Iris Control is used to adjust the light output of the display through the use of a lens-aperture.
The goal of iris control is to manage the level of light produced by a lamp or backlight-based display device so that it can be flexibly adjusted to suit a range of viewing environment characteristics, including throw distance, screen size/gain, room ambient lighting, etc.
There are two variations on iris control. One type is an adjustable f-stop iris integrated into the lens assembly, similar to the lens aperture control found on an single lens reflex camera. With an f-stop system, the level of light can be set to match the viewing conditions. A second variation on iris control is to selectively block light at the lamp source it before enters into the imaging mechanism. This type of iris would typically be driven by a n electronic circuit that is configured to reduce the light output for images that are dark, but allow higher light levels for bright images. Such a control is typically referred to as a dynamic iris.
A dynamic iris should produce a better contrast ratio than a fixed iris. Unfortunately the resulting higher contrast ratios come at a cost -- constant manipulation of black levels. Imaging science is based on peak light output and black level remaining the same for a consistent gamma response, regardless of image content; a dynamic iris does just the opposite.
Like many other "features," a dynamic iris is an enhancement to improve your perception -- but its goal is to manipulate the image. Sometimes the effect can be helpful, but at other times it negatively impacts picture quality. Displays that incorporate a dynamic iris should also provide a user-accessible on/off feature, so that the dynamic iris can be eliminated if the viewer determines it causes more harm than good.
As an alternative to iris control on projection-style displays, a neutral density filter can be fitted into the light path.