Minimum Luminance Contrast for Legibility
Current guidelines for luminance contrast of graphic elements are directed at assuring sufficient luminance contrast for legibility.

Examples:

8.6.2.6.2 Text-background contrast. The contrast between text and its background shall be sufficiently high to ensure readability of the text.
8.6.2.6.3 Color foreground/background difference. In general, the color foreground shall differ from its background by a minimum of 100 delta-E (CIE Yu’v’) distances.
8.6.2.6.4 Contrast. An adequate contrast of at least 7:1 should be maintained between foreground and background colors to enhance color perception and perceived image resolution.

FAA Human Factors Design Standard http://hf.tc.faa.gov/hfds/

This is certainly an important consideration, and such guidelines should be continued and improved. For this purpose the main improvements that are needed are better community agreement on
1) the appropriate statistic(s) for luminance contrast for various graphics and viewing conditions, and
2) the required levels.

Multiple Levels of Luminance Contrast--Managing Attention
Symbols and text that convey high-priority information should have high luminance contrasts. However, if too many graphic elements have high luminance contrast it becomes difficult for the user to locate the most urgent data--the display becomes cluttered.

One solution is to remove some of the information, placing it on another display or another page that can be displayed. This solves the problem of clutter within each page, but it introduces another set of difficult problems relating to the user's management of the several displays/pages and integration of information from several pages in the user's mind.

Another approach that is often useful is to leave less urgent information on the same display, but to give it less graphic emphasis relative to the most critical information. One way to reduce graphic emphasis is to assign less luminance contrast to less critical information.

To use this kind of design we need to choose safe, usable luminance contrasts for each level of urgency. We know of no current guidelines that address appropriate luminance contrasts.

We're working to develop guidance about 1) how many levels of urgency can be effectively distinguished using luminance contrast, 2) what the relative luminance contrasts should be, and 3) what the minimum luminance contrast of the least urgent level should be.