, the light from the bright area will strike the area around the screen and reflect back onto it, washing out the darker areas and reducing the in-scene contrast ratio.

Ideally, the immediate area surrounding the screen should have the lowest possible level of reflectance.

A black flock-lined cove surrounding the screen has a dramatic effect on the in-scene contrast, yet it still allows contemporary styling and colours for the rest of the room (see main image).

The choice of screen material can also affect the in-scene contrast. Screens with a high level of unwanted light rejection, reflected and incidental, are desirable if you do not have sufficient control over the environment. Typically, these are grey screens, though screen technology is constantly evolving.

At the opposite end of the scale, a low-cost screen can actually degrade the image contrast without any outside influences.

The role of a screen is to scatter the incidental light in all directions. A low-quality screen can scatter the light along the surface of the screen, again causing the bright areas to wash out the dark.

With the poorest screens the light can enter the fabric, causing it to glow and wash out the darker areas.

The optics, particularly the lenses, in a projection system can also scatter the light internally and reduce the in-scene contrast. This is yet another factor that is rarely quoted, but high-quality lenses and optics will have this parameter as a main design factor.

If a system uses a flat panel LCD or plasma display your perception of black will still be limited by the environment. The issue here is not so much the effect of light directly on the screen but the effect of the ambient lighting on our ability to see darker details of the image on screen.

The human eye has an impressive dynamic range – the ability to see over a huge variation of light levels. This is primarily achieved through the iris.

In a bright room the iris is closed and sensitivity to dark areas of an image is reduced. In this case the calibration of the black level is important to ensure that every detail in the original image is viewable in the chosen ambient situation.

The brightness control on a display adjusts the brightness of the black, rather than the brightness of the image. It really should be called the ‘blackness control’, but this terminology has stuck since the first days of television.

Set the brightness control too high and the black level will be higher than the display and environment requires, this compresses the difference between the blacks and whites, reducing the contrast ratio.

Set the brightness control too low and darker areas of the image won’t be visible.

A test pattern from a calibration disk such as Digital Video Essentials can be used to correctly set the brightness control for the ambient environment.

Displays that are ISF calibration configuration and control (ISFccc) certified will have an ISF ‘night’ and ISF ‘day’ memory that can be used for storing settings for different ambient lighting conditions.

These settings can be recalled by the lighting control system, or based on the time of day, to get an optimum result for your customer.