请教：相对照度要如何设定?

<P ><FONT size=3>如题，像下方相对照度的图形要如何设定（下图是从网络下载来的）</FONT></P><img src="data/attachment/forum/dvbbs/2004-7/200479202148602.gif" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'data/attachment/forum/dvbbs/2004-7/200479202148602.gif\');}" onmousewheel="return imgzoom(this);" alt="" />